WO1984001371A1

WO1984001371A1 - METHOD OF MAKING REACTION BONDED/HOT PRESSED Si3N4 FOR USE AS A CUTTING TOOL

Info

Publication number: WO1984001371A1
Application number: PCT/US1982/001372
Authority: WO
Inventors: Andre Ezis; Elaine C Beckwith
Original assignee: Ford Werke Ag; Ford Motor Canada; Ford France; Ford Motor Co
Priority date: 1982-09-30
Filing date: 1982-09-30
Publication date: 1984-04-12
Also published as: EP0126063A1; EP0126063A4; EP0126063B1; DE3278499D1; BR8208097A; AU9125482A; JPS59501628A; AU565753B2

Abstract

A method of making a reaction bonded/hot pressed silicon nitride comprising object. To form a protective amorphous glass on crystallites in the final product, an Al2O3/SiO2 ratio must be maintained which is greater than 2. Second phase crystallites are formed prior to hot pressing. A mixture of silicon, SiO2, and .4-2.3 molar percent (by weight of the silicon) of oxygen carrying agents, i.e., Y2O3 and Al2O3, is preformed and reaction nitrided to form discs or billets having at least 60 % alpha Si3N4 and a high proportion of second phase crystallites which displace substantially all silicate glass except for a controlled smll quantity (.2-1% by weight). The reactive amounts of Y2O3, Al2O3 and SiO2 are controlled to assure formation of substantially Y1Si12O24N4 as the second phase crystallite. Al2O3 is controlled in an amount of .4-4% by weight to insure that the small proportion of glass has little or no microporosity associated therewith and serves to protect the oxynitrides against linear oxidation kinetics.

Description

METHOD OF MAKING REACTION BONDED/HOT PRESSED Si₃N FOR USE AS A CUTTING TOOL

BACKGROUND OF THE INVENTION AND PRIOR ART STATEMENT

5 Si3 4 has been known as a ceramic since 1857, but its structure was not determined with some degree of certainty until the 1950*s. Useful objects have been fabricated from this ceramic by essentially two methods or arts: (1) hot pressing Si3 4 powder mixed with pressing _Q aids, or (2) reaction bonding silicon powder mixed with a catalyst by heating in nitrogen. The two modes were com¬ bined in U.S. patent 3,839,540, permitting pressing aids to be added to silicon prior to reaction bonding heating, thus eliminating the need for a catalyst and producing a much 5 more dimensionally accurate product.

In the hot pressing art, pressing aid additives were deemed necessary because pure Si3 4 is almost un- sinterable, even under pressure. Typically, such aids have included the oxides of Mg, Y, Ce, Fe, Ca, Cr, Zr, Zn, Be, o Al and selected rare earths. See U.S. patents 3,830,652; 4,304,576; 3,903,230; 4,046,580; 4,234,343, 4,102,698 and 4,038,092. The pressing aids function as a means to flux the fusion or densification of the Si3 4 powder during hot pressing by the formation of a low temperature liquid. Unfortunately, such liquid more readily forms a type of glassy or amorphous phase in the final product which inhibits high temperature physical properties of the ceramic and reduces the life of a cutting tool made from this material. There has been little recognition in the hot pressing art of the role played by the formation of certain desirable second phase crystallites, such as oxynitrides, from such additives as a displacement for the glass phase to increase tool life. Investigators of hot pressed Si3N₄ have detected the presence of various oxynitrides without

VTI understanding their significance or interrelationship to ceramic tool life and performance. Secondly, there has been little recognition of the role played by the reten¬ tion of a small, controlled quantity of the glassy phase in protecting not only the displacing second phase crystallite, such as oxynitrides, against high temperature oxidation, but also other crystallites. In fact, the prior art has generally followed the axiom that adding oxygen to a powder system for forming a ceramic by nitriding will be detrimental.

This invention is concerned with the additives that must be selected and used in reactive amounts and under conditions that result in the formation of desirable second phase crystallites, advantageously accompanied by a small glass envelope; of particular interest is the use of Y2O3 and AI2O3.

