METHOD OF MAKING HIGH STRENGTH HOT PRESSED Si3N4

Abstract: A method of making high strength silicon nitride. A mixture of silicon and reactive oxygen carrying agents is milled to an absolute maximum particule size of 16 microns or less. The milled mixture is compacted and heated in a nitrogen atmosphere to substantially fully react the materials to form a Si3N4 comprising body. The body is then hot pressed to full density under reduced pressing conditions.

Description: METHOD OF MAKING HIGH STRENGTH HOT PRESSED Si3N BACKGROUND OF THE INVENTION AND PRIOR ART STATEMENT * This invention is directed to the art of making 5 silicon nitride from silicon powder as a starting material; the powder and selected densification aids are subjected to a reactive nitriding gas to form a mixed phase (alpha and beta) silicon nitride body, the body being then hot pressed. The prior art has consistently measured the mean 10 particle size of powder ingredients (including silicon powder and oxygen carrying agents such as MgO) for making a hot pressed silicon nitride body (see U.S. patents 3,839,541; 3,839,540; and 3,591337). This allows a truncated particle size distribution permitting some, but 15 still significant, particles larger than the mean size and, of course, several particles smaller than the mean. Such distribution, based on a mean measurement along with a known comminution mode, has been useful because it correlated well with packing efficiency for making a cold 20 compacted preform with good density. Unfortunately, the presence of several large particles tolerated by a mean particle size measurement, permits an increased number of flaws to form in the resultant silicon nitride product. The average Weibull characteristic strength (modulus of 25 rupture) is about 100 ksi for a silicon nitride product formed with only mean particle size control. t Even though mean particle size control, as per the a. prior art, failed to control flaw formation in the final product, the broad particle size distribution did facili- 30 tate greater packing efficiency when consolidating the powders under pressure. It would be useful if a method were available which not only eliminated much of the present flaws in a silicon nitride hot pressed product which tend to hold the Weibull characteristic strength OMPI level to no greater than 100 ksi, but also would not reduce the packing efficiency of the powder ingredients if a special compression ratio was desired. SUMMARY OF THE INVENTION The invention is a method of making an improved hot pressed silicon nitride comprising object by (a) forming a powder mixture of silicon and reactive oxygen carrying agents, the mixture being dry, wet or jet milled to an absolute particle size distribution having sub- stantially no particle greater than 16 microns, (b) compacting the mixture to form a preform having a density of about 1.0 gm/cm3, (c) heating the preform in a nitriding atmosphere without the use of pressure, normally associated with hot pressing, to substantially fully react said powder ingredients with said atmosphere to form a body consisting substantially of silicon nitride, and (d) hot pressing the nitrided preform to produce a silicon nitride comprising object of desired dimension and density. Preferably, the reactive oxygen carrying agents are selected from the group consisting of 2O3, MgO, Ceθ2, Zrθ2 and I2O3. The amount of the oxygen carrying agents permitted in the mixture is preferably limited to (percent by weight of the mixture): 3.2-16% Y2O3 and .4-4% AI2O3. Optimally, the maximum absolute particle size of the oxygen carrying agents should be substantially less than the maximum silicon particle size. The mixture can additionally contain a silicon nitride composite in an amount of 2-25% by weight of said mixture, said silicon nitride composite increasing the particle packing efficiency, increasing green density to about 1.4 gm/cm^, and decreasing the hot pressing compression ratio. OMPI The silicon nitride object resulting from the practice of this method will possess improved physical properties, specifically four point flexural strengths in excess of 150 ksi at room temperature and hardness (45-N) of 92. The room temperature flexural four point bend strength was determined in an Instron machine (Model 1122) using a self aligning steel fixture with an outer span of 19.05 mm (0.750") and an inner span of 9.525 mm (0.375") respectively. The test bars were 31.75 mm (1.25") long by 6.35 mm (0.250") wide by 3.175 mm (0 S 125") thick. SUMMARY OF THE DRAWINGS Figure 1 is a photograph (1000 X-magnification) of a silicon nitride material produced in accordance with prior technology, and without the teachings of this invention, i.e., a particle size distribution curve containing particles greater than 16 microns. The photograph illustrates a high