Micro-scale abrasion and scratch response of PVD coatings at elevated temperatures

Physical vapour deposited (PVD) coatings are often exposed to high temperatures in service, for example, in cutting tool applications, yet laboratory testing of such coatings is usually performed at room temperature. In this work two tribological test methods were used to rank the performance at room temperature and at 350°C in air of four PVD coatings (two TiN coatings of different thickness, TiCN and AlTiN) deposited by steered-arc evaporation on to tool steel substrates. The tests were the widely-used scratch test, and the micro-scale abrasion test, in which a steel ball rotates against the specimen surface in the presence of fine abrasive particles. The methods used to modify the tests to operate at the high temperature are described; in the case of the micro-scale abrasion test, dry silicon carbide abrasive particles were used and the results are compared with those obtained with dry particles at room temperature and with the more usual water-based slurry. Wear volumes in the abrasion test as low as 10−14 m3 were measured by optical profilometry. Care was taken to separate the effects of testing at high temperature from any permanent changes in material or interface properties induced by exposure to high temperature. The ranking of the coatings by the abrasion test was very similar under all conditions, although some differences in the relative magnitude of wear resistance were noted. The ranking of the coatings by scratch testing changed significantly with temperature, indicating that scratch testing at room temperature of hard coatings intended for use at high temperature may be misleading.