FR2607130A1 - FIBROUS REINFORCED SILICON NITRIDE COMPOSITE CERAMICS - Google Patents

Abstract

TOUGH COMPOSITES OF SILICON CARBIDE FIBERS DERIVED FROM SILICON NITRIDE MATRIX ARE OBTAINED BY PREWINNING THE FIBERS WITH PYROLYTIC CARBIDE AND BY CONTROLLING THE NITRURATION OR ANY OTHER PROCESS FOR FORMING THE MATRIX OF NITRIDE FROM NITORIDE. AT LEAST 5 NANOMETERS OF CARBIDE REMAINS IN THE COMPOSITE AFTER ITS FORMATION. THE RUPTURE OF THESE COMPOSITES IS NON CATASTROPHIC. COMPOSITES HAVING SIMILAR PROPERTIES MAY BE OBTAINED BY PRODUCING AROUND THE FIBERS A SENSITIVELY EMPTY OR PARTLY FILLED WITH SILICON NITRIDE BARBS.

Description

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  The present application concerns an invention over which the Government of the United States of America has certain rights in accordance with contract No. F33615-83-C-5006 / P00004

with the United States Air Force.

  The present invention relates to the field of composites

  ceramics consisting of a continuous phase, called indif-

  fiercely also "matrix", and of a discontinuous phase, also called indifferently "reinforcement". The discontinuous phase is at least mainly in the form of elongated fibers. Such materials are generally presented in the art considered as composites

  reinforced with fibers. The invention relates more particularly to

  composites with a mainly formed matrix

  silicon nitrutre and reinforcing fibers

  formed mainly of silicon carbide.

  Like most other ceramics, nitride

  silicon in itself has low ductility, low extensibility

  lity or other capacity to recover after a tension, so that when it is subjected even during a short period of time to mechanical tensions exceeding its resistance, normally it breaks. The practical uses of ceramic objects generally expose them to discontinuous and non-uniform mechanical loads, so that the mechanical tension in small areas of the ceramic can easily exceed the resistance of the ceramic even when the overall tension is well above below the value that would normally lead to failure in the laboratory test. High stresses in small areas can cause cracks to form, and since the cracks themselves concentrate stresses at their ends, a single initial crack can propagate through one

  object a ceramic, which causes its catastrophic rupture

  than. Although the term "catastrophic" is often used loosely to describe the rupture of materials, in the context of the present application, it is used in a more precise sense, with reference to the conventional measurement of the tension induced in a material by mechanical deformation. For most materials, including ceramics, the stress-strain ratio is linear for - 2 - 2s601, 30 for small strains. The increase in deformation can lead to a value, called limit deformation, for which the rate of increase in tension with increasing deformation begins to fall below the value it had for very low tensions. For conventional unreinforced ceramics, the limit deformation coincides with the rupture of the ceramic, so that the tension falls essentially to zero. The rupture of a body is defined as catastrophic within the framework of the present request if the tension on the body at a deformation% greater than the limit deformation is less than 20% of the stress on the body at a deformation 2% less than

limit deformation.

  A process well known in general terms in the art considered for improving the mechanical stability of usually fragile ceramics such as silicon nitride is the reinforcement of the ceramic with

  inclusions of other materials, often of another ceramic.

  Small ceramic fibers or other particles, due to their almost perfect crystallinity, are

  usually more resistant and sometimes more resistant

  your impact as compact bodies, even with the same nominal ceramic composition, which are made by conventional practical methods such as sintering powders or reaction bonding. The reinforcement is not necessarily made of particles having the same composition as the matrix, and often it is advantageous to use a

  different composition for certain specific properties

  res in which it is greater than the matrix.

  In some cases, but not in all, reinforcement, particularly with strong elongated fibers, prevents the

  catastrophic failure of a composite, even under conditions

  tions which one would expect to cause the rupture of the matrix of the composite alone. This improvement in the breaking strength originating from fiber reinforcement is basically attributed to three mechanisms generally designated by the following terms in the prior art: charge transfer, crack bridging and decohesion.

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  Until recently, most new types of fiber-reinforced composites have been made by researchers trying to improve strength or stiffness. For such purposes, it is necessary to achieve a strong bond between the matrix and the reinforcing fibers, so that obtaining this strong bond has usually been the goal. For example, an improvement in the modulus of rupture of composites having a matrix of silicon nitride formed by sintering has been described by Yajima et al. in US Patent 4,158,687. Continuous silicon carbide fibers formed by a special process described in US Patent 4,100,233 were used as reinforcement, and "polycarbosilane" powder was added to the nitride powder silicon to improve the bond between the matrix and the fibers. By these means, a composite body was obtained containing fibers oriented unidirectionally with a rupture modulus (in this case called "flexural strength") of 610 MPa. The composites formed are presented as having good oxidation resistance, good corrosion resistance, good heat resistance and resistance to high temperatures, but nothing is said about the nature.

composite failure.

  In fiber-reinforced composites with such a tight bond, as illustrated in the Yajima patent, cracks resulting from concentrated mechanical stresses in the matrix tend to spread through the fibers and crack them just as well. Recent researchers have discovered that such harmful propagation of cracks can be prevented by surrounding the reinforcing fibers with a crack deflection area. This crack deflection area must have mechanical properties such that most of the cracks that propagate in the area from the matrix are either stopped or follow a

  path that keeps them away from the reinforcing fibers

  is lying. One of the first researchers to recognize the value

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  possible coating of the fibers with weakly bonded coatings appears to be Warren, as shown in European patent application No. 0 121 797 published on October 17, 1984. On page 4, lines 23-25, this application indicates: "poor bonds between fibers and matrix produce tough composites while good fiber / matrix bonds lead to materials

  fragile and susceptible to defects ". In the form of execution

  tion which is believed to be most relevant to this application, the Warren application teaches the formation of a row of carbon fibers, the coating of these fibers while they are in a row with a layer of pyrolytic carbon, machining the resulting porous body to the desired final shape, overcoating with a layer of silicon carbide chemically deposited in the vapor phase, heating to about 1482 C "to obtain stability

  dimensional between silicon carbide / pyrolytic carbon

  that and the substrate "and, ultimately, the over-coating with

  silicon nitride chemically deposited in the vapor phase.

