WO1996032359A1

WO1996032359A1 - Silicon nitride body having high as-fired surface strength

Info

Publication number: WO1996032359A1
Application number: PCT/US1996/005037
Authority: WO
Inventors: Mohammad Behi; Jean Yamanis; Chien-Wei Li
Original assignee: Alliedsignal Inc.
Priority date: 1995-04-11
Filing date: 1996-04-11
Publication date: 1996-10-17

Abstract

A silicon nitride body is sintered in a silicon nitride crucible at a temperature ranging from about 1700 °C to 2100 °C. Sintering is carried out in the presence of nitrogen gas having sufficiently high pressure to prevent decomposition of the silicon nitride. The sintered body exhibits as-fired surface strength greater than 75 % of its bulk, machined-surface strength.

Description

SILICON NITRIDE BODY HAVING HIGH AS-FIRED SURFACE STRENGTH

BACKGROUND OF THE INVENTION I. Field of Invention

The present invention relates to the field of silicon nitride ceramics and more particularly to a monolithic silicon nitride ceramic having high as-fired surface strength.

2. Description Of The Prior Art

Silicon nitride, Si₃N₄, ceramics are conventionally densified under i) pressureless, ϋ) elevated gas pressure, ϋi) hot-pressing or iv) hot-isostatically pressing conditions. Under any of these conditions silicon nitride, a covalently bonded material, is typically densified via a liquid-phase mechanism using a quantity of sintering aids. The microstructure of liquid-phase sintered silicon nitride, when properly sintered, is characterized by acicular, or needle-like, grains of β-Si₃N which form via a reconstructive process. α-Si₃N particles in the raw material powder dissolve in the oxynitride liquid that forms at or below the densification temperature and precipitates out as β-Si₃N in the form of hexagonal prismatic grains. The sintering aid liquid solidifies upon cooling and forms the grain boundary phase which binds the β-Si N₄ grains together.

In pressureless sintering, a silicon nitride powder compact is fired at about 1700°C under 1 atmosphere of nitrogen gas. Under these conditions, the compact densifies to near theoretical density only if the sintering aids used form liquids at low temperatures, i.e., temperatures lower than about 1550°C. As a result of the low melting temperature of the sintering aids, silicon nitride ceramics sintered under these conditions have poor mechanical properties at high temperatures. For this reason, pressureless sintered silicon nitrides bodies find applications as machine parts where part temperatures are below about 1000°C. Densification of silicon nitride by hot pressing may lead to ceramics with improved operational temperature capability compared to the pressureless sintered materials when more refractory sintering aids are used. However, hot pressing has severe limitations in terms of component size and shape because only simple, right- cylinder or block shapes can be fabricated. Thus, the value of this process for the production of ceramic machine parts is limited.

Densification of silicon nitride by hot isostatic pressing (HIP) permits the fabrication of near net shape ceramic components which may be formulated with refractory sintering aids for high temperature applications. The most effective method for component fabrication by HIP is to embed the component in molten glass which transmits external gas pressures of about 30,000 psi to the silicon nitride compact and drives the densification process. In this process, the molten glass is in contact with the entire external surface of the component and some constituents of the glass may diffuse into the component and alter the grain boundary composition of the ceramic in the difiusion zone. After sintering and cooling down, the ceramic component is still embedded in solidified glass This glass is usually removed by sandblasting, a process which causes impingement damage to the component's exterior surface. As a result, the room-temperature strength of the material when tested in the as-processed state, i.e., the as-processed surface is not removed by machining, is very low and usually less than about 50% of the strength of the same material after machining. For reference, data for the as- processed and machined surface strengths of hot-isostatically pressed silicon nitride have been reported by J.R. Smyth et al. in the "Proceedings of the Annual Automotive Technology Development Contractors' Coordination Meeting", SAE publication P-265, Dearborn, Michigan, November 2-5, 1992, p. 245. Moreover, the as-processed surface strengths of the material at high temperatures are also much lower than the corresponding machined surface strengths and this may be the result of both the damage caused by the sandblasting and the diffusion of glass constituents into the ceramic component as discussed hereandabove. Densification of silicon nitride by gas pressure sintering is the most efficient method for the fabrication of near net shape ceramic components which may be formulated with refractory sintering aids for high temperature applications. Gas pressure sintering of silicon nitride bodies at higher temperatures often leads to a body whose exterior surface microstructure and properties are not the same as in the bulk of the material. The cause for the microstructure difference between the bulk and the exterior surface lies in the interaction of the exterior surface with the surroundings, i.e., the furnace and its heating elements, the crucible in which the body is placed for densification, or both, and is a function of the composition of the silicon nitride green body. For example, when the furnace and the heating elements are made of graphite, the exterior surface of the specimen "sees" the carbon surroundings and the carbon vapor that is present in the furnace at the high sintering temperatures used, i.e., temperatures greater than about 1700°C. Silicon nitride bodies which are sintered under the latter conditions lead to materials whose exterior surface microstructure differs from that of the bulk as a result of reaction of the body with the carbon vapor or loss of some of the sintering aid materials due to volatilization or both. In some cases, this¹ interaction may partially, or even extensively, inhibit the body's densification depending on the material's composition. In order to overcome these undesirable effects on silicon nitride green body densification or exterior surface microstructure, densification is often carried out by embedding the silicon nitride body in silicon nitride powder or powder mixtures consisting of silicon nitride, sintering aids, and other, perhaps, inert powders. However, packing powders often interfere with the densification process by slowing down heat transfer rates to and from the green body and/or by interfering in some other physicoche ical phenomenon. SUMMARY OF THE INVENTION

