A model of the dynamics of boundary film formation

Abstract

The dynamics of formation and loss of the boundary films formed during sliding on steel surfaces were investigated over a range of temperature. Tests are performed on a cylinder-on-disk machine using mineral oil with various concentrations of zinc dial~ldithiophosphate (ZDP). The thickness and refractive index of the boundary films during step load test were monitored in situ with an ellipsometer, and the composition of the films was analyzed by X-ray photoelectron spectroscopy (XPS). As temperature increases, chemical reactivity increases the film formation rate, while the film removal rate increases owing to (a) the decrease of durability of the boundary film material and (b) the reduction of hydrodynamic fluid film thickness due to decreasing viscosity of the lubricant. There is a balance between these two competing mechanisms, and this balance is reflected in the bounda~ film thickness. The boundary films consist of a film of oxide and metallic compound (OMM) covered by an organo-iron compound (OIC). Their relative effectiveness in preventing scuffing depends on temperature and composition. In particular, the OIC is effective in reducing wear of the opposing surfaces by covering the OMM.

A multi-technique characterization of ZDDP antiwear films formed on Al (Si) alloy (A383) under various conditions

“…Our results provide evidence that un-reacted ZDDP (which aligns with peak 2 (figure 1)) decreases significantly when the concentration of ZDDP is decreased from 1.2 to 0.3%. Current U.S. environmental laws regulate maximum P content in passenger vehicles to 0.1%, resulting in a ZDDP concentration of $1% [18], which has also been found to be close to the optimum ZDDP concentration range for the AW regime [55]. Chao et al [55] has suggested that increasing ZDDP concentrations promote thicker films, which are also formed more rapidly.…”

“…Current U.S. environmental laws regulate maximum P content in passenger vehicles to 0.1%, resulting in a ZDDP concentration of $1% [18], which has also been found to be close to the optimum ZDDP concentration range for the AW regime [55]. Chao et al [55] has suggested that increasing ZDDP concentrations promote thicker films, which are also formed more rapidly. In addition, he suggested that lower concentrations of ZDDP will lead to significantly less adsorption on the surface, resulting in the formation of shorter chain polyphosphates.…”

A review of scuffing models

“…For example, the effect of various concentrations of ZnDDP in mineral oil using a cylinder-on-disk machine operating at 0.06 m/s was studied by Choa et al [62]. It was found that the removal of protective surface films is thought to depend on decreasing film durability rather than reduction of the hydrodynamic fluid film thickness caused by decreasing lubricant viscosity.…”

Model of scuffing based on the vulnerability of an elastohydrodynamic oil film to chemical degradation catalyzed by the contacting surfaces

“…This heat transfer results in preheating of the lubricant and aggravating its localized decomposition within the contact. Progressive changes in friction coefficient during scuffing have also been observed [29]. This seems to be further evidence of a period which lasts several revolutions of rotation during which nascent surface spreads over the contacting surfaces.…”

“…Thus in a high speed contact, rotating at many revolutions per second, a high areal concentration of nascent surface may therefore rapidly develop based on a lifetime of each portion of the nascent surface of several revolutions. On the other hand in slower speed laboratory scuffing tests, a distinct progression from low friction to high friction characteristic of scuffing has been observed [29]. A low speed scuffing model has been proposed where a backward flow of heat from the contact centre to the film inlet is significant and affects scuffing [30].…”