Experimental Investigation of Transition in Lubrication Regime for Thin-Film Coated Surfaces

Friction and wear behavior of lubricated sliding contact is determined by the operating lubrication regime. A useful approach to determining the operating lubrication regime is the calculation of the λ ratio, which is defined as the ratio of lubricant fluid film thickness (h), and the composite surface roughness (ơ) of contacting surfaces (λ = h/ơ). Thin-film tribological coatings are increasingly being used for application in lubricated machine elements such as gears and bearings. It is usually assumed by design engineers that application of thin-film coatings has no effect on fluid-film lubrication. This paper presents our experimental investigation of the impact of several (5) commercially available coatings on lubrication regime during a unidirectional sliding contact. Using a ball-on-flat contact configuration and lubricated with PAO basestock oil, tests were conducted in which the λ ratio was varied as a function of time, for both uncoated and coated flat specimen. In test with uncoated flat, the various distinctive lubrication regime of hydrodynamic, mixed and boundary were observed as indicated by the measured friction coefficient (Stribeck Curve). In tests with some of the coatings, especially the carbon based DLCs, there was no obvious distinctive transition in lubrication regime. In other coatings (ex. TiCN), various lubrication regime were also observed; although the rate of transition from one regime to the other was different. The effects of coatings are attributed to their inherent mechanical and tribological properties as well as their impact on the run-in process.