WO2005079207A2

WO2005079207A2 - Boron carbide composite bodies, and methods for making same

Info

Publication number: WO2005079207A2
Application number: PCT/US2004/039319
Authority: WO
Inventors: Michael K. Aghajanian; Allyn L. Mccormick; Bradley N. Morgan; Anthony F. Liszkiewicz Jr.; Jeffrey R. Ramberg; David W. Mckenna
Original assignee: M Cubed Technologies, Inc.
Priority date: 2003-11-25
Filing date: 2004-11-23
Publication date: 2005-09-01
Also published as: JP4945245B2; WO2005079207A3; JP2007513854A; WO2005079207A9

Abstract

A boron carbide composite body produced by an infiltration process that possesses high mechanical strength, high hardness and high stiffness has applications in such diverse industries as precision equipment and ballistic armor. In one embodiment, the composite material features a boron carbide filler or reinforcement phase, and a silicon carbide matrix produced by the reactive infiltration of an infiltrant having a silicon component with a porous mass having a reactable carbonaceous component. In an alternate embodiment, the infiltration can be caused to occur in the absence of the reactable carbonaceous component, e.g., to produce 'siliconized boron carbide'. Potential deleterious reaction of the boron carbide with silicon during infiltration is suppressed by alloying or dissolving a source of boron, or a source of carbon, or preferably both boron and carbon into the silicon prior to contact of the silicon infiltrant with the boron carbide. In a preferred embodiment of the invention related specifically to armor, good ballistic performance can be advanced by loading the porous mass or preform to be infiltrated to a high degree with one or more hard fillers such as boron carbide, and by limiting the size of the morphological features, particularly the ceramic phases, making up the composite body. The instant reaction-bonded boron carbide (RBBC) composite bodies are at least comparable in ballistic performance to current boron carbide armor ceramics but feature lower cost and higher volume manufacturing methods, e.g., infiltration techniques.

Description

TITLE Boron Carbide Composite Bodies, and Methods for Making Same

CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This patent document claims the benefit of Commonly Owned U.S. Provisional Patent Application No. 60/524,916, filed on November 25, 2003. This patent document furthermore is a Continuation-in-Part of Commonly Owned U.S. Patent Application Serial No. 10/271,312, filed on October 15, 2002, which is a Continuation-in-Part of Commonly Owned U.S. Patent Application Serial No. 09/990,175, filed on November 20, 2001, and now allowed, and which claims the benefit of Commonly Owned U.S. Provisional Patent Applications 60/329,358, filed on October 15, 2001, and 60/252,489, filed on November 21, 2000. The entire disclosure of each of these commonly owned patent applications is expressly incorporated by reference.

TECHNICAL FIELD [0002] This invention relates to metal-ceramic composite bodies produced by a metal infiltration process, e.g., silicon infiltrated composite bodies. More particularly, the invention relates to reaction-bonded and siliconized composite bodies having a boron carbide filler or reinforcement, and to ballistic armor structures produced from such boron carbide composite bodies. The instant composite bodies are also extremely rigid, which in combination with their low specific gravity potential makes them attractive candidate materials for applications in precision equipment such as machines used to fabricate semiconductors.

BACKGROUND ART 1. Discussion of Related Art of Others [0003] In many applications, weight is not a critical factor, and traditional materials such as steel can offer some level of protection from airborne threats such as ballistic projectiles and shell fragments. Steel armors offer the advantage of low cost and the fact that they also can serve as structural members of the equipment into which they are incorporated. In recent decades, certain hard ceramic materials have been developed for certain armor applications. These ceramic-based armors, such as alumina, boron carbide and silicon carbide provide the advantage of being lighter in mass than steel for the same ballistic stopping power. Thus, in applications in which having an armor having the lowest possible mass is important, such as (human) body armor and aircraft armor, low specific gravity armor materials are called for. The lower the density, the greater the thickness of armor that can be provided for the same areal density. In general, a thick armor material is more desirable than a thinner one because a greater volume of the armor material can be engaged in attempting to defeat the incoming projectile. Moreover, the impact of the projectile on a thicker armor plate results in less tensile stress on the face of the plate opposite that of the impact than that which would develop on the back face of a thinner armor plate. Thus, where brittle materials like ceramics are concerned, it is important to try to prevent brittle fracture due to excessive tensile stresses on the back face of the armor body; otherwise, the armor is too easily defeated. Rather, by preventing such tensile fracture, the kinetic energy of the projectile perhaps can be absorbed completely within the armor body, which energy absorption manifests itself as the creation of a very large new surface area of the armor material in the form of a multitude of fractures, e.g., shattering. [0004] U.S. Patent No. 5,372,978 to Ezis discloses a projectile-resistant armor consisting predominantly of silicon carbide and made by a hot pressing technique. Up to about 3 percent by weight of aluminum nitride may be added as a densification aid. The finished product features a microstructure having an optimal grain size of less than about 7 microns. Fracture is intergranular, indicating energy-absorbing crack deflection. Moreover, the economics of manufacturing are enhanced because less expensive, less pure grades of silicon carbide can be used without compromising the structural integrity of the material. [0005] U.S. Patent No. 4,604,249 to Lihleich et al. discloses a composition particularly suited for armoring vehicles. The composition is a composite of silicon carbide and steel or steel alloy. Silicon and carbon particulates, optionally including silicon carbide particulates, are mixed with an organic binder and then molded to form a green body. The green body is then coked at a maximum temperature in the range of about 800°C to about 1000°C. The temperature is then rapidly raised to the range of about 1400°C to about 1600°C under an inert atmosphere of at least one bar pressure. In this temperature range, the silicon and carbon react to form silicon carbide, thereby producing a porous body. The pores are then evacuated in a vacuum chamber, and the body is immersed in molten steel or steel alloy. The metal fills up the pores to produce a dense composite armor material. [0006] U.S. Patent No. 3,725,015 to Weaver discloses composite refractory articles that, among other applications, have utility as an armor material for protection against penetration by ballistic projectiles. These compositions are prepared by cold pressing a mixture of a powdery refractory material (which could be boron carbide) and about 10 to 35 parts by volume of a carbon containing substance, such as an organic binder material or elemental carbon carbonaceous material to form a preform, heat-treating the preform to convert the carbonaceous material to carbon, and then contacting the heated preform with a molten metal bath, the bath containing at least two metals and maintained at a temperature between 1700°C and 1900°C. The molten metal infiltrates the preform, the refractory material matrix sinters and at least one of the metallic constituents reacts with the carbon to produce a metal carbide. Because the thermal expansion coefficient of the metal mixture is close to or slightly greater than that of the refractory matrix, the composite shape cools to room temperature essentially free of cracks and residual stress. Weaver states that, while there are no rigid particle size parameters except those dictated by the properties desired in the final product, a maximum size of about 350 microns for the particles of the powdered materials that make up the mixture to be pressed is preferred. Further, he recommends adding to the metal mixture the same metal as the metal constituent of the refractory material. For example, he says that if boron carbide is the refractory material, the incorporation of about 6% of boron in the molten metal mixture prevents the dissolution of boron out of the boron carbide. [0007] U.S. Patent No. 4,104,062 to Weaver discloses a high density, aluminum- modified boron carbide composition that is well suited as protective armor against ballistic projectiles. About 70 to 97 percent by weight of boron carbide powder is blended with about 3 to about 30 percent of aluminum powder. A temporary binder is added to this mixture, and a preform is pressed. This preform is then hot pressed in an oxygen-free atmosphere at a pressure of at least 500 psi (3.5 MPa) at a temperature of from 1800°C to about 2300°C. [0008] U. S. Patent No. 3,857,744 to Moss discloses a method for manufacturing composite articles comprising boron carbide. Specifically, a compact comprising a uniform mixture of boron carbide particulate and a temporary binder is cold pressed. Moss states that the size of the boron carbide particulate is not critical; that any size ranging from 600 grit to 120 grit may be used. The compact is heated to a temperature in the range of about 1450°C to about 1550°C, where it is infiltrated by molten silicon. The silicon is not stated as containing any dissolved boron or carbon. The binder is removed in the early stages of the heating operation. The silicon impregnated boron carbide body may then be bonded to an organic resin backing material to produce an armor plate. [0009] U.S. Patent No. 3,859,399 to Bailey discloses infiltrating a compact comprising titanium diboride and boron carbide with molten silicon at a temperature of about 1475°C. The compact further comprises a temporary binder that, optionally, is carbonizable. Although the titanium diboride remains substantially unaffected, the molten silicon reacts with at least some of the boron carbide to produce some silicon carbide in situ. The flexural strength of the resulting composite body was relatively modest at about 140 MPa. A variety of applications are disclosed, including personnel, vehicular and aircraft armor. [0010] U.S. Patent No. 3,796,564 to Taylor et al. discloses a hard, dense carbide composite ceramic material particularly intended as ceramic armor. Granular boron carbide is mixed with a binder, shaped as a preform, and rigidized. Then the preform is thermally processed in an inert atmosphere with a controlled amount of molten silicon in a temperature range of about 1500°C to about 2200°C, whereupon the molten silicon infiltrates the preform and reacts with some of the boron carbide. The formed body comprises boron carbide, silicon carbide and silicon. Taylor et al. state that such composite bodies may be quite suitable as armor for protection against low caliber, low velocity projectiles, even if they lack the optimum properties required for protection against high caliber, high velocity projectiles. Although they desire a certain amount of reaction of the boron carbide phase, they also recognize that excessive reaction often causes cracking of the body, and they accordingly recognize that excessive processing temperatures and excessively fine-grained boron carbide is harmful in this regard. At the same time, they also realize that excessively large-sized grains reduce strength and degrade ballistic performance. , [0011] Each of the above-described armor inventions suffers from one shortcoming or another. Hot pressing is expensive and shape-limited. Hot pressed or sintered ceramics do not hold dimensional tolerances as well as reaction-bonded silicon carbide ("RBSC"). Iron matrix composite materials are heavy in relation to ceramic armors. The prior RBSC armors having a boron carbide reinforcement, sometimes referred to in this document as "reaction-bonded boron carbide" or "RBBC", lose some of the boron carbide, particularly the finer particle sizes, due to reaction with the silicon infiltrant, as well as yield significant coarsening of the microstructure, as will be shown in more detail below. Even at the relatively low temperature of 1475C, boron carbide can chemically react with molten silicon. As for the composites containing SiC filler, an infiltration temperature of 2200°C is too high, and will likely result in exaggerated grain growth, also deleteriously coarsening the microstructure. [0012] As the preceding synopsis of the patent literature indicates, silicon carbide composites and boron carbide composites made by a silicon infiltration process have been proposed and evaluated as candidate armor materials decades ago. [0013] In the Third TACOM Armor Coordinating Conference in 1987, Viechnicki et al. reported on the ballistic testing of a RBSC material versus sintered and hot pressed silicon carbide materials. Not only was the RBSC substantially inferior to the other silicon carbides, Viechnicki et al. came to the general conclusion that purer, monolithic ceramics with minimal amounts of second phases and porosity have better ballistic performance than multiphase and composite ceramics. (D.J. Viechnicki, W. Blumenthal, M. Slavin, C. Tracy, and H. Skeele, "Armor Ceramics - 1987," Proc. Third TACOM Armor Coordinating Conference, Monterey, CA (U.S. Tank-Automotive Command, Warren, MI, 1987) pp. 27-53). [0014] Accordingly, in spite of the price advantage of RBSC relative to sintered or hot pressed silicon carbide, what the market has preferred has been a sintered or hot pressed monolithic ceramic product. In fact, according to some sources, in recent years RBSC had developed a reputation as not being worthy of serious consideration as an armor material. Until the introduction of the assignee's products, there had been little or no RBSC armor on the market. [0015] The details of a ballistic impact event are complex. One widely held theory of defeating a ballistic projectile is that the armor should be capable of fracturing the projectile, and men erode it before it penetrates the armor. Thus, compressive strength and hardness of a candidate armor material should be important. The above-mentioned armor patent to Taylor et al., for example, suggests a correlation between strength and ballistic performance. They noted that when the size of the largest grains exceeded 300 microns, both modulus of rupture and ballistic performance deteriorated. Keeping the size of the boron carbide grains below about 300 microns in diameter permitted their reaction-bonded boron carbide bodies to attain moduli of rupture as high as 260 MPa, and they recommended that for armor applications the strength should be at least 200 MPa. [0016] There seems to be a consensus in the armor development community that hardness is indeed important in a candidate armor material, and in particular, that the hardness of the armor should be at least as great as the hardness of the projectile. As for the strength parameter, however, those testing armor materials have had a difficult time correlating mechanical strength (both tensile and compressive) with ballistic perfoπnance. In fact, except for hardness, there seems to be no single static property that functions as a good predictor of good armor characteristics in ceramic materials. Instead, the guidance that has been provided from the armor developers to the materials developers based upon actual ballistic tests has been that candidate armors in general should possess a combination of high hardness, high elastic modulus, low Poisson's ratio and low porosity. (Viechnicki et al., p. 32-33) [0017] The instant inventors have re-visited RBSC, and even more particularly, RBBC as a candidate armor material because they believe that such a material can be developed whose anti-ballistic performance is competitive with other armor ceramics, such as the hot pressed armors, but at reduced cost. i

