Abstract
Aims
Wear is difficult to predict in mixed lubricated articulating surfaces and the time of computation is one of the challenges due to the deterministic definition of roughness on a micro-scale. This research aims to efficiently capture the wear and the evolution of the roughness of mixed lubricated bearing surfaces, employing a statistical description of the roughness.
Methods
A numerical model was developed which characterizes the wear of a loaded and lubricated pin-on-plate system, assuming a rough non-wearing pin and a rough wearing plate. The part of the load, which is borne by asperities in contact, is derived from the Greenwood-Williamson approach and the rest, which is carried by the fluid film, is based on the Patir-Cheng flow factors lubrication method. Wear is computed in the areas of direct solid contact only. For simplicity, the depth of the pin and plate are assumed infinite in order to reduce the lubrication problem to one-dimension. The roughness and asperities are described by their Cumulative Distribution Functions (CDFs). As the plate runs-in the pin, the roughness of the plate is worn by the roughness of the pin, and the process is continued until steady wear is attained. The local gap-dependent flow factors influence the load carried by the thin film of the lubricant, whereas, the local gap-dependent overlap of asperities of the pin and the plate determines the true contact load. The sum of fluid and solid contact load is balanced with the applied load, adjusting the separation between the plate and the pin. The plate asperity CDFs are updated assuming Archard's wear model for the solid contact only and the asperity wear is extrapolated to update the roughness of the plate.
Results and conclusion
In this study, material properties and roughness information of silicon nitride coatings are used for the pin and the plate. The evolution of load sharing due to wear reported a decrease of asperity contact load with an increase in fluid load since the total load is balanced with the applied load. A decrease in the contact area ratio is also observed, i.e. the ratio of total contact area to the nominal area reduced due to a lesser contact load. Effect of wear on the plate roughness CDF, indicated the smoothing of asperities during prolonged wear. In addition, a friction study varying the entrainment velocities is conducted to show all the regimes of lubrication. The boundary regime had a solid friction coefficient of 0.16, followed by a decrease in friction coefficient from a value of 0.16 to 0.005 in the mixed regime and an increase to a value of 0.02 in the full film regime. The model is numerically efficient in computing wear for mixed lubricated problems, given the CDFs of two real rough surfaces.
