In recent years, the performanceof steel to resist subsurface-initiated fatigue such as pitting has been greatly improved with the application of high-purity steel and the improvement of heat treatment techniques. However, a relatively new type of surface-initiated fatigue termed as micro-pitting has been found in applications of surface hardened gears and rolling element bearings among others. Micro-pitting, also known as surface distress, is a surface-initiated fatigue phenomenon caused by the plastic deformation of the surface roughness in highly loaded rolling/slidingcontacts under conditions that the lubricating oil film cannot be fully established, i.e. starved and/mixed lubrication. It occurs widely in machine elements of non-conformal contacts such as gears and rolling element bearings. Micro-pitting affects the accuracy, service life, reliability of machine elements and the whole system.

In this paper, the research progress of micro-pitting reviewed from perspectives on the related experimental methods and test rigs, the main driving factors for micro-pitting, and the possible measures that could be considered to prevent micro-pitting in design and engineering practice. Firstly, the current available experimental setup and methods for the study of micro-pitting were introduced. These included the FZG gear test rig, the FE8 bearing test rig, the MPR micro-pitting test rig, the twin-disc setup and the possible TE77 reciprocating sliding tester. The advantages and disadvantages of the related evaluation methods or standards and test rigs compared and summarized. Secondly, to date, the forming mechanism of micro-pitting has not yet been fully revealed, and comprehensive knowledge from multiple subjects needs to be considered for a better understanding, such as materials science of metal and lubricants, tribo-chemistry, and elasto-plastic contact mechanics. The main driving forces and influencing factors that contribute to micro-pitting were summarized in terms of lubrication/interfacial mechanics and lubricant chemistry, e.g., the film thickness to roughness ratio, the slide-to-roll ratio, the contact pressure, and different anti-wear additives. The possible evolution process of micro-pitting, i.e., micro-crack initiation, propagation, and material spalling had been summarized from the initial stage to the surface damagein view of fatigue crack development. Thirdly, some potential measures to suppress micro-pitting for engineering design were given, including the optimization of the geometric shape of tribo-pairs to reduce the maximum contact pressure, the increase of oil film thickness via lubricant selection and/or the reduction of the roughness amplitude in manufacturing, the emphasis on the running-in during the initial stage of operation, and the proper selection of lubricating materials, especially anti-wear additives.

To conclude, the formation mechanism of micro-pitting needs to be further explored, and design criteria and standards for gears and bearings need to be built for engineering practice. The future studies for the understanding of micro-pitting are prospected and may be carried out from the following aspects. First, an efficient, cost-effective, and highly representative testing method needs to be developed for micro-pitting, which could be used to screening lubricants and additives. Second, numerical models for the prediction and analysis of micro-pitting should be built by considering mixed lubrication and the generated tribofilm and/or adsorption layers through physical and chemical effects of additives at lubricated interfaces. Third, the effect of anti-wear additives and friction modifiers on micro-pitting needs to be further studied regarding tribochemical and tribophysical actions of additives at different working conditions. Last but not the least, the competitiveor synergistic relation between micro-pitting and other surface damage and failure modes, such as fatigue or macro-pitting, mildwear, severe wear and scuffing need to be explored. The boundary of the dominating factors is helpful for machine design and for failure analysis.