  Because the carbon fibers which form the original substrate are arranged in a row in a fabric, felt or similar structure before coating, the coating is not uniform around the fibers, as clearly shown in Figure 3 of the drawings of the Warren patent application: at the places where two of the original fibers touch in the original row, the coating cannot

  apparently not penetrate them.

  The layer of pyrolitic carbon deposited according to the teachings of Warren was so weakly bonded that the "fibers were free to move to a different degree from the carbon and / or silicon carbide matrix systems". Due to its low coefficient of thermal expansion, silicon nitride has long been considered one of the most attractive ceramics for use in conditions requiring resistance to

  thermal stresses. However, the low mechanical strength

26071-30

  that to the impact of non-reinforced silicon carbide in practically all useful operating temperatures and also its low resistance to creep at temperatures

  high seriously limit its practical applications.

  One of the previous attempts to improve the properties of silicon nitride by including other materials in it has been described by Parr et al.

  in US Patent 3,222,438. This patent teaches the inclusion

  sion of 5 to 10% of silicon carbide powder in metal silicon powder intended to be transformed into a solid ceramic body by treatment with nitrogen gas at a

  sufficiently high temperature to favor the transformation

  tion of silicon to its nitride. This process, called reaction bonding, produces ceramic bodies of coherent silicon nitride having a significantly improved creep resistance compared to those without the addition of silicon carbide powder. The baking bodies were formed from powder by cold pressing in a matrix, and it was recommended to add cetyl alcohol as a binder and lubricant for the powder.

  before compression. The description of this patent recommends

  strongly, and all claims specify, that the

  reactive bonding temperature exceeds 1420 C, which is the melting point of silicon, during part of the bonding cycle. The modulus of rupture of the composite bodies formed is not given, being simply indicated that it could be "favorably (sic) compared to those already published by others".

  The use of relative silicon carbide fibers-

  short for reinforcing ceramics has been described by Hough in US Pat. No. 3,462,340. The orientation of the fibers by mechanical or electrostatic forces is presented as an advantage according to this patent, but no information is given. quantitative about the mechanical properties of the resulting composites. In addition, the matrix of composites described by this patent is limited to "pyrolytic" materials. The term "pyrolitic" is not clearly

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  defined in this patent but it is apparently limited to materials having all their chemical constituent elements deriving from a gas phase in contact with the hot reinforcing filaments and a mold-shaped substrate which determines the internal shape of the body to be conformed. No method is described or suggested in this patent for obtaining silicon nitride as a "pyrolytic" product according to

this definition.

  A use of very short silicon carbide fibers to reinforce ceramic composites having a silicon nitride matrix has been described by Komeya et al. In US Patent No. 3,833,389. According to the teachings of this patent, the matrix was formed by sintering silicon nitride powder rather than by nitriding a metal silicon powder, and the maximum length of the inclusions of fiber fibers. silicon carbide was 40 microns. It was necessary for a rare earth component to be present in the matrix in addition to the silicon nitride and the highest modulus of rupture (called "breaking strength") was 375 megapascals (hereinafter called MPa). A

  much more recent publication, P. Shalek et al. "Hot-

  pressed SiC Whisker / Si 3N4 Matrix Composites ", 65 American Ceramic Society Bulletin 351 (1986), also used hot-pressed silicon nitride powder with elongated" carbide "beards of silicon carbide as reinforcement,

  but these beards are still no more than 0.5 mm long.

  The use of silicon carbide barbs in other matrices is taught in US Pat. No. 4,543,345 of September 24, 1985, in the name of Wei (matrix of alumina, mullite or boron carbide) and the patent US No. 4,463,058 of July 31, 1984 to Hood et al. (mainly

metal dies).

  A composite with a reinforcement of continuous and oriented silicon carbide fibers is described by Brennan et al. in US Patent 4,324,843. The matrix described by Brennan was a crystalline ceramic prepared by heating a non-crystalline glassy powder of the same composition

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  chemical than the desired matrix in the composite. This

  description of the matrix appears to exclude nitride from

  silicon which is not described in the patent as a material

  possible for the matrix. In fact, the most-

  Broad of this patent requires a matrix of metallic alumino-silicate or of melanrige thereof. Perhaps for this reason, the highest modulus of rupture indicated in this patent for any of the products is less than 100 MPa. Another microstructural variant of silicon nitride-silicon carbide composites is described by Hatta et al. in US Patent 4,335,217. According to this patent, neither the fibers, nor the powder of silicon carbide or silicon nitride are used as initial constituents of

  composite. Instead, a powdered polymer is mixed

  slow containing both silicon and carbon with silicon metal powder, it is compressed and then heated under a nitrogen atmosphere. The polymer gradually decomposes on heat to give silicon carbide, while the silicon powder reacts with nitrogen to form silicon nitride. The composition of the final composite is described as "comprising crystals of beta-silicon carbide, alpha-silicon nitride and beta-silicon nitride ... forming interwoven textures of beta-silicon carbide among said crystals of alpha-silicon nitride and beta-silicon nitride without bond

  chemical to create micro-spaces ... for absorption

  tion of thermal tensions ". The highest modulus of rupture

  reported for these composites was 265 MPa.

  In this Hatta patent, reference is also made to "conventional SiC-Si3N4 composite systems ... produced by baking mixtures of silicon powder with ... SiC fibers in a nitrogen gas atmosphere at a higher temperature at 1220 C ". No further details are given, however, as to how to prepare these

  composites supposed to be classic.

