HIGH STRENGTH SILICON NITRIDE

Abstract: The addition of controlled amounts of Al<u2>uO<u3>u to high purity Si<u3>uN<u4>u powder (containing less than 0.1 weight percent cation impurities and containing Y<u2>uO<u3>u as a densifying additive) enables shorter sintering times to achieve polycrystalline Si<u3>uN<u4>u bodies having densities approaching theoretical density, while a postsintering crystallization heat treatment results in strengths at high temperatures not otherwise obtainable in the presence of Al<u2>uO<u3>u. Resulting Si<u3>uN<u4>u bodies are useful as engine parts and components or a regenerator or recuperator structures for waste heat recovery. In addition, such Si<u3>uN<u4>u bodies containing Al<u2>uO<u3>u exhibit good oxidation resistance.

Description: HIGH STRENGTH SILICON NITRIDE TECHNICAL FIELD This invention relates to a method for producing poly- crystalline bodies of silicon nitride (Si-N.) containing Y 2°3 an( ^ A "'"2 0 3 to f ac: *--*-i tate sintering, and .exhibiting opti ¬ mum mechanical strength. BACKGROUND ART Si N 4 powder characterized by cation impurities of 0.1 weight percent or less, a morphology of predominately crys¬ talline alpha phase and/or amorphous phase and fine particle size (3 microns or less average particle size as determined by B.E.T.), when consolidated with an additive such as MgO or Y 2 °3 and sintered, is known to enable production of poly- crystalline bodies approaching theoretical density. See U.S. Patent 4,073,845, issued to S. T. Buljan et al. on Feb. 14, 1978, and assigned to GTE Sylvania Incorporated. Such powders may be consolidated into dense bodies by either hot pressing at less severe temperature and pressure conditions than are necessary with less pure and less reactive powders, or by cold pressing and sintering, which is not possible with some less pure and less reactive powders. In the fabrication of such polycrystalline bodies, up to 25 weight percent of Y °3 or a lanthanide rare earth oxide such as Ce0 2 is typically added as a sintering or densifying aid. In addition, some workers in the prior art have inten¬ tionally added impurity materials other than the primary densification aid, such as M. Mitomo, "Sintering of Si 3 N 4 with A1 2 0 3 and Y 2 °3"' Yogyo-fcyokai-Shie, 85 (8) 408-412, 19 Others have chosen to introduce impurities by the selection impure starting materials such as R. . Rice et al., "Hot Pressed Si 3 N 4 with Zr-Based Additions", Journal of the American Ceramic Society, 58, (5-6) 264 (1975). Another high temperature property of Si 3 4 which is effected by impurities is the material's resistance to oxid tion. Rare earth oxide-containing Si 3 N 4 materials apparent resist oxidation by formation of a surface silicate glass layer which forms by the oxidation of Si 3 N 4 . This interfac layer impedes further oxidation by acting as a barrier to further oxygen diffusion. The oxygen diffusion rate has ge erally been observed to increase by the addition of modifie or intermediates to the glass structure. It would thus be expected that the presence- of modifier compounds such as th alkali or alkaline earth oxides or intermediates such as 1 in a Si 3 , body would decrease the body's resistance to oxi dation. DISCLOSURE OF INVENTION In accordance with the invention, a method is provided for facilitating sintering of Si 3 N 4 bodies to densities ap- proaching theoretical density while maintaining optimum lev of mechanical strength at both room temperature and elevate temperature, making them particularly useful in application such as vehicular and aerospace engine and related structur parts, regenerators and recuperators for waste heat recover etc. Such method comprises the steps of mixing Si 3 4 powder of high purity (less than 0.1 weight percent cation impuri¬ ties and about 2 to 4 weight percent Si0 2 ) with controlled amounts of Y 2 0 3 and A1 2 0 3 as densifying and sintering aids, and consolidating and sintering the materials to a polycrys talline body, and thereafter subjecting the body to a post- sintering crystallization heat treatment to optimize mechani¬ cal strength. In accordance with the invention, good oxidation resis- tance is maintained