The present invention provides a silicon nitride ceramic body which has as- fired surface strength which exceeds 75% of the bulk, machined-surface strength of said ceramic. Advantageously, the as-fired surface composition of the silicon nitride body is essentially the same as the bulk material composition.

In addition, the invention provides a process for fabricating a silicon nitride body which has high as-fired surface strength and an as-fired surface composition which is essentially the same as the composition of the bulk material. This invention further provides a method for densifying silicon nitride by gas pressure sintering so that the as-fired or exterior surface composition is the same as that of the bulk and the strength is within 25% of the bulk, machined- surface properties of the material. Specifically, this invention provides a method for densification of a silicon nitride body by gas pressure sintering in a silicon nitride crucible or, preferably, in a silicon nitride crucible which has an overall chemical composition which is close to or essentially the same as the composition of the silicon nitride body being sintered.

According to one aspect of this invention, a silicon nitride green body is placed in a silicon nitride crucible having the same composition as the sintered silicon nitride body and gas pressure sintering the green body in said crucible so as to density the green body to a density higher than about 98% of the theoretical density. In this way the resultant sintered silicon nitride body has exterior surface composition which is essentially the same as that of the bulk and the unmachined exterior surface strength is higher than about 75% of the bulk strength. As a result of so controlling the exterior surface composition and strength, net shape and complex silicon nitride parts could be fabricated which would require minimal, if any, diamond grinding of the exterior surface of the part. Silicon nitride parts produced by this process would be less costly to produce because they would not require costly diamond grinding and, in addition, would have excellent oxidation resistance and lifetimes.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be more fully understood and further advantages will become apparent when reference is made to the following description of the preferred embodiments of the invention and the accompanying drawings, in which:

Fig. 1. is a photomicrograph depicting as-fired surface morphology of a silicon nitride sample sintered in a silicon nitride crucible; Fig. 2 is a photomicrograph depicting as-fired surface morphology of a silicon nitride sample sintered in a graphite crucible;

Fig. 3 is an X-ray diffraction pattern of the composition of the as-fired surface of a silicon nitride sample that was processed in a silicon nitride crucible; and Fig. 4 is an X-ray diffraction pattern of the composition of the as-fired surface of a silicon nitride sample that was processed in a graphite crucible.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The fabrication of cost-effective silicon nitride parts requires that these parts be fabricated in net or near-net shape to minimize machining costs. The use of net or near net shape silicon nitride parts requires that the material properties, whether mechanical or physicochemical, be uniform throughout the body. In particular, the exterior surface properties should be as close to the bulk material properties as possible. It is the object of the present invention to provide a silicon nitride ceramic body which has as-fired surface strength that exceeds 75% of the bulk, machined-surface strength.

