Manifestations of reduced tool-chip friction during turning of AISI 1045 steel with PVD-coated carbide inserts

Short duration turning tests were performed with uncoated and various PVD-coated straight grade carbide inserts against AISI 1045 steel, without coolant. Cutting forces, built-up edge (BUE) intensity, and characteristics of the chips (length, curl diameter, and colour) were made and related to measurements of friction coefficient obtained from a custom heavy-load, high-temperature tribometer as well as from reported thermal conductivity values obtained from the literature. At lower cutting speeds, where the size of BUE tends to be larger, it is proposed that reductions in cutting forces and BUE intensity observed for certain PVD-coated inserts pertain more to reduced sticking intensity between tool and chip, for which the friction coefficient results from the tribometer are a helpful guide. At higher cutting speeds, hotter temperatures and greater thermal softening of work material at the tool-chip interface seem to be responsible for reductions in cutting force and BUE size, which is linked to the thermal conductivity of the coatings. As well, delamination of certain coatings was observed both during cutting tests as well as tribometer tests, suggesting the potential merit of the tribometer as a method for screening coatings for adequate substrate-adhesion. Investigations of tool wear during longer duration turning tests suggest that tribometer COF results can be predictive of relative tool wear rates among tool candidates, under cutting conditions that give rise to attrition as the dominant wear mechanism.