2. Discussion of Commonly Owned Patents [0018] U.S. Patent No. 6,503,572 to Waggoner et al., teaches that reaction-bonded or reaction-formed silicon carbide bodies may be formed using an infiltrant comprising silicon plus at least one metal, e.g., aluminum. Modifying the silicon phase in this way permits tailoring of the physical properties of the resulting composite, and other important processing phenomena result: Such silicon carbide composite materials are of interest in the precision equipment, robotics, tooling, armor, electronic packaging and thermal management, and semiconductor fabrication industries, among others. Specific articles of manufacture contemplated include semiconductor wafer handling devices, vacuum chucks, electrostatic chucks, air bearing housings or support frames, electronic packages and substrates, machine tool bridges and bases, mirror substrates, mirror stages and flat panel display setters. [0019] U.S. Patent No. 6,609,452 to McCormick et al. teaches that a fine-grained reaction-bonded composite material can provide excellent ballistic properties, particularly against small arms fire. By "fine-grained" what is meant is that no more than about 10 percent by volume of the moφhological features making up the microstructure of the composite material should be permitted to be much above about 100 microns in size. The composite material preferably is highly loaded in one or more hard reinforcement substances, with silicon carbide being particularly preferred. [0020] The teachings of these commonly owned U.S. Patents are incoφorated herein by reference.

DISCLOSURE OF THE INVENTION [0021] It is an object of the instant invention to produce a composite material that is lightweight, stiff, strong and substantially pore-free. [0022] It is an object of the instant invention to produce a composite material that has utility in precision equipment and nuclear power applications. [0023] It is an object of the instant invention to produce a composite material by a silicon infiltration process that features a significant fraction of boron carbide. [0024] It is an object of the instant invention to produce a reaction-bonded boron carbide composite material in which chemical reaction of the boron carbide phase with the molten silicon infiltrant during processing is attenuated or suppressed. [0025] It is an object of the instant invention to produce a silicon-infiltrated boron carbide composite material that, due to attenuation or chemical reaction between boron carbide and silicon, features a smaller or finer grain size of the boron carbide phase than would be possible absent the diminution in chemical reaction. [0026] It is an object of the instant invention to produce a ballistic armor' whose ballistic performance at least approaches that of commercially available ceramic armors such as alumina or hot pressed boron carbide. [0027] It is an object of the instant invention to produce a ballistic armor less expensively than hot pressed ceramic armors. [0028] These objects and other desirable attributes can be achieved through the application and engineering of boron carbide composite bodies. In accordance with a preferred, but by no means the only embodiment of the instant invention, a molten infiltrant containing silicon and one or more sources of boron is contacted to a porous mass that contains at least some boron carbide, and also containing at least some reactable or "free" carbon. The molten infiltrant infiltrates the porous mass without a pressure or vacuum assist to form a composite body of near theoretical density. The silicon component of the infiltrant reacts with the free carbon in the porous mass to form in-situ silicon carbide as a matrix phase. Further, the tendency of the molten silicon to react with the boron carbide component can be suppressed or at least greatly attenuated by the alloying or doping of the silicon with one or both of a boron source and a carbon source. The resulting composite body thus comprises boron carbide dispersed or distributed throughout the silicon carbide matrix. Typically, some residual, unreacted infiltrant phase containing silicon and small but detectable amounts of boron and carbon is also present and similarly distributed or interspersed throughout the matrix. Thus, these composite materials may be referred to in shorthand notation as Si/SiC/B₄C. [0029] Reaction formed composites featuring a boron carbide reinforcement possess stiffness (e.g., elastic or Young's Modulus) comparable to their counteφarts featuring the usual silicon carbide reinforcement, but exhibit a lower specific gravity for the same volumetric filler loading. Accordingly, such B₄C reinforced SiC composites will find utility in applications requiring low mass and high stiffness, such as equipment requiring precise motion control, often at high accelerations. Further, because of the extreme hardness and low specific gravity of boron carbide, such composites are attractive armor material candidates. [0030] In another embodiment embraced by the instant invention, composite bodies featuring a boron carbide reinforcement can be made by a siliconizing process. Here, the preform contains substantially no carbon that is in a form to react (e.g., no free carbon) with the molten silicon metal, or at least an amount of free carbon that is insufficient to react. The molten metal containing silicon is caused to infiltrate the porous mass or preform without formation of an in-situ silicon carbide phase. [0031] In the armor embodiment in particular, the instant inventors have discovered that a very desirable armor material can be produced when the known hardness requirement is combined with a relatively fine-grained microstructure. To achieve this microstructure, it may be important to minimize the extent of chemical reaction during infiltration, and also to minimize the extent of microstructural development (such as recrystallization or other forms of sintering). To this end, the inventors also recommend limiting the size of the bodies (e.g., particles) making up the hard filler materials of the preform to be infiltrated. In general, the size of the filler particles should be kept below about 300 to 350 microns in diameter, and very good performance in defeating small arms fire is realized when at least 90 volume percent of all moφhological features, or at least of the ceramic phases, are kept below about 100 microns in size. Accordingly, the resulting microstructure of the instant boron carbide composite materials engineered for armor applications features filler particles of limited size, and is a microstructure of limited interconnectivity of the bodies making up the hard phase(s) provided in the porous mass or preform. In other words, the bodies making up the filler material should have no more than a small or slight amount of interconnectedness to one another such as through excessive sintering or recrystallization, or by excessive in-situ SiC formation.

DEFINITIONS [0032] "Areal Density", as used herein, means the mass of an armor system per unit area. [0033] "Ballistic stopping power", as used herein, means the V₅₀ projectile velocity per unit of total areal density. [0034] "Free Carbon", as used herein, means carbon that is intended to react with molten silicon to form silicon carbide. This term usually refers to carbon in elemental form, but is not necessarily limited to the elemental carbon form. [0035] "Inert Atmosphere", as used herein, means an atmosphere that is substantially non-reactive with the infiltrant or the porous mass or preform to be infiltrated. Accordingly, this definition includes gaseous constituents that might otherwise be thought of as mildly reducing or mildly oxidizing. For example, forming gas, comprising about 4 percent hydrogen, balance nitrogen, might be considered to be an inert atmosphere for pwposes of the present disclosure, as long as the hydrogen does not reduce the filler material and as long as the nitrogen does not appreciably oxidize the infiltrant or filler material. [0036] "Mass Efficiency", as used herein, means the areal density of rolled homogeneous steel armor required to give the same ballistic performance as that of the targets of a given areal density being tested, expressed as a ratio. [0037] "Reaction Bonded Silicon Carbide", or "RBSC", refers to a ceramic composite body produced by reaction-bonding, reaction-forming, reactive infiltration, or self-bonding. [0038] "Reaction-Bonded Boron Carbide", or "RBBC", as used herein, means a class or subset of reaction-bonded silicon carbide composites in which the filler or reinforcement of the composite, i.e., the phase being bonded, includes boron carbide. [0039] "Reaction-Bonding", "Reaction-Forming", "Reactive Infiltration" or "Self- Bonding", as used herein, means the infiltration of a porous mass comprising carbon in a form that is available to react with an infiltrant comprising silicon to produce a ceramic composite body comprising at least some silicon carbide produced in-situ. [0040] "Siliconizing", as used herein, means the infiltration of a porous mass with a molten infiltrant containing silicon metal, at least the silicon constituent being substantially non- reactive with the constituents of the porous mass, to produce a composite body having a matrix containing silicon metal. Thus, "siliconized boron carbide" refers to a composite body containing boron carbide and silicon metal, but substantially no silicon carbide formed in-situ from a reaction of the silicon metal. [0041] "Total areal density", as used herein, means the areal density of ceramic armor material plus the areal density of any other material that should properly be considered a part of the assembly of components making up an armor system. Examples of other materials would be fiber reinforced polymeric materials frequently used to back up or encase a ceramic armor plate.

BRIEF DESCRIPTION OF THE FIGURES [0042] Figure 1 is a cross-sectional view of a feeder rail as described in Example 1. [0043] Figures 2A and 2B are front and side views, respectively, of a set-up used to prepare the boron carbide reinforced silicon carbide composite tiles of Example 1. [0044] Figure 3 is an optical photomicrograph of a polished cross-section of the RBBC material produced in accordance with Example 2. [0045] Figures 4A and 4B are top and front views, respectively, of a set-up used to prepare silicon carbide composite breastplates according to Comparative Example 1. [0046] Figure 5 is an optical photomicrograph of a polished cross-section of the SiC- filled RBSC material produced in accordance with Comparative Example 2. [0047] Figure 6 is an optical photomicrograph of a polished cross-section of the RBBC material produced in accordance with Comparative Example 3. [0048] Figures 7A-7C illustrate several applications of the armor material embodiment of the instant invention. [0049] Figures 8A and 8B are optical photomicrographs of RBSC composite materials illustrating a coarse microstructure and one of limited interconnectivity of SiC ceramic constituents, respectively. [0050] Figures 9A and 9B are optical photomicrographs taken at about 50X and about 200X magnification, respectively of polished cross-sections of the siliconized boron carbide composite material produced in accordance with Example 5.

