Friction and Wear Phenomena of Vegetable Oil–Based Lubricants with Additives at Severe Sliding Wear Conditions

The tribological responses of palm oil and soybean oil, combined with two commercial antiwear additives (zinc dialkyl dithiophosphate and boron compound), were investigated at a lubricant temperature of 100°C and under severe contact conditions in a reciprocating sliding contact. The friction coefficient of palm oil with zinc dialkyl dithiophosphate was closest to the commercial mineral engine oil, with a 2% difference. The soybean oil with zinc dialkyl dithiophosphate produced a 57% improvement in wear resistance compared to its pure oil state. The existence of boron nitride in vegetable oils was only responsive in reduction of wear rather than friction. The response of commercial antiwear additives with vegetable oils showed a potential for the future improvement in the performance of vegetable oils.     

含硼、铜润滑油添加剂的摩擦学性能

四球实验和演示仪实验表明,含有硼,铜等元素的润滑油添加剂具有十分优良的极压和抗磨性能,且在高载荷条件下其抗磨性能尤为突出。该添加剂还能有效地防止油品的氧化,减少摩擦引起的温升。硼、铜复合添加剂的上述性能均优于所评价的市售同类品。     

The antiwear synergism between an organotin compound and a ZnDDP

A four‐ball tester was used to evaluate the antiwear performance of an organotin compound, stannic stearate, plus a zinc dialkyldithiophospate (ZnDDP). The results indicate that stannic stearate combined with ZnDDP exhibits excellent antiwear properties in a paraffin‐based oil. The tin compound could also improve the antiwear performance of two commercial engine oils, SH/CD 15W‐50 and SE 15W‐40, which resulted in a reduction of the oil temperature and less oxidation of the oils.     

The antiwear property of zinc dialkyldithiophosphates in used engine oils

The change in the lubrication properties of vehicle engine oils with driving distance was studied by using a cross-pin type of lubricant tester and a Fourier transform IR spectrometer. The effect of the thermal decomposition of zinc dialkyldithiophosphates (ZnDTPs) on their antiwear property was also examined. The amount of the ZnDTP in engine oil decreased and the amounts of the decomposition products increased with increasing running distance but a good antiwear property was still observed. After 60 h of heating at 135°C, the ZnDTP in sample oils had decomposed into a precipitate and the antiwear property of the oil had deteriorated. With sample oils containing ZnDTP and a detergent or a dispersant, the decomposition products dissolved in the oil and the antiwear property was maintained even after 80 h of heating. The results suggest that the decomposition products of ZnDTPs have a good antiwear property and that the good antiwear property of the used engine oils was retained because of the solubilization of decomposition products into the oil with the aid of a dispersant or a detergent.     

Characterization of tribofilms derived from zinc dialkyl dithiophosphate and serpentine by X-ray absorption spectroscopy

The tribological performance of serpentine in combination with ZDDP as additive for base oil was investigated by a Plint high frequency friction tester at room temperature and 100 °C. The tribofilms formed by serpentine and ZDDP were analyzed using the scanning electron microscopy technique equipped with energy dispersive X-ray spectroscopy. X-ray absorption spectroscopy at phosphorus K- and L_3,2-edges, sulfur K- and L_3,2-edges, silicon K-edge, magnesium K-edge, oxygen K-edge, and zinc L_3,2-edge were recorded to determine the chemistry of the tribofilms. It is found that a combination of serpentine with ZDDP helps reduce the friction of oil blend and exhibits better antiwear properties than base oil.     

Evolution of ZDDP-derived reaction layer morphology with rubbing time

Functional additives, particularly extreme pressure and antiwear additives, in formulated oil will compete to adsorb and function in tribological contacts. A low-polarity commercial base oil, poly-α-olefin (PAO), blended with zinc dialkyl dithiophosphates (ZDDP) has been studied. The tribological performance was evaluated using a ball-on-disk test rig under mixed rolling-sliding conditions in the boundary lubrication regime at 90°C. An adapted in situ interferometry technique was used to monitor the additive-derived reaction layer formation. The thickness of the reaction layer evolves with rubbing until reaching a limiting thickness value of approximately 70 nm. The evolution of the topography and mechanical properties of the ZDDP-derived reaction layer with rubbing time were studied using Atomic Force Microscopy. A constant roughening and hardening of the additive-derived layer with rubbing time is observed and related to the different tribological performance of the layer at different rubbing times.     

