Oil film lifetime and wear particle analysis of laser-patterned stainless steel surfaces

In this experimental study, periodic cross-like patterns with different structural wavelengths (6 and 9 µm) were fabricated by laser-interference patterning on stainless steel samples (AISI-304). Tribological tests with a ball-on-disc tribometer in rotational sliding mode were performed under mixed lubrication. Spin coating was used to produce homogeneously distributed oil films with a defined initial oil film thickness in order to ensure reproducible testing conditions for all tested samples. By means of an advanced electrical resistivity circuit, a sharp increase in the coefficient of friction was well correlated with a change in the lubrication regime from mixed to boundary lubrication. This event defines the oil film lifetime at the corresponding cycle count. An oil film lifetime of 1000 sliding cycles was determined for the polished reference samples which is independent of the initial oil film thickness. For the laser-patterned surfaces, the oil film lifetime increases with decreasing initial oil film thickness. Moreover, the cross-like pattern with a structural wavelength of 6 µm shows a significant improvement in the oil film lifetime by a factor of 130 compared to the unpatterned reference. A subsequent analysis of the wear particles was able to demonstrate that the wear particles being produced for the laser-patterned surfaces are typically smaller than half of the used structural wavelength. As a consequence, the produced wear particles can be effectively trapped in the topographic minima positions. Consequently, those particles do not remain in the contact area thus reducing the abrasive wear component and increasing the oil film lifetime.