It is well known that silicon nitride is sintered via a liquid phase mechanism wherein the silicon nitride powder is intimately mixed with one or more sintering aid oxide powders, the mixed powder is formed into a green body and the green body is fired at a high enough termperature to sinter to near full density. As the green body is heated to the sintering temperature the oxide powders melt or react with the silicon nitride to form a liquid phase which becomes the primary medium for mass transport in the densification process. The silicon nitride powder used in the fabrication of silicon nitride bodies is primarily in the α-Si N₄ form. During liquid phase sintering, the α-Si₃N₄ particles dissolve in the liquid and subsequently precipitate as β-Si₃N₄ particles in the form of hexagonal, prismatic grains.

Silicon nitride bodies which can be used under load bearing conditions at operating temperatures in excess of about 1200°C must be formulated with refractory sintering aids which, in turn, require densification temperatures in excess of about 1800°C so as to form the liquid phase which drives the densification process. These high temperature firing conditions require the presence of nitrogen gas at sufficiently high pressure in order to mitigate the decomposition of silicon nitride itself. As mentioned hereinabove, the densification of silicon nitride in the presence of nitrogen, and perhaps other inert gases, is referred to as gas-pressure sintering.

When a silicon nitride green body is fired to temperatures in excess of about 1700°C, the silicon nitride composition, i.e., the mixture of silicon nitride and sintering aid particles, in the vicinity of the exterior surface "sees" a different environment than the material in the interior of the body. This environment comprises the surfaces of the heating elements, the furnace interior, and the crucible that is often used. These surfaces are usually tungsten or graphite At the high sintering termperatures needed for silicon nitride densification, the heating element, furnace and crucible surfaces and the vapors of these surfaces interact with the exterior surface of the silicon nitride being sintered. In addition, the exterior surface of the silicon nitride being sintered gives off vapors. As a result of these physicochemical interactions between the body being sintered and its environment, the chemical composition of the grain boundary phase near the silicon nitride exterior surface is different from that in the interior. Moreover, in some areas the grain boundary phase may be entirely absent and in other areas the interactions may lead to the formation of pits. The effects of these physicochemical interactions are more pronounced when the furnace and heating elements are made of graphite or when the silicon nitride green body is sintered in a graphite crucible. The different chemical composition of the grain boundary phase or its absence at the exterior surface gives rise to a local microstructure which is different form that in the interior and, in particular, the local exterior microstructure may be coarser. The differences in chemical composition and microstructure between the exterior and interior surface translates to differences in mechanical and other properties and, in particular, to lower fracture strength at the exterior surface in the as-fired state. The lower as-fired surface fracture strengths are often associated with the aforementioned pits becoming the fracture origin or critical flaw.

We have discovered that by sintering a silicon nitride green body in a silicon nitride crucible having identical or similar composition leads to sintered silicon nitride bodies which have as-fired surface strengths which exceed 75% of the bulk, as-machined surface strength. We have also discovered that by sintering a silicon nitride green body in a silicon nitride crucible having identical or similar composition, the as-fired surface composition of the sintered silicon nitride body is essentially the same as the bulk material composition. Sintering of silicon nitride in a silicon nitride crucible having identical or similar composition makes the environment, both solid surface and gaseous, which surrounds the silicon nitride body being sintered similar to the final body composition and eliminates or significantly reduces the undesirable physicochemical interactions between the exterior surface and the surroundings mentioned previously because it eliminates the different chemical sources or sinks and the chemical potential driving differentials.

Sintering of silicon nitride in a silicon nitride crucible having identical or similar composition can be carried out in tungsten or graphite furnace When sintering in a graphite furnace, the silicon nitride crucible provides an excellent barrier to the dentrimental effects of the carbon surfaces and carbon vapors emanating from the furnace graphite materials.

In addition, there is provided, in accordance with the invention, a process for fabricating a silicon nitride body which has high as-fired surface strength and as-fired surface composition which is essentially the same as the composition of the bulk material. This invention provides a method for densifying silicon nitride by gas pressure sintering so that the as-fired or exterior surface composition is the same as that of the bulk and the strength is within 25% of the bulk, machined-surface properties of the material. Specifically, this invention provides a method for densification of a silicon nitride body by gas pressure sintering in a silicon nitride crucible or, preferably, in a silicon nitride crucible which has an overall chemical composition which is close to or essentially the same as the composition of the silicon nitride body being sintered.

