CA1122385A - Method for producing dense silicon nitride containing yttrium oxide and aluminum oxide and having high temperature strength and oxidation resistance - Google Patents

Abstract

METHOD FOR PRODUCING DENSE SILICON NITRIDE
CONTAINING YTTRIUM OXIDE AND ALUMINUM OXIDE
AND HAVING HIGH TEMPERATURE STRENGTH AND
OXIDATION RESISTANCE

ABSTRACT OF THE DISCLOSURE
The addition of controlled amounts of A1203 to high purity Si3N4 powder (containing less then 0.1 weight percent cation impurities and containing Y203 as a densifying additive) enables shorter sintering times to achieve polycrystalline Si3N4 bodies having densities approaching theoretical density, while a post-sintering crystallization heat treatment results in strengths at high temperatures not otherwise obtainable in the presence of A1203. Resulting Si3N4 bodies are useful as engine parts and components or a regenerator or recuperator structures for waste heat recovery. In addition, such Si3N4 bodies containing A1203 exhibit good oxidation resistance.

Description

3~35 20735 CN METrIOD FOR PRODUCING DENSE SILICON NITRIDE CONTAINING
YTTRIUM OXIDE AND ALUMINUM OXIDE AND HAVING HIGEI
TEMPERATURE STRENGTH AND OXIDATION RESISTANCE
Field of the Invention This invention relates to a method for producing polycrystalline bodies of silicon nitride (Si3N4) containing Y2O3 and A12O3 to facilitate sintering, and exhibiting ~optimum mechanical strength.
Prior Art ~ Si3N4 powder characterized by cation impurities of 0.1 weight percent or less, a morphology of predominantly crystalline 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 Y2O3 and sintered, is known to enable ~production of polycrystalline 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 Y2O3 or a lanthanide rare earth oxide such as CeO2 is typically ; added as a sintering or densifying aid.
In addition, some workers in the prior art have intentionally added impurity materials other than the primary~
~~densification aid, such as M. Mitomo, "Sintering of Si3N4 with A12O3 and Y2O3", Yogyo-Kyokai-Shi, 85 (8) 408-412, 1977-~
Others have chosen to introduce impurities by the selection ` of impure starting materials such as R. W. Rice et al., "Hot Pressed Si3N4 With Zr-Based Additions", Journal of the American Ceramic Society, 5 , (5-6) 264 ~1975~.

3L1;~3~5 Another high temperature property of Si3N4 which is ~effected by impurities is the material's resistance to oxidation. Rare earth oxide-containing Si 3N4 materials apparently resist oxidation by formation of a surface silicate glass layer which forms by the oxidation of Si3N4.
This interfacial layer impedes further oxidation by acting as a barrier to further oxygen diffusion. The oxygen diffusion rate has generally been observed to increase by the addition of modifiers or intermediates to the glass ~ structure. It would thus be expected that the presence of ' modifier compounds such as the alkali or alkaline earth oxides or intermediates such as A12O3 in a Si3N4 body would decrease the body's resistance to oxidation.
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Summary of the Invention In accordance with the invention, a method is provided for facilitating sintering of Si3N4 bodies to densities approaching theoretical density while maintaining optimum l levels of mechanical strength at both room temperature and ` elevated temperature, making them particularly useful in applications such as vehicular and aerospace engine related structural parts, regenerators and recuperators for waste heat recovery, etc.
~, Such method comprises the steps of mixing Si3N4 powder of high purity (less than 0.1 weight percent cation impurities and about 2 to 4 weight percent SiO2) with controlled amounts of Y2O3 and A12O3 as densifying and sintering aids, and consolidating and sintering the ~I materials to a polycrystalline body, and thereafter sub-30, jecting the body to a post-sintering crystallization heat ' treatment to optimize mechanical strength.

-2-' 1, ~l~Z;3~5 In acc~rdance with the invention, good oxidation ~ resistance is maintained for these Y2O3 and A12O3-containing ;: bodies.
~' ' Brief Description of the Drawing Figure 1 is a dilatometric graph for a simulated intergranular composition showing curves "a" and "b" before ~ and after crystallization;
10 : Figure 2 is a graph of Oxidation Rate Constant Kp(Kg /m -sec) vs. Reciprocal Temperature T x 10 ( K ) for Si3N4-Y2o3 bodies with and without A12O3.