MODES FOR CARRYING OUT THE INVENTION [0051] In accordance with the present invention, a substantially pore-free, mechanically strong composite material is produced that contains boron carbide, preferably in a large volume fraction or combined with one or more exceptionally hard, stiff materials such as silicon carbide to yield a large fraction of very hard, very stiff material as the reinforcement component of the composite. Furthermore, through careful control of the processing conditions, e.g., to suppress reaction of the boron carbide phase, a superior material can be produced, particularly a superior armor product. In addition, the composite bodies produced according to the present invention maintain dimensional tolerances upon thermal processing better than do hot pressed and sintered bodies. [0052] As stated above, silicon carbide and boron carbide, two candidate materials having very desirable hardness for certain applications envisioned by the instant invention, are difficult to fully density by traditional approaches such as by sintering. Such materials are amenable to hot pressing, but hot pressing has its drawbacks, for example, its expense and limitations of the possible geometries that can be produced without extensive machining. [0053] Thus, for economy and manufacturing flexibility, among other reasons, the composite bodies of the instant invention may be produced by a reactive infiltration technique, usually termed "reaction forming" or "reaction bonding", whereby a molten infiltrant comprising silicon is contacted to a porous mass comprising carbon and at least one hard ceramic material that includes boron carbide. The molten silicon-based material infiltrates the interconnected porosity in the porous mass or preform. The molten silicon contains one or more sources of boron in a quantity sufficient to attenuate the tendency of the boron carbide component to chemically react with the molten silicon. Particularly preferred is when the molten silicon also contains one or more sources of carbon, whose presence also appears to help suppress this chemical reaction. Concurrent with the infiltration, the silicon reacts with the carbon in the porous mass or preform to form silicon carbide, which silicon carbide typically has the "beta" SiC polymoφh. The amount of infiltrant is generally provided in such a quantity that the carbon in the porous mass or preform is completely reacted to silicon carbide, with sufficient additional infiltrant supplied to fill any remaining void space between the filler material and the in-situ silicon carbide. The resulting composite materials feature a matrix of the in-situ silicon carbide. Dispersed throughout the matrix is the filler and residual, unreacted infiltrant material. As the residual infiltrant is often interconnected, it is sometimes considered as part of the matrix of the composite. [0054] In terms of the preferred processing conditions, atmospheres that are compatible with this type of infiltration include vacuum or inert atmospheres such as argon, although vacuum is preferred. The vacuum does not have to be "hard" or high vacuum; that provided by a mechanical "roughing" pump is entirely adequate. Although the infiltration tends to be more robust at the higher temperatures, it is also more aggressive, which could give rise to unwanted side reactions, particularly of the boron carbide component. Further, it is more difficult to confine the infiltrant spatially at higher temperatures. Moreover, higher processing temperatures are more likely to give rise to exaggerated grain growth. For all of these reasons, the preferred processing temperatures are those that are generally low yet consistent with reliable infiltration. For infiltrating silicon-based metals into a boron carbide -containing particulate mass in a rough vacuum environment, temperatures in the range of about 1450°C to 1600°C should be satisfactory. [0055] Boron carbide is an especially attractive filler material candidate where the mass of the article is of concern because of its low theoretical density of about 2.45 to 2.55 grams per cubic centimeter. (The range in reported theoretical density may be due to the fact that boron carbide is not a line compound per se, but instead exhibits a limited range of stoichiometry.) Because the Young's Modulus of boron carbide is comparable to that of silicon carbide (about 450 GPa), boron carbide has a higher specific stiffness than does silicon carbide. High specific stiffness is a valuable property in applications such as those requiring precise motion and control of motion, especially where large loads or high accelerations are involved. Moreover, boron carbide is even harder than silicon carbide. Thus, a RBSC composite body featuring boron carbide as a reinforcement or filler material (i.e., "RBBC") may offer higher hardness yet lower specific gravity as compared to a RBSC composite having silicon carbide as the filler material. [0056] In an alternate embodiment, the instant invention includes boron carbide composites made by a "siliconizing" process, similar to the process to make "siliconized SiC". Here, a molten infiltrant comprising silicon, usually commercially pure elemental silicon, is contacted to a porous mass of ceramic material, including at least some boron carbide, that is wettable by the molten infiltrant under the processing conditions, which is generally taken to be a vacuum or inert gas (e.g., argon) environment. The ceramic material containing the boron carbide can be in the form of substantially non-connected particles such as a loose mass of particulate, or may be in the form of a lightly sintered or "bisque-fired" material, or may be heavily sintered, with only a small amount of interconnected porosity. Unlike the RBSC process, here the source of carbon in the porous mass is substantially lacking. Thus, siliconizing is not as robust an infiltration process as is the RBSC process. Accordingly, somewhat higher infiltration temperatures may be required, such as between about 1500°C to about 2000°C, and/or a vacuum environment (as opposed to inert gas environment, for example) may be required. For making siliconized boron carbide for armor applications, however, the present inventors recommend that the higher infiltration temperatures and the heavier sintering of preforms (e.g., making the filler bodies more intercomiected) should probably should be avoided, for reasons that will be discussed in more depth to follow. [0057] Under most of the prior art silicon infiltration conditions, however, boron carbide is at least somewhat reactive with the molten silicon. Although one reaction product of such reaction is more in-situ silicon carbide, where one is attempting to maximize the boron carbide loading, it would be desirable if the boron carbide could remain substantially unaffected by the infiltrant; that is, it would be desirable if the silicon did not react with the boron carbide. In the reaction-bonding embodiment, the instant invention solves this problem by dissolving some boron into the molten silicon, thereby reducing the activity of the silicon for reaction with boron carbide. Although pure silicon will eventually become saturated in boron and carbon as it reacts with the boron carbide phase in the porous mass or preform, this approach is not preferred, unless this porous mass or preform is "sacrificial", and not the ultimate article of commerce being produced. In many instances, reaction of the boron carbide reinforcement of the porous mass or preform with the silicon infiltrant has led to cracking of the resulting silicon carbide composite body. Instead, what is preferred is to provide a source of boron to the silicon-based infiltrant prior to the infiltrant making contact with the boron carbide in the porous mass or preform. Any boron-containing substance that can be dissolved in silicon may be useful in the context of the instant invention; however, elemental boron and boron carbide are particularly preferred. [0060] One can envision any number of techniques for adding a boron source material to the silicon infiltrant. The approach preferred according to the instant invention is to support the preform to be infiltrated on, and to feed the infiltrant into the preform by way of, kiln furniture consisting of a porous preform comprising boron carbide. Specifically, a silicon-containing infiltrant can infiltrate kiln furniture (later referred to as a "feeder rail" or "beam") containing at least some boron carbide. The kiln furniture may be provided in either the porous condition, e.g., as a preform; or in the "already infiltrated" condition, e.g., as a composite body. The preform that ultimately is intended to become an article of commerce upon infiltration, sometime referred to as the "object" preform, is supported on the kiln furniture. The silicon- containing infiltrant dissolves at least some of the boron carbide of the kiln furniture, and may even become saturated with carbon and/or boron. When this molten silicon then continues to infiltrate into the object preform that is in contact with the kiln furniture, the infiltrating silicon will react very little if at all with the boron carbide in the object preform. Any cracking of the kiln furniture as a consequence of silicon reacting with the boron carbide in the kiln furniture should not unduly affect the continued infiltration of the silicon into the object preform. Of course, the supporting kiln furniture is not required to contain boron carbide per se. Many boron-containing substances in which the boron is able to dissolve in the silicon component of the infiltrant should be satisfactory; however, substances such as boron oxide may not be sufficiently refractory under the thermal processing conditions. Further, the boron source is not required to be located in the kiln furniture; it may be alloyed or otherwise introduced into the silicon component of the infiltrant at most any point prior to the molten silicon making contact with the boron carbide of the object preform. For example, the instant inventors have found it useful when building the "lay-up" for infiltration to supply boron carbide particulate to the bottom of the vessel housing the molten silicon infiltrant, dispersed, for example, as loose powder between the feeder rails. Moreover, the inventors have noticed, at least in the RBBC embodiment, that the presence of a boron nitride coating on the porous mass or preform to be infiltrated also helps suppress the boron carbide reaction. [0061] For silicon infiltrations that rely on little to no reactable carbon in the porous mass or preform, such as the boron carbide siliconizing process, in addition to the boron source, it may also be desirable to add a source of carbon to the molten silicon to suppress the tendency for silicon to dissolve carbon from the boron carbide. Of course, boron carbide provides a carbon source as well as a boron source, but it may be desirable to provide an independent carbon source, such as the many forms of elemental carbon. These can be provided in powdered form, e.g., graphite powder, and may be admixed with the material to make the feeder preforms, or may be admixed with the silicon infiltrant, which often is provided in powdered or chunk form. It also may be the case that carbon additions, independent of a boron source, can provide some measure of reaction suppression. [0062] It should be noted that, at a processing temperature of about 1550°C , only a few weight percent of elemental boron, and perhaps only about 1 wt% or so of carbon, will dissolve in molten silicon. Nevertheless, these amounts should be sufficient to substantially suppress the dissolution or reaction of the boron carbide reinforcement with molten silicon. [0063] A preferred embodiment of the instant invention relates to the specific application of the instant boron carbide composite materials as armor for stopping ballistic projectiles. To defeat the incoming projectile, such ceramic armors usually feature at least two layers made up of very dissimilar materials. Namely, such a component of a ballistic armor system features, at a minimum, a ceramic layer and a backing layer, which typically are bonded to one another. As the name suggests, relative to the direction of travel of the projectile, the backing layer is placed behind the ceramic layer. Sometimes, one or more layers of a protective material are also placed in front of the ceramic layer, but these are usually for the puφose of protecting the ceramic from fractures due to routine handling (or mishandling). The puφose of the ceramic layer is to "process" the impinging projectile, such as by flattening, shattering, eroding it, etc. The role of the backing layer is to then "catch" the processed projectile as well as any backward propelled fragments of the ceramic layer. Typically, the backing layer can deform to a large degree without failing catastrophically. The backing layer may be made of metals or alloys such as aluminum, iron or steel, titanium, etc., which for vehicular armor, may be the structure of the vehicle itself. Where lightweight armor is needed, the backing layer typically is a fiber-reinforced polymeric (FRP) material. The fibers employed in these backing layers include polyethylene, aramid and glass fibers. A well-known FRP backing material goes by the tradename "SpectraShield", registered to AlliedSignal Inc. (now owned by Honeywell International Inc., and referring to a roll product consisting of two plies of unidirectional extended-chain polyethylene fiber tapes cross-plied at right angles, resulting in a nonwoven, thermoplastic composite); however, several such FRP backing materials are commercially available. [0064] Armor generally takes the form of a plate, but the plates need not be flat, regular polygons. Often, the armor plates must be shaped to conform to the underlying structure to be protected. Body armor, for example, is often curved in one or more dimensions to better conform to the shape of the wearer, e.g., conform to a human torso. [0065] According to many who are skilled in the armor arts, what is sought in the way of an armor material is one that fractures and erodes the impacting projectile before it can penetrate the armor system. Viechnicki et al. (ibid.) have shown that all that is required in terms of hardness is for the armor to have at least the same hardness as the projectile, but that further increases in hardness over the required "threshold" level do not add significantly to the performance level. [0066] Accordingly, in addition to the motion control applications alluded to above, boron carbide composites should be attractive candidate armor materials, and in fact as the prior art shows, others have attempted to apply boron carbide composite materials as armors previously. Because armor is often specified by total weight, armor systems having low bulk density are sought after because the armor can be made thicker for the same mass, the desirability of which was discussed previously. One implication of the extreme hardness of boron carbide is that a greater amount of non-hard phase, e.g., metal, can be tolerated in a composite body comprising boron carbide and metal, for example, to enhance other properties such as strength or toughness, and still meet the overall hardness required of the composite body. [0067] The overall hardness of the boron carbide composite material of the instant invention is proportional to the hardnesses of the constituents of the composite material, and to their volumetric proportions. In terms of developing a high-performing armor material, this armor embodiment of the instant invention focuses on achieving a sufficiently high volumetric loading of the hard ceramic phases such as boron carbide as to meet overall hardness levels believed to be important, and on limiting the size of the largest grains or crystals, particular the ceramic crystals, making up the composite body. To state it more precisely, substantially all of the moφhological features making up the microstructure of the boron carbide composite body should be smaller than about 350 microns in size. More preferred is that substantially all of these features be smaller than about 212 microns; still more preferred is that at least 90 percent by volume be less than about 100 microns in size. Particularly preferred is for the boron carbide composite body having at least 90 volume percent of its ceramic moφhological features being no greater than about 55 microns in size. [0068] Such an upper limit to the particle size of the filler materials used in the porous mass or preform can be achieved, among other techniques, by sieving the filler bodies. For example, a 170 mesh and 200 mesh (U.S. Standard) screen yields particles having a maximum size of about 90 microns and 75 microns, respectively. Similarly, 45 mesh, 50 mesh and 70 mesh (U.S. Standard) sieve screens pass particles having a maximum size of about 350 microns, 300 microns and 212 microns, respectively. Even more preferred is for the boron carbide composite body having at least 90 volume percent of its moφhological features being no greater than about 55 microns in size. [0069] One technique for maximizing the amount of hard phase in the composite body is to produce a porous mass or preform that is highly loaded volumetrically in the hard phases, typically in the form of filler materials having high hardness. Highly loaded preforms can be produced by utilizing a distribution of filler material particle sizes sufficiently wide so that small particles can nest or fit within the interstices of larger particles. Because these two parameters of maximizing the loading of hard fillers in the preform while capping or limiting the size of the largest particles inherently are at odds with one another, careful attention to processing parameters is required to achieve both in the same material. Fortunately, the instant inventors have been relatively successful in attaining preforms highly loaded in hard filler while limiting the size of the filler bodies in such a way that, for example, at least 90 percent by volume are smaller than about 100 microns in diameter. Even with this more conservative upper limit of about 100 microns on the size of the largest particles, it is still possible to produce preforms that are 65 volume percent or more loaded in hard ceramic phases such as SiC and/or B₄C. [0070] Some of the "larger" hard ceramic fillers used in the Examples to follow have the following particle size distributions: Grade F240 CRYSTOLON^® SiC (Saint-Gobain/Norton Industrial Ceramics, Worcester, MA) has 90 percent by volume of all of its constituent particles being smaller than about 55 microns, and 97 percent smaller than about 70 microns. Grade F320 CRYSTOLON^® SiC has 90 volume percent of its particles being smaller than about 37 microns, and 97 percent finer than about 49 microns. These results were calculated based on the Eppendorf-Photosedimentometer. According to sieve analysis, 220 grit TETRABOR^® B₄C (ESK, Kempten, Germany) has 85 volume percent of its particles being smaller than about 75 microns, and substantially all of its constituent particles being smaller than about 106 microns. [0071] It may be that limiting the fine grain size as specified by the instant invention is really a proxy for high mechanical strength, or at least for placing a lower limit on mechanical strength of the composite material. Because limiting the grain size is a necessary but not a sufficient condition for achieving high strength in brittle materials, achieving a high strength target traditionally has been taken as something of a metric for the quality of the ceramic or composite body produced. With brittle composite materials in general and brittle composite materials produced by infiltration in particular, a number of defects can seriously impair the mechanical strength of the resulting composite body. These include non-uniform filler material distribution in the preform, incomplete infiltration of the preform, e.g., leaving porosity and/or unreacted carbon or other reactants in the preform, and excessive grain growth during thermal processing, either of the filler material or of any silicon carbide produced in situ. Such defects probably would also impair the anti-ballistic performance of the material. [0072] It may be the case that the microstructures of the boron carbide composite materials of the instant invention result in fracture in a different (e.g., transgranular versus intergranular) mode than do the prior art composite bodies made by silicon infiltration techniques that have the larger, more interconnected microstructures. Whatever the exact reason or operative mechanism, the instant inventors have discovered that RBBC materials of limited grain size and limited connectivity of the ceramic phase(s) are very effective at stopping ballistic projectiles, particularly from small arms fire. [0073] Because the hard filler component of the boron carbide composite bodies of the instant invention is so much harder than the silicon component (Knoop Hardness of about 2900- 3580 kg/mm² for B₄C, for example, versus about 1100 kg/mm²(Vickers) for Si, respectively), the overall hardness of the composite body is strongly dependent upon the relative amounts of each phase. Thus, when the end-use article of the instant composite material is armor for protection against ballistic projectiles, it may be important that the composite body contain a large volume fraction of the hard phase(s), particularly where the residual infiltrant phase component is softer than silicon, a scenario that will be discussed in more detail below. In a reaction-formed silicon carbide composite material, some silicon carbide is produced in situ. Thus, it is possible to form a composite body that is highly loaded in silicon carbide by infiltrating silicon into a porous mass containing large amounts of carbon. For reasons that also will be discussed in more detail below, this approach is not preferred. Instead, what is desired is to reactively infiltrate a porous mass or preform that is highly loaded not with carbon but rather with the hard ceramic phase(s) of the filler material(s). In an alternate embodiment, a preform highly loaded with hard filler materials but little or no reactable carbon is infiltrated with molten silicon (e.g., "siliconizing") or silicon-containing metal. [0074] When the porous mass or preform comprises boron carbide, no additional carbon source is required to produce silicon carbide, because the silicon of the molten infiltrant can react with the boron carbide. This was the approach taken by Taylor et al in the '564 Patent, discussed above (as well as by others). When the objective is to maximize hardness and/or boron carbide loading of the resulting composite body, this approach may be undesirable. Specifically, boron carbide has a higher hardness and lower specific gravity than does silicon carbide. Thus, reaction of boron carbide with silicon to produce silicon carbide (plus silicon borides) trades a substance of high hardness for a substance of lower hardness and higher specific gravity. Accordingly, one may want to minimize the reaction of the boron carbide component. Thus, where boron carbide is to be used as a filler material in a reaction-bonded composite body where high hardness and low specific gravity of the body are desired, as they are in armor systems, a source of carbon other than the boron carbide should be present in the porous mass or preform. [0075] In a siliconizing process, there is little to no reactable carbon present in the porous mass. However, in the absence of at least some carbon, the molten silicon may tend to dissolve carbon from the boron carbide of the porous mass. Accordingly, it may be desirable to supply a source of carbon to the molten silicon, perhaps even up to the saturation point, prior to contact of the molten silicon with the porous mass to be infiltrated. Above this concentration, the excess carbon will likely precipitate out as in-situ SiC. Again, by first contacting the molten silicon with a carbon source such as elemental carbon or boron carbide, the silicon can be caused to take some carbon into solution. The carbon source can be provided by the same techniques as are used to provide the boron source to the silicon infiltrant. This issue of molten silicon dissolving carbon from the boron carbide of the porous mass or preform generally is not a