  Most of the non-patent literature in the field of silicon nitride and silicon carbide composites which, in general terms, covers the same information as the patents indicated above, has been summarized by Fischbach et al. in their final report to the Department of Energy under the references Grants ET-78-G-01-3320 and DE-FG-01-78-ET-13389. These researchers found that the types of fibers which are said to have been used with great success by Yajima in US Patent 4,158,657 were not very satisfactory with regard to their binding due to the tendency of the of these fibers to separate from the outer layer of fibers

during nitriding.

  Metallic coatings for ceramic reinforcing fibers described in US Pat. No. 3,869,335 of March 4, 1975 in the name of Siefert. These coatings are believed to be effective because the ductility of the metals allows absorption of the crack propagation energy by deformation of the metal. Composites with metallic coated fibers are satisfactory for use at relatively low temperatures, but at elevated temperatures the metallic coatings can melt and therefore seriously weaken the composite. The matrices taught

  by Siefert were of glass, with possibilities of use-

  sation at lower temperatures than ceramics.

  So, for ceramics, temperature limitation,

  brought by metallic coatings on reinforcements

  is a serious drawback.

  US Patent Application No. 700,246 filed February 11, 1985 by Rice teaches the use of boron nitride as a coating for ceramic fibers for the purpose of producing a crack deflection area when the coated fibers are incorporated into the composites. We are specifically talking about silicon carbide, alumina, graphite and silica fibers, silicon carbide, cordierite, mullite and zirconium oxide. The results have been very variable. The tenacity of silicon carbide fiber composites in silica matrices has been

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  considerably increased by a coating of about 0.1 micron of boron nitride, but the same type of coated fibers used in zirconium oxide or cordierite produces little improvement in the toughness of the composite compared with composites uncoated fibers. One reason for the effectiveness of boron nitride has been

  suggested, and there is additional relevant information

  in European patent application No. 0 172 082 filed by the European Propulsion Company, published on February 19, 1986. This application teaches that boron nitride applied to fibers by gas phase reactions between gases containing boron and nitrogen, as well as the carbon coatings produced by certain types of pyrolysis, is deposited on the fibers in lamellar form with relatively weak bonds between the layers or strata. Thus, a crack which penetrates into the coating will normally see its direction of propagation change if necessary, so that the crack will propagate along an interface between the layers of the coating. These interfaces are parallel to the fiber surface so that the crack is usually prevented from entering the fiber. The carbon and silicon carbide fibers in the silicon carbide matrices are specifically described in this application and other matrices are suggested, such as alumina formed by decomposition.

aluminum butylate.

  The advantages of the crack deflection area around the reinforcing fibers in a different matrix are

  illustrated by John J. Brennan in "Interfacial Characteriza-

  tion of Glass and Glass-ceramic Matrix / NICALON SiC Fiber Composites ", in a report presented at the conference on Tailoring Multiphase and Composite Ceramics, held at Pennsylvania State University, July 17-19, 1985. This publication teaches that certain conditions of treatment lead to composites in which a carbon-rich layer is formed around the SiC reinforcing fibers, and the carbon-rich layer acts as a deflection zone of

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  cracks. Likewise, the advantages of boron nitride coatings on the reinforcing fibers are described by B. Bender et al. in an article titled "Effect on Fiber

  Coatings and Composite Processing on Properties of Zirconia-

  Based Matrix SiC Fiber Composites ", 65 American Ceramic Society Bulletin 363 (1986). As would be expected from the titles of these reports, neither

  teaches to use silicon nitride as a matrix.

  J.W. Lucek et al. in an article entitled "Stability of Continuous SiC (0) Reinforcing Elements in Reaction Bonded Silicon Nitride Process Environments", Metal Matrix, Carbon and Ceramic Matrix Composites, NASA Conference Publication No. 2406, pages 27-38 (1985>, describe matrices of silicon nitride reinforced with silicon carbide fibers having a diameter of about 10-25 microns. These SiC fibers were derived from starting materials formed from organosilicon polymers. With these silicon carbide fibers, we did not obtained highly resistant and non-brittle composites. Lucek et al reported, based on information provided by others, that some of the fibers they had used had been pre-coated with boron nitride. the fibers actually have

  been coated has been questioned since the original report.

  Lucek et al., Due to government security restrictions

  imposed on them in return for supplying the fibers intended to be coated, did not attempt to characterize the composites they had prepared to a sufficient degree to determine whether boron nitride, or any other material, was actually present around the silicon carbide fibers derived from the polymers (DP) they had used, after the composites were

  made from fibers intended to be coated.

  It is known that the SiC DP fibers are subject to a certain recrystallization, with concomitant shrinkage in volume, and to partial volatilization, probably preceded by a chemical reaction which gives volatile products, when heated in the temperature range - l - 2607130 necessary for the formation of reaction-linked silicon nitride (NSLR). In contrast, silicon carbide fibers subjected to chemical vapor deposition (DCV), also studied by Lucek et al., Are much less prone to harmful changes during nitriding. Lucek et al. determined that the tensile strength of their SiC DP fibers intended to be coated with boron nitride deteriorated significantly less after exposure of the fibers to the nitriding temperature and atmosphere than the tensile strength of similar fibers not coated. However, they further determined, by flexural testing of composites made with different SiC fibers, that (1) the strength of NSLR composites reinforced with both types of SiC DP fibers was significantly lower than that of fiber reinforced composites SiC DCV, (2) the strength of such composites made with DP fibers intended to be coated was even even lower than that of similar composites having uncoated DP fibers and (3) the tensile failure of

  composites with both types of DP fibers was essential-

  slightly catastrophic. It can be assumed that an unidentified phase of the process for manufacturing fibers in composites destroys and / or modifies the properties of any coating carried by these fibers, so that when it is bonded in the NSLR matrix, the coating does not acts more

  effectively to deflect cracks.