for these Y 2 0 3 and Al 2 0 3 -containing bodies. BRIEF DESCRIPTION OF THE DRAWING Figure 1 is a dilatometric graph for a simulated inter- granular composition showing curves "a" and "b" before and after crystallization; Figure 2 is a graph of Oxidation Rate Constance K (K 2 '/m4-sec) vs. Reciprocal Temperature 1 x 104(°K-1) for™ " S ^3 N 4" Y 2°3 bod: -- es w i th a nd without Al 2 0 3 . BEST MODE FOR CARRYING OUT THE INVENTION For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following dis¬ closure and appended claims in connection with the above description of some of the aspects of the invention. The Si 3 4 starting material may be amorphous material, amorphous material which has been partly crystallized by heat treatment, or may be a mixture of substantially com¬ pletely amorphous material and substantially completely crys¬ talline material. A method for obtaining the Si 3 N 4 powder of the requisite purity, morphology and particle size is des¬ cribed in detail in U.S. patent application S.N. 625,330, filed October 23, 1975, assigned to the present assignee and now abandoned. The presence of Al 2 0 3 in the composition facilitates consolidation to full density during pressureless sintering or hot pressing, as indicated by lower temperatures, shorter times and, in the case of pressureless sintering, lower nitrogen overpressures used to control Si 3 N 4 vaporization. The A1 2 0 3 should be present in the composition in the amount of at least about 0.25 weight percent, below which enhancement of sintering is negligible, preferably about L_-___ ( OMPI to 2.5 weight percent. Such Al 3 O 3 may be present as an im¬ purity in other starting material, or added as a starting material or precursor, such as Al(OH) 3 , or may be introduced through abrasion of Al 3 O 3 mills and/or milling media. The Y 2 °3 mav be ad<3ed in the amount of from about 2 to 25 weight percent, although for SiO 2 content of at least 3 weight per.cent, 3 to 13 weight percent Y 2 O 3 * s Preferred for optimum oxidation resistance, as taught in copending U.S. patent application S.N. , and assigned to the present assignee. As taught therein, Y 2 °3 ^ s mos t prefer¬ ably added in the amount of about 3 to 6 weight percent in. order to optimize oxidation resistance. While a general procedure is outlined for hot pressing, it is to be understood that alternate processes for produ- cing Si 3 N 4 bodies are also suitable for the practice of the invention, for example, hot isostatic pressing or any pressureless sintering step preceded by a suitable consoli¬ dation step such as dry pressing, isotatic pressing, extru¬ ding, slip casting, injection molding, etc. See U.S. Patent 4,073,845 for a general procedure for pressureless sintering of silicon nitride bodies. A general procedure for hot pressing will now be des¬ cribed. Silicon nitride powder consiting of 30 to 40 weight percent amorphous silicon nitride, remainder crystalline silicon nitride, with about 95 percent of the crystalline silicon nitride being the alpha phase, 100 parts per million cation impurities and about 2 to 4 weight percent SiO 2 , is mixed with Y 2 O 3 us i n 9 a solution of toluene and about 3 volume percent methanol to form a slurry and the slurry is milled with Al 3 O 3 , or Si 3 N 4 grinding media for about 1 hour to affect a uniform ball milled blend of the starting powders. Where Al 3 O 3 grinding media are chosen for wet milling it is to be expected that from about 0.5 to 1.5 weight percent Al 3 O 3 will be picked up by abrasion of the milling media. Such amounts constitutes sufficient Al 3 O 3 to result in signi¬ ficant enhancement of the sintering process, enabling signi- ficantly shorter sintering times. Where Zr0 2 or Si.,N 4 mil¬ ling media are employed, A1 2 0 3 may be added to the starting material in the form of a powder, or alternatively Si 3 N 4 powder containing A1 2 0 3 as an impurity may be used. The slurry is dried and milled in the dry state for an additional 3 to 50 hours, and then screened through a coarse mesh, e.g. 50, screen. Where alumina milling media are employed during dry milling, it may be expected that about 0.5 to 1.5 weight percent additional Al 