In Situ Observation of Wear Process Before and During Scuffing in Sliding Contact

Abstract

In this study, a direct observation of a point contact area was conducted to understand the scuffing phenomenon. The contact area was produced between a rotating sapphire disc and a stationary steel ball and it was lubricated using n-hexadecane. The image detected by a colour digital CCD camera, load, and friction were synchronously recorded by a computer during the test. It was found that wear debris produced in the contact area played an important role in the wear process, which includes running-in and scuffing. In the test, debris particles accumulated in the inlet zone, and some particles entered the contact area to cause abrasive wear of the ball surface, even in the light-load stages. During the running-in process, the abrasive wear by debris particles changed the conformity between the sliding surfaces. In the high-load stage, just before the occurrence of scuffing, the whole contact area was flattened at once. When scuffing occurred, the contact area suddenly expanded. The conformity of the contact area was dramatically changed during its expansion. The flattening of the whole contact area and dramatic expansion with changing the conformity seemed to play important roles in scuffing.

Role of Wear Particles in Scuffing Initiation

“…Enthoven et al further proposed that scuffing is caused by a critical rate of production of wear particles and its accumulation in the inlet of the lubricated contact. Our previous study on in-situ observation of scuffing process of steel [5] also indicated that wear particles produced in the contact area plays an important role in the scuffing process. Nikas's numerical simulation [6,7] showed that the presence of soft and ductile particles in lubricated contacts can cause surface softening/melting and local scuffing.…”

“…Each frame of the contact images was precisely synchronized with friction coefficient data. Instrumentation of this tribometer is described in [5].…”

“…Figure 1 shows that the friction coefficient increases suddenly with great fluctuations from around 8.5 s while before this moment the friction coefficient increases slowly with low fluctuations. [5] has observed that scuffing occurs as severe plastic flow in the lubricated sliding. Figures 1 and 2 show that scuffing also occurs by severe plastic flow in the dry sliding scuffing.…”

Boundary and Extreme Pressure Lubrication

“…Sand contamination of lubricated gear wheels not only causes accelerated wear by abrasion but also a temperature rise in the lubricant due to elevated friction, which is associated with increased roughness and more extensive metallic contact between the gear teeth [142]. Wear particles generated in lubricated sliding contacts also play an important role in controlling the onset of scuffing [141]. The critical temperature appears therefore to be a function of many parameters not just pressure, heat of adsorption and sliding speed.…”

In Situ Observation of Heat Generation Behaviour on Steel Surface During Scuffing Process

“…The authors developed in situ observation systems for the contact area comprising a rotating sapphire disc and a stationary steel pin. Yagi et al [36] directly observed the contact area, reporting local plastic flow on the steel surface during scuffing. Li et al [37] indicated that plastic flow firstly occurred at the point where the wear particles accumulated.…”