concentration of defects, here iron silicide, in the fully hot pressed product, which is a strength determining flaw in the final product; Figure 2 is a photograph of a diamond ground surface of the material of Figure 1 showing in greater clarity the large grain beta silicon nitride particles associated with the presence of iron silicide, a portion having been pulled out as a result of the grinding operation. DETAILED DESCRIPTION To eliminate flaws that might occur in the final product, as illustrated in Figures 1 and 2, the particles that cannot nitride must be eliminated. A considerable amount of such flaws are associated with the presence of free silicon particles after the nitriding step has been completed. These free silicon particles are available to form silicides with the impurities that are present in the OMPI WIPO starting powder ingredients. When iron silicide is subjected to high temperature, such as in the final nitriding stage and during hot pressing, it forms a liquid into which the alpha silicon nitride dissolves. Cooling or solidification of these iron rich solutions causes a growth of very large sized beta silicon nitride particles surrounded by considerable porosity and consistently associated with the iron silicide solidified adjacent thereto (see Figure 1 at A). Large beta Si3 4 grains or defects can also be produced by other silicon metal impurities, such as chromium, manganese, etc., in like fashion. These localized areas of silicide, accompanied by large grain beta silicon nitride and porosity (see Figure 2 at B), form a weak area or zone within the final hot pressed product which limits the overall Weibull characteristic strength level (a modulus of rupture) of the silicon nitride material to consistently about 100 ksi or less, and with relatively low associated values (Weibull modulus) of 8 or less. The presence of free silicon is a direct result of inadequate nitriding; nitriding is a diffusion process dependant upon the distance the gas phase must penetrate to obtain chemical reaction. Oxygen is known to reduce nitrogen diffusion and therefore interfere with efficient nitriding of silicon. Oxygen carrying agents used in reaction bonded silicon nitride technology impede this nitriding process. Silicon particles that could be nitrided efficiently in the absence of oxygen carrying agents are found to no longer nitride efficiently in the presence of oxygen carrying agents using state of the art nitriding technology. When there are large sized particles in the mixture, the diffusion distance is not sufficient to entirely penetrate such particles and incomplete nitriding results, allowing such free silicon to remain in isolated pockets within the nitrided product. It is difficult, and in most cases O PI undesirable, to remove impurities within the starting ingredients which may combine with such free silicon. For example, up to 1.0% iron, either as metallic or combined, is present in silicon powder purchased commercially on the open market. The silicon also contains silicon dioxide as an oxide film on each of the grains which impedes the nitriding process and is difficult to strip or reduce. The iron tends to break up the Siθ2 film on the silicon, facilitating nitriding. Iron in the form of Fe2θ3 is sometimes added to facilitate the breakup of the Siθ2 layers. Moreover, the expense of attempting to purify the starting ingredients beyond that which is commercially available is highly exorbitant and is not a desirable alternative solution. A preferred method for eliminating such flaws in the final product of a hot pressed silicon nitride body according to the invention herein is as follows. Forming Mixture 1. A mixture of powdered silicon and reactive oxygen carrying powder agents is prepared. The reactive oxygen powder carrying agents is defined herein to mean powder ingredients that are effective to form second phase crystallites, particularly oxynit ides, when reacted with the silicon under a heated nitrogen atmosphere. The powder agents can be advantageously selected from the group consisting of 2°3 * AI2O3, MgO, Ceθ2 Zrθ2, Hfθ2, and other rare earths. Use of these agents will improve physical characteristics and formation of a second phase crystallite which will be uniformly dispersed and substantially displace the detrimental glassy phase normally formed, except for a controlled and limited amount of such glassy phase. For purposes of the preferred method, 2O3 and AI2O3 are used as the reactive oxygen carrying agents, along with Siθ2, which is normally inherent as an oxide -^T EA OMPI WIPO film on the silicon powder. Y2°3 i s normally required to be present in an amount of about 3-16% by weight of the mixture and AI2O3 is normally required to be present in an amount of about .4-4%. For purposes of the preferred method, a uniform powder