Method for Producing Ceramic Articles of Increased Fracture Toughness

The present invention relates to a method of producing ceramic articles, having improved fracture toughness properties, by powder metallurgical techniques.

Structural ceramics such as silicon nitride, silicon carbide, partially stabilized zirconia and partially stabilized alumina are being developed for high temperature applications in heat engines of various types (automotive, gas turbine), tools and dies, and other components because of their high strength at elevated temperatures, and errosion and corrosion resistance. However, because of the inherent brittleness (low fracture toughness) of ceramics, their application potential is not as widespread as it could be if their fracture toughness and ductility were enhanced.

The fracture strength of a ceramic is expressed by the relationship:

where K.sub.c is the fracture toughness (resistance to crack propogation) and c is the size of the critical crack that leads to failure. For a fixed flaw size distribution, therefore, strength is dependent on fracture toughness. Improvements in fracture toughness would undoubtedly enhance the use of such ceramics, especially where resistance to any thermal cycling is desired.

It has been proposed by F. F. Lange, "The Interaction of a Crack Front with a Second Phase Dispersion", Philosophical Magazine, 22 (1970) pp. 983-92 and further developed by A. G. Evans, "The Strength of Brittle Materials Containing Second Phase Dispersion", Philosophical Magazine, 26 (1972) pp. 1327-44, that the strength (hardness) and fracture toughness of ceramics can be increased by crack pinning by dispersed particles of a second phase. The concept is based on an observation of an increase in the length of a crack front when it interacts with two or more inhomogeneities. The theory further developed by Evans showed that significant improvement in fracture energy (toughness) can be achieved by a presence of closely spaced second phase particles.

Evidence of improvement of fracture toughness by a dispersion of voids throughout a solid material was provided by G. F. Hurley and F. W. Clinard, Jr. "Fracture Toughness and Hardness of Neutron-Irradiated A1.sub.2 O.sub.3, MgAl.sub.2 O.sub.4 and Y.sub.3 Al.sub.5 O.sub.12 ", DOE/ER-0048/1 (1981) pp. 51-57. They observed an increase in fracture toughness by a factor of 2 in a single crystal alumina which had been neutron irradiated. The irradiation produced an array of voids. They also found a good correlation between theoretically predicted fracture toughness and that observed, assuming the voids to be impenetrable obstacles. While neutron irradiation, with the resultant production of voids can increase the fracture toughness of ceramics, as a technique for producing ceramic with high toughness, such irradiation has several drawbacks. It is costly, it produces an anisotropic distribution of voids, and it renders the material radioactive.

In the present method, bubbles of a gas, such as helium, are homogeneously dispersed throughout a ceramic article to increase the fracture toughness of the ceramic, with the article formed by a powder metallurgical technique.

Ceramic articles of improved fracture toughness are prepared by a process wherein ceramic particles are impregnated with gas ions, the gas ion-containing particles cold compacted into a form and the compacted form is then heated, under pressure, to a temperature in excess of about 1000.degree. C. to produce a ceramic article having gas bubbles substantially uniformly dispersed throughout the article.

The gas ions are preferably inert gas ions, such as helium ions, which are impinged onto and impregnated into ceramic particles, such as silicon carbide, silicon nitride, stabilized zirconia or alumina, of a particle size of between about one-half to five micrometers in diameter. Cold isostatic pressing produces a stabilized form which is then sintered or hot isostatically pressed into an article, whereby the gas ions in the particles form gas bubbles in the ceramic article, the gas bubbles substantially uniformly dispersed throughout the ceramic article produced.

The drawing is a flow chart schematically illustrating the process of the present invention.

Ceramic articles having improved fracture toughness are produced by the present process wherein ceramic particles are impregnated with gas ions, the impregnated particles compacted, and the compact heated such that the gas ions form bubbles homogeneously dispersed throughout the formed ceramic article.

The present process forms ceramic articles of improved fracture toughness from ceramic compositions such as oxides, nitrides, borides and carbides of silicon, zirconium, aluminum, magnesium, yttrium, and other complex ceramic compounds such as SiAlON. Examples of such ceramic compositions are silicon nitride, silicon carbide, partially stabilized zirconia, alumina, SiAlON, and the like.