problem in the reactive infiltration systems, e.g., RBBC, because there is usually more than enough reactable carbon present to saturate the silicon and suppress this reaction. [0076] Techniques for maximizing the volumetric loading of filler materials in the porous mass or preform are well known, and usually take the form of blending a plurality of filler material bodies, for example, particles, having a distribution of sizes in such a way that smaller particles tend to fill the interstices between larger particles. There are limits to the size distribution, however, to the extent of distribution of particle sizes. For example, where there is a potential for chemical reaction, as there is for boron carbide in these silicon infiltration systems, smaller particles tend to be more reactive than larger particles due to their large total surface area. At the other end of the scale, at some point, large-sized filler material particles will begin to reduce the strength of a composite body that fails by a brittle fracture mechanism due to the introduction of critical-sized flaws into the material. Further, whether it is strength-related or not, there is anecdotal evidence in the prior art that RBSC bodies containing large or relatively large grains were not superior armor materials. Accordingly, the instant invention overcomes this problem by providing a technique whereby the relatively fine boron carbide particles can be infiltrated in a reaction-bonding operation, and not be consumed in a reaction with the incoming silicon infiltrant. The ability to make a fine-grained RBBC is not only beneficial for armor applications, but also for many precision equipment applications. Specifically, while the higher strengths afforded by the fine grain size composite material may not be essential, the fine grain size permits finer features to be ground or machined into the material. [0077] Although most any of the known techniques may be employed to produce a porous preform that can be infiltrated by a molten infiltrant comprising silicon, the techniques that seem to be better able at producing preforms, particularly relatively thin preforms, that are highly loaded with one or more fillers are those that utilize a liquid phase, for example, sediment casting, slip casting or thixotropic casting. But other well-known ceramic processing techniques such as dry pressing may also be entirely satisfactory, depending on the particulars of the composition and article being formed. [0078] Recently, it has become known to alloy the infiltrant metal used to make a reaction-formed silicon carbide body so that the metal phase of the formed body includes a constituent other than silicon. (See, for example, commonly owned U.S. Patent No. 6,503,572.) For example, in the instant boron carbide composite system, the infiltrant may comprise an alloy of silicon, boron and copper to yield a phase in the formed boron carbide composite body comprising metallic copper or copper alloy or a copper-silicon intermetallic compound. Such bodies containing an alloy infiltrant phase often are softer but tougher than similar bodies having essentially pure silicon as the infiltrant phase. In spite of the hardness reduction, reaction- bonded boron carbide composites having an alloyed infiltrant phase might still function well as armor materials. For example, the property of compressive strength or toughness may be an important factor contributing to good anti-ballistic character, particularly when combined with high hardness. For example, enhanced toughness might contribute to improved multi-hit capability of the resulting armor product, and/or might contribute to enhanced durability which is important even for routine handling in the field. The siliconizing process should also be amenable to the addition of other (non-silicon) metals to the infiltrant. [0079] Again, the porous mass of the instant invention always contains some amount of boron carbide. In the absence of proactive techniques such as doping of the silicon infiltrant with a source of boron, and with a source of carbon if necessary, the boron carbide component of the porous mass will tend to react with the molten silicon to produce silicon carbide plus silicon boride(s). In the instant invention, the system has been designed such that the boron carbide does not react to any great degree with the molten silicon. Thus, the boron carbide component can be considered to be a substantially inert filler material. In addition to the boron carbide, the porous mass can incoφorate one or more other such filler materials. By this is meant a filler material that is substantially non-reactive with the molten infiltrant under the local processing conditions. One such filler material that is especially preferred is silicon carbide, as molten silicon more easily wets silicon carbide than other inert materials, such as oxides. However, it should be possible to admix at least some amount of other filler materials that may not be as wettable as boron carbide or silicon carbide under the local processing conditions and still achieve wetting and infiltration of the overall porous mass by the molten silicon. Examples of such alternative filler materials include titanium diboride, silicon nitride and aluminum nitride. It may even be possible to admix a quantity of "non-wettable" filler materials (e.g., aluminum oxide) and still accomplish wetting and infiltration of the porous mass by molten silicon, particularly if the porous mass contains a source of reactable carbon, and most particularly if this carbon source is interconnected, such as in the form of a coating on the filler bodies. [0080] The filler material making up the porous mass to be infiltrated may be provided in a number of different moφhologies, including particulates, platelets, flakes, whiskers, continuous fibers, microspheres, aggregate, etc. Particulates are often preferred for reasons of economy and availability. [0081] While not possible through visual inspection, it is possible using x-ray diffraction techniques to distinguish a silicon carbide matrix that is reaction-formed from any silicon carbide that may be present as a reinforcement or filler material. Specifically, the reaction- formed silicon carbide typically is of the beta polymoφh, at least under the instant processing conditions. In contrast, most commercially available silicon carbide, particularly the commodity grades, is the alpha form that is so commonly used as a filler material. Accordingly, one can provide at least approximate quantitative data as to the relative amounts of each that are present in the composite body. [0082] A wide range of sizes of filler material bodies can be successfully infiltrated using the reaction-forming process, e.g., bodies ranging from several millimeters in size down to bodies on the order of a micron in size. Again, when the goal is to produce a body having attributes of a ballistic armor, the filler bodies, and in fact, all of the moφhological features making up the ceramic component of the composite body should be kept below about 300-350 microns, and preferably below about 212 microns in size. [0083] Previously, the inventors recommended that the size of the moφhological features (e.g., crystallites, etc.) be kept below about 100 microns or so. However, if certain reaction conditions are kept under control as described in the following paragraphs, then it may be possible to use particles somewhat larger than about 100 microns and still achieve good ballistic performance. [0084] In addition to limiting the maximum size of the bodies of filler making up the porous mass, the porous mass of filler material should not be exposed to excessive temperatures, especially during infiltration. In this regard, the instant inventors have successfully infiltrated a porous mass of boron carbide particulate (plus added carbon) at a temperature of about 1550°C without causing reaction of the boron carbide with the boron-doped silicon infiltrant. Here, "excessive" also refers to temperatures at which ceramic grains can grow appreciably. For example, the transformation of silicon carbide from the beta to the alpha crystallographic form occurs at about 2050°C. The crystallographic transformation is often accompanied by extensive grain growth, which can be observed as a coarsening of the microstructure. Depending upon the exact conditions, it may be possible to heat to a slightly higher temperature (perhaps about 2100°C) and still avoid this recrystallization; however, it is not known what will happen to the boron carbide component at this temperature. Still, it would be advisable not to conduct the infiltration, or post-process the infiltrated mass, at temperatures in excess of about 2000°C. Again, as afar as producing the instant boron carbide composites is concerned, the lowest temperatures that accomplish the objectives generally are to be preferred. [0085] Moreover, in the reaction-bonding composite systems, a high volume fraction of hard phase(s) should not be accomplished through production of large amounts of the in-situ silicon carbide phase, but instead through the engineering of highly loaded masses of the hard ceramic filler material. For example, the porous mass to be infiltrated preferably contains free or elemental carbon as the carbon source to form the in-situ silicon carbide. The amount of this free carbon should be limited to (generally) no more than about 10 percent by volume of the porous mass, and preferably, no more than about 5 or 6 percent. Thus, in general, the amount of silicon carbide produced in-situ should be limited to no more than about 24 volume percent of the final composite body, and preferably no more than about 12 to 14 percent. Among the problems that result from excessive reaction during the infiltration process are temperature spikes due to the exothermic nature of the chemical reaction of silicon and carbon. Such temperature spikes can cause cracking due to localized thermal expansion. Also, the conversion of elemental carbon to silicon carbide entails a volumetric expansion of about 2.35 times. Thus, large amounts of reaction are also detrimental from the standpoint that the large volumetric change can also lead to cracking. [0086] What the instant inventors have noticed, however, is that many of the prior art reaction-bonding publications expressly disclose processing conditions that the inventors identify as entailing "excessive reaction", as warned against immediately above. What results is excessive grain growth and coalescence or fusing of individual grains or moφhological features (e.g., grains) into larger ones. See, for example, Figure 8A. In contrast to this coarse microstructure, the instant inventors have produced silicon-infiltrated composite materials for armor having good ballistic performance and that have microstructures similar to what is shown in Figure 8B. This microstructure is characterized by minimal chemical reaction, little to no recrystallization of the SiC, and minimal coalescence, sometimes referred to as "clumping". It should be pointed out that these two figures feature SiC and not B₄C as the filler particles, but that does not negate the point being illustrated. Thus, the instant inventors assert that as long as one can continue to produce ceramic-rich composite materials by a silicon infiltration technique with these microstructures exhibiting minimal interconnectivity of the hard filler particles (which were initially provided as discrete entities), one may increase the filler particle size somewhat above 100 microns or so and still obtain acceptable ballistic performance in an armor application. For example, it may be possible to increase the size of the filler particles up to about 212 microns, or perhaps even into the 300-350 micron range. [0087] Accordingly, the resulting microstructure of the instant armor-grade boron carbide composite materials is one of limited interconnectivity of the bodies making up the boron carbide, and possibly other hard phase(s), provided in the porous mass or preform. In other words, the bodies making up the filler material should have no more than a small or slight amount of interconnectedness to one another such as through excessive sintering or recrystallization, or by excessive in-situ SiC formation. [0088] Although not required, the carbon source added to the porous mass or preform for the reaction-bonding embodiment of the invention usually takes the form of elemental carbon, such as graphite. For many applications, particularly those requiring high stiffness, it is desirable that the silicon carbide of the resulting composite body be at least partially interconnected. This outcome is more readily achieved if the carbon in the porous mass or preform is interconnected. Further, interconnected carbon in the porous mass or preform assists the infiltration process in terms of speed and reliability. In a preferred embodiment, the carbon is introduced to the porous mass as a resin. This mixture may then be molded to the desired shape. Curing the resin renders the porous mass self-supporting, e.g., as a preform. During subsequent thermal processing, or during an intervening firing step, typically in a non-oxidizing atmosphere, the resin pyrolyzes to carbon in interconnected form to yield a preform containing at least about 1 percent by volume of carbon. The resin infiltration and pyrolysis cycle may be repeated one or more times if an increase in the carbon content is needed. [0089] Reaction-bonded boron carbide composite bodies are generally cheaper to manufacture than hot pressed boron carbide bodies. Not only may a plurality of RBBC bodies be thermally processed simultaneously, but the tooling (typically graphite) lasts longer than that used in hot pressing operations. [0090] As mentioned previously, the present RBBC composite materials can be produced to net size and shape better, for example, as a curved tile for a body armor application, than can a hot pressed boron carbide armor tile, as expressed or measured by the achievement of precise net dimensional tolerances. The instant inventors also expect the instant siliconized boron carbide materials to show better dimensional reproducibility than hot pressed boron carbide. [0091] The tighter dimensional tolerances represent a performance advantage. Specifically, production armor, especially armor for weight-sensitive applications, typically is specified or certified as meeting some minimum ballistic protection level, as measured by a V₅₀ projectile velocity number at a specified maximum weight or areal density. (As a point of information, the ballistic test terminology in this patent document has the same meaning as the same terminology found in MIL-STD-662F.) Because the objective is high ballistic performance and low areal density, both of which parameters are related to thickness but varying oppositely of one another, one wants as uniform a thickness of the armor plate as possible. This is especially true in view of the fact that the V50 value must be achieved at the lower limit of the permissible thickness range, i.e., the thinnest permissible plate, while the maximum weight is determined by the upper limit of the thickness range. [0092] As long as the overall shape of a ceramic armor plate is within specifications, it is at least theoretically possible to restore non-uniformities developed during thermal processing by means of grinding or machining. Such post-processing operations, however, are usually expensive and rarely are they commercially viable in the body armor market. Accordingly, the ceramic armor body should have uniform thickness in the as-thermally processed condition. [0093] Conformity of the shape of the formed ceramic armor body to the intended shape is also important. The ability to make ceramic armor plates having complex shaped curves that faithfully reproduce the desired shape can have significant value in meeting the form and fit requirements of the armor product. The RBBC materials, exhibiting better thickness uniformity than sintered or hot pressed armor ceramics, are also expected to exhibit better shape fidelity than the sintered or hot pressed product. Similar results are expected for the siliconized boron carbide material. [0094] The following non-limiting examples further illustrate the instant invention. EXAMPLE 1 [0095] This example demonstrates the production via reactive infiltration of a Si/SiC composite body containing a boron carbide reinforcement, i.e., Si SiC/B₄C. More specifically, this Example demonstrates the infiltration of a silicon-containing melt into a preform containing an interconnected carbon phase derived from a resinous precursor, and silicon carbide and boron carbide particulates. [0096] Preforms were prepared by a sedimentation casting process. Specifically, about 28 parts of water were added to 100 parts of ceramic particulate and 8 parts of KRYSTAR 300 crystalline fructose (A.E. Staley Manufacturing Co.) to make a slurry. The ceramic particulate content consisted of about equal weight fractions of 220 grit TETRABOR^® boron carbide (ESK GmbH, Kempten, Germany, distributed by Micro Abrasives Coφ., Westfield, MA) having a median particle size of about 66 microns and 500 grit CRYSTOLON green silicon carbide (St. Gobain/Norton Industrial Ceramics) having a median particle size of about 13 microns (Grade 500 RG). The solids and liquids were added to a plastic jar and roll mixed for about 40 hours. The slurry was de-aired in about 760 mm of vacuum for about 5 minutes. About 15 minutes prior to casting, the slurry was re-roll mixed to suspend any settled particulates. [0097] A graphite support plate was placed onto a vibration table. A rubber mold having a cavity of the desired shape to be cast was wetted with a surfactant (Sil-Clean, Plastic Tooling Supply Co., Exton, PA). The wetted rubber mold was then placed onto the graphite plate and allowed to dry. The slurry was poured into the cavity. Vibration was commenced. [0098] The residual liquid on the top of the casting was blotted up with a sponge periodically during sedimentation. After the particulates had fully settled (about 3 hours), vibration was halted. The graphite plate, the rubber mold and the castings inside were transferred from the vibration table to a freezer maintained at a temperature of about minus 20°C. The casting was thoroughly frozen in about 6 hours, thereby forming a self-supporting preform. [0099] From the freezer, the frozen preform was demolded and placed onto a graphite setter tray. The graphite tray and preform were then immediately placed into a nitrogen atmosphere furnace at ambient temperature. The furnace was energized and programmed to heat to a temperature of about 50°C at a rate of about 10°C per hour, to hold at about 50°C for about 8 hours, then to heat to a temperature of about 90°C at a rate of about 10°C per hour, to hold at about 90°C for about 8 hours, then to heat to a temperature of about 120°C at a rate of about 10°C per hour, to hold at about 120°C for about 4 hours, then to heat to a temperature of about 600°C at a rate of about 50°C per hour, to hold at about 600°C for about 2 hours, then to cool down to about ambient temperature at a rate of about 100°C per hour. This firing operation pyrolyzed the fructose, yielding a well-bonded preform containing about 2.7 percent by weight carbon. [0100] The above-mentioned steps were employed to produce two "beam" or feeder rail preforms and a number of tile preforms. Each tile preform had a mass of about 174 grams and had overall dimensions of about 100 mm square by about 9 mm thick. Each rail preform had a cross-section as depicted in Figure 1 and measured about 220 mm long by about 15 mm wide by about 25 mm thick. During infiltration of the tile preforms, these rails would serve as a conduit for conducting molten infiltrant toward and into the tile preforms. [0101] Next, a set-up to confine the infiltration process was prepared. [0102] Referring to Figures 2A and 2B, the interior surfaces of a Grade ATJ graphite tray 31 (Union Carbide Coφ., Carbon Products Div., Cleveland, OH) measuring about 790 mm by about 230 mm by about 51 mm deep were spray coated with a boron nitride slurry or paint 33 using a Model 95 Binks spray gun. The boron nitride paint was prepared by diluting about 1800 grams of LUBRICOAT boron nitride paste (ZYP Coatings, Oak Ridge, TN) with deionized water to a volume of about 1 gallon (3.7 liters). Two relatively light coats of this boron nitride paint were applied, with brief ambient temperature drying in air between coats. [0103] The boron nitride-coated tray was then placed into a larger graphite chamber 35 having interior dimensions of about 825 mm long by about 270 mm wide by about 320 mm in height. The chamber also featured means for supporting a parallel row of graphite dowel rods. [0104] Referring now specifically to Figure 2B, two plies of PANEX^®30 low oxidation carbon cloth 44 (Grade PW03, plain weave, 115 g/m², Zoltek Coφ., St. Louis, MO) weighing about 48 grams and measuring about 790 mm by about 230 mm was placed on the floor of the coated graphite tray 31, 33. Four boron carbide rail preforms 42, each having a mass of about 190 grams and a length of about 200 mm, were placed on top of the cloth and arranged parallel to the length dimension of the tray. Silicon in lump form 21 (Grade LP, Elkem Metals Co., Pittsburgh, PA) and comprising by weight about 0.5 percent Fe (max) and the balance Si, was then distributed more or less uniformly over the carbon cloth between the individual preform rails. Calculations showed that about 1510 grams of silicon infiltrant would be required to completely react the elemental carbon and fill the interstices in the cloth, feeder rail preforms and tile preforms; however, about 10% additional silicon was provided to the set-up. [0105] Graphite dowel rods 49 measuring about 0.25 inch (6 mm) in diameter and spray coated with boron nitride 33 were placed into graphite holders or supports 47. A total of fifteen square tile preforms 41 (only four are shown in the Figure) similarly spray coated with boron nitride 33 were placed across the two rails edgewise in each half of the tray. As the boron nitride tended to act as a barrier material hindering over-infiltration, the surface of the tiles that were to contact the boron carbide preform rails were left uncoated. [0106] The top of the chamber was covered with a loose-fitting (non-hermetically sealing) graphite lid 34 featuring a number of approximately 1 cm diameter through-holes 36 to permit atmosphere exchange. The holes were covered with a piece of graphite felt 38 which was held in place with a graphite block 40 which served as a dead load, thereby completing the setup. [0107] The completed set-up was then placed into a vacuum furnace at about ambient temperature (e.g., about 20°C). The air was evacuated using a mechanical roughing pump, and a rough vacuum of less than about 100 millitorr residual pressure was thereafter maintained. The lay-up was then heated from ambient temperature to a temperature of about 1350°C at a rate of about 200 °C per hour. After maintaining a temperature of about 1350°C for about 1 hour, the temperature was further increased to a temperature of about 1550°C at a rate of about 200°C per hour. After maintaining a temperature of about 1550°C for about 1 hour, the temperature was decreased to a temperature of about 1450°C at a rate of about 100°C per hour. Without holding at this temperature, the lay-up temperature was further decreased to a temperature of about 1300°C at a rate of about 25°C per hour, which was immediately followed by a cooling at a rate of about 200°C per hour to approximately ambient temperature. [0108] Following this heating schedule, the chamber and its contents was recovered from the vacuum furnace, disassembled and inspected. The silicon infiltrant had melted and infiltrated through the carbon cloth, thereby converting the carbon cloth to silicon carbide cloth. The molten silicon infiltrant had also infiltrated through the rail preforms and into the square tile preforms, and reacting with the elemental carbon therein, to form dense, silicon carbide matrix composite bodies having a boron carbide reinforcement. Because each tile preform was supported by the rails in line contact, only low-to-moderate hand force was sufficient to separate the Si/SiC/B C composite tiles from the feeder rail composite material.