  At the present time, DCV silicon carbide fibers are very expensive, but it is believed that if significant demand develops, DP fibers could be made at a cost much lower than that of DCV fibers. There are also fundamental advantages to the smaller diameter of the DP fibers: the smaller fibers are more flexible and flexible, in particular for the reinforcement of complex shapes which require resistance in more than one direction and which have thin sections. It is difficult in practice

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  to arrange a single thickness of fibers in a plane in one

  row which will give a substantially isotropic reinforcement.

  It is therefore more common to use fibers within a single layer in substantially parallel rows and to superimpose layers of such fibers with different

  directions for obtaining sensitive mechanical properties-

  isotropic. Naturally, if one wishes to have an object whose thickness is a little greater than that of a layer of DCV fibers, such an arrangement is impossible with such fibers, but it could be obtained with DP fibers whose diameter may be less than a tenth that of DCV fibers. The smaller, softer DCV fibers can also be more easily arranged with sharp bends in the desired composite. On the other hand, the fundamentally higher thermal stability of DCV type fibers makes their use safer in

  composites intended for use at high temperatures.

  For these reasons, it is advantageous to provide strong and stubborn NSLR composites having reinforcements made of DP fibers or formed of other small diameter SiC fibers, as well as reinforcements using larger fibers, and the composites of two types can be made depending on the

present invention.

  A generalization * which appears clearly from the prior art recalled above is that the properties of composites of silicon nitride and silicon carbide, like those of composites in general, are very sensitive to the details of the microstructure of the composite. (A similar conclusion is given in the previously cited Fischbach document). The micro-structural details are in turn sensitive to the chemical and physical characteristics of the starting materials and to the process used to transform

  said starting materials in a coherent composite body.

  Until now, mechanical tenacity could hardly have been predicted

  new and different composite micro-structures.

  Silicon carbide fibers having at least 1 mm in length can be used more advantageously than short fibers for reinforcing composites having a matrix of silicon nitride, in particular a matrix formed by reaction bonding. The terms "silicon carbide fibers" or "SiC fibers" as well as the grammatical variations of these terms, must be understood, within the framework of the present application, as including any material in fibrous form having at least 55% of its atoms constituents formed of silicon and carbon. This expressly includes the DP type of fibers already described above, which are known to be amorphous under certain conditions and to contain substantial amounts of oxygen and nitrogen atoms, and fibers having a nucleus in any other material such as carbon. The moduli of composite products reinforced with silicon carbide fibers are particularly high if substantial regions of the composite produced contain long fibers which are substantially rectilinear and mutually parallel, with an orientation transverse to the direction (s) of the largest deformations exerted on the current composite

of use.

  At a minimum, the strength and the number of individual fibers in the matrix must be large enough for the fibers to be collectively capable of carrying the load on the composite after the matrix has broken. Thus, if necessary, a total load transfer from the matrix to the fibers can occur without mechanical failure due to a tensile overload. In mathematical terms, if 1c is the maximum load that the composite C can undergo without breaking the matrix, Vf is the proportion of fiber in the cross section of the composite transverse to the application of the force, and t is the resistance to f

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  traction per unit area of the fibers, in this case the ratio lc / Vf must be less than or equal to tf. For security purposes, it is preferable that the ratio lc / vf is significantly less than tf. As can be seen from the relationship given above, if the total strength of the composite is increased and fibers having the same tensile strength are used, it may be necessary to increase the proportion of fibers to meet this requirement. criterion. For example, if the total composite has a tensile matrix rupture at 552 MPa and the fiber proportion is only 30%, it is necessary to have a fiber tensile strength of at least 1 , 84 GPa. If the proportion of fibers is raised to 60%, fibers having a tensile strength of 0.94 GPa would be suitable. The proportion of fibers should normally be between 20 and 80% in

volume in composites.

  The resistance of the composites according to the invention to catastrophic failure can be particularly increased by providing, in the final composite, zones for deflection of the cracks surrounding practically all the reinforcing fibers. A crack deflection area is a

  region with significantly different mechanical properties

  both of the matrix and of the reinforcing fibers. In general, crack deflection occurs most effectively in materials with new potential surfaces which can be formed with less energy than that necessary to form other possible new surfaces by breaking the material of the deflection zone. The surfaces whose formation requires such a lower energy can result from the anisotropy of surface energy within the crystals, from the grain boundaries in polycrystalline materials,

  or other similar phenomena.

  The crack deflection area often differs from

  chemical composition both with respect to the reinforcing fibers-

  ment than the matrix, but a difference in morphology could also be sufficient. A characteristic that

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  particularly preferred for a crack deflection zone is the presence of at least one surface of

  preferred slip extending in an approximate direction

  parallel to the surfaces of the reinforcing fibers.

  The preferred sliding surface can be, and often is, at the interface or in the vicinity of the interface of the area of deflection of cracks and either of the fibers or of the matrix, as a result of phenomena such as modifications of interface composition, coefficient differences

  thermal expansion or morphological differences.

  Such an interfacial sliding surface is considered to be part of the crack deflection zone in

as part of this application.

  Examples of materials suitable for forming zones of

  deviation include the carbon that has been deposited chemical-

  ment in the vapor phase (also called pyrolytic carbon), boron nitride and the 2H (d), 27R, 16H, 21R, 12H and 32H polytypes of the aluminum-nitrogen-silicon-oxygen system. The

  Pyrolytic carbon is particularly preferred.

  In the co-copending United States patent application no. 893,747, there are described in detail composites having reinforcing fibers whose diameter is greater than 100 microns. This application focuses on composites

  having fibrous reinforcements of smaller diameters.