In the reaction bonding art, oxides of aluminum have been mixed with silicon powder to promote a sialon product as a result of nitriding, see U.S. patent 4,038,092 an the article by K.H. Jack and W.J. Wilson, "Ceramics Based on the Si-Al-O-N and Related Systems", Nature (London) Physical Science, 233 (80) 25-29 (1972). Oxides of yttrium have been added to silicon powder prior to nitriding to promote subsequent sintering (see U.S. patent 4,285,895), or added to silicon nitride formed by the reaction of Si, Al2°3 ^{anα" N}2' ^{for tiie} purpose of facili¬ tating sintering, see U.S. patent 4,184,884. However, users of AI2O3 or Y2°3 i*¹ such reaction bonding techniques again have failed to recognize the role that can be played by the formation of certain silicon oxynitrides derived from the additives as a displacement for the glass phase. This lack of understanding is evident particularly in U.S. patent 4,285,895 wherein it is stated that a chemical analysis of the nitrided body exhibited 91% Si3N₄, 9% Y2O3, and minor phases of free Y, Si, O2 and N2 (see column 6, lines 1-6), but no oxynitrides or silicates. This prior art also again failed to recognize the role played by a controlled quantity of glassy phase that is effective in protecting the crystallites against high temperature oxidation. The generation of the right oxynitrides or second phase crystallites will increase tool life, not by an increase in strength, but by an increase in hardness and chemical stability.

INVENTION SUMMARY The invention is first a method of making a silicon nitride comprising object that contains a controlled amount of a high alumina content silicate coating the crystallites of the object. The silicate functions to increase the wear resistance of the material when used as a cutting tool material.

The method comprises (a) forming a mixture of silicon nitride, a densification aid, silica, alumina, or other reaction product of silica and alumina, said alumina and silica being proportioned to provide an alumina/silica ratio greater than 2, said densification aid being selected and proportioned to insure maintenance of the required alumina/silica ratio during subsequent heat treatment, and (b) heat treating said mixture to form a heat fused, sub¬ stantially crystallized silicon nitride comprising object and second phase crystallites with substantially full theoretical density, said silicon nitride and second phase crystallites being coated with a high alumina content silicate film.

Preferably, the densification aid is 2°3 and is present in said mixture in an amount of at least 2% by weight of the mixture, said amount not only facilitating hot pressing of the object to substantially full density, but also providing substantially full reaction of said Y2O3 with said silica to form second phase crystallites in said object. Preferably, the mixture is prepared by forming a first mixture of silicon, silica, alumina, said densification heating the first mixture in a nitriding atmosphere to form a second mixture of silicon nitride, reaction product of silica and alumina, and second phase crystalliltes, said second mixture being subjected to step (b).

Preferably, the first mixture essentially contains by weight 2-19% 2θ3_# .4-5% AI2O3, 1-3% SiO₂, and the remainder silicon.

The invention is secondly a method of making a reaction bonded/hot pressed silicon nitride comprising object by the steps of: (a) forming a compact from a mixture of powdered silicon and reactive oxygen carrying agents, the latter being effective to form upon heating at least one second phase crystallite dispersed throughout the compact, said reactive oxygen carrying agents being present in said mixture in an amount to substantially fully react said silicon upon heating in nitrogen, whereby the compact will consist essentially of Si3N4 and second phase crystal¬ lites; (b) heating the compact in a nitriding atmosphere, without the use of pressure normally associated with hot pressing, to produce a silicon nitride comprising body consisting essentially of Si3 4 and said second phase crystallites, said body having a size greater than and a density less than the object to be formed; and (c) hot pressing the body to produce a silicon nitride comprising object of required dimension and density useful for a cutting tool and consisting essentially of the chemistry in said body.

The inventive method significantly reduces the formation of glass as a second phase in the Si3N4 product; the glass is displaced by crystallites formed, for the first time, prior to hot pressing as a result of the full reaction of available silicon and an effective amount of oxygen carrying agents, the available silicon being that not used in forming Si3N₄. The combination of Si3 4 and second phase crystallites (a) reduces the wear rate of the resulting object when used as a cutting tool, (b) increases the hardness and high temperature stability of the object, and (c) achieves considerable processing economy by lowering the required hot pressing temperature and removing mixture proportion limitations normally imposed by lower viscosity requirements for powder in hot pressing. Preferably, the silicon is 98% pure or more with less than 1.5% by weight metal contaminants and less than .05% by weight carbon. Silica is present as a surface oxide on the starting silicon and is preferably introduced into the mixture as an oxide surface coating on the silicon powder as a result of milling the mixture (1-3% by weight of silicon). The reactive oxygen carrying powder agents, when considering the broadest aspect of this invention directed to economical reduction of the glassy phase and the formation of a second phase crystallite, may be selected from the group consisting of oxides of silicon yttrium, cerium, zirconium, hafnium, and other rare earths (i.e., G 2θ3, a2θ3, SC2O3). The reactaive oxygen carrying agents are present in the mixture in a molar percent of silicon in the range of .4-2.4. The second phase crystal- lite is preferably limited to 2.9-14.4% by weight of the nitrided body which remains essentially the same hot pressed object.

However, it is most advantageous to restrict the selection of the oxygen carrying powder agents to 3-19% ^Y2°3 and .4-5% I2O3 to obtain additional improvements, particularly the formation of yttrium silicon oxynitride crystallites (optimally YιSiθ2N), the former further reducing the required temperature during hot pressing and the latter promoting the formation of a particular pro- tective amorphous silicate that coats the silicon oxynitride and prevents catastrophic oxidation failure at high temperature when used as a cutting tool.

