High Strength Silicon Nitride

The present invention relates to a dense silicon nitride material having a novel composition and, in particular, relates to an improved silicon nitride material incorporating strontium carbonate, as a green state material, and yttrium oxide which are used as sintering aids to produce the dense silicon nitride material having high strength over a wide range of temperatures.

Sintered silicon nitride materials are well-known in the art. However, silicon nitride materials are usually limited in that the materials do not exhibit high strength and long stress rupture life when utilized in a high temperature (greater than 1000.degree. C.) environment. It is known that sintering aids have been utilized in order to increase the density of the sintered material which in turn increases the mechanical strength. It is also known that the incorporation of yttrium oxide (Y.sub.2 O.sub.3) is and remains one of the most utilized sintering aids to increase the properties of sintered silicon nitride materials. Furthermore, it is known to use strontium oxide (SrO) as a sintering aid with silicon nitrides, however, strontium oxide is unstable and reacts with the atmosphere and therefore degrades prior to the final formation of the silicon nitride material. In addition, strontium oxide is much more expensive than strontium carbonate which is stable.

It is therefore an object of the present invention to produce a dense silicon nitride material which has improved mechanical strength and stress rupture life at high temperatures.

It is another object of the present invention to produce a silicon nitride material which has been processed using an appropriate combination of strontium carbonate and yttrium oxide as starting sintering aids.

More specifically, in accordance with the present invention, there is provided a densified silicon nitride having a final composition comprising (i) 82.0-97.5% silicon nitride (ii) 0.5-6.0% of strontium oxide and (iii) 2.0 to 12.0% of yttrium oxide. The silicon nitride is made by mixing the three constituents; silicon nitride, yttrium oxide and strontium carbonate along with an appropriate organic binder, if necessary, for forming the shape. The article can then be formed by any appropriate method such as hot pressing, cold isostatic pressing, injection molding, extrusion, uniaxial diepressing or slip casting. Thereafter, the article is thermally treated in a vacuum in order to decompose the strontium carbonate into carbon dioxide and strontium oxide. The article is then cooled to room temperature in the vacuum then stored in a low humdity environment such as a dessicator which keeps the article from contacting moisture. Densification by hot isostatic pressing is achieved by encapsulating the article in a metal (e.s. Nb) can under vacuum utilizing electron beam welding or other encapsulation technique, and heating to a temperature greater than 1600.degree. C. in an inert atmosphere while pressurized to 192.9 MPa (28 KSI) (hot isostatically pressed) before cooling to room temperature and ambient pressure. The article can also be hot isostatically pressed to high density using other encapsulants such as high temperature glass frit systems. In addition, the article can also be densified by sinter/hot isostatic pressing process which eliminates the need to use an encapsulant.

According to the present invention, a family of dense silicon nitride materials are prepared from mixtures comprising silicon nitride, yttrium oxide and strontium carbonate. The mechanical characteristics of the silicon nitride material is improved in that a dense body which is made from approximately 0.5-6.0 wt. % of strontium oxide (after conversion from strontium carboante) 2.0-12.0 wt.% yttrium oxide, and the balance silicon nitride is highly densified and has an improved mechanical flexure strength at high temperatures. FIG. 1 compares the flexural strength of one of the improved silicon nitride materials (8 wt.% Y.sub.2 O.sub.3, 2 wt.% SrO and 90 wt. % Si.sub.3 N.sub.4 densified by Nb encapsulation) with the state of the art silicon nitrides. This densified silicon nitride has the flexural strength and other typical properties shown in Table I. As shown within FIG. 1, the improved silicon nitride has significantly higher flexural strength over a range of temperatures.

Table II shows the results of thirteen different densified article compositions and their corresponding densified properties. These articles were all densified by the Nb encapsulation and HIP process. Each of these articles were fabricated using green-body articles having a composition of 0.5-6.0% (after decomposition from strontium carbonate) by weight of strontium oxide, 1.8 to 12.0% by weight yttrium oxide and the balance silicon nitride. The starting material for strontium oxide is strontium carbonate at a weight percentage of from 0.7-8.6%. Strontium carbonate is utilized in order to stablize the sintering aid in the green forming stage and thereby retain the integrity of the article through densification of the final silicon nitride material.

Green-body articles having a composition in the range given above can be densified in one of two ways. The first process requires an encapsulant while the second does not, each process can be used for near net shape forming of articles.

