MIXED LUBRICATION, WEAR AND CONTACT FATIGUE OF ROUGH SURFACES

Full-film hydrodynamic lubrication is an ideal which is rarely achieved in practice. In most practical rolling/sliding contacts such as those in gears, bearings and replacement human joints the roughness of the surfaces plays an important part in their lubrication, wear and surface fatigue. In gears, for example, the combined roughness of even the best quality ground surfaces is usually of the order of the oil film which can be generated hydrodynamically. A similar situation occurs in roller bearings at low speed or high temperature and in replacement human joints because of the low effective viscosity of the lubricant. This leads to a lubrication regime which has been loosely described as mixed (meaning a mixture of hydrodynamic and boundary lubrication), or more specifically as micro-elastohydrodynamic lubrication (micro-EHL) in which high ripple pressures occur and colliding asperities on the two surfaces act as individual, transient elastohydrodynamic/solid contact encounters. This project addresses two important engineering consequences of mixed lubrication: (1) mild wear/wear particle generation, and (2) near-surface rolling contact fatigue/micropitting. Much progress has been made in numerical modelling of micro-EHL, but such analyses, if they are to be of practical use, must now be developed to include the detailed behaviour of solid contact events in terms of a wear model. The project will therefore produce an analysis of contact duration, traction, adhesion, flash temperatures and particle removal. The second major development required, in order to advance the study of near-surface distress due to fatigue, is the effect of initial plastic deformation of asperities during the crucial running-in phase when machine surfaces are brought into heavy contact for the first time. This will include a detailed treatment of plastic deformation, permanent set and residual stress field of asperity features.Our specific objectives are to:1. Develop a consistent mixed hydrodynamic/solid contact treatment for real surfaces which includes the detailed modelling of transient dry asperity contacts.2. Develop a model of mild wear particle generation based on the detailed friction, adhesion, deformation and temperatures occurring at transient contacts.3. Produce a model of near-surface fatigue, relevant to the phenomenon of micropitting and rolling contact fatigue, which includes the permanent set and residual stress field obtained from elastic/plastic contact analysis.4. Carry out controlled experiments under realistic engineering load and speed conditions to validate each of these modelling developments.