Abstract: There is disclosed an Al.sub.2 O.sub.3 based ceramic which includes 5% to 50% by weight of a hard dispersed phase of at least one compound selected from metal carbide, metal nitride, metal carbonitride, metal oxy-carbide, metal oxy-nitride and metal carbo-oxy-nitride. The compound is represented by M(CNO), wherein M is at least one metal selected from the group consisting of Ti, Zr and Hf. The ceramic optionally contains 1% to 25% by weight of ZrO.sub.2, and balance Al.sub.2 O.sub.3 matrix, which contains an additive dissolved in the Al.sub.2 O.sub.3 grains so as to form solid solution therewith. The additive is contained in an amount of 0.01% to 12% by weight with respect to the Al.sub.2 O.sub.3 matrix and is at least one oxide of an element selected from the group consisting of Y, Mg, Cr, Ni, Co, and rare earth elements. The Al.sub.2 O.sub.3 grains have an average grain size no larger than 1.0 .mu. m while the hard phase constituent has an average grain size no larger than 0.6 .mu. m.

Abstract: There is disclosed an Al.sub.2 O.sub.3 based ceramic which includes 5% to 50% by weight of a hard dispersed phase of at least one compound selected from metal carbide, metal nitride, metal carbonitride, metal oxy-carbide, metal oxy-nitride and metal carbo-oxy-nitride. The compound is represented by M(CNO), wherein M is at least one metal selected from the group consisting of Ti, Zr and Hf. The ceramic contains no greater than 25% by weight of ZrO.sub.2, and balance Al.sub.2 O.sub.3 matrix. which contains an additive dissolved in the Al.sub.2 O.sub.3 grains so as to form solid solution therewith. The additive is contained in an amount of 0.01% to 12% by weight with respect to the Al.sub.2 O.sub.3 matrix and is at least one oxide of an element selected from the group consisting of Y, Mg, Cr, Ni, Co, and rare earth elements. The Al.sub.2 O.sub.3 grains have an average grain size no larger than 1.0 .mu.m while the hard phase constituent has an average grain size no larger than 0.6 .mu.m.

Abstract: There is disclosed a silicon nitride based sintered material which contains 0.1% to 20% by volume of zirconium oxide, 0.1% to 14% by volume of zirconium nitride, 3% to 15% by volume of a binder phase of an Mg-Si-O-N or Mg-Si-Zr-O-N system, and balance .beta.-silicon nitride. The sintered material may include an oxide layer of an average thickness of 10 to 1,000 .mu.m in a surface thereof and having a zirconium oxide concentration increasing toward the surface thereof. A process specifically adapted to manufacture the above sintered material is also disclosed.