Dynamics of Tribofilm Formation in Boundary Lubrication Investigated Using In Situ Measurements of the Friction Force and Contact Voltage

Abstract

The complex dynamics of tribofilm formation on boundary-lubricated steel surfaces were investigated in real time by combining in situ measurements of the temporal variation of the coefficient of friction and contact voltage. Sliding experiments were performed with various blends consisting of base oil, zinc dialkyl dithiophosphate (ZDDP) additive, and two different dispersants at an elevated oil temperature for a wide range of normal load and fixed sliding speed. The evolution of the transient and steady-state coefficient of friction, contact voltage, and critical sliding distance (time) for stable tribofilm formation were used to evaluate the tribological performance of the tribofilms. The blend composition affected the load dependence of the critical sliding distance for stable tribofilm formation. Tribofilm friction was influenced by competing effects between the additive and the dispersants. Among various formulations examined, the tribofilm with the best friction characteristics was found to be the blend consisting of base oil, a small amount of ZDDP, and a bis-succinimide dispersant treated with ethylene carbonate. The results of this study demonstrate the effectiveness of the present experimental approach to track the formation and removal of protective tribofilms under boundary lubrication conditions in real time.

Wear Mechanisms, Composition and Thickness of Antiwear Tribofilms Formed from Multi-Component Lubricants

“…The ball-on-disk tribometer (Falex-6, thrust washer, Bruker Co., Billerica, MA, USA) used in a previous study of the friction characteristics of tribofilms formed from similar blends [38] was also used to perform the wear experiments of this study. Both the ball and disk specimens consisted of AISI 52100 steel (1.04 C, 0.35 Mn, 0.275 Si, 1.45 Cr, and 96.89 Fe, all wt%).…”

“…The wear experiments were carried out under conditions of different loads in the range of 1.22-10.15 kg (i.e., 11.97 to 99.57 N), or mean Hertzian contact pressures in the range of 0.785-1.589 GPa (computed for 193 GPa elastic modulus, 0.3 Poisson's ratio, and 8 cm ball diameter), oil temperature of 100 ± 5 • C, linear sliding velocity fixed at 0.19 m/s, and test time set at 2 h. Calculations confirmed that the foregoing test conditions resulted in sliding in the boundary lubrication regime. Further details about the properties, polishing, and cleaning of the specimens can be found elsewhere [38].…”

“…Disk specimen selection was based on the lowest and highest load that, for a given blend, yielded a stable, nonzero contact voltage. Since the highest load was close to the critical load, more than one disks were selected for each blend due to the bi-modal contact voltage response (i.e., zero and nonzero contact voltage responses [38]) obtained when sliding occurred under the critical load. A total of 19 disks corresponding to 35 combinations of blend and load were used in the XPS studies.…”

“…Dispersants are used to sustain the stability and cleanness of oil blends by inhibiting the agglomeration of wear debris [34], or nanoparticles used as lubricant additives [35]. For instance, different chemical interactions and tribological characteristics were found when detergents and dispersants were incorporated in a base oil that contained ZDDP [24,[36][37][38]. Succinimides represent a common class of dispersants, whether simultaneously used with additives or alone in fully formulated lubricants [24,34,35,39].…”

“…However, while incorporating a succinimide dispersant in base oil can effectively disperse the wear debris, it cannot provide significant wear protection in the absence of an antiwear additive [34]. Chemical interactions between ZDDP and different nitrogenous dispersants have been reported to produce different friction behaviors, illuminating varying levels of competitive effects of these additives on friction characteristics [38]. Indisputably, understanding these antagonistic effects between additives and dispersants is critical for developing lubricant formulations that provide a balance between tribofilm antiwear properties and lubricant stability and cleanness.…”