Method for manufacturing a cutting tool insert made of a silicon nitride body

A method for producing a cutting tool of a silicon nitride sintered body having a cutting edge, comprising the following steps:

• preparing a silicon nitride sintered body containing silicon nitride and at least one sintering assistant selected from the group consisting of MgO, Y.sub.2 O.sub.3, Al.sub.2 O.sub.3, ZrO.sub.2, Aln, CaO, CeO.sub.2, and SiO.sub.2, whereby said sintered body includes a member selected from the group consisting of columnar .beta.-Si.sub.3 N.sub.4 and .beta.'-SIALON, removing said .beta.-Si.sub.3 N.sub.4 and .beta.'-SIALON at least from a surface of said cutting edge of said silicon nitride sintered body by a treatment with abrasive grains, said treatment being selected from the group consisting of blasting, barreling, and ultrasonic vibration, and applying a surface coating to said cutting edge surface from which said columnar .beta.-Si.sub.3 N.sub.4 and .beta.'-SIALON has been removed, said surface coating having at least one layer of at least one component selected from the group consisting of carbide, nitride, carbonitride, nitride carbonate of Ti and Al.sub.2 O.sub.3, said at least one layer having a thickness of at least 0.1 .mu.m and not more than 10 .mu.m, except for said layer of Al.sub.2 O.sub.3 which has a thickness of at least 0.4 .mu.m and not more than 10 .mu.m.