|Ph.D Student||Chen Zhou|
|Subject||Elastic-Plastic Coated Surface Contact|
|Department||Department of Mechanical Engineering||Supervisor||Professor Emeritus Izhak Etsion|
|Full Thesis text|
At the heart of analyzing the contact behavior of coated rough surfaces is to understand the single coated asperity contact, which, in the present research, is modeled by the contact between a rigid flat and a deformable coated sphere (coated spherical contact). The elastic-plastic coated spherical contact is investigated in the case of hard coatings using the finite element method. This contact problem includes a full slip contact under normal loading and a full stick contact under combined normal and tangential loading.
For the full slip contact under normal loading, various aspects are addressed. The plasticity evolution in a coated sphere is demonstrated. Based on the plasticity evolution, a new criterion for the substrate plasticity level (SPL) is suggested. It is found that the plasticity evolution is more complex for smaller coating thickness and larger elastic moduli ratio of the coating and substrate materials. Several elastic-plastic contact parameters are investigated. A transition interference is presented dividing the elastic-plastic regime of deformation into two sub-regimes. An appropriate normalization of the contact parameters is proposed enabling universal dimensionless relations of the contact load, contact area vs. the interference. Yield inception in a coated sphere is also explored. Various modes of yield inception at single or simultaneous double or triple locations on the symmetry axis are observed and are presented in a yield map as functions of the dimensionless coating thickness and material properties.
For the full stick contact under combined normal and tangential loading, the static friction in a coated spherical contact is studied. With the increase of the dimensionless coating thickness, the static friction coefficient first increases to its maximum value at a certain dimensionless coating thickness, thereafter decreases, and eventually levels off. The static friction coefficient as a function of the dimensionless coating thickness is provided for a wide range of material properties and normal loading.
Finally, an elastic-plastic coated rough surface contact model is developed by incorporating existing single coated asperity contact models in a statistical multi-coated-asperity surface model. To determine the mode of the coated surface contact deformation, an equivalent plasticity index is defined, which mainly depends on the coating material properties and substrate roughness. It is found that a coated surface with a thicker, stiffer, harder coating and a rougher substrate has a higher surface separation and smaller real contact area under a given load.