Superhard oxidation-resistant Ti1-xAlxBy thin films grown by hybrid HiPIMS/DCMS co-sputtering diboride targets without external substrate heating

Abstract

Ti1-xAlxBy films (0.40 ≤ x ≤ 0.76, and 1.81 ≤ y ≤ 2.03) combining good mechanical properties and high-temperature oxidation resistance are demonstrated. Layers are grown using a hybrid high-power impulse and dc magnetron co-sputtering employing two target configurations (AlB2-HiPIMS/TiB2-DCMS and TiB2-HiPIMS/AlB2-DCMS) and no external substrate heating. Near-stoichiometric B content are achieved by co-sputtering two diboride targets. Time-resolved ion mass spectrometry analyses reveal that the ionization of the DCMS flux (Al) is much higher during TiB2-HiPIMS/AlB2-DCMS. The effect is caused by the difference in the first ionization potentials and the ionization probabilities of sputtered metals and results in lower B/metal ratios in films grown in this configuration. The B/metal ratio in the single-phase Ti1-xAlxBy decreases with increasing Al content, which is explained by the change between angular distribution of Ti and Al atoms. Alloying with Al improves the high-temperature oxidation resistance: the thickness of the oxide-scale forming after 1 h anneal at 800 °C decreases more than 15 times upon increasing x from 0.36 to 0.74. Ti1-xAlxBy films with 0.58 ≤ x ≤ 0.67 offer the best compromise between high-temperature oxidation resistance and mechanical properties with an average oxide scale thickness 90–180 nm and the hardness of 34–38 GPa.