The formation of a critically small amount of a protective amorphous glass, that possesses a low diffusion coefficient, is promoted by the presence of an oxide reactive with Siθ2 and Y2O3, particularly .4-5% I2O3. The reactive oxide to form the glass may be selected from the group consisting of AI2O3, MgO, Ceθ2, Fe2C>3, CaO, Cr2θ3, Zrθ2, BeO, and rare earth oxides. The glass is important to obtaining the optimum increase in the wear life of the object when used as a cutting tool or in high temperature oxidizing environments; the permitted glass forms a thin coating (i.e., 8-10 angstroms thick) on the crystallites reducing the transport of oxygen necessary to undesirable linear oxidation kinetics of the crystallites, such as the yttrium silicon oxynitrides. Proportioning the mixture to form at least 80% of the second phase crystallite as ^γl^sil°2^N (by restricting the amount of Y2O3 to 8-12%) causes the controlled amount of glass to have an improved diffusion coefficient free of microporosity. In addition, the nitriding cycle and level of hot pressing temperature affect the formation of ιSiιθ2 phase. The nitriding cycle should be designed to keep the starting composition of the nitriding gas substantially constant, and an exo- thermic reaction should be minimized as much as possible as by utilizing prereacted oxynitrides in the compacted mixture. The nitriding reaction should be designed to convert as much of the silicon to Si3 4 as possible, i.e., 99.5%, and of that amount at least 60% is in the alpha form. The hot pressing temperature should be designed to obtain a high alpha to beta Si3N₄ conversion, beta Si3N₄ having long, needle-like grains that produce a better knitted structure with better strength; preferably, this is about 1650°C (3000°F).

G--/.PI Optimally, the oxygen carrying agent/Siθ2 ratio is maintained at 1.1-6.4%. Controlling the ratio of 2θ₃/Siθ2 desirably promotes reduction of the N-melilite phase (Y2θ3.Si3 4) to no greater than .5% by weight of the body, and a density in said body of at least 2.3 g/cm³. The purity of the starting Y2°3 and AI2O3 is advantageously selected to be at or above 99.99% and 99.5%, respectively. A desirable average particle size for the mixture is obtained by milling so that at least 50% of the mixture is about 4.0 microns or less, and 90% of the mixture is less than 23 microns.

Heating to nitride preferably is carried out to an ultimate temeprature of (2000-2600°F) 1090-1430°C (prefer¬ ably 2560°F) for a time sufficient to produce a body consisting essentially of silicon nitride (preferably at least 60% by volume in the alpha form), 2.9-14.4% second phase crystallites, i.e., yttrium silicon oxynitride (preferably the YιSiθ2N phase), and the remainder refrac¬ tory silicate, free silicon, and unreacted Y2°3- The nitrided body is preferably characterized by a density of 2.3-2.78 gm/cm³.

Hot pressing is advantageously carried out in an environment uncontrolled as to inertness (ambient atmos¬ phere in a carbonaceous vessel) at an ultimate pressing temperature equal to or less than 1670°C, optimally

1371-1650°C (2500-3000°F), under an ultimate pressure of 3600-3800 psi, and for a period of .25-3.0 hours.

The compression ratio is substantially reduced to 1.0:1 to 1.8:1. The viscosity of the mixture used to nitride is permitted to be high as a result of these hot pressing conditions.

The resulting object is preferably characterized by the silicon nitride being substantially in the beta or beta prime form, the silicon oxynitrides are enveloped by a yttrium silicate, glass in a thickness of 8-10 angstroms and having no microporosity. The object preferably possesses a hardness of 88.5-92.0 on the 45-N scale, a density of 3.2-3.35 g/cm³, a fracture strength of about 85,000 psi at 1200°C in air in a 4-point bend test, and an oxidation resistance that prevents weight pickup by the object after 450 hours in air at 1000°C.

DETAILED SPECIFICATION A preferred method for making a silicon nitride comprising object according to this invention is as follows.

