XANES Study of Tribofilm Formation With Low Phosphorus Additive Mixtures of Phosphonium Ionic Liquid and Borate Ester

Abstract

The development of low phosphorus engine oils is important to minimize phosphorus-induced exhaust catalyst poisoning and resulting in harmful emissions. In this study, low phosphorus oil formulations were prepared by using an ashless additive mixture of borate ester (SB) with ionic liquid composed of a phosphonium cation and phosphate anion (P_DEHP) at 350 and 700 ppm phosphorus. Tribological properties of this binary additive system were evaluated using a reciprocating cylinder on a flat test configuration. Favorable interaction between P_DEHP and SB resulted in a significant reduction in friction coefficient and wear volume, in particular for P_DEHP(700P) + SB oil blend. Time-scale analysis of tribofilm formation was determined by running the tribological experiments for 5, 15, and 60 min duration. Electrical contact resistance (ECR) results revealed that the addition of P_DEHP at 350 ppm of phosphorus to SB at 500 ppm of boron can reduce the incubation time from 300 to 100 s for stable tribofilm formation. X-ray absorption near-edge spectroscopy (XANES) analysis of tribofilms indicates that the tribofilm mechanism for additive mixtures of P_DEHP and SB initially involves the formation of boron oxide-based films, which later interact with phosphorus to form boron phosphates in addition to iron phosphates. Incorporation of the high amount of boron phosphates in addition to boron oxide/acid and iron phosphates in the tribofilms contributed to the improved tribological performance of P_DEHP(700P) + SB oil. XANES results reveal that tribofilms formed due to the interaction of SB and P_DEHP evolve to a cross-linked structure, wherein the chain length of polyphosphates is increased with the increase in rubbing time.

Current Knowledge on Friction, Lubrication, and Wear of Ethanol-Fuelled Engines—A Review

“…Similar studies aiming to develop extreme pressure (EP), antiwear (AW), and FM additives that are empowered by fuel contamination and work well in lubricants formulated with higher levels of detergents and dispersants are of the utmost importance to increase engine reliability and efficiency. For example, because products of the antiwear additive (AW) ZDDP compromise the efficiency of the catalytic converter in the exhaust system, environmental pressures have led to a rush for alternative AW additives that produce lower levels of sulphur and phosphorous emissions [63]. The interaction of alternative AW additives and ethanol should be urgently investigated in order to determine whether they can better protect surfaces in ethanol-fuelled engines while reducing damage to the exhaust system.…”

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

“…As a result, molybdenum dialkyl dithiocarbamate (MoDTC) was introduced as another promising additive that can form a low-friction tribofilm [ 4 , 13 ] possessing a lattice structure consisting of MoS 2 nanosheets, although it has been reported that MoS 2 tribofilms are much easier to remove than ZDDP tribofilms [ 18 ]. Effective low-friction/antiwear tribofilms have also been produced from phosphorous- and sulfur-containing ionic liquids [ 4 , 19 , 20 , 21 ] and ceramic nanocrystals [ 22 ], through the synergy of transition metal oxide nanoparticles and conventional sulfur-containing oil additives [ 23 ], borate-modified blends [ 24 ], and borate-, phosphorous-, and sulfur-containing white oil and gear oil [ 25 , 26 , 27 ]. The good stiffness and strength, easy-to-shear ability, and potential to form antiwear tribofilms of two-dimensional transition metal carbides, nitrides, and carbonitrides, collectively known as MXenes, have also made them attractive additive candidates for tribological applications [ 28 , 29 , 30 ].…”