The first densification process is referred to as the encapsulation/hot isostatic pressing (HIP) process and comprises the steps of mixing the three constituents having the desired particle size by milling or other techniques. The mixed material is formed into the desired shape either by cold isostatic pressing, uniaxial die-pressing, injection molding, compression molding, extrusion or slip casting. In the case of cold isostatic pressing the milled ceramic powder is placed within a rubber die (or a polymeric envelope) and subjected to pressures of 275.6-413.4 Mpa (40-60 KSI) at room temperature. In the case of uniaxial pressing the ceramic powder (with or without a small amount of organic binder) is compressed to high pressure 17.2-68.9 MPa (2.5-10.0 KSI) in a metal die. In the case of injection molding, compression molding or extrusion, an organic binder at an appropriate percentage is blended with the ceramic powder to form a molding feed stock (or molding mix) and the molded into a shaped article using an injection molder, compression molder or extruder. The organic binder in the article is subsequently removed by an appropriate thermal cycle. In the case of slip casting, the milled ceramic powder is dispersed in water with the aid of a dispersant to form a low viscosity slurry or slip. This slip is then cast into a mold to form the shaped article, which will be dried prior to further processing. The formed green body, by any one of the above processes, is heated in a vacuum at 1100.degree.-1250.degree. C. for 3-10 hours. The heating step results in the decomposition of the strontium carbonate to the strontium oxide. Thereafter, the formed green body is cooled to room temperature while still in the vacuum in order to protect the green body from contact with moisture. The green body is then enclosed in an encapsulant (metal can or glass) while remaining under the vacuum. Once sealed within the encapsulant, the green body is subjected to temperatures of approximately 1600.degree.-1900.degree. C. and pressure of 10.3-206.7 MPa (1.5-30 KSI) for 1 to 8 hours in an inert atmosphere. Thereafter, the densified article is allowed to cool to room temperature.

The flexural strengths of thirteen compositions from within the specified range, densified by the Nb encapsulation/HIP method have already been listed in Tables I and II. One of the thirteen compositions (8 wt. % Y.sub.2 O.sub.3, 2 wt. & SiO, and 90 wt. % Si.sub.3 N.sub.4), was also densified by the glass encapsulation/HIP method, which is a near-net-shape process. The strength of this material approaches that of the material densified by the Nb encapsulation/HIP method (Table I). Table III shows the flexural strength as well as other properties of the glass encapsulated/HIP'ed material.

The second densification process is referred to as the sinter/hot isostatic pressing (Sinter/HIP) process and as stated does not require an encapsulant and is better suited to forming near net shape articles economically. With this process, the shaped green body can be formed by the methods described above, i.e. slip casting, injection or compression molding, cold isostatic pressing; etc. and vacuum heat treated to decompose the strontium carbonate to strontium oxide while removing the carbon dioxide.

The shape green body is then placed in a crucible or container and buried in a mixture of powders of silicon nitride (Si.sub.3 N.sub.4), yttrium oxide (Y.sub.2 O.sub.3), aluminum oxide (Al.sub.2 O.sub.3) and boron nitride (BN). The crucible is then placed in a furnace and heated in nitrogen atmosphere at a temperature in the range of 1800.degree.-1900.degree. C. and pressure in the range of approximately 0.7-2.1 MPa (100-300 psi) for 2 to 6 hours. Thereafter, the pressure is increased to a pressure in the range of approximately 10.3-206.7 MPa (1.5-30 KSI) for a period of 1 to 8 hours. Following the Sinter/HIP process, the article is allowed to cool to atmospheric conditions.

Table IV shows net shape articles which have been formed by a variety of green-forming processes; slip casting, injection molding or cold isostatic pressing, and densified utilizing either the encapsulation/HIP or sinter/HIP process. As disclosed, the final net shape articles can range from a simple billet to a complex shaped turbocharger turbine rotor which includes a wide variation of cross-sectional sizes.

It is presently believed that the materials used, herein i.e. silicon nitride, yttrium oxide, and strontium carbonate reduced to strontium oxide, when mixed within the specified weight percentages, and densified according to the described proceses, combine to form a grain boundary composition having superior flexure strength and adhesion to the silicon nitride grains. In particular, mixtures having bulk compositions of 89-91 weight % silicon nitride, 7-10 weight % Yttrium oxide and 1-3 weight % strontium oxide have been processed and bulk flexure strengths from 560 to 662 MPa at 1400.degree. C. have been observed.

While a number of specific test compositions, processes and near net shape articles have been set forth and described, it is understood that these are provided by way of example only and that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims: