Effect of Carbonaceous Hard Coatings Overcoat on Friction and Wear Properties for Al Alloy Sliding Bearing in Oil Lubrication

Diamond-like Carbon (DLC) film and Carbon Nitride (CNx) film are expected to be applied to automotive engine bearings because they show low friction and high wear resistance. Whereas many fundamental tribological re-searches of DLC and CNx have been conducted, practical researches of these coatings on engine bearings are still not enough. Therefore, clarifying friction and wear properties of the coatings on real engine bearings are strongly required. Especially, since lubrication conditions of engine bearings vary in automotive drive, clarifying the effect of lubrication conditions on tribological properties is important. For the reasons, we focus on the clarification of the effect of lubrication condition on friction and wear properties of the bearings coated with several DLC coatings. In this research, tribological properties of three different types of bearings, aluminium-alloy bearings without any coatings, a-C:H coated bearings and CNx coated bearings, are compared. The CNx coating was deposited by Ion Beam Assisted Deposition (IBAD) method on a-C:H coated bearings. As results, it is clarified that CNx coated bearings show the lowest friction coefficient, μ = 0.10, between three bearings under low sliding speed conditions. Under high sliding speed conditions, CNx after running-in also shows the lowest friction co-efficient. In the view point of wear, CNx coating reduces specific wear rate to 4.5 × 10−9 mm3/Nm from the value of a-C:H coating, 7.9 × 10−8 mm3/Nm. From these results, it is clarified that CNx coating on a-C:H coated bearing achieves low friction and high wear resistance coating on engine bearings.