Micro-spalling or surface damage is a surface failure mechanism commonly found in heavily loaded, non-conformal, roll-slip lubricated contact modern mechanical components (such as bearings and gears). This damage is caused by asperity-level rolling contact fatigue, which occurs due to repeated asperity stress fluctuations during rolling contact, and can be characterized by numerous micro-cracks and micro-spallings that form on the rolling surface, generally occurring when the oil film thickness is insufficient Under poorly lubricated conditions (low Λ values) that completely separate the rolling surfaces, the load is carried by the asperity-asperity contact and the lubricant, respectively. Because the current trend is to use thinner lubricants to maximize the efficiency of mechanical components, the focus is on understanding the phenomenon of micro-spalling and designing rolling surfaces that are more resistant to micro-spalling and withstand higher power densities. Micro-spalling has now been identified as a surface contact fatigue phenomenon that involves a competition between light wear and rough peak fatigue. By correcting the running-in of the surface or removing the layer of fatigued material, slight abrasion can reduce the formation of micro-spalling pits. It has been confirmed that additives such as anti-wear, anti-friction and extreme pressure have an important role in enhancing or delaying the formation of micro-flaking. Additives that prevent wear on rough rolling surfaces can enhance the formation of micro-spalling pits, which generally maintain a high surface roughness magnitude and thus maintain a high friction factor or increase the friction factor, greatly increasing the risk of micro-spalling. In contrast, additives that allow some degree of running-in wear or reduce the coefficient of friction often reduce the risk of micro-spalling. The literature has focused on exploring the role of ZDDP antiwear additives, which are beneficial to sliding friction but potentially detrimental to rolling friction. A recent study showed that the degree of micro-spalling is more dependent on the degree of running-in wear than the thickness of the final formed friction film as described in [5]. In this case, adequate running-in wear will greatly reduce the risk of micro-peeling. However, in the absence of additives, other factors (such as operating conditions, steel surface, metallurgical properties) receive more attention. If the Λ value is very low and antiwear additives are lacking, harsh contact conditions generally lead to a higher risk of micro-spalling and even wear. Reference [13] believes that the initiation and expansion of micro-spalling is mainly controlled by the working stress; Reference [14] believes that increasing the slip-roll ratio will produce a long sliding distance, thereby accelerating the micro-spalling. In any case, until a certain threshold is reached, light abrasion predominates and micro-spalling damage is reduced. In addition, negative sliding (slower moving surfaces) is generally believed to be detrimental to the occurrence and extent of micro-spall damage due to increased pressurized oil effects, which aid in crack opening, although some studies have shown the opposite The conclusion is that, due to less wear, positive sliding causes micro-spalling damage to develop faster than negative sliding. In addition to operating conditions, the surface morphology and the role of materials are mainly studied. Studies have shown that surface roughness is the dominant cause of micro-peeling and that rough-smooth contact is detrimental to smoother surfaces. In this case, the rough surface induces fatigue microcirculation of the smooth surface, thereby promoting micro-spalling damage. Stress fluctuations caused by another surface roughness generally only occur on smooth surfaces. In addition, the orientation of the asperities relative to the rolling direction has an important influence on the degree of micro-spalling. Compared with the longitudinal asperities, the transverse asperities are more harmful; the lateral arrangement of asperities induces stress fluctuations and accelerates the micro-spalling damage. Another important consideration is the steel and its properties (eg hardness). Bearing and gear surfaces should have high enough hardness (58~66 HRC) to withstand high Hertz contact stress (>1 GPa). Rolling contact fatigue life is generally proportional to the hardness level, starting with Olver's study of severe micro-spalling damage, and previous studies have shown that surface hardness plays a major role when micro-spalling damage occurs. In this case, the micro-spalling damage is so severe that the rapid material loss is not due to conventional wear, but to rolling contact fatigue, which leads to high wear rates and finally dimensional loss. Severe micro exfoliation wear is accelerated when the hardness of the specimen is softer than that of the counterpart, and the hard counterpart maintains a high plasticity index (the ability to induce plastic deformation on the counterpart), further damaging the soft specimen. When considering only mild spalling damage (i.e. surface fatigue competes with light wear), Oila et al. showed that harder steel surfaces lead to an earlier origin of micro spalling, however its propagation rate is significantly slower than that of soft surfaces. Recently, Vrcek et al. developed a method to study micro-spalling and wear performance using a disk-disk arrangement, and the results showed that for two harder surfaces also at higher hardness levels, the most severe due to minor minor wear occurred. Micro peeling damage. In addition, if the rough counterpart is softer, the hardness difference can completely eliminate micro-peeling damage. However, further research is required to gain a better understanding of the effect of hardness on surface damage (i.e. micro-spalling and wear phenomena) for the selection of materials and their heat treatment