. Detailed Description of 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 ~ disclosure and appended claims in connection with the above l description of some of the aspects of the invention.
20 ~ The Si3N4 starting material may be amorphous material, amorphous material which has been partly crystallized by heat treatment, or may be a mixture of substantially completely amorphous material and substantially completely ~i crystalline material.
The presence of A12O3 in the composition facilitates consolidation to full density during pressureless sinter-ing or hot pressing, as indicated by lower temperatures, shorter times and, in the case of pressureless sintering, ,1 lower nitrogen overpressures used to control Si3N4 30 ~1 vaporization.
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31~5 The A12O3 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 1 to 2.5 weight percent. Such A12O3 may be present as an impurity in other starting material, or added as a starting material or precursor, such as Al(OH)3, or may be introduced through abrasion of A12O3 mills and/or milling media.
~ The Y2O3 may be added in the amount of from about u 10 ~ 2 to 25 weight percent, although for SiO2 content of at least 3 weight percent, 3 to 13 weight percent Y2O3 is preferred for optimum oxidation resistance. As taught therein, Y2O3 is most preferably 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 producing Si3N4 bodies are also suitable for the practice ~l of the invention, for example, hot isostatic pressing or 20 'I any pressureless sintering step preceded by a suitable consolidation step such as dry pressing, isostatic pressing, extruding, slip casting, injection molding, etc.
, See U.S. Patent 4,073,845 for a general procedure for Z~ pressureless sintering of silicon nitride bodies.
~' A general procedure for hot pressing will now be , described. Silicon nitride powder consisting of 30 to 40 weight percent amorphous silicon nitride, remainder ' crystalline silicon nitride, with about 95 percent of ~I the crystalline silicon nitride being the alpha phase, 30 i 100 parts per million cation impurities and about 2 to 4 weight percent SiO2, is mixed with Y2O3 using a solution of toluene and about 3 volume percent methanol to form a slurry and the slurry is milled with A12O3, or Si3N4 ~' '', `', .~
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;3~35 , grinding media for about 1 hour to effect a uniform ball milled blend of the starting powders. Where A12O3 grinding media are chosen for wet mllling it is to be expected that from about 0.5 to 1.5 weight percent A12O3 will be picked up by abrasion of the milling media. Such amounts ! constitute sufficient A12O3 to result in significant `~ enhancement of the sintering process, enabling significantly shorter sintering times. Where ZrO2 or Si3N4 milling media are employed, A12O3 may be added to the starting material 10 ; in the form of a powder, or alternatively Si3N4 powder containing A12O3 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 A12O3 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 20 `,i powder is then prepressed at about 2000 psi and then the die is placed in a chamber containing argon, and a pressure 'i of about 500 psi is applied up to about 1200C, and then i pressure and temperature are increased simultaneously so , that the ultimate pressure and temperature are achieved at about the same time. The densification process is monitored using a dial gauge indicating ram travel within 'i the die body. A rate of downward movement of the ram ,I cross head below about 0.004 inches per hour indicates Il completion of densification. The assembly is then cooled 30 ll over a period of about 1 to 2 hours. Ultimate pressures il and temperatures of from about 3,000 to 5,000 psi and 1675C to 1800C for a time of about 2 to 5 hours are ', , adequate conditions for the achievement of essentially full S densification of the silicon nitride body. ', . . .
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3~5 ; To show the effect of impurities in general and A12O3 I in particular on consolidation time during hot pressing, I
two samples containing 13 weigh-t percent Y2O3 were hot pressed using as starting Si3N4 material high purity and ~; low purity Si3N4 powders, respectively. Impurity levels ¦ are shown in Table I in weight ~.

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;/ TABLE I
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High Purity Si3N4 Low Purity Si3N4 ` Al 0.002 0.423 Fe -- 1.15 Mn -- 0.027 C -- 0.458 Mg 0. 0007 0 . 013 Ca 0.0006 0.224 Z
Mo 0.01 --., 20 ~I Hot pressing time and other conditions are shown in ~1~ Table II.
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i, TABLE I I
A12O Hot Pressing Hot Pressing Hot Pressing Si3N4 Powder (weighi~ %) time (min.) T ~ . (C) Pressure (psi) ~High Purity 0.004 290 1750 5000 '1 Low Purity 0.800 195 1750 5000 ~, I As may be seen from the table, for the same temperature 30 ',and pressure, hot pressing time was reduced from 290 to 195 minutes where A12O3 was increased from about 0. 004 to about 10.800.
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To show the effect of the presence of A12O3 upon pressureless sintering temperatures and nitrogen pressures ~necessary to reach full densification, four polycrystalline ~Si3N4 bodies containing 6 weight percent Y2O3 and O, 1.5 ~and 2.5 weight percent A12O3 were pressureless sintered.
; Results are shown in Table III.
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TAsLE III
SampleDensity Weight -O ~aximum N2 Pressure No.(-OTheoretical) A12O3Sintering(psig) Te~. (C~ _ ~; 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 A12O3 upon sintering temperatures and N2 overpressure. As A12O3 ~'increased from 0 to 2.5 weight percent, temperature 20 ~ decreased from 1970+ C to 1825C, 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 A12O3-containing intergranular phase will ,generally be in a predominantly amorphous state. Because this amorphous state has poor mechanical properties above about 1200C, it must be crystallized by a post-sintering heat treatment in order to obtain optimum high temperature Istrength. Such heat treatment should be carried out in a 30 ~non-oxidizing atmosphere such as nitrogen or a rare gas, I
such as lle, Ne, or Ar, at a temperature of about 1250C
to 1600C for at least about 1 hour but preferably at least about 5 hours, in order to avoid substantial oxidation of the body.

