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 Ti_1-xAl_xN 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 Ti_1-xAl_xN coatings, as well as their wear behavior at elevated temperatures. First principle density functional theory (DFT) calculations are conducted for face-centered cubic c-Al_1-yMo_yN, c-Ti_1-yMo_yN, and c-Ti_1-x-yAl_xMo_yN 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-Ti_0.55Al_0.42Mo_0.03N, c-Ti_0.53Al_0.41Mo_0.06N, and c-Ti_0.50Al_0.38Mo_0.12N 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 °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-Ti_1-x-yAl_xMo_yN during dry-sliding high temperature ball-on-disk tests (against 6 mm alumina balls at 700 °C) is significantly reduced to 1–2.0 × 10⁻⁶ mm³/N m, as compared to the Mo-free Ti_0.56Al_0.44N counterpart. This is the result of the presence of solid lubricant MoO₃ phases in the wear tracks of the arc evaporated c-Ti_1-x-yAl_xMo_yN 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 Ti_0.56Al_0.44N significantly improves their high temperature wear resistance by simultaneously keeping the outstanding coating characteristics of pure Ti_1-xAl_xN.