Modeling and Parametric Study of Fatty Acids Boundary Lubrication in Nominally Flat Contacts of Metallic Surfaces

This article develops a first-principle based mathematical model for fatty acids supported boundary lubrication in nominally flat metallic-surface contacts. It defines the problem with five measurable parameters: contact load, sliding velocity, bulk temperature, boundary film breakdown temperature, and root mean square (RMS) surface roughness. It characterizes the state of contact and lubrication with two variables. One is the system friction coefficient and the other is the proportion of the real area of contact in which the lubrication is broken down. The model is used to study the lubrication performance under light-load and high-speed operating conditions with a series of parametric analyses. The results and analyses suggest that the state of contact and lubrication is sensitively related to the surface roughness, boundary film breakdown temperature, and sliding velocity and temperature of the system. The results also show that a high surface thermal conductivity is very beneficial, whereas an increase in its hardness may weaken the lubrication. Relevancy to engine piston/cylinder contacts and lubrication is elaborated.