High temperature tribological performance of CrAlYN/CrN nanoscale multilayer coatings deposited on gamma-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 gamma-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 degrees 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 degrees C. However at higher test temperatures of 300 and 650 degrees 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. (C) 2009 Published by Elsevier B.V.