Sputter-deposited Al–Au coatings

Transition metal nitride-based, wear-resistant hard coatings on cutting tools and other substrates often lack self-lubricating properties at elevated temperatures and distinct colorations allowing product differentiation. In the present work, the possibility of achieving these objectives using coatings based on the purple-red Al2Au phase within the Al–Au system was investigated. Coatings within this system have been deposited onto cemented carbide and Inconel substrates using unbalanced magnetron sputtering. The coatings were characterized with respect to their topography and morphology, chemical and phase composition, hardness, optical, oxidation and tribological properties. Al2Au-containing coatings have been deposited with dense, fine-grained structures yielding a hardness of 4 GPa and pink coloration. Vacuum annealing at temperatures of 500 °C results in a chance to a pronounced purple coloration. The coatings are stable up to about 850 °C, where the onset of oxidation occurs. Low friction coefficients when testing against alumina counterparts were achieved in the temperature range between 500 and 700 °C. The concept of applying Al2Au-containing coatings as a colored self-lubricating layer on top of a hard coated cemented carbide tool warrants further investigations.