Wear behaviour of r.f.-sputtered TiN, TiB 2 and Ti(B,N) coatings on metal forming tools in a model wear test

Abstract

Within the last few years grey cast iron tools have been widely used for sheet metal forming owing to their low friction coefficient and excellent emergency running properties. Low hardness values and therefore reduced abrasive wear resistance are the main restrictions for this material. As shown in previous work, TiN coatings reduce the abrasive wear of cast iron test pieces under several tribological conditions. In this paper we deal with thin hard coatings of the system Ti-B-N deposited onto cast iron by means of an r.f. sputter process with r.f.-biased substrates. The coatings were tested with regard to their fundamental properties such as hardness, adhesion and structure. The most promising TiN, TiB 2 and Ti(B,N) coatings were tested under metal forming conditions. It is shown how the graphite lamellae of grey cast iron substrates disturb the coating structure. The main wear mechanism results from cracking of the coatings in the areas of graphite lamellae. The cracking leads to highly abrasive wear debris containing hard coating and substrate particles. Therefore a 1 μm thick pure titanium layer is deposited prior to the hard coating to reduce the influence of the graphite on the growth of the hard coating. As a main result the wear behaviour of the coated tools is remarkably improved without a significant change in coating hardness or adhesion.