According to one aspect of this invention, a silicon nitride green body is placed in a silicon nitride crucible having a composition essentially identical or similar to the composition of the sintered silicon nitride body. Gas pressure sintering of the green body in the crucible is carried out so as to densify the green body to a density higher than about 98% of the theoretical density. In this way the resultant sintered silicon nitride body has exterior surface composition which is essentially the same as that of the bulk, and the unmachined exterior surface strength is higher than about 75% of the bulk strength. As a result of so controlling the exterior surface composition and strength, net shape and complex silicon nitride parts, formed by processes such as slip casting, injection molding, powder pressing or other forming processes, are fabricated which require minimal, if any, diamond grinding of the exterior surface of the part. Silicon nitride parts produced by this process are less costly to produce because they do not require costly diamond grinding and, in addition, have excellent oxidation resistance and service life

The following examples are presented to provide a more complete understanding of the invention. The specific technique, conditions, materials. proportions and reported data set forth to illustrate the principles and practice of the invention are exemplary and should not be construed as limiting the scope of the invention.

Example 1

A plaster mold with a cavity (7.5" diameter x 3.75" height) was prepared for drain casting a silicon nitride crucible. A batch of 2000 grams of silicon nitride powder, containing 5.1 wt.% La₂O₃, 1.7 wt% Y₂O₃, 1.2 wt% SrO, 0.6 wt% SiC and 91.4 wt.% Si.N^ was mixed with deionized water (DI) water. The slurry, containing 70% solids and 0.1 to 0.2% Darvan C dispersant and adjusted to 9.8 pH using ammonium hydroxide, was milled for 24 hours with silicon nitride media. The milled slurry was poured into a plaster mold and allowed to cast from two to four hours to form the silicon nitride crucible. The green crucible was sintered, in one run, at 1800, 1950, and 2000°C for 2, 3 and 1.75 hours, respectively, under 1500 psig of a nitrogen and argon gaseous mixture.

A total of five silicon nitride billets were slip cast and fired. The powder composition, the milling and firing conditions were the same as those used in the fabrication of the silicon nitride crucible as described above. Three billets were densified in the silicon nitride crucible and two in a graphite crucible. All the samples achieved better than 99% of theoretical density.

The fired billets were sliced into modulus of rupture (MOR) bars, having dimensions of 3x4x50mm, and tested by 4-point bending at room temperature Sample densities, the number of bars tested, the average as-fired surface strengths, and the standard deviations are shown in Table 1. Table 1 As-Fired Surface Strength of Samples Sintered in Silicon Nitride and Graphite Crucibles

Crucible Sample No. Density No. of MOR Average 4-pt Standard Material (g ml) Bars Bend Strength (ksi) Deviation (ksi)

Si_N₄ 1 3.28 8 114.4 8.2

Si₃N₄ 2 3.29 4 124.6 12.7

Si₃N₄ 3 3.29 5 122.3 4.4

The Following are Comparative Examples

Graphite 4 3.30 12 80.1 15.6

Graphite 5 3.30 12 63.9 1.6

Comparing the average strength data for samples 1, 2, and 3 in Table 1, i.e., the as-fired strength data for the samples that were fired in a silicon nitride crucible, to the corresponding strengths of samples 4 and 5, i.e., comparative 0 samples, it is evident that the as-fired surface strengths of the samples that were sintered in a silicon nitride crucible are be much higher. These data show the great advantage of firing silicon nitride bodies in silicon nitride crucibles and, particularly, in silicon nitride crucibles having the same composition as the silicon nitride bodies being fired. 5

Example 2 Modulus of rupture (MOR) bars were sliced from the bulk (i.e., away for the exterior surfaces) of the billets which were densified in the silicon nitride crucible as reported in Example 1. Some of these bars were tested by 4-point 0 bending at room temperature. The number of bars tested, the average strengths, and the standard deviations are shown in Table 2. Table 2 Bulk, machined-surface Strength of Samples Sintered in Silicon Nitride

