Investigation of wear initiation in CVD coated cermet tools during stainless steel turning

CVD (chemical vapor deposition) coated cutting tools typically exhibit cooling cracks caused by the mismatch in coefficients of thermal expansion (CTEs) between the coating and cemented carbide. These cracks act as weak points during stainless steel turning, contributing to premature tool wear. As a strategy to suppress cooling cracks formation, we investigate replacing the conventional cemented carbide with a cermet substrate, which has a CTE closely matching that of the coating. Therefore, in this work we analyze and compare the influence of a cermet substrate on wear initiation in CVD coated tools during AISI 316Ti stainless steel turning. Two coatings were evaluated: (i) nanolayered κ-Al2O3–TiN/Ti(C,N) and (ii) (0001)-textured α-Al2O3/Ti(C,N). Both coatings were free of cooling cracks when deposited on the cermet and exhibited a compressive residual stress state. In the κ-Al2O3–TiN coating, fracture occurred between the nanolayers at 0.3 m of cutting, similar to that observed when the same coating was deposited on cemented carbide. In the α-Al2O3/Ti(C,N), the use of cermet promoted more gradual coating degradation, with cermet exposure first observed after 16 m of cutting, compared with only 1 m for the corresponding cemented carbide tool. These results indicate that suppression of cooling cracks delays wear in α-Al2O3/Ti(C,N) coatings. However, coating removal still occurred after a relatively short cutting distance (16 m) and once the coating was fully removed, the low fracture toughness of the cermet became a critical factor influencing tool wear.