1. Compacting

Preliminary to this step, a mixture of powdered silicon, Siθ2, and reactive oxygen carrying powder agents is prepared and milled. Reactive oxygen carrying powder agents is defined herein to mean powder ingredients that are effective to form second phase crystallites, parti¬ cularly oxynitrides, when reacted with the silicon under a heated nitrogen atmosphere. The powder agents can be advantageously selected from the group consisting of Siθ2* Ϊ2°3' Ce2θ3, Zrθ2 Hfθ2, and other rare earths. Use of these agents will improve physical characteristics and formation of a second phase crystallite which (a) will uniformly be dispersed, and (b) substantially displace the detrimental glassy silicate phase normally formed except for a controlled and limited amount of the latter. Use of critical amounts of Y2O3 and AI2O3 will provide additional improvements, including (a) formation of the Y Siθ2N as the predominant oxynitride phase resulting from nitriding, permitting much lower hot pressing temperatures leading to increased economy, (b) formation of a critically small amount of a protective amorphous glassy silicate coating the crystallite or oxynitride which is effective to prevent high temperature oxidation of the crystallite during high temperature cutting tool use.

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CMΓ- For purposes of the preferred method, a uniform powder mixture is prepared with* 2000 grams silicon (86.6 weight percent of mixture), 278 grams 2°3 d² weight percent of mixture and 13.9% of silicon), and 32 grams I2O3 (1.4 weight percent of mixture and 1.6% of silicon). The usable range for the oxygen carrying agent is .4-2.3 molar percent of the mixture and .42-2.4 molar percent of silicon. Y2°3 is normally used in the range of 3-19% by weight of the silicon and 3.2-15.6% by weight of the mixture. The glass forming oxide, such as AI2O3, is used in a range of .4-5% by weight of the silicon, .4-4.0% by weight of the mixture. Siθ2 is present usually as an oxide on the silicon powder and increased to 1-3% by weight of the silicon by milling. The oxide that is added to be reactive with Siθ2 and 2°3 to form the protective amorphous glass can be selected from the group consisting of .MgO, Ceθ2, AI2O3, Fe2θ3, CaO, Cr2C>3, Zrθ2, BeO, and other rare earth oxides.

Silicon is selected to have 98% or greater purity and a starting average particle size of 8-9.2 microns. The major trace metal contaminants experienced with such purity include, as a maximum: Fe - 1.0%, Al - .5%, and Mn - .09%. Nonmetallic contaminants include, as a maximum: carbon - .05% and O2 - less than .5%. Yttria is selected to have a purity of at least 99.99% with an average crystal size of o

.0438 microns (438A). Alumina is selected to have a purity of at least 99.5% with an average particle size of .3-.5 microns.

The mixture is comminuted and blended by being charged into an inert milling jar along with grinding media in the form of Burundum cylinders (85% AI2O3 and 11% Siθ2, 2% MgO, 1.2% CaO, .8% of [Tiθ2, Fe2°3, Na2θ, K 0] which adds AI2O3 to the mixture by attrition), milled for 48 hours at 64 rpm, then the mixture is separated from the media by use of a #10 mesh screen. The milling-is- preferably dry, but can be wet, with some accompanying disadvantages.. The oxygen carrying agents must be in a reactive form with a high surface area and small crystal¬ line size. The resulting milled mixture will have at least 50% with an average particle size of about 4 microns, and 90% with an average particle size of less than 23 microns. The oxygen level after milling in air will be increased to 1.6 weight percent of the silicon, and be present as an oxide coating on the silicon in an amount of 3.0 weight percent. The oxide coating should never be stripped off. The ratio of oxygen carrying agent/Siθ2, such as Y2θ3/Siθ2, is controlled to be in the range of 1.1-6.4, and preferably about 4.

A measured quantity of the milled mixture is loaded into a cold press die arrangement and pressed at ambient conditions by use of 1400-1500 psi to form a compact of a size about 6 inches by .6 inch, and a density of 1.4 g/cm³. 2. Heating to Nitride The compact is heated in a nitriding atmosphere, without the use of pressure normally associated with hot pressing, to produce a silicon nitride comprising body consisting of Si3N , at least one dispersed second phase crystallite (i.e., silicon yttrium oxynitride), .2-1% silicate (by weight of the body) , and up to .5% by weight of free silicon and unreacted oxygen carrying agents (here Y2O3 and AI2O3). The body will have a size greater than and a density less than the object to be formed.

To carry out the heating, the compact is placed in an enclosed furnace, preferably evacuated to a pressure of less than 1 micron and heated at a fast rate, i.e., 500°F/hr. (278°/hr.) to 1200°F (649°C). The furnace is then filled with a gaseous mixture consisting of 72% by weight nitrogen, 3% hydrogen, and 25% helium, at a pressure of about 2.7 psig. The total O2 and H2O content in such gaseous mixture is less than 4 ppm. The temperature of the furnace is then increased, in steps, to: 1200-1700°F at 500°F/hr., to 1700-2000°F at 200°F/hr. (lll°C/hr.), and to a nitriding temperature of 2000-2600°F (1093-1427°C) at a slower rate. The temperature is held constant thereafter. Fresh nitrogen is intermittently supplied to the furnace to replace the nitrogen consumed in forming Si3N4 and oxy¬ nitrides. Nitrogen is added when the pressure drops below 2.4 psig and brought back up to maximum pressure of 2.7 psig. The temperature is increased back up to 2560°F after adding nitrogen. The material is cooled to room tempera¬ ture at a rate of 250°F/hr. (56°C/hr.).

