Running−in and the Evolution of Metallic Surfaces Subjected to Sliding and Rolling Contact
Creators: name: Karpinska, Agnieszka
Publisher: name: Imperial College London
Year: 2010

description: The thesis concerns an integrated approach to investigating running−in of metallic surfaces under rolling/sliding conditions in the presence of the ZDDP anti−wear additive. Running−in occurs in the initial period of the contact process and involves rapid mechanical and chemical changes of rubbing surfaces, bulk material and lubricant, leading to reaching a steady state, in which an increased load−carrying capacity of the surfaces is achieved. Previous research suggested that anti−wear additives, such as ZDDPs, can prevent or postpone the effective running−in, preserving the relative surface roughness and maintaining the severity of asperity interactions. The friction, surface topography and the ZDDP tribofilm growth were measured in a number of rolling/sliding experiments carried out with an MTM tribotester. The specimens were varied according to their surface finish and hardness. To assess the influence of running−in on the change of lubrication regime, Stribeck curves were obtained at certain intervals during the test. Additionally, chosen worn surfaces and wear debris were analysed by a range of electron microscopy techniques. A novel method of wear calculation of rough surfaces was employed, based on a comparison of the 3D topography scans of the initial and worn surfaces. ZDDP films were chemically removed from the specimen surfaces using ethylenediaminetetraacetic acid (EDTA) sodium salt solution to ensure the correct results of the optical topography measurements. Studying running−in required the tests to be periodically interrupted, e.g. for topography measurements, and such interruptions were found to affect the wear, roughness and friction results; however, the main trends remained unchanged. A combination of AISI 1013 carbon steel and AISI 52100 bearing steel specimens was tested first, and the former were found unsuitable for use in this study, possibly due to the influence of the silicon carbide particles embedded in their surface on the wear mechanisms involved. As a result, the specimens made of AISI 52100 steel, but of different hardness, were used in the further testing. A transition from boundary to mixed lubrication regime as a result of running−in, evident in the friction and electrical contact resistance results, was found only in the tests carried out without the additive. In the presence of ZDDP, the friction was higher than in the corresponding base oil tests, and the highest increase was observed in the mixed regime. The friction changes with time did not correlate with the increase of the tribofilm thickness, which suggested that the friction depended on the surface coverage with the tribofilm rather than its thickness. However, the possible influence of the film roughness on the surface topography was not investigated. While the rate of the film formation could be related to the value of the lambda ratio, the tribofilm removal was found to depend on the direction of the roughness lay. The asperity−level surface conformity was assessed using a cross−correlation analysis technique and it was found to be influenced by the orientation of the surface finish. Additionally, the EHL analysis of the profiles of a rough surface taken at different times during the test was used to observed the evolution of the contact pressure distribution with running−in. The optical phenomena involved in the spurious wear effect, previously found in the results of WLI topography measurements of surfaces with ZDDP films, were studied with a WLI scanning microscope and a confocal system. However, this effect was not observed in the results from the tempered AISI 52100 steel specimens, possibly because of the presence of the temper oxide on the surface. The reflectivity of the top layer of the film was low compared to the surface under the film. Under certain measurement conditions, the observed effect was assumed to be caused predominantly by a difference of the refractive index between the tribofilm and air. Thus, on the basis of the height difference between the surface with a film and after the film removal, and corrected for the difference in refractive index, the estimated thickness of the tribofilm was found for every measurement point. The resultant film thickness was within the range expected of a typical ZDDP tribofilm, and for a rough surface it was higher on the tops of asperities. A closer examination of the film properties and optical system design could be a foundation of a new film thickness measurement method, which would require accurate information to be obtained from both top and bottom surfaces of the film.