| abstract |
Cutting tools and cutting tool inserts having improved mechanical and chemical wear resistance under demanding conditions of machining speed, temperature, or workpiece hardness comprise a composite silicon nitride substrate having at least one hard adherent coating layer of a refractory material. The composite silicon nitride substrate body consists essentially of particles of a refractory material uniformly distributed in a matrix consisting essentially of a first phase of silicon nitride and a refractory second phase comprising silicon nitride and an effective amount of a densification aid selected from the group consisting of yttrium oxide, hafnium oxide, the lanthanide rare earth oxides, and mixtures thereof. |