Comparison of two methods used for high temperature tribological testing of protective thin coatings

This study aims to contribute to the better understanding of results obtained by different methods used for high temperature tribological evaluation of hard coatings. For these purposes tribological testing directly at high temperatures and testing of annealed coatings at room temperature were compared. In this study, high temperature tribological behavior of 3 μm thick TiAlN coating (3050 HV0.05) was evaluated using high temperature pin-on-disk tribometer. Coating was prepared on EN X38CrMoV5 steel samples using cathodic arc deposition. Coated samples were tested against Al2O3 ball, in air atmosphere, at room temperature, 300 °C, 500 °C, 600 °C, and 700 °C, and after being previously annealed (PA) at these temperatures. Stylus profilometry, confocal microscopy, focused ion beam, and energy dispersive spectroscopy were employed for evaluation of the wear tracks. At room temperature (RT) the steady-state COF was 0.72. Tribo-tests at 300 °C and 500 °C resulted with steady COF which slightly increased to a maximum value of 0.7 and 1, respectively. COF for tests at 600 °C and 700 °C, after reaching a max value of 0.95 and 0.85, declined for both cases. Each of the tribo-tests on PA samples displayed COF values similar to RT tests, but with pronounced oscillations. Tribo-tests at RT and on PA samples produced similar wear tracks that displayed combination of both adhesive and abrasive wear mechanism, while abrasive and oxidative wear mechanism was observed at high temperatures. At 600 °C coating degradation due to oxidation of substrate initiated, and at 700 °C coating was completely damaged. It is suggested that declining COF at 600 °C and 700 °C is due to formation of Fe-O and Cr-O inside of wear tracks. Additionally, oscillations of COF on PA samples are suggested to be the consequence of adhesive wear of coatings. Finally, detailed analysis revealed that, apart from their COF, tribo-tests on PA samples are quite similar to RT tests, but significantly differ from high temperature tests.