Tribological properties and wear behaviour of sputtered titanium-based hard coatings under sheet-metal-forming conditions

Abstract

In recent years, grey cast iron tools have been widely used for sheet metal forming because of their low friction coefficient and excellent emergency running properties. Low hardness and therefore reduced wear resistance are the main restrictions for this material. Tool steels are harder, but against steel workpiece material a tendency to adhesive wear might cause problems under insufficient lubrication. As shown in previous work, TiN coatings reduce the abrasive and adhesive wear of cast iron and steel samples under several tribological conditions. The present paper deals with thin hard coatings within the system TiBN, deposited on grey cast iron and tool steel by means of a d.c. or r.f. sputter process with r.f.-biased substrates. The coatings were analysed with regard to their fundamental properties such as hardness, adhesion and structure. The most promising titanium-based hard coatings were tested in a tribological model wear test and under metal-forming conditions. For the grey cast iron tools it is shown how the graphite lamellae disturb the coating structure. In this case the main wear mechanism is cracking of the coatings in the areas of graphite lamellae. The cracking leads to highly abrasive wear debris containing hard coating and substrate particles. The influence of the graphite on the coating growth can be suppressed by means of a thin pure titanium layer deposited prior to the hard coating. As the main result the wear behaviour of the coated grey cast iron tools is remarkably improved without a significant change in coating hardness or adhesion. On steel tools no influence of the base material on the coating was observed. For the coating a low tendency of adhesion to the workpiece material and high hardness values were obtained as well, which leads to a superior wear resistance of coated steel tools under sheet-metal-forming conditions.