The nitrided body will preferably consist of silicon nitride (at least 60% of which is in the alpha form), 3-15% silicon yttrium oxynitride in the YιSiθ2 phase, and the remainder a yttrium silicate glass (which may be theorized to be Y Siθ2), and up to .5% of silicon and unreacted Y2°3« Some trace amounts of the Yιo^s-i-6⁰24^N2 phase may be present at times. The N-melilite phase (Y2θ3.si3 ) will be present in an amount no greater than .5% by weight of the body. The body will have a density of at least 2.3 gm/cm³. The minimum alpha/beta Si3N4 ratio is 1.5. This body is an intermediate product or commodity that has independent utility as starting block for other shaping techniques, one of which is hot pressing. » Hot Pressing

The nitrided body is hot pressed to produce silicon nitride comprising object of required dimension and density. A pressing fixture having graphite walls is used to carry out hot pressing. The walls and nitrided body are both coated with a slurry of boron nitride and dried. The pressing fixture with the nitrided body therein is placed in the hot pressing furnace. The heating and pressing is carried out preferably in increments: (1) a mechanical loading of 100 psi is applied at room temperature to the body; (2) temperature is increased to 1800°F (982°C) and pressure increased to 500 psi; (3) the temperature is then increased to 2500°F (1371°C) and pressure simultaneously increased to 2500 psi; (4) the temperature is finally increased to the hot pressing temperature of 3000°F

(1649°C) and pressure increased to 3700 psi, the latter conditions being maintained until at least 99% or desirably 99.5% of theoretical full density is achieved. This usually requires .25-3.0 hours at the hot pressing tem- perature. The object is then cooled at any rate, even quenched, to room temperature.

The resulting object will consist essentially of beta silicon nitride, 2.9-14.4% by weight silicon oxy¬ nitrides (predominantly YιSiθ2 ) enveloped by a silicate phase having a thickness of 8-10 angstroms and having no microporosity. The object preferably possesses a hardness of 88.5-92.0 on the 45-N scale, a density of 3.2-3.35 gm/cm³, a fracture strength of about 85,000 psi at 1200°C in a 4-point bend test, and an oxidation resistance that prevents weight pickup by the object after 450 hours in air at 1000°C. Some oxynitrides of the Yιo^si6°24^N2 and Y4Si2O 2 phase can be present up to 10% of the crystallite second phase. Examples A series of cutting tool samples were prepared and tested as to physical parameters and machining performance to illustrate how variations in processing and chemistry facilitate or deny obtaining the advantages of this invention. The results are summarized in Table I. Column 2 indicates whether the sample was prepared from silicon powder or silicon nitride powder. Samples 1-3 were prepared in accordance with conventional prior art hot pressing practices wherein powder additives were introduced to Si3 4 powder (Siθ2 is present as a surface oxide on the starting Si3N4 powder and as a result of milling) and directly subjected to hot pressing in the powder form. Viscosity of the mixture is of some concern for samples 1-3 and limits the amount of additives that can be introduced without inhibiting milling and pressing. Accordingly, no mixture with greater than 9% oxygen carrying agents was prepared. The ingredients for these samples was based on a weight percentage of the silicon nitride mixture. The -Si.3^N4 powder was 99% pure with a surface oxide (Siθ2) content of 1-3%. The Y2°3 powder was 99.9% pure.

Milling was carried out prior to hot pressing by use of AI2O3 milling media until the average particle size was 2-7 microns. A wetting lubricant was added during milling (methanol) in a ratio of 1:1 with the silicon nitride powder. The mixture was milled for a time dependent on mill speed, particle size of the starting powder, and the average particle size to be achieved, preferably about 2.5 microns. The mixture was then dried and screened (-100 mesh).

Samples 4-13 were prepared by placing the indicated proportions of silicon powder and oxide additives (see column 3) in a polypropylene nalgene jar containing Burundum cylinders as the grinding media (10,000 grams of the cylinders as a batch), each having 13/16" as length or diameter. The starting silicon was 98.5% pure with Fe (.6 weight percent), Al (.11 weight percent), and Mn (.06 weight percent) as the major trace metal contaminants. All other trace metals were less than .05 weight percent. The oxygen level of the starting silicon was .46 weight percent, carbon level .03 weight percent, and the sulfur level .002 weight percent. The starting silicon powder was 100% less than 45 microns, having an average particle size of 8 microns. The yttria used was 99.99% pure with an o average crystal size of 438A and a surface area of 6.3 m2/g_m. T e alumina used was 99.5% pure with an ultimate crystal size of .3-.5 microns. The powder mixture was milled for 48 hours at a speed of 64 rpm and then separated from the grinding media by passing the powder mixture through a #10 mesh screen. No attrition was found from the grinding media. The resulting powder was a uniform mixture of silicon powder and oxide additives reduced in particle size to an average of 3.1 microns. The oxygen level of silicon powder was increased to 1.6 weight percent, which corresponds to an Siθ2 content of 3.0 weight percent. The starting Y2θ3/Siθ2 ratio was 7.2 (the alumina was also a small source of Siθ2).

