Role of Surface Micro-Texturing in Acceleration of Initial Running-in during Lubricated Fretting

Surface texturing is considered to be an effective means of enhancing the properties of a tribological contact not only in a normal uni-directional sliding condition but also in fretting under lubricated conditions. Well-regulated “micro dimples” were formed on a flat surface using the method of micro fabrication. In this study, a bearing steel (HV760) was used for the specimens with ball-on-flat configuration. The frictional force and relative movement between the specimens were measured simultaneously during fretting with a frequency 7.35 Hz, for fretting up to 2 × 105 cycles under a lubricated condition of 350 neutral oil (typically used for grease lubricated contacts). The normal load, and fretting stroke were varied in the range of 4.9 N to 22.1 N, 12 μm to 215 μm respectively. The initial running-in process (namely, the phenomenon of a significant reduction in the coefficient of friction seen in the early stages of fretting) was the main focus of the study, with comparisons being made between specimens with flat surfaces. The main findings were that a micro texturing surface resulted in a reduced number of cycles to complete the running-in process (when compared to the flat surface), but that the wear scar was wider in the micro texturing surface. It is proposed that the micro texturing surface provides a less stiff contact than the flat surface and that the lubricant can become entrapped in the dimples in the contact, and thus provide enhanced entrainment of the lubricant into the contact