Colliding asperities a tribological event on micro scale

University dissertation from Luleå tekniska universitet

Abstract: In order to predict and optimize energy efficiency, fuel consumption and service life, friction and wear need to be predetermined with higher accuracy than what is possible today. This prediction and optimization is crucial for the development of sustainable mechanical components and systems with excellent environmental performance. Better and more reliable models for predicting friction, wear and scuffing risk in boundary lubricated contacts will be developed in this project. This includes a model for asperity-asperity collision with components of contact mechanics, thermodynamics and physics. In the boundary lubricated contact, loads are mostly carried by asperities. This makes the real area of contact is so different from the nominal contact area, a small fraction of the nominal contact area supporting the load will cause high contact stress and large deformation. Surfaces of machine components operating under high stress in long period can easily cause damage. Therefore, an elastoplastic analysis ofasperity collision was conducted with the Finite Element Method. The contact area and contact stress were studied based on the change of parameters as adhesive friction coefficient, interference and collision velocity. The plastically deformed area and residual stress after collision were also depicted in figures. Friction will generate heat in the sliding contact, and eventually cause a temperature rise. Due to the heat is generated at asperities, heat flux is not continuous and the temperature both increase to a relatively high value and decrease to a small value in very short time. This kind of temperature is often called flahtemperature, and it is important to study because it can affect the viscosity of the lubricant, the formation of tribolayer and in turn it will affect the mechanical properites of the surface. The flash temperature was analyzed based on the previous study of the elastoplastic asperity collision, the times for flash temperature to reach maximum value were given and thermal expansion was also included. The FEM model can conduct a study regardless of the geometry and material properties of the surface asperity, but due to the very fine mesh required at the interface, it is not suitable to carry out an analysis of the rough surface contact. Therefore the Boundary Element Method was adopted to have a thorough study of the rough surface contact. The features of the analysis coudcuted in the FEM model, such as strain hardening and friction, should be replicable in the BEM model. In the end, an Engineering tool for the rough surface contact will be developed.

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