Generation of wear particles in lubricated contacts during running-in

If tribological contacts are not operating under full film lubrication, wear particles may be released which could significantly influence the lubrication performance and therefore the lifetime of machine elements. In this thesis, a model for the generation of wear particles in mixed lubrication will be described with emphasis on the running-in phase. The operating conditions and material were chosen to simulate the contact between a steel rolling element and cage (steel or brass) as can be found in rolling bearings. This thesis includes an experimental wear particle analysis and an efficient numerical algorithm (Boundary Element Methods, BEM) for the simulation of the generation of wear particles. First an efficient algorithm was developed for the calculation of friction in mixed lubrication based on a load sharing concept. The model was used to calculate the surface stresses that are needed for the wear model. It combined a finite difference based numerical elasto-hydrodynamic solver for the calculation of the hydrodynamic film and a BEM based solver for contact stress analysis. The friction model was successfully validated using measurements under various conditions. A (subsurface) stress-based BEM wear model was developed to calculate the size of wear particles. A critical Von Mises stress criterion was employed for the disruption of particles. The friction and wear models were combined to predict the generation of wear particles under mixed lubricated conditions. The model calculations show how friction reduces during running-in due to a smoothening of the surfaces. Steel and brass particles were collected and analyzed using Dynamic Light Scattering, Scanning Electron Microscopy and Atomic Force Microscopy and used for model validation. The model fitted the experimental data reasonably well for both, the steel and the brass materials. Finally, the model was extended to the growth and wear of tribo-films to also make it applicable if anti-wear additives are present. This effect was validated using experimental data from the literature. The thesis is divided into two parts. The first part (Part A) is a summary of the work. The second part (Part B) consists of the journal articles in which the details are described.