2 0 3 may be picked up in the batch. The screened powder is then loaded into a graphite hot pressing die whose interior surfaces have previously been coated with boron nitride powder. The powder is then prepressed at about 2000 psi and then the die is placed in a chamber containing argon, and a pressure of about 500 psi is applied up to about 1200 °C, and then pressure and temperature are increased simulteneously so that the ultimate pressure and temperature are achieved at about the same time. The densification process is monitored using a dial guage indicating ram travel within the die body. A rate of downward movement of the ram cross head below about 0.004 inches per hour indicates com¬ pletion of densification. The assembly is then cooled over a period of about 1 to 2 hours. Ultimate pressures and temperatures of from about 3,000 to 5,000 psi and 1675 °C to 1800 °C for a time of about 2 to 5 hours are adequate condi- tions for the achievement of essentially full densification of the silicon nitride body. To show the effect of impurities in general and A1 2 0 3 in particular on consolidation time during hot pressing, two samples containing 13 weight percent ^2^3 were -- 0 -- pressed using as starting Si 3 N 4 material high purity and low purity Si 3 N 4 powders, respectively. Impurity levels are shown in Table I in weight %. OMPI TABLE I High Purity Si 3 4 Low Purity Si 3 N 4 Al 0.002 0.423 Fe 1.15 Mn 0.027 C 0.458 Mg 0.0007 0.013 Ca 0.0006 0.224 Mo 0.01 Hot Pressing time and other conditions are shown in Table II. TABLE II Si 3 N 4 A1 2 0 3 Hot Pressing Hot Pressing Hot Pressi Powder (weight %) time (min.) Temp.(°C) Pressure(ps High Purity 0.004 290 1750 5000 Low Purity 0.800 195 1750 5000 As may be seen from the table, for the same temperatur and pressure, hot pressing time was reduced from 290 to 195 minutes where A1 2 0 3 was increased from about 0.004 to about 0.800. To show the effect of the presence of A1 2 0 3 upon pres¬ sureless sintering temperatures and nitrogen pressures necessary to reach full densification, four polycrystalline Si 3 N 4 bodies containing 6 weight percent Y 2 °3 an( ^ -*' ^ ' ^ an 2.5 weight percent Al 2 o 3 were pressureless sintered. Resul are shown in Table III. OM 3ΑBLE III Sample Density (% Weight % Maximum Sintering N 2 Pressure No. Theoretical) A1 2 0 3 Temp. (°C) (psig) 1 100.0 2.5 1825 105 2 100.0 1.5 1825 115 3 98.0 0 1950 130 4 98.2 0 1970+ 140 Results clearly show the beneficial effect of l 2 0 3 upon sintering temperature and N 2 overpressure. As A1 2 0 3 increased from 0 to 2.5 weight percent, temperature decreased from 1970+ °C to 1825 °C, overpressure decreased from 140 to 105 psig, and density increased from 98 to 100 percent of theoretical. After densification by pressureless sintering or hot pressing, the Al 2 0 3 -containing intergranular phase will gen¬ erally be in a predominately amorphous state. Because this amorphous state has poor mechanical properties above about 1200 °C, it must be crystallized by a post-sintering heat treatment in order to obtain optimum high temperature strength. See copending U.S. Patent application S.N. filed concurrently herewith, and assigned to the present assignee. Such heat treatment should be carried out in a non- oxidizing atmosphere such as nitrogen or a rare gas, such as He, Ne, or Ar, at a temperature of about 1250 °C to 1600 °C for at least about 1 hour but preferably at least about 5 hours, in order to avoid substantial oxidation of the body. In order to demonstrate crystallization of the inter¬ granular phase, three samples of Si 3 N 4 plus 12 weight percent Y 2°3' alj0ut weight percent Si0 2 and about 2 to 3 weight percent A1 2 0 3 were prepared by pressureless sintering at 1775 °C for 3 hours in a nitrogen atmosphere. X-ray dif¬ fraction analysis showed only beta Si 3 N 4 , indicating an amorphous interganular phase. Heat treatment conditions and crystalline phases shown to be present after heat treatment are shown in Table IV. TABLE IV Sample Temp Time No. (°C) (Hrs.) Phases Present 1 1400 °C 5 Beta Si 3 N 4 , 10 2 O 3 . 9Si0 2 . Si 3 N 2 1540 °C 5 Beta Si 3 N 4 , 10Y 2 O 3 . 