mixture is prepared with 2000 grams of silicon (86.6% by weight of the mixture), 278 grams of 2O3 (12% by weight of the mixture), and 32 grams of I2O3 (1.4% by weight of the mixture for purposes of this invention). The silicon starting powder that is usually commercially available is 98% pure or greater. The average starting particle size of such silicon powder is usually in the range of 10-20 microns, which permits particles of 100, 150, and up to 540 microns to be present as well as smaller particles lower than 16 microns. The major trace metal contaminants normally experienced with such commercially available silicon powder include: iron in an amount of 1.0%, aluminum up to .5%, manganese up to .09%, and calcium up to .09%. Nonmetallic contaminants normally include: carbon up to .05%, and oxygen less than .5%. The yttria is usually available on a commercial market with a purity of 99.99% and average crystal size of about .0438 microns (438 angstroms) with particles as large as 40 microns. The alumina usually has a purity of 99.5% and is usually added separately, but can be added by attrition during the milling operation. The alumina will have an average particle size of about .3-.5 microns with some particles as large as 50 microns. Milling to Strict Particle Size Control 2. The mixture is then comminuted and blended by being charged into a dry milling apparatus or by being subjected to jet milling. The dry milling normally is carried out by use of a ball milling device, where balls or other equivalent milling media are introduced to the mixture and the apparatus rotated so as to cause the particles to grind against the milling media over an extended period of time. Dry milling has to be coupled with classification to ensure no particles are larger than 16 microns. The wet milling normally is carried out by use of a wet milling device, where balls or other equivalent milling media and a liquid are introduced to the mixture and the apparatus rotated so as to cause the particles to be reduced to the desired size. Jet milling is a method by which the particles are caused to impinge upon each other under jets of gas (air or inert) so as to cause comminution. For purposes of this preferred method, the mixture is charged into an inert milling jar along with grinding media in the form of Burundum cylinders (85% AI2O3, 11% Siθ2, and 4% other ingredients) for a period of 48 hours at 64 rpm. The mixture is then separated from the media by use of a #10 mesh screen. The dry milled ixture is air classified and/or sieved to insure that no particle in the mixture is greater than 16 microns and optimally no particle should be greater than 3-4.5 microns. The milling is carried out for a sufficient period of time to insure that no particle in the mixture is greater than 9 microns and optimally no particle should be greater than 3-4.5 microns. In order to determine whether the mixture is of such limited particle size, x-ray sendimentation, centrifugation, sieving, and laser light scattering tests may be employed. By reducing the absolute maximum particle size, the particle size distribution becomes such that packing efficiency is detrimentally affected. Without the intermingling of somewhat large and small particles, the packing of the entire powder mixture becomes limited. The poor packing does not detrimentally affect the chemical OMPI reaction. The thicker preforms that result from a poor particle size distribution results in very long hot pressing strokes. If a small compression ratio is desired, the poor packing efficiency can be remedied by adding 2-25% of silicon nitride composite powder. This silicon nitride composite will have a particle size distribution which when combined with the silicon mixture will give the desired packing efficiency. The silicon nitride composite is made according to the teachings of this invention (using a maximum particle size of 16 microns or less) for forming a mixture, compacting, and heating to nitride. The nitrided body is then dry milled to the desired particle size districution. The silicon nitride composite powder must be the same chemistry as the resulting nitrided body without the addition of composite. This insures a completely homogeneous body with the same chemistry throughout. This homogeneous approach (adding silicon nitride composite of the same chemistry as the resulting nitrided body) eliminates areas of local high silicon nitride concen¬ tration without second phase chemistry (low density areas) that is obtained by the addition of silicon nitride to a silicon powder mixture. The larger sized particles of the silicon nitride composite are useful and present no bad effects; no free silicon results from the use of the silicon nitride composite. Compacting 3. A measured quantity of the milled mixture is then loaded into a cold