Effect of Mo on the thermal stability, oxidation resistance, and tribo-mechanical properties of arc evaporated Ti-Al-N coatings

The general tendency in industrial machining and forming operations toward further reduced operation time and increased efficiency requires continuous improvements of protective coatings regarding their tribo-mechanical properties. Alloying arc evaporated Ti1-xAlxN with small amounts of molybdenum (Mo) has shown very promising results to further improve the tribological properties of the industrially widely used Ti-Al-N. Therefore, the authors study in detail the effect of Mo on the thermo-mechanical properties of Ti1-xAlxN coatings, as well as their wear behavior at elevated temperatures. First principle density functional theory (DFT) calculations are conducted for face-centered cubic c-Al1-yMoyN, c-Ti1-yMoyN, and c-Ti1-x-yAlxMoyN showing increasing energy of formation with increasing Mo incorporation for all structures investigated. However, the thereby (DFT) obtained/calculated lattice parameters for comparable compositions perfectly matches with those of the single-phase face-centered cubic structured c-Ti0.55Al0.42Mo0.03N, c-Ti0.53Al0.41Mo0.06N, and c-Ti0.50Al0.38Mo0.12N coatings studied. These coatings have an as deposited hardness of about 28 GPa, which can be maintained upon vacuum annealing up to T-a = 800 degrees C. At higher temperatures, their hardness decreases as the formation of the weaker w-AlN and Mo3Al phases occurs. In contrast to that, the abrasive wear of Mo-containing c-Ti1-x-yAlxMoyN during dry-sliding high temperature ball-on-disk tests (against 6mm alumina balls at 700 degrees C) is significantly reduced to 1-2.0 x 10(-6) mm(3)/N m, as compared to the Mo-free Ti0.56Al0.44N counterpart. This is the result of the presence of solid lubricant MoO3 phases in the wear tracks of the arc evaporated c-Ti1-x-yAl(x)Mo(y)N coatings investigated-confirmed by a combined energy dispersive spectroscopy and x-ray photoelectron spectroscopy analysis. Based on their findings, the authors can conclude that the addition of up to 12 at.% Mo (of the metal fraction) to Ti0.56Al0.44N significantly improves their high temperature wear resistance by simultaneously keeping the outstanding coating characteristics of pure Ti1-xAlxN. Published by the AVS.