Influence of Yttrium on the Thermal Stability of Ti-Al-N Thin Films

Abstract: Ti_1-xAl_xN coated tools are commonly used in high-speed machining, where the cutting edge of an end-mill or insert is exposed to temperatures up to 1100 °C. Here, we investigate the effect of Yttrium addition on the thermal stability of Ti_1-xAl_xN coatings. Reactive DC magnetron sputtering of powder metallurgically prepared Ti_0.50Al_0.50, Ti_0.49Al_0.49Y_0.02, and Ti_0.46Al_0.46Y_0.08 targets result in the formation of single-phase cubic (c) Ti_0.45Al_0.55N, binary cubic/wurtzite c/w-Ti_0.41Al_0.57Y_0.02N and singe-phase w-Ti_0.38Al_0.54Y_0.08N coatings. Using pulsed DC reactive magnetron sputtering for the Ti_0.49Al_0.49Y_0.02 target allows preparing single-phase c-Ti_0.46Al_0.52Y_0.02N coatings. By employing thermal analyses in combination with X-ray diffraction and transmission electron microscopy investigations of as deposited and annealed (in He atmosphere) samples, we revealed that Y effectively retards the decomposition of the Ti_1-x-yAl_xY_yN solid-solution to higher temperatures and promotes the precipitation of c-TiN, c-YN, and w-AlN. Due to their different microstructure and morphology already in the as deposited state, the hardness of the coatings decreases from ~35 to 22 GPa with increasing Y-content and increasing wurtzite phase fraction. Highest peak hardness of ~38 GPa is obtained for the Y-free c-Ti_0.45Al_0.55N coating after annealing at T_a = 950 °C, due to spinodal decomposition. After annealing above 1000 °C the highest hardness is obtained for the 2 mol % YN containing c-Ti_0.46Al_0.52Y_0.02N coating with ~29 and 28 GPa for T_a = 1150 and 1200 °C, respectively.