The purpose of this paper is to clarify the wear mechanism which occurs at high cutting speed. The alloys tool of coated cemented carbides with Al₂O₃+TiC by CVD was studied and discussed from the point of view of the relationship between flank wear and the condition of the occurrence of deposited metals on the tool surface, obtained by cutting tests and dynamic diffusion tests. In cutting tests by turning, the cutting speed was varied from 150 m/min to 400 m/min. The contact planes of both rectangular samples in a dynamic diffusion tests, inclined in 10 degrees to the horizontal plane, were kept at high temperatures ranging from 973 K to 1473 K for 30 minutes in an argon-gas environment. The main results obtained are as follows :
(1) The best machining conditions of this tool were in cutting speed from 200 m/min to 250 m/min, and feed from 0.25 mm/rev to 0.35 mm/rev. (2) At high cutting speed, much deposited metal was observed on the tool edge, and this metal decreased the machined surface accuracy. (3) In the dynamic diffusion tests, the deposition between the coated layer and the work piece occurred at temperature of 1073 K or more. The coated layer was chipped from the matrix of cemented carbides at high temperature of 1373 K or more.