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In order -to demonstrate crystallization of the inter-; granular phase, three samples of Si3N4 plus 12 weight ~I percent A12O3 were preapred by pressureless sintering at 1775C for 3 hours in a nitrogen atmosphere. X-ray diffraction analysis showed only beta Si3N4, indicating an amorphous intergranular phase. Heat treatment conditions and crystalline phases shown to be present after heat , threatment are shown in Table IV~

,Sample No.Temp(C) Time(Hrs) Phases Present 1400C 5 Beta Si3N4 ~ lY2O3 9Si2 Si3N4 2 1540C 5 Beta Si3N4, lOY2O3 9SiO2 Si3N4 3 1650C 5 Beta Si3N4 1, 1650C was too high a temperature to crystallize the ~second phase.
In order to further demonstrate crystallization of the intergranular material, a powder mixture of 52 weight percent ,IY203 - 28 weight percent A12O3 - 20 weight percent SiO2 was 20 ~¦prepared. This is a simulation of the second phase composi-lltion of the Si3N4 body containing 6 weight percent Y2O3 with 'il2 weight percent A12O3. The SiO2 was added since it is a natural species on the surface of the starting Si3N4 powder ~typically at about the 3 weight percent level. The mixture waS melted at 1750C and then quickly cooled to room ¦temPerature. As melted, the Y2O3 - A12O3 - SiO2 composition contained the nonequilibrium phase 7Y2O3 9SiO2 (according ~to R. R. Wills et al., J. Mat. Sci., Vol 11, pp. 1305-1309, Il1976) plus a large amount of amorphous material as evidenced 30 Iby x-ray diffraction.
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il5 Dilatometer tests were carried out on the intergranular phase simulated composition prepared above. The curves are shown in Figure 1. Curve "a" indicates that the composition prior to crystallization has a glass transition of about 800C and a dilatometric softening point of about 890C. 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, while the softening point is the temperature at which the pressure of the dilatometer probe causes deformation of the sample. For comparison, curve "b", after heating at 3.3C
per minute to 1400C and cooling at the same rate, shows some evidence of a glass transition temper~ature at as high as 1200C and a softening point at about 1380C. This indicates that the glass phase crystallized at about 1400C. !
It is recognized that crystallization of the second phase in the presence of A12O3 leads to a substantial improvement in high temperature strength of Si3N4 bodies.
An unexpected benefit of the present process is that oxidation resistance is substantially retained in the above bodies containing A12O3.
Fig. 2 is a diagram of Oxidation Rate Constant Kp(Kg /m4-sec) for Si3N4 bodies containing 4, 6, 8, 10 and 12 weight percent Y2O3 versus Reciprocal Temperature in T x 104(K 1). The Oxidation Rate Constants for Si3N4 bodies containing 2 weight percent A12O3 and 4 to 12 weight percent Y2O3 are represented by the band or window defined by the lines labeled "minimum" (4 percent Y2O3) and "maximum" (10 to 12 percent Y2O3). It may be seen that the bodies containing 2 weight percent A12O3 have only slightly increased rates of oxidation over the 6 percent Y2O3 body containing essentially no A12O3.

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While there has been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined `
. by the appended claims.

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Claims (3)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for producing a polycrystalline body consisting essentially of a first phase of silicon nitride grains, and a second substantially completely crystalline intergranular phase consisting essentially of SiO2, Y2O3 and A12O3, the method comprising: mixing particulate starting materials of Si3N4 powder, Y2O3 powder, and A12O3 powder, said A12O3 powder in the amount of about 0.25 to 2.5 weight percent of the materials, the Si3N4 powder containing up to 0.1 weight percent of cation impurities and from 2 to 4 weight percent of SiO2 as an impurity on the surface of the Si3N4 particles; consolidating the powder mixture by pressing at a pressure of up to about 2000 psi, sintering the con-solidated powder mixture by simultaneously applying heat and pressure of up to about 1800°C and up to about 5000 psi;
and subsequently heat-treating the sintered body in a non-oxidizing atmosphere at about 1250°C to 1600°C for a time of at least about 1 hour to achieve substantially complete crystallization of the intergranular phase.

2. The method of claim 1 wherein SiO2 is present in the starting material in the amount of at least about 3 weight percent and Y2O3 is added to the starting material in the amount of about 3 to 13 weight percent.

3. The method of claim 2 wherein the heat treatment is carried out for at least about five hours.