High temperature tribological performance of CrAlYN/CrN nanoscale multilayer coatings deposited on γ-TiAl

Nanoscale multilayer nitride coatings deposited by advanced PVD techniques have shown particular promise in improving the tribological properties of a number of modal alloy steels [P.E. Hovsepian, D.B. Lewis, Q. Luo, W.D. Munz, P.H. Mayrhofer, C. Mitterer, Z. Zhou, W.M. Rainforth, TiAlN based nanoscale multilayer coatings designed to adapt their tribological properties at elevated temperatures, Thin Solid Films 485 (2005) 160-168]. In this study, we report the effect of temperature on the friction and wear behaviour of a CrAlYN/CrN multilayer coating with a CrAlYON/CrON topcoat deposited on γ-TiAl. Deposition was performed by unbalanced magnetron sputtering following a high power impulse magnetron sputtering (HIPIMS) pre-treatment of the polished substrate. A series of pin-on-disc type experiments, sliding against a polycrystalline alumina counterpart, were carried out at four temperatures: 20, 120, 300 and 650 °C. An increase in the average steady-state dynamic friction coefficient was observed between the material couple, from 0.56 at room temperature to 0.65 at 120 °C. However at higher test temperatures of 300 and 650 °C a decrease in these values was observed to 0.59 and 0.40, respectively. Scanning electron microscopy and energy dispersive X-ray analysis showed evidence of oxidation at the worn surface of all test temperatures investigated, whilst laser confocal microscopy indicated the formation of an interactive tribo-layer above the plane of the original test surface. Focused ion beam sectioning has been used to prepare site specific samples of the tribo-layers for transmission electron microscopy. The evolution of the wear scar composition and structure and its influence on the reduction in dynamic friction coefficient at elevated temperatures is discussed.