On the wear mechanisms of uncoated and coated pcBN tools during turning of 17–4 PH martensitic stainless steel

Abstract

Polycrystalline cubic boron nitride (pcBN) is a very promising tool material for turning martensitic stainless steels at high cutting speeds (v_c > 200 m/min). The competitive advantage of pcBN over cemented carbide increases as the cutting speed is increased. Changing the speed might lead to a shift in the wear balance and hence the knowledge about tool wear below v_c = 200 m/min might not be applicable at v_c = 600 m/min. The coatings designed for the lower speed range might also not be performing in the same way at higher speeds. This paper investigates the wear mechanism of uncoated and (Ti,Al)N coated pcBN tools when turning 17–4 PH in a hardened condition at speeds v_c = 200–600 m/min. Both scanning and transmission electron microscopy are used to study the worn tools. The in-depth analysis reveals that adhesive wear is only active at low speeds. Increasing the speed does however lead to more wear by diffusion and oxidation. The cBN is preferentially worn out, leaving the TiC binder at the tool-chip interface. Oxidation results in the accelerated wear of the pcBN but also in the formation of metal oxides within the adhered build up layer. The (Ti,Al)N coating does not significantly extend the tool life within this speed range, but it suppresses the adhesive wear mechanism preventing premature tool failure.