Wear resistant Ta-Al-N coatings deposited via reactive HiPIMS

Ta-N (Ta at.% about 40, N at.% about 60) and Ta-Al-N (Ta at.% about 25, Al at.% about 15, N at.% about 60) coatings were deposited on Si coupons via reactive HiPIMS from a pure Ta target and a Ta/Al (50/50 at.%) source respectively. Working parameters (p = 1 Pa, N2/(Ar+N2) = 0.5, P = 500 W, T = 200 °C) were kept constant with the exception of the applied substrate bias voltage, which was varied from -50 to -100 V to improve mechanical and tribological properties. In particular, comparing the two sets of coatings, hardness and elastic modulus were considerably higher for Ta-Al-N films, while their wear resistance (w) was about 20 times lower. In the light of these results, the authors tried reducing N2/(Ar+N2) ratio (form 0.5 to 0.05) during sputtering to optimize w. The best result was achieved at N2/(Ar+N2) = 0.05 (i.e. 5% N2). For this specimen, the variation in ionization fraction and transport behavior of different species led to a clear deviation of deposited film composition [1]. Indeed, going down to the 5% of N2, the coating composition changed (very low Al and N at.%), microstructure varied, mechanical properties slightly improved (H from about 27 (about 15 at.% Al content) to 30 GPa (about 1 at.% Al content)), wear resistance raised significantly (more than 60 times greater). Focusing on the wear tracks (ball-on-flat reciprocating mode), the authors observed roll-like debris for all specimens. Combining SEM-EDS and Raman analyses, they found: i high w Ta-Al-N sample (about 15 at.% Al): as soon as the nitride started oxidizing, it came off the film surface (debris were oxidized but no oxygen was detected into the wear track); ii low w coatings (Ta-N and Ta-Al-N containing about 1 at. % Al): the oxide layer peeled off only when the oxygen level was sufficiently high(greatly oxidized debris plus oxygen detected into the wear track). Therefore, the formed TaxOy seemed to act as a lubricant. [1] B. C. Zheng, Z. L. Wu, B. Wu, Y. G. Li, M. K. Lei, A global plasma model for reactive deposition of compound films by modulated pulsed power magnetron sputtering discharges, Journal of Applied Physics 121 (2017) 171901