Focus on the concept of pressure-velocity-time (pVt) limits for boundary lubricated scuffing

The mechanism of scuffing has been the object of tribological investigations for many years, but the factors that cause its activation are still poorly recognised by engineers and scientists. An industrial approach to this problem requires the determination of the maximum values of operational parameters that secure against the occurrence of scuffing if they are not exceeded. The example most commonly used in practice (especially by bearing, polymers or gears producers) is the pV (pressure x velocity) criterion, which offers the possibility of calculating the limits for friction pairs that are working under different kinetic configurations of materials and tribological conditions. However, such an approach is a significant simplification, since it does not take into account the unequivocal parameters of the tribosystem, such as the duration or frequency of its operation close to its mechanical limitations. These parameters can have a significant impact on the stability of the properties of lubricants, particularly in the context of an elastohydrodynamic film or boundary layer forming. This is why the concept of pVt (pressure x velocity x time) limits with regard to scuffing performance is postulated and elucidated in this paper. Taking into consideration the multi-parametric approach to the technological creation of the surface layer features, investigations regarding the dependence of scuffing performance on morphological, rheological and physical-chemical properties were performed. In order to better understand the fundamentals of the scuffing process, a series of systematic tribological double-blind trials were carried out with an active lubricant (olefin sulphide) on poorly lubricated cylinder/plane interfaces. The scuffing process was analysed at the final phase under boundary lubricated conditions, specifically for ground and burnished steel cylinders (AISI 4130) and polished planes of cast iron (EN-GJL-300).