EXAMPLE 2 [0109] The technique of Example 1 was substantially repeated, except that no silicon carbide particulate was used in fabricating the preform, and the particle size distribution of the boron carbide was modified such that substantially all particles were smaller than about 45 microns. Following the pyrolysis step, the preforms contained about 75 percent by volume of the boron carbide particulate and about 4 percent by volume of carbon. [0110] After infiltration, the ceramic material contained nominally 75 vol. % B₄C, 9 vol. % reaction-formed SiC, and 16 vol. % remaining Si (i.e., an Si SiC/B₄C composite). A polished section was examined using a Nikon Microphot-FX optical microscope. An optical photomicrograph of the material is shown in Figure 3. It is clearly evident that, by careful selection of processing conditions, including addition of a source of boron to the silicon infiltrant, little growth and interlocking of the particles has occurred, thus allowing a relatively fine microstructure to be maintained. For instance, the photomicrograph shows little visible reaction between the Si and B₄C as a result of the infiltration process.

EXAMPLE 3 [0111] The technique of Example 2 was substantially repeated, except that, before supplying the silicon infiltrant to the lay-up, a monolayer of TETRABOR^® boron carbide particulate (220 grit, ESK) was sprinkled onto the carbon cloth between the feeder rails. The amount of silicon was concomitantly increased to account for the added boron carbide, and to maintain an excess supply of silicon of about 10 percent, as in Example 1.