The ceramic compositions are provided in a finely divided powdered state with particles in a size of between 0.5-5 micrometers (.mu.m) in diameter. The small solid particles are of a size which enables impregnation thereof with gas ions at reasonable ion energies.

The ceramic particles are impregnated with gas ions, under vacuum, to produce gas ion-containing ceramic particles. The gas ions used may vary depending upon the ceramic particle components, although inert gases such as helium, argon, neon or krypton are preferred, so as to preclude chemical reaction with the ceramic particles. In instances where a ceramic nitride is used, however, nitrogen might be used as the source of gas ions, while where a ceramic oxide is used, oxygen might be used as the source of gas ions. The impregnation of the ceramic particles with gas ions is effected by impingement of the particles with a high energy beam of accelerated gas ions, under vacuum, at ambient temperature, and preferably with tumbling of the particles. Typically, an ion impingement of the ceramic powder will be effected with a few hundred KeV accelerated beam of gas ions. Preferably, a 150 KeV beam of ions is used, although the energy of the beam may be varied depending upon the particle size of the ceramic material and the accelerated gas specie used. The particles are continuously tumbled during impingement to uniformly expose all of the powder particles to the ion beam using known accelerators and vacuum tumbling systems, such as a rotating cup, for example as described by L. R. Fleischer, J. A. Spitznagel and W. J. Choyke, "Helium Embrittlement of CRT Materials Simulated by Ion Implantation and Hot Isostatic Pressing of Metal Powders", CONF-750989, Vol. II, 1976, pp. 372-386.

The impregnation of the ceramic particles with gas ions is effected at ambient temperatures and the impingement carried out for a period of time sufficient to provide the quantity of ions in the particles that will provide the desired quantity of gas bubbles in the ceramic article produced.

The gas ion-containing particles are then compacted into ceramic articles by first cold compacting to stabilize the same and then hot compacting or sintering. In the cold compacting step, the gas ion-containing particles are compacted at ambient temperature by the application of pressure equally about the supply of gas ion-containing particles. Cold isostatically pressing of the gas ion-containing particles forms a monolithic shape or form which can be subsequently hot compacted. The cold compacting is effected at pressures sufficient to compress the particles to a density of about 65 to 80 percent of theoretical density to provide a form for subsequent hot pressing.

Hot compacting of the gas ion-containing ceramic particles is then carried out with or without pressure to achieve substantially maximum density of the particles. Hot isostatic pressing, at superatmospheric pressure, or sintering at atmospheric pressure, is effected at an elevated temperature, a temperature in excess of about 800.degree. C. Heating and compacting of the form of gas ion-containing particles causes the gas ions to form gas bubbles throughout the article formed, with the gas bubbles substantially uniformly distributed throughout the ceramic article.

The inert gas bubbles formed homogeneously throughout the ceramic article should have a low c/r.sub.o ratio, where 2c is equal to interbubble spacing and 2r.sub.o is the bubble diameter. It is preferred that the c/r.sub.o ratio be between 0.1 to 5. Bubble size during the formation of the ceramic articles can be controlled by process parameters such as time, temperature and amount of gas ions present in the ceramic particles. The size of the gas bubbles dispersed throughout the ceramic article must be less than about 1 micrometer in diameter, with much smaller bubbles preferred, such as bubbles of a size of between about 100-400 Angstroms in diameter.

The process is schematically illustrated in the drawing which illustrates a supply of ceramic particles (Step 1) of a diameter of 0.5-5 micrometers, into which gas ions are injected (Step 2) under vacuum. The supply of gas ion-containing ceramic particles are cold compacted (Step 3) into a form that stabilizes the particles, and the form is then heated with or without pressure (Step 4) to produce a ceramic article containing gas bubbles. The finished article (Step 5) comprises a ceramic article containing inert gas bubbles substantially uniformly dispersed throughout the article which induces increased fracture toughness.

The present process, by providing ceramic articles having gas bubbles substantially uniformly dispersed throughout the article, improves the fracture toughness of the article.