Abrasive wear of multilayered/gradient CrAlSiN PVD coatings: Effect of interface roughness and of superficial flaws

PVD coatings have been used to improve the wear resistance of various tribological systems. In general, analytical models that describe the abrasive wear behaviour of ceramic materials do not take into account the presence of superficial flaws in ceramic coatings. Flaws are pores or protuberances present in the coating surface formed during the coating process. In this work, the percentage of superficial flaws was measured using optical microscopy and was correlated to the abrasive wear resistance of the investigated coatings. Two different coating designs were analyzed, multilayer and gradient. In both coating architectures, the chemical composition at the coating-substrate interface was kept constant at 75% of CrN and 25% AlSiN, and the CrN/AlSi ratio was varied throughout the coating thickness. The mechanical properties of the coatings were evaluated using Knoop microhardness tests and progressive load scratch tests. The presence of flaws in the coatings was connected to the substrate roughness values. Free-ball microabrasive wear tests were carried out using SiC abrasive slurry with a concentration of 20 wt%. Wear rates of the coatings increased linearly with e.Lc−3/4.H−1/2, where e was the superficial flaws fraction, Lc the critical load measured in the scratch tests and H the microhardness. The coating wear rates were intensified up to 72% by increasing the interface roughness. The wear rate of 2.6 µm thickness coatings was about 120% higher than the 3.1 µm thickness coatings. This work showed that the percentage of superficial flaws was a key parameter that affected the wear rate and critical load of the coating. This result was related to the fact that superficial flaws acted as stress concentration sites that raised the local stress leading to premature failure of the coating during the wear process. The combination of these techniques afforded a powerful tool with which to develop an efficient and reliable methodology for characterizing tribological coatings.