  In the drawings: FIG. 1 shows the program in duration and in temperature of the short nitriding according to the present invention; FIG. 2 illustrates the tension-strain relationship for certain composites of silicon carbide fibers, uncoated or with different coatings, in an NSLR matrix; FIG. 3 shows the tension-strain relationship for a composite made according to the invention, with a non-catastrophic failure; FIG. 4 shows the tension-strain relationship for another composite according to the invention and compares it with that

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  a composite made using the same initial components but with different nitriding conditions which give a different result; Figures 5 and 6 are micro-photographs of a cross-section taken through composites of the type whose weakening curves are given in Figure 4. Reinforcement fibers Two effective types of DP silicon carbide fibers for making the composites according to the invention are commercially available under the brand name Nicalon fibers from Nippon Carbon Ltd., Tokyo, Japan. These fibers, described by the manufacturer as being derived from polycarbosilane precursors, have a diameter between 8 and 20 microns and are available in two qualities: standard and ceramic. The latter, which has a higher bulk density and less oxygen as an impurity, is generally preferred over the standard quality for the manufacture of composites according to the present invention. Another effective type of fiber is that supplied by Dow Corning (not yet commercially) under the designation MPDZ, lot no. 33050-22-1. These fibers are said to be derived from

  methylpoly (disilylazane). MPDZ fibers provide

  sites with greater tenacity when used according to the invention than do Nicalon fibers. Other fibers, which have not been specifically tested, could

be as effective.

Crack deviation zones

  Crack deviation zones can be favored

  carefully created by at least two different processes: (1) coating of reinforcing fibers with a suitable material to serve as a crack deflection zone then preserving and / or improving the properties of the coating during nitriding, or ( 2) creation of the crack deflection zone during nitriding from the fiber materials, the matrix, and / or the nitriding atmosphere. Either method is

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  able to give good results; which one you prefer depends on the expected results and the type of fibers used. Coating: a suitable coating consists of pyrolytic carbon according to the method taught by J. V. Marzik, "CVD Fibers", Proceedings of the Metal and Ceramic Matrix Composite Processing Conference, Vol. II, p. 39-65 (Conference held at the Battelle laboratory in Colombus from 13 to 15 November 1984). Normally an initial thickness of at least 1 micron is preferred, but the most important characteristic of the coating is its ability to deflect cracks after treatment and not the initial thickness.

of the coating.

  In situ generation: It has been observed that the diameter of the SiC DP fibers normally decreases during nitriding. The mechanism of the size reduction is not clear, but it can be reasonably attributed to a loss of surface mass, a reduction in the void volume of the fibers and to chemical reactions. The reduction in diameter can produce an empty or almost empty space around the fibers after a few nitriding cycles, as illustrated in some of the examples below. In other examples, the space between the dense parts of the reinforcing fibers and the mass of the matrix is filled with a variable quantity of materials which are presumed to be derived from the reaction between the fiber, the matrix

  and / or the nitriding atmosphere during nitriding.

  In at least one case, the area around the fibers contained

  silicon nitride beards, although in short supply

  health to fill the space. An effective crack deflection area can be either a thin space or an area filled with porous material, such as barbs

randomly oriented.

  Green body, elimination of the binder and sintering The arrangement of the fibers in the desired shape for the final composite can be carried out by any means conventional in the technique considered. When we think

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  that the final product must be subjected to tensions during its use basically in a single direction, the fibers must be arranged, as much as possible, transverse to this direction so that the expected tension will have to bend the fibers to deform the body that these fibers strengthen. For applications where the voltages are applied in different directions,. it may be advantageous to use several layers, the fibers having a parallel orientation in each layer, but a different orientation from one layer to another. In many cases, however, it will be appropriate to use fibers as short as one millimeter in length with a

  relatively random orientation.

  Many final product structures can be efficiently assembled from thin, flat "ribbons" containing oriented fibers. To make such ribbons, a sufficient number of fibers or fiber tows are made to support the desired width by any suitable mechanical means, in a monolayer, with the fibers substantially rectilinear and parallel to each other. This row of fibers is supported in any suitable manner so that it can be coated with a slurry of silicon powder and at least one polymeric binder in a suitable solvent. The preferred silicon powder is of technical quality, with a nominal purity of 99%, and an average particle size of about 3 microns. (Suitable material has been obtained from Elkem Metals Co., Marietta, Ohio). Although many natural and synthetic polymeric materials, such as polyvinyl acetate, vegetable gums, etc., can also be used as

  polymeric binders, the preferred one is poly (vinyl-

  butyral) plasticized, sold under the brand Butvar 891

  by Monsanto Chemical Co., Springfield, Massachusetts.

  About 26 parts by weight of silicon, 8.6 parts by weight of polymer (including the plasticizer), 2 parts by weight of acetone, and 63.4 parts were mixed together.

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  by weight of a suitable solvent such as isopropyl alcohol

  than. The mixture was applied as a coating by any

  what means, such as manual application, spraying

  tion, a brush display, a curtain coating, etc. on the prepared row of silicon carbide fibers to a depth sufficient to cover the fibers after drying. The combination of fibers and slurry is dried at about 20 ° C for about 2 hours in the ambient atmosphere, giving a coherent flexible ribbon from which the

  solvent has been substantially removed.

  The ribbons thus obtained can be spread by

  conventional means to take any desired final shape.

  To make composite samples for testing purposes, suitable lengths of tape thus prepared were cut, stacked on top of each other, while maintaining a common direction of orientation to the fibers in the pieces of cut tape. , and they have been mechanically pressed in a direction perpendicular to the plane of the ribbon segments in the pile, preferably under a pressure of at least 0.4, but not exceeding 0.7, MPa and at a temperature of about 100 C. In a classic example, compressed blocks 50 mm wide were thus prepared,

  mm long and 6-8 mm thick.

