The structure and composition of aluminum wear debris generated by unlubricated sliding in different environments

Abstract

The sliding behavior of high purity aluminum was investigated, with a focus on the chemical composition and structure of wear debris generated in different environments. Sliding tests were carried out with a pin-on-disk tribometer. The pin was a 440 C stainless steel ball and the disk was 99.99% aluminum. Relative humidity in air varied from 20% to 95%. The sliding behavior depended strongly on the test environment, especially the relative humidity. Wear debris was characterized by scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The debris from air tests differed both chemically and structurally from the original contacting material. A high concentration of oxygen was detected by EDS in the wear debris and on the wear track. XPS was employed to gather information on the bonding of the oxygen incorporated with aluminum during sliding. XRD analysis suggests that the metallic component of the wear debris produced in air retains high elastic strains. Weight changes of the wear debris during heat treatment were measured by TGA and these tests suggest that the debris includes hydroxide. The endothermic reaction detected by DSC provides evidence for the loss of water during heating. The results provide new information about tribochemical phenomena during the sliding of aluminum. These involve more than simply forming and removing a film of Al 2O 3.