9Si0 2 . Si 3 N 3 1650 °C 5 Beta Si 3 N 4 1650 °C was too high a temperature to crystallize the second phase. In order to further demonstrate crystallization of th intergranular material, a powder mixture of 52 weight per¬ cent Y 2 0 3 -28 weight percent Al 2 0 3 - 20 weight percent Si0 2 was prepared. This is a simulation of the second phase co position of the Si 3 N 4 body containing 6 weight percent Y 2 ° with 2 weight percent A1 2 0 3 . The Si0 2 was added since it a natural species on the surface of the starting Si 3 N 4 pow der typically at about the 3 weight percent level. The mixture was melted at 1750 °C and then quickly cooled to room temperature. As melted, the y 2 °3 ~ A -*-2°3 ~ S: -* 0 2 com P o tion ' contained the nonequilibrium phase 7Y 2 °3 • 9s ->- 0 2 (according to R. R. Wills et al., J. Mat. Sci., Vol. 11, p 1305-1309, 1976) plus a large amount of amorphous material as evidenced by x-ray diffraction. Dilatometer tests were carried out on the intergranul phase simulated composition prepared above. The curves ar shown in Figure 1. Curve "a" indicates that the compositi prior to crystallization has a glass transition of about 800 °C and a dilatometric softening point of about 890 °C. The transition temperature is that at which the thermal expansion changes from a relatively low value of a solid glass to the relatively high value of a liquid phase, whil the softening point is the temperature at which the pressu of the dilatometer probe causes deformation of the sample. For comparison, curve "b", after heating at 3.3 °C per min to 1400 °C and cooling at the same rate, shows so e evidence of a glass transition temperature at as high as 1200 °C and a softening point at about 1380 °C. This indicates that the glass phase crystallized at about 1400 °C. It is recognized that crystallization of the second phase in the presence of Al 2 0 3 leads to substantial improve¬ ment in high temperature strength of Si 3 N 4 bodies. See copending Patent Application S.N. , filed concur¬ rently herewith, and assigned to the present assignee. An unexpected benefit of the present process is that oxidation resistance is substantially retained in the above bodies containing Al 2 0 3 . Fig. 2 is a diagram of Oxidation Rate Constant Λ K (K / " see) for Si 3 4 bodies containing 4, 6, 8, 10 and 12 weight percent Y 9 0, versus Reciprocal Temperature in x 10 (°K ). The Oxidation Rate Constants for Si 3 N 4 bodies containing 2 weight percent A1 2 0 3 and 4 to 12 weight percent Y 2 0 3 are represented by the band or window defined by the lines labeled "minimum" (4 percent Y 9 °3^ an( ^ "maximu " (10 to 12 percent Y 2 0 3 ). It may be seen that the bodies contain¬ ing 2 weight percent A1 2 0 3 have only slightly increased rates of oxidation over the 6 percent containing essen¬ tially no A1 2 0 3 . INDUSTRIAL APPLICABILITY The Si_ 4 bodies of the present invention are useful as engine parts and components or regenerator or recuperator structures for waste heat recovery.

Claims: CLAIMS 1. A method for producing a polycrystalline body con sisting essentially of a first phase of Si 3 N 4 grains and a second intergranular phase comprising Y 2 °3 and s i°2' t * ιe 5 method comprising consolidating particulate starting mate¬ rials and sintering the consolidated materials, the method characterized by adding A1 2 0 3 in the amount of at least about 0.25 weight percent to the materials prior to conso¬ lidation to facilitate sintering, and heat treating the -*_0 sintered body to achieve substantially complete crystalli¬ zation of the intergranular phase. 2. The method of Claim 1 wherein A1 2 0 3 is added in the amount of about 1 to 2.5 weight percent. 3. The method of Claim 1 wherein Si0 2 is present in 15 the amount of at least about 3 weight percent and ' Y 2 °3 ^ s added in the amount of about 3 to 13 weight percent and the heat treatment is carried out within thr range of about 1250 °C to 1600 °C for a time of at least about 1 hour. 4. The method of Claim 3 wherein the heat treatment 20 carried out for at least about 5 hours. IJU SA *

Inventors: QUACKENBUSH C, SMITH T
Applicants: GTE LABORATORIES INC, GTE LABOR INC
Application year: 1979
Main classification: C04B35
IPC codes: C04B35, C04B41