  The compressed blocks, or other bodies of any shape, are then treated to remove the polymeric binder which they contain. Preferably this is done by heating the bodies in an inert gas atmosphere with a rate of temperature rise of approximately 1150 C per hour to obtain a final temperature of approximately 1150 C, maintaining at this temperature for approximately 15 minutes, then cooling by natural convection at an estimated speed of between 100 and 200 C per hour. During the heating process, the flow rate of the inert gas must be kept at a volume sufficient to cause any significant gaseous decomposition product formed and the bodies must be kept under pressure. By this process, the original content of polymeric binder is practically

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  completely removed from the bodies, but due to the sintering of the silicon powder particles, the bodies remain coherent. Nitriding The bodies freed from the binder are transformed into their final ceramic form by heating the bodies in an atmosphere of nitrogen gas having a chemical purity of at least 99.998%. Preferably, the nitriding is continued as long as it is necessary to transform practically all the elementary silicon contained in the body into silicon nitride. Because silicon has a much lower melting point than silicon nitride, the presence of residual elemental silicon can limit the use of final composites at high temperatures. A complete absence of silicon is preferred, as revealed by an X-ray diffraction analysis, which analysis could detect as little as 0.1 atomic percent. In the process of forming reaction-bonded silicon nitride (NSLR) without reinforcement, which has been established for a long time, it has become common to use an extended nitriding cycle to maximize the amount of alpha-crystalline form silicon nitride in the final product. It was believed that crystals of the alpha form produced a more resistant product than those of the beta form which predominate in products prepared under higher nitrogen gas pressure and with time.

  corresponding shortcuts. Milking cycles

  100 hours with a maximum temperature of 1410 C

  are common for making NSLR monolithic bodies.

  Such long nitriding cycles, as used for monolithic NSLRs, can sometimes be effective in making composites according to the invention, but they are rarely preferred, assuming that they are sometimes. Instead, we generally prefer much shorter cycles that can reach only 8 hours at a maximum temperature of 1350 C. If we want to create

- 21 - 26713

  a crack deflection zone containing barbs, an intermediate length cycle, such as 48 hours, may be preferred. Specific modes of implementation of

  the invention are given in the examples below.

  The nitriding of the preparation of the composites according to the invention is normally carried out in a vacuum oven with cold walls, controlling either the flow or the pressure. For controlled flow nitriding, which is the preferred method for in situ generation of crack deflection zone, the binder-free composite samples are first heated under vacuum to a temperature of about 11000C . Then nitrogen gas is admitted into the furnace chamber until the pressure

  total oven has reached a desired initial value.

  The gas flow is then interrupted and the temperature is raised at the rate of about 100 C per hour. As the temperature rises, the pressure in the furnace rises first, then drops as the nitrogen is transformed into non-volatile silicon nitride. The pressure drop is controlled by a detector and when the pressure has dropped to a predetermined excitation value, a solenoid valve controlled by the detector allows the introduction of additional nitrogen gas into the furnace at a flow rate constant during the rest of the nitriding. A nitriding cycle with pressure control is preferred for use with carbon-coated fibers. According to this process, the pressure of the nitrogen gas is controlled throughout the nitriding to a predetermined fixed value. One could use initial nitrogen gas pressures between 0.55 and 200 atmospheres, and durations between 6 and 48 hours, with final temperatures between 1325 and 1400 C for such nitriding cycles as part of the implementation of the invention. The only necessary conditions are the maintenance of sufficient tensile strength in the

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  fibers to meet the strength criterion already given above and that the properties of the interface between the fibers and the matrix, including any existing crack deflection zones, are adequate to avoid catastrophic failure of the composites produced after nitriding. A nitriding cycle at controlled pressure which is especially preferred for composites using coated Nicalon fibers is a cycle in which the initial nitrogen gas pressure is at least 1 atmosphere, the duration does not exceed 15 hours between the first heating above room temperature and the start of cooling, and a maximum temperature not exceeding 1375 C, of which no more

10 hours above 1200 C.

  After the termination of the nitriding treatment, the composite ceramic bodies are preferably cooled at a speed not exceeding 200 ° C. per hour. The end result is a ceramic body resistant to thermal and mechanical shock and suitable for long-term service at temperatures up to around 1200 C, at least in a non-oxidizing atmosphere. These composites contain 20 to 50% by volume of silicon carbide filaments, and it is believed that the density of the silicon nitride composing the matrix

  composites is between about 1.8 and 2.0 g / cm3.

  The mechanism of the initial elasticity rupture of

  composites made according to the invention is not catastrophi-

  than. With all of the prior art SiC reinforced NSLR composites known to the Applicant, the first reduction in ability to withstand loads under tension usually results in total failure, with obvious detrimental consequences for the integrity of any structure

made of such materials.

  The scope and diversity of the invention may be

  more appreciated from the following examples.

Examples 1-5

  In all of these examples, SiC DP fibers of ceramic quality obtained from Nippon Carbon were used. Uncoated fibers were used in Example 1; the

- 23 -673

  coatings and the corresponding example numbers are

  indicated in Table I below.

TABLE I

  CHARACTERISTICS OF THE COATING AND FIBERS

  N Characteristics of the coating Composition Thickness Morphology Resistance to Microns the tensile strength of Gigapascals fibers

  _____________________________________________________________

  1 None __ 2.3 + 0.5 2 Alumina, stoichiometric 0.1-0.15 Bridging 2.3 + 0.5 coarse fibers

3 Silicon carbide

  ci.um, rich in carbon 0.5-0.6 Bridging of Fragile fibers

4 Pyro carbon

  lythic 1.00 Continuous 2.0 + 0.6 Silicon nitride, stoichiometric 0.2+ 0.1 Cracked 2.0 The coating thicknesses shown in Table I were estimated by scanning electron microscopy

(MBE).