In samples 4-13 a portion of the powder mixture (about 340 grams) was cold pressed at 1400 psi into 6" diameter discs, .05-.06" in thickness. The compacts were then nitrided and reacted to form Si3N4 and second phase crystallites with some small proportion of silicate glass. The nitriding cycle employed is similar to that described in U.S. patent 4,127,684.

The nitrided bodies were removed from the furnace and sandblasted to remove any white coating appearing on the bodies. The chemistry of the second phase material for each of the samples prior to hot pressing is characterized in column 4 of Table I. In samples 4-13 the nitriding cycle produced, except for sample 8, some proportion of second phase crystallites. The controlled amount of silicate glass that was not displaced is indicated as a proportion of the second phase material. ^' All of the samples were hot pressed using the pressure and ultimate temperature indicated for the preferred embodiment herein. Pressing was carried out for 1-2 hours to obtain a complete conversion of alpha to beta Si3N4« The hot pressed object was then cooled to room temperature and analyzed for physical parameters. The discs were then diamond sawed into cutting tools of TNC-434 configuration and subjected to a machining operation in¬ volving continuous cutting of a simple cylindrical surface of a grey cast iron stator member having a hardness of 179-222 (brinnel scale with .003 kg. load). The number of pieces cut in a period of one hour and the flank wear on the cutting tool (measured in inches) is given in columns 7 and 8, respectively.

Sample 10, hot pressed at the higher temperature of 1750°C, had a mottled appearance characteristic of microporosity in the material. Areas of high concentration of yttrium were evident in sample 5 showing microporosity as indicated by a heavy mottled appearance and non- uniformity. It is important that the Al2θ3/Siθ2 ratio be greater than 2. When other densification aids (oxides) are present in the mixture, the ratio no longer is the ratio of the starting AI2O3 and Siθ2» A portion of the Siθ2 becomes available to react with the densification aid, leaving a much smaller amount of Siθ2 to be calculated into the ratio. Thus samples 3, 7, 9, 10, 11 and 13 possess an l2°3/Siθ2 ratio greater than 2, but samples 1, 2, 4, 5, 6, 8 and 12 do not. You will note the machining results show a bias in favor of the samples having the right ratio. Ideally, the ratio should be obtained without the need or presence of a densification aid; the state of the art does not allow for this and thus greater care in proportioning is needed to achieve the correct silicate in the final product. Such silicate provides a desirable interface between the material and metal being cut when used as a cutting tool. The silicate also facilitates dissolution of silicon nitride during hot pressing to obtain a finer grained structure more effective as a cutting tool material. Table I demonstrates that proper proportioning of the selected oxygen carrying agents in a silicon mixture (such as 3-19% Y2O3), which is then nitrided to substan¬ tially fully react the ingredients, will cause the formation of crystallites that displace glass. Table I also demonstrates that further improvements in oxidation resistance at 1000°C and flank wear take place by adding AI2O3 in a controlled amount of .4-5% which promotes the formation of low diffusion coefficient glass having little or no microporosity associated therewith.

Claims

We claim:

1. A method of making a reaction bonded/hot pressed silicon nitride comprising object, comprising:

(a) forming a compact from a mixture of powdered silicon and reactive oxygen carrying agents, the latter being effective to form upon heating at least one second phase crystallite dispersed throughout the compact, said reactive oxygen carrying agents being present in an amount to substantially fully react said silicon upon heating in nitrogen, whereby said compact will consist essentially of Si3N4 and second phase crystallites;

(b) heating said compact in a nitriding atmos¬ phere without the use of pressure normally associated with hot pressing to produce a silicon nitride comprising body consisting essentially of Si3N4 and second phase crystal- lites, said body having a size greater than and a density less than the object to be formed; and

(c) hot pressing said body to produce a silicon nitride comprising object of required dimension and density useful for a cutting tool and consisting essentially of the chemistry in said body.

2. The method as in Claim 1, in which said nitrided body contains no greater than .5% by weight free silicon and unreacted oxygen carrying agents.

3. The method as in Claim 2, in which said second phase crystallites in said nitrided body include .2-1.0% by weight of a protective amorphous silicate.

