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.
micro-pittingrolling-sliding contactfailure analysislubricant and additivelubrication
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