Dynamic changes of surface films under boundary lubrication: A study using ellipsometry.

Abstract

Wear and friction behavior in boundary lubrication are strongly influenced by surface films on the sliding parts. These surface films change dynamically while sliding takes place. In spite of this importance few studies have focused on the progression of change of those films. This study focused on simultaneously monitoring dynamic changes on the surface and friction forces between the sliding parts, with the objective of finding cause-effect (surface film - surface failure) relations that could lead to a better understanding of the process of friction and wear. The main technique used for the monitoring of the surface while sliding was ellipsometry. A special Ellipsometer was used (Division-of-Amplitude-Photopolarimeter Mueller -Matrix Ellipsometer). The wear test rig used was a cylinder-on-disk type specially designed to be coupled with the ellipsometer. The friction force was also measured during the test. The specimens were made of AISI 1080 steel and AISI M2 steel. The disk roughness and hardness were varied to study their influence. Bearing rollers (AISI 52100 steel) were used as the second sliding body. The initial Hertzian contact pressure (maximum) was 900 MPa. Specimens were submerged under low viscosity hydrocarbons (Diesel type fuels with and without additives) functioning as lubricants. Other analyses performed were: (1) measurement of wear scars on the cylinders, (2) evaluation of the final roughness on disks, and (3) microscopic analysis of the disk surface using Nomarski differential interference contrast. The main conclusions of the study are: (1) Films are formed mainly as patches corresponding to the localized contact areas during the sliding of the surfaces. The surface roughness is not significantly changed by these films. (2) No connection was found between the apparent thickness, or amount of coverage and the protective properties of formed surface film. This implies that mechanical and chemical characteristics of these films are more important than their thickness or covered area. (3) Significant changes on the surface are registered with the Ellipsometer when scuffing occurs, indicating the capacity of the technique to monitor dynamic changes taking place on the surface under boundary lubrication. However the assumption of continuous films, used in previous works, has to be revised based on these findings.