4. The method as in Claim 1, in which said reactive oxygen carrying agents are present in said mixture in a molar percent of silicon in the range of .4-2.4.

5. The method as in Claim 1, in which said reactive oxygen carrying powder agents consist sub¬ stantially of Y2O3 in an amount comprising 3-19% by weight of said silicon, and said second phase crystallites are comprised of yttrium silicon oxynitrides..

6. The method as in Claim 1, in which said reactive oxygen carrying powder agents are selected from the group consisting of oxides of silicon, yttrium, cerium, zirconium, hafnium, and other rare earths.

7. The method as in Claim 6, in which said silicon is at least 98% pure with less than 1.5% by weight metal contaminants and less than .05% by weight carbon.

8. The method as in Claim 6, in which said oxide of silicon is present as a surface oxide on the silicon powder in the range of 1-3% by weight of the mixture.

9. The method as in Claim 1, in which said body contains 2.9-14.4% by weight second phase crystallites.

10. The method as in Claim 1, in which said oxygen carrying agents include an oxide capable of forming a protective amorphous glass coating on the oxynitrides, said oxide being selected from the group consisting of A1₂0₃, MgO, Ceθ2, Fe2*θ3, CaO, Cr2θ3, Zrθ2, BeO, and other rare earths.

11. The method as in Claim 1, in which said oxygen carrying agents include .4-5% AI2O3.

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12. The method as in Claim 10, in which said protective amorphous glass is 8-10 angstroms thick.

13. The method as in Claim 10, in which the chemistry .of said compact is proportioned to form an increased amount of Y^Siθ₂N second phase crystallite thereby reducing microporosity associated with said protective amorphous glass and improving the diffusion coefficient of said glass.

14. The method as in Claim 1, in which said oxygen carrying agents consist of 3-19% Y2°3' .4-5% AI2O3, and 1-3% Siθ2, said Y2°3 an<3 AI2O3 having a purity respectively of at least 99.9% and 99.5%.

15. The method as in Claim 6, in which the oxygen carrying agents are present in an amount to constitute a ratio of oxygen carrying agent to Siθ2 of 1.1-6.4.

16. The method as in Claim 13, in which the heating of step (b) is carried out to maintain a substantially constant nitrogen content in said atmosphere and the exothermic reaction of said heating is controlled.

17. The method as in Claim 14, in which in the nitrided body of step (b) the N-melilite phase (Y2θ3.Si3N₄) is present in an amount no greater than .5% by weight of the body.

18. A method of making a reaction bonded/hot pressed silicon nitride comprising cutting tool which has increased oxidation resistance at 1000°C, comprising the steps of: (a) forming a compact from a mixture of powdered silicon, Si0₂r ^2°3 ^an^ ^{an oχ}i^de reactive with said Si0₂ and 2°3 to form upon heating a controlled amount of a protective amorphous silicate glass, said 2O3 being present in said mixture in an amount of 3-19% by weight of said silicon, said oxide being present in an amount of

.4-5% by weight of said silicon, said Siθ2 being present in amount of 1-3% by weight of said silicon and in a Y2θ3/Siθ2 ratio of 1:1-6.4;

(b) heating said compact in a nitriding atmos- phere, without the use of pressure normally associated with hot pressing, to produce a silicon nitride comprising body consisting of Si3 4, at least one yttrium silicon oxynitride present in an amount of 3-15% by weight of said body and dispersed throughout, up to .5% free silicon and residual Y2°3' 1% oτ less silicate glass coating said yttrium silicon oxynitride, at least 60% of said Si3 4 being in the alpha form, said body having a size greater than and a density less than said tool;

(c) hot pressing said body to produce a silicon nitride comprising cutting tool of required dimension and density consisting essentially of the chemistry developed previously during said heating step.

19. The method as in Claim 18, in which said oxide of step (a) is AI2O3.

20. The method as in Claim 18, in which said oxide of step (a) is selected from the group consisting of A1₂0₃, MgO, Ce0₂, BeO, Fe₂0₃, CaO, Cr2θ₃, Zr0₂, and other rare earth oxides.

21. The method as in Claim 18, in which said oxide is selected to form a silicate glass in step (b) having a low diffusion coefficient for oxygen which prevents catastrophic linear oxidation kinetics of the yttrium silicon oxynitride in said tool when used in machining at 1000°C or greater.

22. The method as in Claim 18, in which said mixture is proportioned to form yttrium silicon oxynitride comprised at least 80% by weight of Y Siθ2N.

23. The method as in Claim 18, in which said silicate formed on said silicon oxynitride in step (b) is 8-10 angstroms thick.