pressed die arrangement and pressed at ambient conditions by use of typically 1400-1500 psi to form a compact of a size of 6 x .6" and a density of about 1.0 gm/cm 3 . With the addition of 2-25% of the silicon nitride composite, the density of the compact becomes at least 1.4 gm/cm 3 . -gfREA£ OMPI Heating to Nitride 4. The compact is heated in a nitriding atmos¬ phere without the use of pressure normally associated with i hot pressing to produce a silicon nitride comprising body "* 5 consisting of mixed phase (alpha or beta) silicon nitride, at least one dispersed second phase crystallite (silicon oxynitride), .2-1% silicate (by weight of the body), and unmeasurable amounts of free silicon and unreacted oxygen carrying agents (here 2O3 and AI2O3). The body will have 10 a size greater than and a density less than the object to be formed. As the result of the use of a critical particle size no greater than 16 microns, conventional nitriding techniques can be used to nitride silicon in the presence of oxygen carrying agents as used in reaction bonded 15 silicon nitride technology (see nitriding techniques in Mangels U.S. patent , the disclosure of which is incorporated herein). Because of the use of an absolute maximum particle size of 16 microns, the nitrided body will preferably 20 consist of silicon nitride which has at least 60% alpha phase silicon nitride, 3-15% silicon oxynitride in the YlSiθ2 phase, and the remainder silicate glass (which may be theorized to be aluminosilicate. The resulting thickness of the nitrided bodies 25 will be approximately the same thickness size as that of the compact going into the nitriding step. Hot Pressing 5. The nitrided body is then hot pressed to produce a silicon nitride comprising object of required 30 dimension and density. A pressing fixture having graphite walls is used to carry out the 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 up to the final hot pressing temperature of 3000°F and pressure of 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 temperature. The object is then cooled at any rate, even quenched, to room temperature. The resulting hot pressed object will have a strength level (modulus of rupture) in the range of 150-190 ksi and a density in the range of 3.28-3.32 gm/cm 3 . The hardness is also increased as a result of the practice of this invention from a typical hardness level (45-N scale) of 90 to 92. The system using the silicon nitride additive to the mixture, will experience less side wall drag (between sides of the preforms and the die walls) which will result in less distortion in the final product. The nitrided preforms formed from a mixture containing a silicon nitride composite will exhibit a smaller height dimension for the preforms and thus reduce the hot pressing stroke. OMPI

Claims: I claim: 1. A method of making a hot pressed silicon nitride comprising object useful as a cutting tool, comprising the steps of: (a) forming a powder mixture of silicon and reactive oxygen carrying agents, said mixture being dry or jet milled to an absolute particle size distribution having substantially no particle greater than 16 microns; (b) compacting said mixture to form a preform having a density of 1.1-1.4 gm/cm 3 ; (c) heating said preform in a nitriding atmosphere without the use of pressure to substantially fully react the ingredients of said mixture with said atmosphere to form a body consisting substantially of silicon nitride; and (d) hot pressing said nitrided body to produce a silicon nitride comprising object of desired dimension and density useful as a cutting tool. 2. The method as in Claim 1, in which said maximum particle size is 3-4.5 microns. 3. The method as in Claim 1, in which said reactive oxygen carrying agents are selected from the group consisting of Y2O3, AI2O3, Zrθ2, Ceθ2, MgO, and other rare earth oxides, each being present in an amount to substantially fully react said silicon upon heating to reactive temperature. 4. The method as in Claim 3, in which the amount of certain of said oxygen carrying agents permitted in said mixture is limited by percentage weight of the mixture to 3.2-16% Y2O3 and .4-4% AI2O3. 5. The method as in Claim 4, in which said mixture additionally contains Si3N4 composite in an amount of 2-25% of said mixture, thereby increasing packing efficiency. 6. The object resulting from the practice of Claim 4, having a modulus of rupture in the range of 150-190 ksi and a hardness (45-N) of at least 92.

Inventors: BECKWITH ELAINE C, EZIS ANDRE, BECKWITH, ELAINE, C., EZIS, ANDRE
Applicants: FORD FRANCE, FORD MOTOR CANADA, FORD MOTOR CO, FORD WERKE AG, FORD FRANCE S.A., FORD MOTOR COMPANY, FORD MOTOR COMPANY LIMITED, FORD MOTOR COMPANY OF CANADA, LIMITED, FORD-WERKE AKTIENGESELLSCHAFT
Application year: 1982
Main classification: C04B35
IPC codes: C04B35