  The composites using fibers of each of the types shown in Table I were first prepared by

- 24 -

  winding or spreading the tow by hand unidirectionally to form a row of fibers

  essentially parallel over a thickness of a tow.

  The ends of the fibers have been fixed with tape to maintain the alignment of the fibers during coating

and other treatments.

* A coating material was prepared in the form of a slurry having the following composition, all the parts being expressed by weight: 2propanol 63.4 parts Poly (vinyl butyral) 4.3 parts Butyl benzylphthalate 4.3 parts ( plasticizer) Acetone 2 parts Powdered metal silicon 26 parts Powdered metal silicon was used having an average particle size of approximately 3 microns and a purity of approximately 99%, the main impurity being constituted by 0.7% of iron . The slurry was sprayed on one side of the previously prepared row of carbon fibers to a minimum thickness of about 15 microns. The slurry had sufficient viscosity to remain on the fibers. After the application of the spray mixture on a first face, the coated composite was dried at approximately 20 ° C. for approximately 2 hours. The row of fibers is then coherent so that it is possible to turn it over without disturbing the alignment of the fibers. The opposite face of the row was coated using the same slurry to a minimum thickness of around 5 microns and then dried under the same conditions as the first face. The dried composite, coated on both

  faces, was flexible and, in what follows, it is called "ribbon '".

  We cut 75 mmn squares from this ribbon. About 8 of these squares were stacked with the direction of the fibers remaining the same in all the squares and the stack was then compressed at 100 C in a 3f matrix

- 25 -

  of steel under a pressure of about 21 MPa. The polymer bonded composite formed by the first pressing was then transferred to a hot graphite pressure matrix and further compacted at 1150 C under a pressure of about 21 MPa and under an atmosphere of argon gas flowing for about 15 minutes. This second hot pressing was used to sinter the silicon matrix, to pyrolyze it and to remove the components forming the fugitive organic binder: poly (vinyl butyral) and butyl benzylphthalate. The result of this process was a coherent composite having a matrix of

  silicon metal and silicon carbide fibers.

  The sintered silicon bodies were then nitrided under the conditions indicated in FIG. 1 to form the final composites. As a result of this treatment, the metallic silicon matrix has been substantially completely transformed into silicon nitride, with an increase in weight of 66.5% and an increase in volume of 22%. The increase in volume is however absorbed by the pores of the silicon matrix forming the composite, so that it does not result in any modification in its external dimensions during the

  transformation into silicon nitride.

  The silicon nitride composite was sliced into test samples along two sets of perpendicular planes, each of which was parallel to the direction of the included silicon carbide fibers. These samples were used for strength tests, performed in a three-point bending configuration with a gap of 62 mm, a constant movement speed of 0.127 mm / minute. The gap-to-thickness ratios have been understood

  between 16: 1 and 46: 1, depending on the thickness of the sample

  lon. The surfaces of the samples such as nitrided have not been modified, except to the extent necessary for the

  cutting samples to the size desired for the accomplishment

  strength test. The samples were used both for quantitative strength assessments, as shown in Table 2 and for

  qualitative estimates of failure behavior.

2 6 0i 13 0

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  The short nitriding cycle produced composites with non-catastrophic failure with the carbon-coated fibers (Example 4). The same coated fibers produced composites with catastrophic failure when using controlled flow nitriding conditions for 48 hours. All. other types of coated and uncoated fibers, i.e. examples 1 to 3 and 5, gave composites with even catastrophic failure

  with short nitriding cycles. Examination of the interfaces

  these in composites by Auger spectrometry and the MBE indicated the presence of a carbon layer at least 60 nanometers thick around the SiC fibers in the

  non-fragile composite of Example 4.

  The deformation limits, the maximum tolerable deformations and the stresses at the deformation limits of the composites of examples 1 to 5 (all with nitriding conditions as indicated in FIG. 1), except for example 3 which was nitrided for 48 hours with a maximum nitrogen pressure of 0.7 atmosphere and a maximum temperature of 1380 C, are given in Table 2 below. The figures in this table have been

  determined at a traveling speed of 0.12 mm / minute.

TABLE 2

  VOLTAGE AND DEFORMATION LIMITS FOR PRODUCTS FROM

EXAMPLES 1 TO 5

  Example Proportion Limit of Voltage Limit ma-

  fiber number deformation maximum x-tension

in volume% MPa portable MPa

- - - - -

1 0.27 0.187 252 252

2 0.20 0.191 376 376

3 0.07 0.110 194 194

4 0.21 0.214 285 390

5 0.18 0.260 180 180

  SNLR unreinforced 0.00 0.140 315 315

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  The weakening curves of the composites produced

  by examples 1 to 5 are represented in FIGS. 2 and 3.

  The non-catastrophic failure of the composite of Example 4 allows it to reach a maximum bearable tension at 0.396% deformation almost equal to twice its deformation limit, while, for all the other composites shown in Figure 2, the voltage limit was equal to

the maximum bearable voltage.

  All the composites made according to examples 1 to 5 showed a significant degree of charge transfer from the matrix to the fibers during the tests, but in each of them, except for example 4, the effective crack deflection zone was apparently insufficient around the fibers to give a non-catastrophic break. This means that post-treatment strength retention by fibers, load transfer, and effective crack deflection areas around the fibers are all important factors in achieving the most stubborn composites. Example 6 In this example, uncoated Nicalon fibers of ceramic quality were used, as in Example 1, but the nitriding cycle was increased to 48 hours. The composite product according to this example offers improved toughness compared to the product of Example 1. Microscopic examination of the composite indicates the formation of an area partially filled with a material distinct from both the fibers and the matrix, around the fibers in the composite, while a

  such a zone is not observed in the product of Example 1.

