Study On Friction And Wear Properties Of CrN And AlCrN PVD-Coatings Under Rotational Wear Condition

CrN and ternary Cr-Al-N coatings deposited by Physical Vapor Deposition (PVD) technique have been successfully used to the surfaces of tools, dies, bearings, piston rings and other wear-resistant parts for their excellent integrated properties. Particularly, the AlCrN coating with high content of Al had excellent performance in high temperature oxidation resistance and good abrasive wear resistance, and thus had become the most promising PVD ternary nitride coating for wear and corrosion protection. Rotational fretting wear is caused between the contact pairs under alternating load with relative rotate motion. It widely exists in industrial equipments, civil mechanical and medical devices, which usually cause accidents and thus incalculable loss. The coating materials can be used for fretting wear and fatigue protection, CrN and AlCrN coatings have shown great potential in rotational fretting wear protection, which had highly increased the great anti-wear resistance of high-end main bearings and ball valves. However, so far there are few reports on the rotational fretting wear properties of these two coatings.In this paper, the rotational fretting behaviors of CrN and AlCrN coatings were studied under different normal loads (Fn=10 N,20 N,40 N) and rotating angular displacement amplitudes (θ=0.25°,0.5°,1°,2°) by using a new developed rotational fretting wear tester with a contact configuration of ball-on-flat. The Si3N4ball was used as counterpart materials. The wear scars were evaluated using scanning electron microscope (SEM), surface profiler, energy dispersive x-ray spectroscopy (EDX), and the rotational fretting wear mechanisms of the two coatings were discussed in detail. The main conclusions are drawn as follows:1. Rotational fretting parameters have important influence on interface friction state of both the CrN and AlCrN coatings. Their rotational fretting region of the coatings greatly depends on normal load and rotating angular displacement amplitude. When normal load decreases and angular displacement amplitude increases, rotational fretting running state of the coatings transfers from the partial slip to the gross slip. In the same condition, AlCrN coating was more prone to run on the partial slip as compared with CrN coating.2. The variation of the friction coefficients of the CrN and AlCrN coatings are greatly depend on the slip status. The friction coefficients of the two coatings under the condition of gross slip are higher than those under the partial slip. The friction coefficients of the coatings increase with the decrease of the normal load or the increase of displacement amplitude. The number of cycles corresponding to the ascent state significantly increases with increasing normal load, but decreases with increasing angular displacement. Under the high normal load of 40 N, the steady friction coefficient values of CrN coating are obviously lower than those of AlCrN coating at different angular displacement amplitudes. As the angular displacement amplitude increases, higher value with quite different curve trends has been observed for the friction coefficients of AlCrN coating, while no visible change for the friction coefficients of CrN coating.3. When friction contact interface of the two coating is running in the regime of partial slip, the coatings exhibit slight wear with little amount of wear debris. When running in the gross slip, the wear of the coating is two-body wear combining with three-body wear. Transferring to the gross slip by increasing the angular displacement amplitude will lead to heavier wear with more debris accumulation at the edges of the wear scar. Under the high normal load of 40 N, the angular displacement amplitude has significant influence on the wear properties of the two coatings. With the angular displacement amplitude increasing from 1°to 2°, the two coatings exhibit difference in wear characteristics and wear debris removal behavior at the center of the wear scar:compaction and accumulation of wear debris is observed inside the wear scar of CrN coating, while a relative higher efficient of wear debris removal besides a plastic deformation accumulation in middle of the wear scar is found for the AlCrN coating. The increase of angular displacement amplitude in the gross slip significantly aggravate the wear of the two coatings, though the wear mechanisms of the coatings are mainly abrasive wear and mild oxidative wear.