24. The method as in Claim 18, in which said hot pressing is carried out at an ultimate temperature of 1200-1650°C in an environment uncontrolled as to inertness.

25. The method as in Claim 18, in which the average particle size for at least 50% of said mixture is about 3.0 microns, and for at least 90% of the mixture is about 23 microns.

26. The method as in Claim 22, in which said YlSiθ2N crystallite comprises 3-15% of the weight of said body, at least 80% of said Y Siθ2N phase converting to another silicon oxynitride phase during hot pressing while retaining said silicate coating therearound.

27. The method as in Claim 26, in which said hot pressing ultimate temperature is in the range of 2500-3000°F (1371-1650°C) and the ultimate pressure is in the range of 3600-3800 psi.

28. The method as in Claim 27, in which said ultimate pressure and temperature is maintained for a period of .25-3.0 hours.

29. The object resulting from the practice of the method of Claim 18, said object being characterized by the pressure of 1-7.5% by weight second phase crystallite, a hardness of 88.5-92.0 on the 45-N scale, and a density of 3.2-3.35 g/cm³.

30. The object as in Claim 29, in which said object additionally is characterized by a fracture strength of about 90,000 psi at 1200°C in air in a 4-point bend test and an oxidation resistance that prevents weight pickup by the tool after 450 hours in air at 1000°C.

31. The object as in Claim 30, in which the object has a visual homogeneity.

32. The object as in Claim 31, in which said silicate coating on said oxynitride crystallites has substantially no microporosity associated therewith.

33. A method of making a silicon nitride comprising body useful as a starting material in the hot pressing of objects, comprising the steps of:

(a) forming a compact from a mixture of powdered silicon, Si0₂, and other reactive oxygen carrying agents, the latter being effective to form upon heating at least one second phase crystallite dispersed throughout said compact, said Siθ2 and reactive oxygen carrying powder agents being present in said mixture in an amount to substantially fully react said silicon upon heating in nitrogen whereby said compact will consist of Si3N4, second phase crystallites, .2-1% silicate by weight, with up to .5% by weight of free silicon and unreacted oxygen carrying agents; and - 26 "

^5 (b) heating the compact in a nitriding atmos¬ phere, without the use of pressure normally associated with hot pressing, to produce a silicon nitride comprising body consisting of Si3N4, at least one dispersed second phase crystallite, .2-1% by weight of the body silicate, and up

20 to .5% by weight of free silicon and unreacted oxygen carrying agents.

34. The method as in Claim 33, in which said reactive oxygen carrying powdered agents consist of Y2°3 n an amount of 3-16% by weight of the silicon and AI2O3 in an amount of .4-5.0% by weight of the silicon, the ratio of

5 Y2θ3/Siθ2 in said mixture being 1:1-6.4.

35. The method as in Claim 34, in which heating is carried out to an ultimate temperature of 1090-1430°C (2000-2600°F).

36. The body resulting from the practice of Claim 34, in which said Si3N4 has at least 60% by volume in the alpha form and said yttrium silicon oxynitride consisting substantially of Y Siθ2N.

37. The body as in Claim 36, characterized by a density of at least 3.25 gm/cm³.

38. A method of making a silicon nitride com¬ prising object useful as a cutting tool material for the machining of metals, comprising the steps of:

(a) forming a mixture of silicon nitride, a 5 densification aid, silica, alumina, or other reaction product of silica and alumina, said alumina and silica being proportioned to provide an alumina/silica ratio greater than 2, said densification aid being selected

CL-'PI and proportioned to insure maintenance of the required alumina/silica ratio during subsequent heat treatment; and

(b) heat treating said mixture to form a heat fused, substantially crystallized silicon nitride com¬ prising object and second phase crystallites with substantially full theoretical density, said silicon nitride and second phase crystallites being coated with a high alumina content silicate film.

39. The method as in Claim 38, in which said mixture is hot pressed in step (b) and in which said densification aid is Y2O3 present in said mixture in an amount of at least 2% by weight of the mixture, said amount not only facilitating hot pressing the object to sub¬ stantially full density, but also providing substantially full reaction of said Y2°3 with said silica to form second phase crystallites in said object.

40. The method as in Claim 39, in which said mixture is prepared by forming a first mixture of silicon, silica, alumina, said densification heating the first mixture in a nitriding atmosphere to form a second mixture of silicon nitride, reaction product of silica and alumina, and second phase crystallites, said second mixture being subjected to step (b).

41. The method as in Claim 40, in which said first mixture essentially contains by weight 2-19% Y2°3_/ .4-5% AI2O3, 1-3% Siθ2, and the remainder silicon.

42. The product resulting from the practice of the method of Claim 41, in which said object has more uniform and finer grained crystallites and each crystallite is coated with a high alumina content silicate.

OMPI ^b