TOF-SIMS and XPS characterization of diamond-like carbon films after tests in inert and oxidizing environments

Abstract

In this study, we used time-of-flight secondary ion mass spectrometry (TOF-SIMS), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy techniques to ascertain the chemical nature of the sliding surfaces and wear debris particles of acetylene-grown diamond-like carbon (DLC) films after tests in dry nitrogen and oxygen. Between these two gasses, nitrogen is relatively inert and hence is not expected to enter into significant tribochemical reactions with sliding DLC surfaces, but oxygen is highly reactive toward carbon and hydrogen (which make up these DLC films) and hence may trigger some tribochemical reactions. The tribological tests were performed in a ball-on-disk tribometer at room temperature (22 °C) and under a 5 N load. The sliding velocity and distance were 0.25 m/s and 500 m, respectively. Disk material was made of AISI M2 steel, while balls were made of 9.5-mm-diameter AISI 440C steel. They were both coated with DLC, hence the sliding occurred between DLC coated surfaces of the balls and the disks. The friction coefficient of DLC against itself in dry nitrogen was rather high- and un-steady (i.e., fluctuating between 0.1 and 0.6) and the wear life was very short (i.e., less than 100 m). Raman spectroscopy of wear debris particles generated during tests in dry nitrogen suggested that some graphitization had taken place, but the XPS and TOF-SIMS of the sliding surfaces did not reveal much tribochemical interactions on their sliding surfaces. The friction coefficient of DLC against itself in dry oxygen was much lower (i.e., 0.06–0.12). The Raman spectroscopy of wear debris particles also suggested some graphitization after tests in dry oxygen. TOF-SIMS and XPS revealed high levels of oxidation for these debris particles suggesting that tribo-oxidation was perhaps a major mechanism controlling the friction and wear behavior of DLC in oxygen. Based on the combined results of Raman, TOF-SIMS and XPS, we concluded that the chemical nature of test environments plays a major role in the friction and wear of acetylene-grown DLC films. Much lower friction and wear of these films in dry oxygen is most likely due to graphitization as well as tribo-oxidation of their sliding surfaces.