Characterization of micro-spalling and wear of different
Source:NETAuthor:UN Addtime:2022-04-20 Click:
Microspalling or surface damage is a surface failure mechanism commonly found in modern mechanical components (such as bearings and gears) with heavy-duty, non-conformal, roll-slip lubricated contact. This damage is caused by rolling contact fatigue at the rough-peak level, which occurs due to repeated rough-peak stress fluctuations during rolling contact and can be characterized by numerous microcracks and microspalling on the rolling surface, generally occurring under poor lubrication conditions (low Λ values) where the oil film thickness is not sufficient to completely separate the rolling surface and the load is carried by the rough-peak-rough-peak contact and the lubricant, respectively. bearing. Since the current trend is to use thinner lubricants to maximize the efficiency of mechanical components, the focus is on understanding the microstripping phenomenon and designing rolling surfaces that are more resistant to microstripping and can withstand higher power densities.
Microspalling is now recognized as a surface contact fatigue phenomenon that involves a competition between light wear and rough peak fatigue. Minor wear can reduce the formation of microspalling pits by correcting the surface run-in or removing the fatigue material layer. It has been proven that: e.g. anti-wear, friction reducing, extreme pressure type 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 microspalling pits by generally maintaining high surface roughness amplitudes and thus maintaining high or increasing friction factors, greatly increasing the risk of microspalling. In contrast, additives that allow some degree of run-in wear or reduce the friction factor often reduce the risk of microspalling. The literature has focused on exploring the role of ZDDP anti-wear additives, which are beneficial for sliding friction but may be detrimental for rolling friction. A recent study showed that the degree of microflaking depends more on the degree of run-in wear than on the thickness of the final friction film formed, as described in the literature . In this case, sufficient run-in wear would significantly reduce the risk of microflaking.
However, in the absence of additives, other factors (e.g. operating conditions, steel surface, metallurgical properties) receive more attention. If Λ values are very low and anti-wear additives are missing, the harsh contact conditions generally lead to a higher risk of micro-spalling or even wear. The literature  suggests that the initiation and extension of microspalling is mainly controlled by the working stress; the literature  suggests that increasing the slip-roll ratio generates long sliding distances and thus accelerates microspalling. In any case, minor wear dominates and reduces microspalling damage until a certain threshold value is reached. In addition, it is generally believed that negative sliding (slower moving surfaces) is detrimental to the occurrence of microstripping damage and the extent of microstripping damage due to the increased pressurized oil effect that helps open cracks, although some studies give the opposite conclusion that positive sliding causes microstripping damage to develop faster compared to negative sliding due to less wear.
In addition to the operating conditions, the focus was on the role of surface morphology and materials. It is shown that surface roughness is the dominant cause of microspalling and that rough-smooth contact is detrimental to smoother surfaces. In this case, the rough surface induces fatigue microcirculation on the smooth surface, thus promoting microspalling damage. Stress fluctuations induced by another surface roughness generally occur only on smooth surfaces. In addition, the orientation of the roughness peaks relative to the rolling direction has an important effect on the extent of microspalling, with transverse roughness peaks being more detrimental than longitudinal roughness peaks; the transverse alignment of the roughness peaks induces stress fluctuations and accelerates microspalling damage.
Another key consideration is the steel and its properties (e.g. hardness). Bearing and gear surfaces should have a sufficiently high hardness (58 to 66 HRC) to withstand high Hertz contact stresses (>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-spall 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, loss of dimensions. Severe micro-spalling wear is accelerated when the hardness of the specimen is softer than the hardness of the counterpart, and the hard counterpart maintains a high plasticity index (the ability to cause plastic deformation in the counterpart), further damaging the soft specimen. When considering only minor spalling damage (i.e., surface fatigue in competition with minor wear), the study by Oila et al. showed that harder steel surfaces lead to an earlier origin of microspalling, yet their expansion rate is significantly slower than that of softer surfaces. Recently, Vrcek et al. developed a method to study microspalling and wear properties using a disk-disk arrangement, showing that the most severe microspalling damage occurs for two harder surfaces that are also at higher hardness levels due to smaller minor wear. In addition, if the rough counterpart is soft, the hardness difference completely eliminates the microspalling damage. However, further research is needed to understand more about the effect of hardness on surface damage (i.e., microspalling and wear phenomena) for the selection of materials and their heat treatment.
The study by Aleks Vrcek et al. focused on the importance of surface hardness differences in surface damage (i.e., microspalling and wear damage) under poor lubrication conditions. Two heat treatments (i.e., surface induction hardening (SIH) and total hardening (TH)) were performed using three bearing steels, highlighting the benefits of applying SIH heat treatments to introduce beneficial residual compressive stresses in the surface and subsurface regions for fatigue of the parts. The results suggest that when microspalling occurs, the selection of the appropriate heat treatment is more important than the selection of a better bearing steel composition, provided that the surface hardness level remains consistent.
Aleks Vrcek et al. characterized the surface damage (i.e., microspalling and wear) of different steel grades under boundary lubrication conditions using a disk-disk test arrangement in which rough SIH-treated counterparts made from three bearing steel grades were brought into contact with smooth specimens of TH-treated G3 and SIH-treated G55 steel, respectively. Based on the test results, the following conclusions were drawn:
1) The faster moving rough surfaces undergo only slight wear and plastic deformation, regardless of the relative surface hardness values of the smooth surfaces. However, the slower moving smooth surfaces experienced different damage patterns depending on the surface hardness difference 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 specimens, which depends only on the relative hardness.
2) For smooth specimens, three main surface damage patterns are identified: if the specimen is hard, only slight wear occurs; if the specimen is the same hardness as the counterpart, micro spalling and slight wear exist simultaneously; if the specimen is soft, the surface undergoes severe micro spalling wear, and the wear rate can be as high as 50 times that of the first two states.
3) At similar hardness levels, the SIH-treated G55 specimen has better surface fatigue resistance than the TH-treated G3 specimen. The transition from micro-exfoliation to severe micro-exfoliation wear occurs when the hardness difference is about 140 HV1 (G55) and 30 HV1 (G3).
4) Further metallurgical tests are required to investigate the morphology of subsurface cracks in the specimens tested at different hardness differences and the potential reasons for the superior fatigue performance of G55 over G3.