Coated composite silicon nitride cutting tools

摘要

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 intermediate hard adherent coating layer of a refractory material and an outer adherent coating layer. The intermediate adherent coating layer has a different chemical composition than the outer adherent coating layer. 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.