Increased thermal stability of Ti–Al–N thin films by Ta alloying

Abstract

To increase the thermal stability of protective coatings has always been a major topic for application oriented materials development. Here, we study the impact of Ta on the thermal stability of magnetron sputtered Ti1−xAlxN thin films using a combined ab initio and experimental approach.With increasing Ta content in Ti1−x−yAlxTayN, a decreasing energy of formation, as well as an increasing interaction of the transition metal d-states go along with an increase in cohesive energy, which point toward a higher stability of the quaternary system. The ab initio predicted increasing bonding strength of Ti1−x−yAlxTayN with increasing Ta content was corroborated by structural and mechanical investigations of Ti1−x−yAlxTayN thin films with y=0, 0.03, 0.05, and 0.1, respectively. With increasing Ta content, a hardness increase of 25%, from ~30GPa to ~40GPa, can be observed after deposition. Annealing experiments in vacuum show that the decomposition process of the supersaturated solid solution towards their stable constituents c-Ti1-zTazN (with z=y/(1−x)) and hexagonal (wurtzite structure, w) w-AlN, is effectively retarded from ~900 to ~1200°C with increasing Ta content. The involved decomposition into c-Ti–Ta-rich and c-Al-rich domains results in hardness maxima of ~38 to ~42GPa for y=0 and 0.1, respectively. Oxidation experiments for 20h at 850 and 950°C yielded fully oxidized Ta-free coatings, whereas the addition of only 3at.% Ta to the metal sublattice results in the form of a layered oxide scale and remaining unoxidized nitride layer thicknesses of ~95 and ~87% (with respect to the as deposited films), respectively.