Friction coefficient and sliding wear of AlTiN coating under various lubrication conditions

AlTiN coatings are known for having a high friction coefficient and a high wear rate during tribo-testing under ambient air. In this study, the friction and wear behavior of AlTiN (Al:Ti=3:2) coating is examined using a ball-on-disk tribo-test with a WC/6%Co ball in an oscillatory relative motion between the ball and the coating. The tests are conducted under various lubrication conditions: ambient air, dehumidified air, tap water, deionized water, oil and acetone. Scanning electron microscope (SEM) pictures were used to capture the wear track. The results indicated that the high friction coefficient of AlTiN coating was caused by the formation of a tribo-film on the wear track, which only occurred under the environments containing water or water vapor. For the other conditions without the presence of water, the tribo-film was not observed on the wear track and low friction coefficients were obtained. For the cases in which a tribo-film formed, the removal of the tribo-film caused friction coefficient to revert back to the low value that existed before the tribo-film formation. Energy-dispersive X-ray spectroscopy (EDX) has been used to analyze the composition of the tribo-film and wear debris. The results showed that the tribo-film was mainly composed of Al, Ti, W, C, Co and O. However, the O concentration in the tribo-film was much higher than the concentration expected solely from the metal oxides. It is believed that the high O concentration in the tribo-film was caused by the interaction of wear debris with the water or water vapor from the environment. This is believed to be an essential requirement for the formation of the tribo-film. The wear debris accumulated at the end of the wear track mainly had an ultra-fine grain size. The typical wear of the AlTiN coating was believed to be caused by spalling, ploughing and oxidation. However, AlTiN coating wore much faster under a deionized water lubrication condition, which is related to the corrosion wear.