A deterministic-statistical model for tribo-contacts in boundary lubrication with lubricant/surface physicochemistry

by 1965- Zhang, Huan

The boundary-lubricated surface contact is truly an interdisciplinary process involving deformation, heat transfer, physicochemical interaction, and random-process probability. The objective of this thesis is to develop a surface contact model as a theoretical platform upon which to carry out the boundary lubrication research with a balanced consideration of all the four key aspects of the contact process. The modeling consists of three successive steps – (1) elastoplastic finite element analysis of frictional asperity contacts, (2) modeling of contact systems with friction, and (3) modeling of a boundary lubrication process. Finite element analysis of frictional asperity contacts – A finite element model is developed and systematic numerical analyses carried out to study the effects of friction on the deformation behavior of individual asperity contacts. The study reveals some insights into the modes of asperity deformation and asperity contact variables as functions of friction in the contact. The results provide guidance to analytical modeling of frictional asperity contacts and lay a foundation for subsequent work on system contact modeling. Modeling of contact systems with friction – Analytical equations are developed relating asperity-contact variables to friction using contact-mechanics theories in conjunction with the finite element results. A system-level model is then derived from the statistical integration of the asperity-level equations. The model is a significant advancement of the Greenwood-Williamson types of system models by incorporating iv contact friction. It also serves as the platform in the final step of model development for the boundary lubrication problem. Modeling of a boundary lubrication process – On the basis of the above mechanical modeling, an asperity-based model is developed for the boundary-lubricated contact by incorporating other key aspects involved in the process. Four variables are used to describe an asperity contact under boundary lubrication conditions, including micro-contact area, friction force, load carrying capacity and flash temperature. In addition, three probability variables are used to define the interfacial state of an asperity junction that may be covered by various types of boundary films. Governing equations for the seven key asperity-level variables are derived based on first-principle considerations of asperity deformation, frictional heating, and formation/removal of boundary lubricating films. These coupled asperity-level equations, some of which are nonlinear, are solved iteratively and the solution is then statistically integrated to formulate the contact model for boundary lubrication systems. The results obtained from the model suggest that it may provide a framework for future investigation of the boundary lubrication process by integrating research advances in contact mechanics, tribochemistry, and other related fields. v