Application of phase contrast imaging atomic force microscopy to tribofilms on DLC coatings

Abstract Tribofilms formed in the rubbing surfaces are closely related to wear mechanisms and steady-state friction in sliding contacts. However, their small thickness, inhomogeneity and discontinuity are the factors that hinder the evaluation of their micromechanical properties. The phase contrast images in tapping mode atomic force microscopy allow an estimation of inhomogeneity in micromechanical properties of the sample surfaces. The purpose of this investigation is to examine how the phase contrast images contribute to the characterization of thin tribofilms. Surfaces of diamond-like-carbon (DLC) coatings before and after friction contact against a steel ball slider were investigated in this study. The chemical characteristics of the worn surfaces were studied by micro-Raman spectroscopy, Auger electron spectroscopy (AES) and secondary ion mass spectroscopy (SIMS) and these results were discussed in association with the phase contrast images. A three-dimensional simulation of the real contact area was also conducted and the pressure distribution obtained by this simulation was compared with the phase contrast image obtained at the same area. The phase contrast images revealed a significant inhomogeneity of the worn surfaces. Thin tribofilms were formed at the real contact regions and their thickness increased at the locations experiencing higher contact pressure. The tribofilms that represented as darker grey scale values indicated that they were less stiff than the initial DLC coating. The comparison of the phase contrast images with the results of micro-Raman, AES and SIMS analyses led to a speculation that the tribolfilms, composed almost of carbon element, may be graphite films or films mainly possessing graphitic property. The phase contrast imaging in atomic force microscopy showed promise as an effective tool for better understanding micromechanical properties of worn surfaces and wear mechanisms.