Abstract: Coated abrasion resistant composite ceramic cutting tools comprise a composite substrate body consisting essentially of a particulate material selected from the group consisting of the refractory metal carbides, nitrides, carbonitrides and combinations thereof uniformly distributed in a matrix, coated with at least one adherent coating layer comprising a refractory metal carbonitride. The particulate material is of an average particle size ranging between about 0.5 microns to about 20 microns and comprises from about 5 to about 60 volume percent of the composite cutting tool. The matrix consists essentially of a modified silicon aluminum oxynitride having about 2 to about 25 volume percent of a modifier selected from the oxides of silicon, yttrium, magnesium, hafnium, zirconium, beryllium, the lanthanides and combinations thereof.

Abstract: A composite article and cutting tool are prepared by densification to form a body consisting essentially of particles of hard refractory material uniformly distributed in a matrix consisting essentially of a first phase and a second phase, said first phase consisting essentially of crystalline silicon nitride and said second phase being an intergranular refractory phase comprising silicon nitride and a suitable densification aid comprising magnesium oxide and silicon dioxide.

Abstract: High purity fine particle size amorphous Si.sub.3 N.sub.4 powder compacts may be pressureless sintered to high density by the addition of from 100 to 1000 parts per million Ti to the powder prior to compaction.

Abstract: Polycrystalline ceramic bodies of Si.sub.3 N.sub.4 when prepared with a nucleating agent to obtain an intergranular phase which is substantially crystalline, exhibit improved mechanical strength at elevated temperatures over bodies having a glass-containing intergranular phase.

Abstract: Synthetic cordierite (2Mg0,2Al.sub. 2 O.sub. 3.5SiO.sub. 2) ceramic bodies exhibiting low thermal expansions are characterized by a microstructure containing less than 10 percent by volume glass and by microcracks of certain size and frequency of occurrence and may be produced, for example, by (1) dry ball milling talc, ball clay and alpha alumina in noncontaminating media, (2) consolidating the mixture to form a green body, (3) heating the green body at a rate of up to 150.degree.C per hour to a temperature within the range of 1350.degree. to 1425.degree.C, (4) holding the body at this temperature for a time of from 1/2 to 10 hours, and (5) cooling the body to below 1,000.degree.C at a rate of up to 150.degree.C per hour. Such bodies possess good to excellent thermal shock resistance, and chemical durability and are useful, for example, as components in gas turbine engines.