Crucibles

Si₃N₄ 1 3.28 3 148.2 4.1

Si₃N₄ 2 3.29 6 141.1 3.2

Si₃N₄ 3 3.29 3 137.5 2.9

Comparing the average strength data for samples 1, 2, and 3 in Tables 1 and 2, i.e., the strength data for the samples that were fired in a silicon nitride crucible, it is evident that the as-fired surface strength for these samples is 77.2%, 88.3%, and 88.9% of the bulk, machined-surface strength of the corresponding 0 samples. The achievement of high as-fired surface strength as exhibited by samples 1, 2 and 3 makes it possible to fabricate silicon nitride parts in net or near-net shape which require minimal, if any, diamond machining. The lower cost of net or near-net shape silicon nitride parts facilitates the introduction of these parts in internal combustion or turbine engines. 5

Example 3 The as-fired surfaces of the samples in Example 1 were examined under an optical microscope. Figs. 1 and 2 are photomicrographs of the as-fired surfaces of the samples that were fired in the silicon nitride and graphite crucibles, 0 respectively.

Fig. 1 represents the as-fired surface morphology of the samples processed in silicon nitride crucible. The exterior surface of the sample is very "clean" and does not have any flaws as a result of the protective environment that the silicon nitride crucible provided during the sintering process. The consequence of the clean as-fired surface, the silicon nitride body has high as-fired surface strength, i.e., greater than 75% of its bulk, machined-surface strength.

Fig. 2 represents the as-fired surface morphology of samples that were fired in the graphite crucible (comparative samples). The photomicrograph shows that the surface of these samples is dramatically altered and is adorned by large flaws which become fracture origins and lead to low strengths. The surface deterioration is probably caused by reaction of the silicon nitride surface with carbon vapor and depletion of the grain boundary from the surface by volatilization during the sintering process.

Example 4 The composition of the as-fired surface of the samples in Example 1 was examined by X-ray diffraction (XRD) analyses. Fig. 3 shows the composition of the as-fired surfaces of the samples processed in a silicon nitride crucible (Table 1). The as-fired surface of these samples consists of β-Si₃N₄ and amorphous grain boundary phases. The surface of the samples was well protected from any undesirable reaction during the sintering process. Therefore, extraneous phases did not form on the surface and the composition of the as-fired surface is identical to the bulk composition. In the absence of side reactions and formation of extraneous phases, the as-fired surface strength of the samples is better than 77% of machined- surface bulk strength (Table 2).

Fig. 4 represents the XRD pattern of the as-fired surface of the samples that were processed in a graphite crucible (Table 1). Comparing this to the pattern of the as-fired surface illustrated by Fig. 4 shows that the as-fired surface of the graphite-fired samples contains one or more new but unknown phases. These unknown phases are the products of the reaction of the sample's external surface with carbon vapor from the graphite crucible at elevated temperatures (1800- 2000°C). Formation of these phases introduces many potential fracture origins (pits) on the as-fired surface of the samples which cause the samples to fail at lower stress, i.e., the samples which are fired in the graphite crucible have lower strength as the data of Table 1 show.

Claims

What is claimed is:

1. A sintered silicon nitride body having as-fired surface strength greater than 75% of its bulk, machined-surface strength, said silicon nitride body having been sintered in a silicon nitride crucible at a temperature ranging from about 1700°C to 2100°C in the presence of nitrogen gas having sufficiently high pressure to prevent the decomposition of silicon nitride.

2. A sintered silicon nitride body as recited by claim 1, said silicon nitride crucible having substantially the same composition as that of said body.

3. A sintered silicon nitride body as recited by claim 1, said body having grain boundary composition in the as-fired surface which is essentially the same as that in the bulk.

4. The process of using a silicon nitride crucible to sinter silicon nitride in the presence of nitrogen gas of sufficiently high pressure to prevent silicon nitride decomposition.

5. A sintered silicon nitride body as recited by claim 1, wherein said nitrogen gas has a pressure ranging from about 0.1 to 15 MPa.

6. A sintered silicon nitride body as recited by claim 2, said silicon nitride crucible having a composition consisting essentially of that of said body.

7. A sintered silicon nitride body, as recited by claim 6, said silicon nitride crucible having a composition consisting of that of said body.

8 In a process for sintering silicon nitride, the improvement wherein said sintering is carried out in a silicon nitride crucible in the presence of nitrogen gas having pressure sufficiently high to prevent decomposition of silicon nitride. 9. A process as recited by claim 8, wherein said sintering is carried out in the presence of nitrogen gas having pressure sufficiently high to prevent decomposition of silicon nitride.