Phase composition and thermal stability of arc evaporated Ti1 − xAlxN hard coatings with 0.4 ≤ x ≤ 0.67

Abstract

Within this work, the properties of arc evaporated Ti1−xAlxN hard coatings were systematically investigated by varying the Al content between 0.4≤x≤0.67 and the bias voltage between −40 and −100V. All coatings deposited from targets with an Al fraction of x=0.60 or higher showed dual-phase structures consisting of cubic and wurtzite phases, where formation of the cubic phase was promoted by a higher bias voltage. Due to an increasing amount of the wurtzite phase with increasing Al content, hardness and wear resistance at room temperature significantly decreased. At elevated temperatures, high Al contents proved to be beneficial, since the formation of an Al-rich oxide scale leads to a reduction of friction and wear rate. Annealing treatments and subsequent X-ray diffraction revealed a lower thermal stability of the coatings deposited with higher bias voltage. The coatings with a moderate Al content yielded the best performance in milling tests, despite the existence of a low fraction of the wurtzite phase.