COMPARATIVE EXAMPLE 1 [0112] This example demonstrates the fabrication of a silicon carbide composite armor plate highly loaded in a fine-grained silicon carbide filler. The example furthermore shows the re-use of some of the components of the thermal processing apparatus. [0113] An armor "breastplate" and four "feeder rail" preforms were prepared by a sedimentation casting process. Specifically, about 24 parts of de-ionized water were added to 100 parts of CRYSTOLON green silicon carbide (Saint-Gobain/Norton Industrial Ceramics, Worcester, MA) and about 6 parts of KRYSTAR 300 crystalline fructose (A.E. Staley Manufacturing Co., Decatur, IL) to make a slurry. The silicon carbide particulate consisted of about 65 parts by weight of Grade F320 (median particle size of about 29 microns, blocky moφhology) and the balance Grade 500 RG (median particle size of about 13 microns, rounded moφhology). The solids and liquids were added to a plastic jar and roll mixed for about 40 hours. The slurry was de-aired in about 760 mm of vacuum for about 5 minutes. About 15 minutes prior to casting, the slurry was re-roll mixed to suspend any settled particulates. [0114] A graphite support plate was placed onto a vibration table. A rubber mold having a cavity of the desired shape to be cast was wetted with a surfactant consisting of a 10 weight percent aqueous solution of JOY dishwashing detergent (Proctor and Gamble, Cincinnati, OH). The wetted rubber mold was then placed onto the graphite plate and allowed to dry. The slurry was poured into the cavity. Vibration was commenced. [0115] After the particulates had fully settled (about 3 hours), vibration was halted. The residual liquid on the top of the casting was blotted up with a sponge. The graphite plate and the castings in the rubber mold thereon were transferred from the vibration table to a freezer maintained at a temperature of about minus 15°C. [0116] Once the casting had frozen thoroughly, the rubber mold was removed from the freezer and the frozen casting contained therein was demolded and placed onto a graphite setter tray for drying and bisque firing. The setter tray was contoured to the shape of the outer face of the breastplate preform. The graphite tray and preform were then placed into a nitrogen atmosphere furnace at ambient temperature. The furnace was energized and programmed to heat to a temperature of about 90°C at a rate of about 40°C per hour, then to hold at about 90°C for about 2 hours, then to further heat to a temperature of about 600°C at a rate of about 100°C per hour, to hold at about 600°C for about 2 hours, then to cool down to about ambient temperature at a rate of about 200°C per hour. This firing operation pyrolyzed the fructose, yielding a well- bonded preform containing about 2 percent by weight carbon. [0117] The carbon content of the breastplate preform was increased by re-infiltrating with a 70 percent by weight aqueous solution of KRYSTAR 300 crystalline fructose. Specifically, the preform was submerged in the fructose solution for a total of about 20 hours. For about the first 2 hours, an oveφressure of about 60 psi (410 kPa) of air was applied to the solution in an effort to force the solution into the preform more quickly. After halting the pressure application for about 15 minutes, it was resumed at the same pressure. After maintaining the oveφressure for about another 2 hours, the pressure was let back to ambient and the preform was permitted to soak in the solution for the balance of the 20 hours. The breastplate preform was then removed from the fructose solution and wiped with a damp cloth to remove excess fructose solution. The preform was then re-pyrolyzed according to the same thermal schedule as described above. The second pyrolysis step added about 3 percent to the overall mass of the preform. [0118] The breastplate preform had a mass of about 700 grams and had overall dimensions of about 318 mm long by about 241 mm wide by about 4.4 mm thick. The breastplate was slightly curved in the length and width dimensions. The feeder rail preforms were the same size in terms of cross-section as those used in Example 1 to infiltrate square tile preforms. [0119] A lay-up for infiltration was then prepared. [0120] Referring to Figures 4A and 4B, the interior surfaces of a Grade ATJ graphite tray 31 (Union Carbide Coφ., Qarbon Products Div., Cleveland, OH) measuring about 790 mm by about 230 mm by about 51 mm deep was spray coated with a boron nitride slurry or paint 33 in substantially the same fashion as was described in Example 1. [0121] The boron nitride-coated tray was then placed into a larger graphite chamber 35 measuring just slightly larger lengthwise and widthwise than the tray, but being of sufficient height to accommodate the long dimension of the armor plate. The chamber also featured means 37 for supporting a parallel array of graphite dowel rods 39.

Infiltration of the Carbon Cloth and Silicon Carbide Feeder Rails [0122] Referring now specifically to Figure 4B, a single PANEX^® 30 low oxidation carbon cloth 44 (Grade PW03, plain weave, 115 g/m², Zoltek Coφ., St. Louis, MO) weighing about 25 grams and measuring about 790 mm by about 230 mm was placed on the floor of the coated graphite tray 31,33. Four silicon carbide rail preforms 32, each having a mass of about 190 grams, were placed across the width of the cloth 44, and arranged in pairs, one pair on each half of the tray. Sufficient silicon (Grade LP, Elkem Metals Co., Pittsburgh, PA, lump form) and comprising by weight about 0.5 percent Fe (max) and the balance Si, was spread over the surface of the carbon cloth to ensure complete infiltration of the cloth, rails and any preform resting on the rails. The top of the chamber was covered with a loose-fitting (non-hermetically sealing) graphite lid 34 featuring a number of 1 cm diameter through-holes 36 to permit atmosphere exchange. The holes were covered with a piece of graphite felt 38 which was held in place with a graphite block 40 which served as a dead load, thereby completing the lay-up. [0123] The completed lay-up was then placed into a vacuum furnace at about ambient temperature (e.g., about 20°C). The air was evacuated using a mechanical roughing pump, and a rough vacuum of less than about 100 millitorr residual pressure was thereafter maintained. The lay-up was then heated from ambient temperature to a temperature of about 1350°C at a rate of about 200°C per hour. After maintaining a temperature of about 1350°C for about 1 hour, the temperature was further increased to a temperature of about 1550°C at a rate of about 200°C per hour. After maintaining a temperature of about 1550°C for about 1.5 hours, the temperature was decreased to a temperature of about 1450°C at a rate of about 100°C per hour. Without holding at this temperature, the lay-up temperature was further decreased to a temperature of about 1300°C at a rate of about 25°C per hour, which was immediately followed by a cooling at a rate of about 200°C per hour to approximately ambient temperature. [0124] Following this heating schedule, the chamber and its contents was recovered from the vacuum furnace. The silicon infiltrant had melted and infiltrated through the carbon cloth and the rail preforms, thereby converting the carbon cloth to silicon carbide cloth, and forming dense, silicon carbide composite feeder rails. From gravimetric analysis, it was determined that there was about 770 grams of uninfiltrated silicon remaining pooled on the silicon carbide cloth. The contents of the graphite chamber were then re-used to fabricate silicon carbide composite armor breastplates.

Infiltration of Breastplate Preforms [0125] About another 1775 grams of silicon 21 (Grade LP, Elkem Metals Co., Pittsburgh, PA) and comprising by weight about 0.5 percent Fe (max) and the balance Si, was distributed on the silicon carbide fabric between the silicon carbide composite (e.g., infiltrated) rails. Graphite dowel rods 39 measuring about 0.25 inch (6 mm) in diameter and spray coated with boron nitride 33 were placed into graphite holders or supports 37. Four breastplate preforms 11 similarly spray coated with boron nitride 33 were placed across the two rails edgewise in each half of the tray (see Figure 4A). The surface of each preform contacting the rails was left uncoated. The top of the chamber was covered as previously described to complete the lay-up. [0126] The graphite chamber and its contents were then thermally processed in substantially the same manner as was used to infiltrate the carbon cloth and silicon carbide feeder rail preforms. [0127] Following this heating schedule, the graphite chamber and its contents was recovered from the vacuum furnace and disassembled. The silicon infiltrant had melted, infiltrated through the composite feeder rails and into the armor breastplate preforms to form dense, silicon carbide composite bodies. Because each breastplate was supported by the rails in line contact on its width dimension, only light hand-applied force was required to separate the formed breastplate composite bodies from the feeder rails. Only a light sandblasting was required to remove several nodules of silicon infiltrant that had exuded through certain points in the boron nitride coating on the breastplates.

COMPARATIVE EXAMPLE 2 [0128] The technique of Example 2 was substantially repeated, except that silicon carbide particulate was substituted for the boron carbide particulate. As in Example 2, however, the particle size distribution of the silicon carbide blend was such that substantially all particles were smaller than about 45 microns. Following the pyrolysis step, the preforms contained about 75 percent by volume of the silicon carbide particulate and about 4 percent by volume of carbon. [0129] After infiltration with molten Si, the resultant bodies consisted of 84 vol. % SiC (75 original and 9 reaction formed) and 16 vol. % Si (i.e., an Si/SiC composite). A typical microstructure (optical photomicrograph) of the material is shown in Figure 5. [0130] In the optical photomicrograph, it is not possible to differentiate between the original SiC and the reaction-formed SiC. As with the reaction bonded B₄C of Example 2, the reaction bonded SiC ceramic shown in Figure 5 displays little interlocking and clustering of the SiC, thus allowing a relatively fine microstructure to be maintained.

