Micro-spalling or surface damage is a surface failure mechanism, commonly seen in modern mechanical components with heavy loads, non-conformal, and roll-slip lubrication contact (such as bearings and gears). This kind of damage is caused by rolling contact fatigue at the rough peak level. Its occurrence is due to the repeated rough peak stress fluctuations during rolling contact. It can be characterized by countless microcracks and microspalling formed on the rolling surface. Generally, it occurs under poor lubrication conditions (low Λ value) where the oil film thickness is insufficient to completely separate the rolling surface. The loads are respectively borne by the rough peak-rough peak contact and the lubricant. As 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 can withstand higher power density.

Nowadays, micro-spalling has been confirmed as a surface contact fatigue phenomenon, which involves the competition between slight wear and rough peak fatigue. Slight wear can reduce the formation of micro-spalling pits by correcting the running-in of the surface or removing the fatigued material layer. It has been confirmed that anti-wear, anti-friction and extreme pressure additives play an important role in enhancing or delaying the formation of micro-spalling. Additives that prevent wear on rough rolling surfaces can enhance the formation of micro-spalling pits. Generally, they maintain a high surface roughness amplitude, thereby maintaining a high coefficient of friction or increasing the coefficient of friction, which greatly increases the risk of micro-spalling. In contrast, additives that allow for a certain degree of running-in wear or reduce the coefficient of friction often reduce the risk of micro-spalling. The relevant literature has mainly explored the role of ZDDP anti-wear additive, which is beneficial to sliding friction but may be harmful to rolling friction. A recent study indicates that the degree of micro-spalling is more dependent on the degree of running-in wear rather than the thickness of the final formed friction film as described in reference [5]. In this case, adequate running-in wear will significantly reduce the risk of micro-spalling.

However, in the absence of additives, other factors (such as operating conditions, the surface of the steel, and metallurgical properties) receive more attention. If the Λ value is very low and there is a lack of anti-wear additives, the harsh contact conditions generally lead to a higher risk of micro-spalling or even wear. Reference [13] holds that the initiation and expansion of micro-spalling are mainly controlled by working stress. Reference [14] holds that increasing the slide-roll ratio will result in a longer sliding distance, thereby accelerating micro-spalling. In any case, before reaching a certain threshold value, slight wear dominates and can reduce micro-spalling damage. In addition, it is generally believed that negative sliding (on slower moving surfaces) is detrimental to the occurrence and degree of micro-spalling damage. This is because it increases the effect of pressurized oil, which helps to open cracks. Although some studies have reached the opposite conclusion that due to less wear, positive sliding causes micro-spalling damage to develop faster compared to negative sliding.

In addition to the operating conditions, the surface morphology and the role of materials were mainly studied. Research shows that surface roughness is the dominant cause of micro-spalling, and rough-smooth contact is harmful to smoother surfaces. In this case, the rough surface induces fatigue microcirculation on the smooth surface, thereby promoting micro-spalling damage. Stress fluctuations caused by the roughness of another surface generally only occur on smooth surfaces. In addition, the orientation of the rough peaks relative to the rolling direction has an important influence on the degree of micro-spalling. Compared with the longitudinal rough peaks, the transverse rough peaks are more harmful. The lateral arrangement of rough peaks induces stress fluctuations and accelerates micro-spalling damage.

Another key consideration is the steel and its properties (such as hardness). The surfaces of bearings and gears should have a sufficiently high hardness (58 to 66 HRC) to withstand higher Hertz contact stress (>1 GPa). Rolling contact fatigue life is generally proportional to the hardness level. Starting from Olver's study of severe micro-spalling damage, previous studies have shown that when micro-spalling damage occurs, surface hardness plays a major role. In this case, the micro-spalling damage is so severe that the rapid material loss is not due to traditional wear but to rolling contact fatigue, which leads to a high wear rate and finally dimensional loss. When the hardness of the specimen is softer than that of the mating part, it will accelerate severe micro-spalling wear. The hard mating part maintains a high plasticity index (the ability to cause plastic deformation on the mating part), further damaging the soft specimen. When considering only minor spalling damage (i.e., when surface fatigue and minor wear are in a competitive state), Oila et al. 's research indicates that harder steel surfaces lead to an earlier origin of micro-spalling, yet their 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 disc-disk arrangements. The results showed that for two harder surfaces at the same higher hardness level, the most severe micro-spalling damage occurred due to minor wear. In addition, if the rough mating parts are relatively soft, the hardness difference can completely eliminate the micro-spalling damage. However, in order to gain a deeper understanding of the influence of hardness on surface damage (i.e., micro-spalling and wear phenomena) in order to select materials and their heat treatment, further research is needed.

The research focus of Aleks Vrcek et al. lies in the importance of the difference in surface hardness in surface damage (i.e., micro-spalling and wear damage) under poor lubrication conditions. Three types of bearing steels were subjected to two heat treatments (i.e., surface induction hardening (SIH) and full hardening (TH)), highlighting the benefits of introducing beneficial residual compressive stress in the surface and subsurface areas through SIH heat treatment to alleviate the fatigue of the parts. The results suggest that when micro-spalling occurs, under the condition that the surface hardness level remains consistent, choosing the appropriate heat treatment is more important than choosing a better composition of bearing steel.

Aleks Vrcek et al. characterized the surface damage (i.e., micro-spalling and wear) of different steel grades by disc-disc test arrangement under boundary lubrication conditions. Rough pairs made of three bearing steel grades and treated with SIH were in contact with smooth specimens of G3 steel treated with TH and G55 steel treated with SIH respectively. Based on the test results, the following conclusions are drawn:

1) Fast-moving rough surfaces only undergo minor wear and plastic deformation, regardless of their relative surface hardness values compared to smooth surfaces. However, smooth surfaces with slower movement undergo different damage patterns, depending on the difference in surface hardness between the specimen and the counterpart. In addition, the material of the counterpart has no significant effect on the micro-spalling or wear of the G3 steel sample, which only depends on the relative hardness.

2) For smooth specimens, three main surface damage mode states are identified: if the specimen is relatively hard, only slight wear occurs; If the hardness of the sample is the same as that of the counterpart, micro-spalling and slight wear will exist simultaneously. If the sample is relatively soft, the surface undergoes severe micro-spalling wear, and the specific wear rate can be as high as 50 times that of the first two states.

3) At similar hardness levels, the G55 specimens treated with SIH have better surface fatigue resistance than the G3 specimens treated with TH. When the hardness difference is approximately 140 HV1(G55) and 30HV1(G3), a transformation from micro-spalling to severe micro-spalling wear occurs.

4) To study the morphology of subsurface cracks on the specimens during tests under different hardness differences and the potential reasons why the fatigue performance of G55 is superior to that of G3, further metallurgical tests are required.