Abstract: A cutting tool composed of silicon nitride comprising chiefly a .beta.-silicon nitride crystal phase and containing a grain boundary phase of rare earth elements, silicon, aluminum, oxygen and nitrogen, wherein the cutting surface is constituted by at least a fired surface or a surface formed by removing and polishing the fired surface by an amount of not larger than 10 .mu.m, and has in the mirror surface thereof a micro-Vicker's hardness which is higher than that of the interior thereof and is not smaller than 16 GPa. The cutting tool has a cutting surface that can be easily worked enabling the cost of working to be decreased, and has a highly hard layer that is suited for cutting cast iron and the like materials featuring increased durability and extended cutting life.

Abstract: An Al.sub.2 O.sub.3 -TiC sintered product comprising Al.sub.2 O.sub.3 as a main component and containing TiC in an amount of from 20 to 40% by weight, wherein the average particle diameter of Al.sub.2 O.sub.3 particles in the sintered product is larger by 5 to 50% than the average particle diameter of TiC particles, the average particle diameter of the sintered product as a whole is not larger than 1 .mu.m, the average particle diameter of TiC particles is not larger than 0.9 .mu.m, and the total amount of the grain boundary phases other than Al.sub.2 O.sub.3 and TiC is not larger than 1.0% by weight. The Al.sub.2 O.sub.3 -TiC sintered product can be used as a substrate 1 for a magnetic head, and has portions such as grooves 4 and stepped portions formed in the surface of the air bearing surface 2 of the slider by being irradiated with ions. The sintered product exhibits excellent surface quality after machined by being irradiated with ions and can be machined at an increased rate.

Abstract: 1. Disclosed is a ceramic sintered body comprising, as main components, (a) 60 to 80% by weight of alumina and (b) 20 to 40% by weight of titanium carbide or a combination of titanium carbide and titanium oxide, wherein said ceramic sintered body further comprises (c) 1 to 10 parts by weight, per 100 parts by weight of the sum of the components (a) and (b), of zirconium oxide, at least a part of which is composed of a tetragonal crystal, and (d) 0.4 to 11% by weight, based on the total ceramic sintered body, of a crystal phase of a composite oxide consisting of magnesium oxide and aluminum.This ceramic sintered body is excellent in the chipping resistance, machinability and surface smoothness, and the ceramic sintered body is suitable as a material of a substrate for a thin film head.