Mechano-chemical model: Reaction temperatures in a concentrated contact

Abstract

Successful boundary lubrication is essential in the design and operation of many mechanical components. The lubrication process is complex and it involves contact mechanics, fluid mechanics, tribochemistry, and material deformation and fracture. Two schools of thought have emerged over the years in examining the mechanisms and modeling of boundary lubrication. The chemical school believes that chemical reactions at the rubbing surfaces control the efficacy of the lubrication process. The mechanical school believes that while chemistry is a factor, hydrodynamics, elastohydrodynamics (EHD), and micro-EHD can account for most of the load-bearing mechanisms, so at least in design, they are the principal issues. This paper attempts to bring the two schools together to examine a common set of experimental data. The experiments involve running wear tests on a four-ball wear tester using microliters of lubricant until seizure. Lubricant degradation and breakdown are therefore a factor in the wear test. Eventually we would like to compare the chemical kinetic model with the mechanical contact model in describing and predicting the effectiveness of the lubrication process, i.e. the time to seizure. The chemical kinetics model assumes that oxygen consumption by the lubricant to make friction polymers controls the process. The mechanical model suggests that if temperatures in the contact exceed a certain limit, scuffing will occur. The key to both models is the temperatures in the contact. This paper describes the two models and focuses on the temperatures in the contact. The temperatures calculated from the two models differ significantly. The temperatures predicted by chemical kinetics are about 100 °C higher than the mechanical model. The identification of the discrepancy and the magnitude of the difference highlight the difference between the two approaches. It is hoped that this paper will bring forth further research effort to this critical issue. Various possible explanations were offered for the temperature difference. A plausible explanation was proposed and initial calculations suggest that by taking into account of the wear process, the temperatures calculated by the mechanical model can reach the temperatures estimated by the chemical model.