COMPARATIVE EXAMPLE 3 [0131] This example demonstrates the production of a composite body by a reactive infiltration process, the composite body featuring a boron carbide reinforcement. The processing was similar as that of Example 1, with the following exceptions. [0132] The carbon cloth and feeder rails were infiltrated first by themselves; a separate thermal processing was employed to simultaneously infiltrate a total of eight tiles from the infiltrated rails. In place of the boron carbide component, the feeder rail preforms featured silicon carbide as the exclusive reinforcement. More precisely, the feeder rail preforms had substantially the same composition as was described in Comparative Example 1. A single ply of carbon cloth was used instead of two plies. For the first infiltration (of cloth and rails) the amount of the silicon infiltrant was somewhat in excess of that quantity calculated as being needed to completely react the elemental carbon and fill the interstices between the reinforcement bodies, e.g., particulate and fiber, making up the rails and cloth. The bodies resulting from this first silicon infiltration were silicon carbide composite cloth and feeder rails. From gravimetric analysis, it was determined that there was about 800 grams of uninfiltrated silicon remaining pooled on the silicon carbide cloth. [0133] For the subsequent thermal processing for infiltrating the eight preform tiles, about 602 grams of the lump silicon 21 (Grade LP, Elkem Metals Co., Pittsburgh, PA) was distributed on the silicon carbide fabric between the silicon carbide composite (e.g., infiltrated) rails. Eight preform tiles, boron nitride coated as in Example 1, were placed onto the infiltrated rails and supported with boron nitride coated graphite dowel rods as in Example 1. [0134] For both infiltration runs, the heating schedule was substantially the same as described in Example 1. [0135] Following this second infiltration, the chamber and its contents was recovered from the vacuum furnace. The silicon infiltrant had melted, infiltrated through the silicon carbide composite rails and into the tile preforms to form dense, Si/SiC/B₄C composite bodies. Upon recovery of the infiltrated tiles, it was observed that there was a zone about 1-2 cm in diameter extending from each contact point with each rail up into the tile. These zones were of a slightly different shade than the balance of the infiltrated tile, and each featured a crack about 2 cm long extending from the normal shade/off-shade boundary toward the interior of the composite tile. [0136] In Figure 6, a typical microstructure is shown were Si-B₄C reaction has occurred. Coarsening of the structure (i.e., large ceramic clusters within the Si matrix) is clearly evident. If Si-B₄C reaction is allowed to occur, as was the case in some previous work, the microstructure significantly coarsens. (See for example, the above-referenced U.S. Patents to Bailey and to Taylor et al.) A coarse microstructure leads to a ceramic with a larger flaw size, and thus lower strength.

Characterization of Mechanical and Physical Properties [0137] After the fabrication step, various mechanical and physical properties of the instant reaction-bonded ceramic composite materials were measured. Density was determined by the water immersion technique in accordance with ASTM Standard B 311. Elastic properties were measured by an ultrasonic pulse echo technique following ASTM Standard D 2845. Hardness was measured on the Vickers scale with a 2 kg load per ASTM Standard E 92. Flexural strength in four-point bending was determined following MIL-STD-1942A, except for the composite material of Comparative Example 1, where ASTM Procedure No. D790 was used. Fracture toughness was measured using a four-point-bend-chevron-notch technique and a screw- driven Sintech model CITS-2000 universal testing machine under displacement control at a crosshead speed of lmm/min. Specimens measuring 6 x 4.8 x 50 mm were tested with the loading direction parallel to the 6 mm dimension and with inner and outer loading spans of 20 and 40 mm, respectively. The chevron notch, cut with a 0.3 mm wide diamond blade, has an included angle of 60° and was located at the midlength of each specimen. The dimensions of the specimen were chosen to minimize analytical differences between two calculation methods according to the analyses of Munz et al. (D.G. Munz, J.L. Shannon, and R.T. Bubsey, "Fracture Toughness Calculation from Maximum Load in Four Point Bend Tests of Chevron Notch Specimens," Int. J. Fracture, 16 R137-41 (1980)) [0138] Results of density, Young's modulus, flexural strength and fracture toughness of the instant reaction-bonded ceramics are provided in Table I. When appropriate, the results are provided as a mean +/- one standard deviation.

Table I. Property Reaction Reaction Bonded SiC Bonded B₄C Density (kg/m ) 3060 2570 Young's Modulus (GPa) 384 +/- 2 382 +/- 6 Flexural Strength (MPa) 284 +/- 14 278 +/- 14 Fracture Toughness (MPa-m^{1 2}) 3.9 +/- 0.5 5.0 +/- 0.4

[0139] The density of the SiC-based material is about 6% lower than monolithic SiC due to the presence of the Si phase, which has relatively low density. This reduced density is important for applications, such as armor, that are weight specific. The B₄C-based material has very low density and is similar to that of monolithic B₄C. [0140] The Young's moduli of the reaction bonded SiC and reaction bonded B₄C ceramics are essentially the same, and compare favorably with other high performance ceramic materials. The specific results are as predicted based on the Young's modulus values for dense SiC, B₄C and Si of -450, -450 and 120 GPa, respectively. In particular, on a weight specific basis, the reaction bonded B₄C has a very high Young's modulus. [0141] Hardness is a very important parameter for armor materials. Previous work has demonstrated that high mass efficiencies are only obtained versus hard armor piercing projectiles when the projectiles are fractured, and that to effectively fracture the projectile, an armor must have high hardness. (See, for example, MX. Wilkins, R.L. Landingham, and C.A. Honodel, "Fifth Progress Report of Light Armor Program," Report No. UCRL-50980, University of CA, Livermore, Jan. 1971; also C. Hsieh, "Ceramic-Faced Aluminum Armor Panel Development Studies," Appendix 9 of Report No. JPL-D-2092, Jet Propulsion Laboratory, Feb. 1985.) [0142] However, it is difficult to compare the many hardness data in the open literature because results can be highly dependent on test method and technique. Therefore, for the instant invention many different commercial materials were obtained. Hardness measurements were then made on both the commercial materials and the new reaction bonded ceramics of the instant invention in an identical manner so that true comparisons could be made. The results are provided in Table II.

[0143] The reaction bonded SiC and B₄C ceramics have very high hardnesses that are well in excess of both tool steel and WC/Co projectiles. In both cases, the Si/SiC and Si/SiC/B C composites have hardnesses that more-or-less reflect the weighted average hardness of the constituents. In particular, because of the very high hardness of monolithic B₄C, the reaction bonded B₄C has a very high hardness value.

Ballistic Testing [0144] A first round of ballistic testing focused on evaluating the SiC-filled RBSC composite material of Comparative Example 1 to a commercially available hot pressed boron carbide. Candidate ceramic armor materials were provided in the form of square tiles measuring about 100 mm on a side. Among the tiles tested were some that were of substantially the same composition as the silicon carbide breastplates of Comparative Example 1. [0145] The Comparative Example 1 ceramic composite material consisted of about 80 percent by volume of silicon carbide, balance silicon. Its bulk density was about 3.0 g/cc, and its Young's Modulus was about 360 GPa. Further, a RBSC body very similar in composition and processing to this Comparative Example 1 material had a four-point flexural strength of about 270 MPa. [0146] To produce an armor target for testing, the ceramic tile is attached to a SpectraShield^® polymer composite backing layer (AlliedSignal Inc., Morristown, NJ). This material is supplied as a 54 inch (1370 mm) wide roll consisting of 2 plies of unidirectional fibers embedded in a resin matrix, with the fibers of one ply being orthogonal to the fibers of the other ply. A number of 12-inch (305 mm) wide sheets were cut from the roll. The appropriate number of these sheets were then laminated and consolidated in an autoclave at an applied pressure of about 150 psi (1.3 MPa) at a temperature of about 121°C for about 60 minutes, thereby forming a rigid polymer composite plate. Following consolidation, a backing plate measuring about 12 inches (305 mm) square was cut from the 54 by 12 inches (1370 by 305 mm) plate using a band saw or water jet. An approximately 5 inch (120 mm) square region in the center of the backing plate was lightly abraded using 120 grit sandpaper. After cleaning the surfaces to be bonded with isopropyl alcohol, a candidate armor tile was bonded to the center of the backing plate using two plies of 76 microns thick urethane film adhesive. The bond was cured under full vacuum in an oven maintained at a temperature of about 121°C for about 30 minutes, thereby forming a ballistic test coupon. [0147] The weight of the backing plate was varied according to the number of laminates used; the weight of the ceramic tile was varied according to the thickness dimension to which the ceramic tile was ground. In each instance, however, the total areal density (ceramic tile plus backing material) was maintained at roughly the same amount. [0148] A target for ballistic testing was assembled as follows: The ballistic test coupon was placed in front of 28 plies of KM2 (600 denier) blanket with rip-stop nylon and camouflage cordura covers to simulate the outer tactical vest (OTV) of a body armor. The OTV simulant and test coupon were located in front of a 100 mm thick block of Roma Plastiline modeling clay that had been conditioned at a temperature of about 35°C for about 6 hours. The test coupon and OTV simulant were secured to the clay block with duct tape, and the clay block was backed up by a steel support structure that was secured to the test table, thereby completing the target. [0149] The targets were shot at zero degrees obliquity using two different types of 7.62 mm projectiles at varying velocities. Table III shows the comparative ballistic test results against the first threat; Table IV reports the results against the other threat. The basic unit of ballistic penetration resistance used in this testing is the V₅₀, the velocity of the projectile at which partial penetration and complete penetration of the target are equally likely. Normalizing the V₅₀ with respect to the total areal density yields a parameter referred to in this disclosure as "ballistic stopping power". Table III

Table IV

[0150] These results were quite encouraging, and indicated that reaction bonded SiC armor could be made competitive from a performance perspective to some of the leading commercially available (e.g., hot pressed) ceramic armors. Accordingly, the instant inventors continued to pursue development of this approach, leading to the instant boron carbide composite materials.

Ballistic Testing: Round Two [0151] In this second round of ballistic testing, the instant RBBC materials of Example 2 were evaluated as candidate armors, and compared to the SiC-filled RBSC material of Comparative Example 2, as well as to commercial hot pressed B₄C (the control). In one series of tests, the reaction bonded SiC and commercial hot pressed B₄C were tested versus ball rounds as the ballistic projectile; and in a second set of tests, the reaction bonded B₄C and hot pressed B₄C were tested versus armor piercing (AP) rounds. [0152] As in the first round of ballistic testing, 100 mm x 100 mm ceramic tiles were bonded to 300 mm by 300 mm fiber-reinforced polymer plates. Ballistic Properties [0153] The results of ballistic testing are provided in Tables V and VI. In Table V, test results versus a 7.62 mm M80 ball round for reaction bonded SiC and commercial hot pressed B₄C (control) are provided. In Table VI, test results versus a 7.62 mm AP M2 round for reaction bonded B₄C and commercial hot pressed B C are provided. In each case, the tables provide the areal density of the system, the mass efficiency of the target, and the normalized mass efficiency relative to the hot pressed B₄C control. The mass efficiencies in the tables were determined based on available data for rolled homogeneous steel armor (RHA) versus the same threats. Specifically, the mass efficiency was calculated as the areal density of RHA required to give the same performance divided by the areal density of the tested targets.

Table V

Table VI

[0154] The ballistic results show that the armor designs employing lower cost reaction bonded ceramics had mass efficiencies equivalent to armors of the same design using hot pressed ceramics. This has enabled the production of cost effective armor products for various applications. In Figures 7 A and 7C, for example, the aircraft armor and personnel armor tiles were fabricated from SiC-filled RBSC. The vehicle armor plate of Figure 7B was fabricated EXAMPLE 4 [0155] The procedure of Example 1 was substantially repeated, with the following exceptions. [0156] In the preform, the ceramic particulate consisted of nominally 45 micron and nominally 12 micron (median particle sizes) boron carbide particulate (ESK GmbH, Kempten, Germany) mixed in a nominally 70:30 ratio. [0157] Immediately following the freezing step, the gates from the sedimentation casting step were removed, such as with a band saw. [0158] Prior to carbonizing at about 600°C, residual water from casting was removed in a drying step. Specifically, the frozen preform was placed on a graphite setter tray and placed into an air atmosphere convection oven maintained at a temperature of about 110°C. After maintaining the preform at a temperature of about 110°C for about 30 to 60 minutes, the temperature of the oven was raised to about 180°C. After maintaining a temperature of about 180°C for at least one hour, the setter tray and preform were removed from the oven and permitted to cool to about 20°C, and the oven was cooled to a temperature of about 110°C again. [0159] In the setup for conducting the reactive infiltration, instead of spray coating the interior surfaces of the graphite tray 31 with boron nitride slurry or paint, these interiors were lined with SAFFIL alumina fibre sheet material (Saffil Ltd., Cheshire, UK) that had been soaked with the boron nitride slurry, and then dried. [0160] The silicon in lump form that was distributed on the carbon cloth had mixed in it boron carbide particulate (same kind as used in the preform) and THERMAX carbon black powder (Grade N-991, Cancarb, Medicine Hat, Alberta, Canada). To infiltrate about 17.4 kg worth of preform material required about 10.6 kg of silicon, about 0.42 kg of the boron carbide particulate, and about 0.11 kg of the carbon black.

