Effect of steel hardness on soot wear

Abstract

Due to incomplete combustion, high levels of soot can accumulate in engine lubricants between drain intervals. This soot can promote wear of engine parts such as timing chains and cam followers. One standard approach to reducing wear is to increase the hardness of the rubbing components used. According to the Archard wear equation, wear rate should be broadly inversely proportional to hardness. To explore this approach for controlling soot wear, wear tests have been conducted in a High Frequency Reciprocating Rig (HFRR) with HFRR steel discs of various hardness against a hard steel ball. Carbon black (soot surrogate) dispersions in model lubricants based on solutions of ZDDP and dispersant in GTL base oils have been studied. Wear volumes have been measured and wear scars and tribofilms analysed using scanning white light interferometry and SEM-EDS. It is found that, while most oils show wear that reduces with increasing hardness, for blends that contain both ZDDP and carbon black, wear rate markedly increases with disc hardness as the latter approaches the hardness of the ball. The results support the prevalence of a corrosive-abrasive wear mechanism when carbon black and ZDDP are both present in a lubricant and suggests that selection of very hard surfaces may not be a useful way to control soot

Material‐Dependent Antagonistic Effects between Soot and ZDDP

“…Carbon black may contribute to the wear process by: 1) abrading off the tribofilm as previously reported, [3,9] 2) impairing the tribofilm integrity as shown in Figures 6 and 7, and 3) promoting sulfidation in tribofilm formation. Qin et al showed the increase in sulfur utilization and enhancement of sulfidation upon addition of carbon during the high-temperature metal sulfidation process.…”

“…Such an antagonistic effect was later confirmed between soot extracted from used diesel engine oils and three types of phosphate‐based antiwear additives . In another report, interesting relationships were observed between wear rate and surface hardness in lubrication of an oil containing both CB and ZDDP in lubricating self‐mated AISI 52100 steel: the wear rates for the steel coupons with hardness HV ≤ 300 or ≥800 were significantly higher than those of intermediate hardness (HV = 400–600 HV) . The author attributed the increased wear on the softer steel (≤300 HV) as being due to the abrasion of surface/tribofilm by relatively hard CB particles (≈500 HV) whereas the higher wear rate of the harder steel (≥800 HV) was thought to be due to the increased high contact stress and frictional heating promoting tribochemical reactions.…”

“…However, the actual mechanism is still being sought. Several wear mechanisms have been hypothesized in the literature, e.g., soot abrading tribofilm, soot deactivating antiwear functionality by absorbing them onto surface, soot promoting oil degradation, soot competing with AW additives to adsorb onto metal surfaces, soot transitioning Fe 3 O 4 to more‐wear‐prone FeO, and soot accumulating at the contact inlet to restrict oil supply…”

“…However, the actual mechanism is still being sought. Several wear mechanisms have been hypothesized in the literature, [1] e.g., soot abrading tribofilm, [2,3] soot deactivating antiwear functionality by absorbing them onto surface, soot promoting oil degradation, soot competing with AW additives to adsorb onto metal surfaces, [4] soot transitioning Fe 3 O 4 to more-wear-prone FeO, [5] and soot accumulating at the contact inlet to restrict oil supply. [6] Spikes' research group recently reported antagonistic interaction between zinc dialkyldithiophosphate (ZDDP) [7] and carbon black (CB, as a soot surrogate) for rolling-sliding of a self-mated 52100 steel contact.…”

“…[8] In another report, interesting relationships were observed between wear rate and surface hardness in lubrication of an oil containing both CB and ZDDP in lubricating self-mated AISI 52100 steel: the wear rates for the steel coupons with hardness HV ≤ 300 or ≥800 were significantly higher than those of intermediate hardness (HV = 400-600 HV). [3] 1901956 (2 of 10) www.advmatinterfaces.de tests were conducted using base oils containing CB, ZDDP, and CB+ZDDP to lubricate several different steel alloys. Results suggested that the antagonistic effects between ZDDP and soot strongly depend on the steel alloy composition.…”