  The presence of this zone seems to play a role in the difference in tenacity. The general physical properties of this composite were not, however, as good as that of the product of Example 4. It is believed that. this difference comes from the degradation of the tensile strength of

  fibers during the relatively long nitriding cycle.

Example 7

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  This example is similar to Example 6 except that standard quality Nicalon fibers were used rather than ceramic quality fibers. In this case, the area around the fibers after nitriding contains randomly oriented silicon nitride barbs which were only very poorly consolidated, assuming

  that they are, with the silicon nitride matrix.

  Qualitatively, the composites in this example were found to be slightly more tenacious than those in Example 6 and considerably more tenacious than those in Example 1, but they nevertheless suffered a catastrophic failure when they

are under stress.

  Examples 8 and 9 For these examples, uncoated MPDZ fibers were used instead of Nicalon fibers. For Example 8, a 48 hour controlled flow nitriding cycle was used and, for Example 9, an 8 hour controlled pressure cycle. The rest of the treatment was the same as in Example 4. The weakening curves of the resulting products are given in FIG. 4. The composite resulting from the longest nitriding cycle, gives a non-catastrophic rupture. It is believed that the difference from Example 6 is due to higher strength of the MDPZ fibers to loss of tensile strength under nitriding conditions. With MDPZ fibers, short nitriding cycles, such as that used in Example 4, give composites subject to catastrophic failure. This is believed to be due to the fact that the shorter nitriding time does not result in the formation of a space around the fibers in the composite, as does the longer cycle. The difference is apparent in the cross sections of

  composites shown in Figures 5 and 6.

  Although these examples relate to rows of unidirectional fibers or filaments, it should be understood that

  the invention is not limited to such an embodiment.

  The composites prepared according to the invention are useful in all current applications of silicon nitride

- 29 - 2

  including, but not limited to, sheaths for thermocouples, rod feet for die casting under reduced pressure, crucibles, seals for furnaces, and caps for non-ferrous metal foundries, particularly aluminum ; degassing tubes and liner plates for primary aluminum foundries; precision mounting devices and fixing devices for soldering, soldering and other heat treatments for the manufacture of electronic and semiconductor components, jewelry and other metallic or glass objects requiring heat treatment; wear-resistant fasteners for optical devices, nozzle guides and electrode holder for electrodischarge machining, or guides and jig for electrochemical machining, welding nozzle and insulator; components of pumps, valves and containers for the treatment of corrosive chemicals and abrasive mixtures, artificial teeth and dental bridges; weakly detectable structural fairings and similar structures for aircraft and other vehicles that will be exceptionally difficult to detect by radar; engine components capable of operating at higher temperatures than engines with

  all-metal combustion chambers.

  Composites having reaction bonded silicon nitride are particularly useful as structural ceramics due to their combination of low weight with high rigidity and low coefficient of

  thermal expansion. This combination of properties

  comes particularly for structures to be used in space, o (1) the absence of oxygen eliminates one of the major limitations of NSLR in terrestrial conditions, namely the sensitivity of NSLR to oxidative degradation at high temperatures, (2) the low weight is particularly advantageous due to the cost of launching weight in space, and (3) a low coefficient of thermal expansion is particularly useful for structures exposed to sunlight on one side

- 30 -

  only. NSLR composites are also useful in electrical applications due to their strength and dielectric properties, and as biological replacement materials due to their resistance to corrosion, their lack of toxicity and their ability to

bind well to animal tissue.

  The superior strength and tenacity of the composite bodies according to the invention makes them useful in additional applications which cannot be filled with the bodies of silicon nitride subjected to catastrophic ruptures.

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Claims (7)

  1- Ceramic composite comprising reinforcing fibers   cement of silicon carbide at least 1 mm long in a matrix formed mainly of silicon nitride, characterized in that: (a) the reinforcing fibers collectively have a sufficient tensile strength to support the load on said composite at the breaking point of the matrix without breaking in traction of the fibers, (b) a crack deflection zone having mechanical properties substantially different from those of both the reinforcing fibers and the matrix occupies a predominant part of the space over a thickness of 5 nanometers around the reinforcing fibers, (c) the reinforcing fibers collectively occupy from 20 to 80% by volume of the composite, (d) the composite offers a non-catastrophic rupture under mechanical stress; and (e) said carbide reinforcing fibers.   silicon having a diameter between 8 and 25 microns.   2- The composite of claim 1, wherein said silicon carbide reinforcing fibers are derived from the partial decomposition of polymer fibers organosilicon.   3- Composite according to claim 1 or 2, further characterized in that at least half of the volume of said   crack deflection zones is an empty space.   4- The composite of claim 1 or 2, further characterized in that said crack deflection zones   are formed of silicon nitride barbs.   - Composite according to any one of claims 1   to 4, further characterized by an ability to withstand a   maximum load of at least 350 megapascals with a deformation tion of fibers of at least 0.3%.   6- Composite according to any one of claims 1   to 5, in which said matrix is formed mainly from -32 -   reaction-bonded silicon nitride.   7- Process for practically total transformation of a sintered silicon matrix into a silicon nitride matrix by bonding by reaction in the presence of nitrogen gas, characterized in that: (a) said silicon matrix is reacted under pressure nitrogen gas at least 0.55 bar for a period not exceeding 48 hours between the first heating above room temperature and the highest temperature reached during the heating cycle; and (b) the highest temperature reached during the   heating cycle does not exceed 1400 C.   8- The method of claim 7, wherein: (a) said nitrogen gas pressure is at least 0.9 bar; (b) the time between the first heating above room temperature and the highest temperature reached during the heating cycle does not exceed 15 hours; (c) the highest temperature reached during the heating cycle does not exceed 1375 C; and (d) the total duration of exposure of the matrix during the nitriding cycle at temperatures above 1200 ° C. does not exceed 10 hours.