EXAMPLE 5 [0161] This Example demonstrates the production of a boron carbide/silicon composite body by the infiltration of a silicon-based metal into a preform that is substantially free of reactable carbon. In other words, this Example demonstrates the formation of a siliconized boron carbide composite body. [0162] An appropriately sized graphite tray was lined on its interior surfaces with the boron nitride soaked alumina fiber sheet material described in Example 4, and dried. About 70 grams of the boron carbide ceramic particulate blend described in Example 4 was then loosely shaped, not by forming a preform, but instead simply by pouring dry particulate into the lined graphite tray, and organizing the dry powder in a sort of a pyramid shape. A crater was then formed in the middle of this pile of ceramic particulate sufficiently large to hold about 125 grams of lump silicon metal (Elkem). The lined graphite tray and its contents was then placed into a larger graphite chamber with a loose-fitting graphite lid, which in turn was placed into a vacuum furnace at ambient temperature and pressure. The furnace was sealed, a rough vacuum was drawn, and the temperature was raised from about 20°C to a temperature of about 1525°C at a rate of about 200°C per hour. After maintaining a temperature of about 1525°C for about 4 hours, the temperature was decreased at a rate of about 200 °C per hour. Below a temperature of about 100°C, the pressure in the vacuum furnace was let back up to ambient, the furnace was opened, and the graphite chamber was removed and disassembled. [0163] It was observed that the silicon had melted and infiltrated the porous mass of boron carbide particulate to form a composite body. A ground and polished cross-section of the composite material was examined metallographically in an optical microscope. These photomicrographs are presented as Figures 9A and 9B, respectively, and they show boron carbide particulate dispersed in a matrix of metal, presumed to be predominantly silicon metal.

EXAMPLE 6

[0164] This Example demonstrates the production of a siliconized boron carbide composite body. [0165] A boron carbide preform is prepared by a dry pressing operation. Specifically, about 3 percent by weight of a 1 wt% methyl cellulose aqueous solution is added to 220 grit TETRABOR boron carbide particulate (ESK GmbH, Kempten, Germany) and thoroughly mixed in, such as with an Eirich mixer. A quantity of this mixture is then loaded into a steel die and pressed under a load of about 4000 pounds (1800 kg) to produce a tile measuring about 4 inches (100 mm) square and about 3/8 inch (10 mm) thick. The pressed tile is then dried at about 100°C to remove the water, and then bisque fired in argon at about 0.5 atmosphere to a temperature of about 2050°C for about 10 minutes to remove the methyl cellulose temporary binder and to lightly sinter the boron carbide particles. [0166] The bisque fired boron carbide preform is then infiltrated in substantially the same manner as in Example 1. The resulting siliconized boron carbide body contains a matrix of silicon metal along with some boron and carbon, and boron carbide embedded by the silicon metal matrix. The boron carbide is substantially unaffected chemically by the infiltration process, e.g., shows no signs of dissolution or chemical attack by the molten infiltrant. EXAMPLE 7 [0167] This Example demonstrates another embodiment for making siliconized boron carbide. [0168] A preform composition is batched and sediment cast as in Example 1 to prepare a tile preform, except that methyl cellulose is substituted for the fructose. The sediment cast tile is then heated in an argon atmosphere to a temperature of about 120°C using the heating schedule of Example 1, to remove the water. After holding at a temperature of about 120°C for about 4 hours, the preform is then heated to a temperature of about 2050°C at a rate of about 200°C per hour to remove the temporary binder and lightly sinter the B C and SiC particles. [0169] The bisque fired boron carbide preform is then infiltrated in substantially the same manner as in Example 1, except that the infiltration is conducted at a temperature of about 1750°C. The resulting siliconized boron carbide body contains a matrix of silicon metal along with some boron and carbon, and boron carbide embedded by the silicon metal matrix.

INDUSTRIAL APPLICABILITY [0170] The boron carbide composite materials of the instant invention possess exceptional hardness and stiffness, low specific gravity and relatively high flexural strength. Although the instant disclosure has focused primarily on the potential application of the instant materials as anti-ballistic armor, they should also find many applications where rigidity and low specific gravity are important materials properties, such as in the robotics, tooling, and other precision equipment industries. The instant composite materials might also be attractive as abrasives or wear-resistant parts. Where the possibility of boron contamination is not a concern, the boron carbide composite materials of the instant invention may find applications in the semiconductor fabrication industry, such as in air bearing housings or support frames, machine tool bridges and bases, mirror stages and flat panel display setters. The instant composite materials might make desirable mirror substrates. Further, these boron carbide composites may find applications in the nuclear industry, specifically, in applications where neutron absoφtion is important. [0171] The ceramic armors of the instant invention, possessing the desirable properties of low specific gravity and high hardness, should be particularly useful against small arms fire, e.g., as body armor, and as aircraft armor. The present boron carbide materials might also find application as armor for marine vessels and ground-based vehicles, e.g., for heavier threats. [0172] An artisan of ordinary skill will readily appreciate that numerous variations and modifications can be made to the invention as disclosed and exemplified above without departing from the scope of the invention as set forth in the appended claims.

Claims

CLAIMS What is claimed is:

1. A composite material, comprising: a matrix component comprising an alloy comprising silicon having dissolved therein at least one substance comprising boron and at least one substance comprising carbon; and a reinforcement component comprising boron carbide, said reinforcement phase distributed throughout said matrix, said boron carbide being substantially unaffected by said alloy.

2. The composite material of claim 1 , produced by a process comprising providing a molten infiltrant comprising said silicon having dissolved therein said boron and said carbon; and infiltrating said molten infiltrant into a porous mass comprising said boron carbide.

3. The composite material of claim 1, wherein said alloy further comprises at least one substance comprising carbon dissolved in said silicon.

4. The composite material of claim 1 , produced by a reaction-bonding process.

5. The composite material of claim 1 , produced by a siliconizing process.

6. The composite material of claim 1, containing substantially no beta-SiC.

7. The composite material of claim 1 , containing less than about 2 vol% of said beta-

SiC.

8. The composite material of claim 2, wherein there is substantially no chemical reaction of Si with carbon of said porous mass to form SiC in said composite body.

9. The composite material of claim 1, wherein said composite material comprises no more than about 30 percent by volume of said infiltrant phase.

10. The composite material of claim 1, wherein substantially all of said filler material crystallites are smaller than about 106 microns in diameter.

11. The composite material of claim 1 , wherein at least 90 volume percent of said filler material crystallites are smaller than about 55 microns in diameter.

12. The composite material of claim 1, wherein said reinforcement component makes up at least 65 percent by volume of said composite material.

13. The composite material of claim 1 , wherein said boron carbide makes up at least 65 percent by volume of said composite material.

14. A component of a ballistic armor, said component comprising at least one ceramic layer and at least one backing layer placed behind and bonded to said ceramic layer; said ceramic layer comprising at least one boron carbide composite body comprising (a) a matrix comprising an alloy comprising silicon and at least some boron dissolved in said silicon; and (b) at least one reinforcement material comprising a plurality of boron carbide filler bodies dispersed throughout said matrix; wherein said boron carbide composite body has a hardness of at least about 926 kg/mm² as measured with a Vickers indenter using a 1 kg load, and further wherein said boron carbide composite body is characterized by a fine-grained microstructure (i) exhibiting no more than a small or slight degree of interconnectivity of the bodies making up the filler material(s), and (ii) made up of moφhological features, wherein substantially all of said moφhological features are smaller than about 350 microns in size.

15. The ballistic armor component of claim 14, further comprising at least some beta- SiC distributed through said matrix.

16. The ballistic armor component of claim 14, containing substantially no beta-SiC.

17. The ballistic armor component of claim 14, wherein said alloy further contains at least one substance comprising carbon dissolved in said silicon.

18. The ballistic armor component of claim 14, wherein substantially all of said moφhological features are smaller than about 212 microns.

19. The ballistic armor component of claim 14, wherein at least 90 percent by volume of said moφhological features are smaller than about 100 microns.

20. The ballistic armor component of claim 14, wherein said at least one filler material comprises a plurality of crystallites, and further wherein at least 90 volume percent of said filler material crystallites are smaller than about 100 microns in diameter.

21. The ballistic armor component of claim 14, wherein said composite material comprises no more than about 30 percent by volume of said infiltrant phase.

22. The ballistic armor component of claim 14, wherein said matrix comprises no more than about 24 percent by volume of silicon carbide produced in-situ.

23. The ballistic armor component of claim 20, wherein substantially all of said filler material crystallites are smaller than about 106 microns in diameter.

24. The ballistic armor component of claim 20, wherein substantially all of said filler material crystallites are smaller than about 90 microns in diameter.

25. The ballistic armor component of claim 20, wherein at least 90 volume percent of said filler material crystallites are smaller than about 55 microns in diameter.

26. The ballistic armor component of claim 20, wherein at least 85 volume percent of said filler material crystallites are smaller than about 75 microns in diameter.

27. The ballistic armor component of claim 20, wherein at least 97 volume percent of said filler material crystallites are smaller than about 70 microns in diameter.

28. The ballistic armor component of claim 14, further comprising a ballistic stopping power of at least 90 % that of armor grade boron carbide when tested under the following conditions: • 7.62 mm projectile • SpectraShield^® polymer composite backing material • total armor system areal density of about 30 kg/m² • an outer tactical vest simulant comprising 28 plies of 600 denier KM2 blanket having rip-stop nylon • armor grade boron carbide plate thickness of about 0.69 cm.

29. The ballistic armor component of claim 14, wherein said composite material comprises reaction-bonded boron carbide.

30. The ballistic armor component of claim 14, wherein said composite material comprises siliconized boron carbide.

31. The ballistic armor component of claim 14, wherein said backing layer comprises at least one fiber-reinforced plastic material.

32. The ballistic aπnor component of claim 31 , wherein said fiber comprises at least one material selected from the group consisting of polyethylene, aramid and glass.

33. The ballistic armor component of claim 14, wherein said backing layer comprises at least one metal selected from the group consisting of aluminum, iron and titanium.

34. A method for making a composite body, comprising: providing a porous mass or preform comprising at least one filler material comprising boron carbide; providing a molten infiltrant comprising silicon having dissolved therein at least one substance comprising boron and at least one substance comprising carbon; then, in a vacuum or inert atmosphere, contacting said molten infiltrant to at least one of said porous mass or preform; infiltrating said molten infiltrant into said porous mass or preform, and continuing said infiltrating to a desired extent, thereby forming a composite body comprising said boron carbide dispersed in a matrix comprising said infiltrant; and solidifying said molten infiltrant.

35. The method of claim 34, wherein said molten infiltrant contains at least some dissolved boron and some dissolved carbon prior to contact with said porous mass or preform comprising boron carbide.

36. The method of claim 34, wherein said preform is produced by a liquid-phase processing technique.

37. The method of claim 34, wherein an amount of said dissolved boron and said dissolved carbon is sufficient to suppress reaction of said silicon with said boron carbide.

38. The method of claim 34, wherein said infiltrating is conducted at a temperature no greater than about 2000°C.

39. The method of claim 34, wherein said infiltrating is conducted at a temperature in the range of about 1450°C to about 1750°C.

40. The method of claim 34, wherein said porous mass or preform contains substantially no free carbon.

41. A method for making a composite body, comprising: providing a porous mass or preform comprising at least one filler material comprising boron carbide; providing a molten infiltrant comprising silicon having dissolved therein at least one substance comprising carbon; then, in a vacuum or inert atmosphere, contacting said molten infiltrant to at least one of said porous mass or preform; infiltrating said molten infiltrant into said porous mass or preform, and continuing said infiltrating to a desired extent, thereby forming a composite body comprising said boron carbide dispersed in a matrix comprising said infiltrant; and solidifying said molten infiltrant.