Silver-Organic Oil Additive for High-Temperature Applications

Modern lubricants face the task of providing lubrication over a wide range of temperatures, and extreme engine temperatures can exceed the thermal degradation limits of many engine oils. Soft metal additives can extend the life of engine oils at very high temperatures by providing solid lubrication to contacting surfaces. We report a new silver–organic complex which contains a high metal content and minimal supporting organic ligands. This silver pyrazole–pyridine complex is evaluated as a friction-reducing and anti-wear additive in engine oil at testing temperatures which thermally degrade the base oil. Two sets of ball-on-disk tests are performed: the first at a constant temperature of 200 °C and the second while increasing the chamber temperature from 180 to 330 °C. At 200 °C, the wear is considerably reduced compared with the base oil when the silver-organic additive is present at 2.5–5.0 wt%. Furthermore, the silver-based additive at 20 wt% in oil induces a remarkable friction reduction during the temperature ramp test, so much, so that the tribological transition from the oil as the primary lubricant to its degradation, and to the silver additive as the primary lubricant, is imperceptible.     

抗磨添加剂对不同服役阶段润滑油摩擦学性能的影响

采用黏度测试仪测定新油及3种不同服役阶段润滑油的黏度,采用UMT-II摩擦磨损试验机考察其摩擦学性能,并同时考察3种在用润滑油添加抗磨添加剂后的摩擦学性能。研究结果表明:润滑油的黏度随着运行里程数的增加呈现先降后增的趋势;随润滑油运行里程数的增加,润滑油的摩擦因数增大,导致试验钢球的磨损量也增加;抗磨添加剂对不同服役阶段的润滑油的抗磨性能影响程度不同,在磨合磨损期和正常磨损期,加入抗磨添加剂后并不能改善润滑油的抗磨性能,而在异常磨损期,抗磨添加剂的加入可较好地改善润滑油的抗磨性能。     

Reduced Phosphorus Concentration Effects on Tribological Performance of Passenger Car Engine Oils

Phosphorus is present in engine oils in the form of the antiwear and antioxidation additive zinc dialkyldithiophosphate (ZDDP). Its effects on wear and friction were studied at different temperatures using a high-frequency reciprocating rig (HFRR). The electrically insulating tribofilm formation was measured using an electrical contact resistance (ECR) technique. The wear and friction performance of a fully formulated fresh oil containing 0.05 wt% phosphorus was compared with the corresponding used oil drained from a vehicle. The results show that the wear performance of fresh oils having phosphorus concentration from 0.02 to 0.1 wt% is very similar. Further reduction of phosphorus concentration below 0.02 wt% leads to high wear. The coefficient of friction increases with increased phosphorus concentration at temperatures above 80°C but decreases with increased phosphorus concentration at temperatures below 80°C. The used oil and the fresh 0 wt% P oil running on the original fresh steel surface exhibit higher wear than when both oils were evaluated on a previously formed film from a fresh oil containing 0.05 wt% phosphorus.     

Antiwear Tribofilm Formation on Steel Surfaces Lubricated With Gear Oil Containing Borate,Phosphorus, and Sulfur Additives

The formation of antiwear tribofilms plays a critical role in the longevity of automotive gears. The focus of this experimental study was on the lubrication efficacy of gear oils with different contents of borate-, phosphorus-, and sulfur-containing additives leading to the formation of protective tribofilms. Experiments were performed with AISI 52100 steel balls sliding against AISI 52100 steel disks in baths of different oils at ambient (～32 °C) and elevated (～100 °C) temperatures under load and speed conditions favoring sliding in the boundary lubrication regime. Friction coefficient responses accompanied by electrical contact voltage measurements provided real-time information about the formation and durability of the antiwear tribofilms. The wear resistance of the tribochemical films was quantified by wear rate data obtained from surface profilometry measurements of wear tracks on the disk specimens and sliding tests performed at ambient temperatures after the formation of the tribofilms during elevated-temperature sliding. Results indicate a strong dependence of tribofilm formation on temperature and type of additives. The slightly lower friction and higher wear resistance obtained at elevated temperatures with blended oils is attributed to the increased chemical reactivity of additives containing borate, phosphorus, and sulfur, leading to the formation of durable tribofilms. Relatively higher wear resistance and faster tribofilm formation were obtained with the borate-enriched gear oil formulations.     

Industrial Gear Oils: Tribological Performance and Subsurface Changes

This study examined the tribological performance of three gear oils (Oils A, B and C), in relation to surface and microstructural changes. Oil A contains molybdenum dithiophosphate friction modifier, Oil B contains amine molybdate combined with zinc dialkyl dithiophosphate antiwear additive, while Oil C contains phosphonate and a commercial gear oil package. Following sliding tests of a hardened AISI 52100 steel ball on a spheroidized AISI 52100 steel disc, the worn surfaces were chemically studied using Raman and energy-dispersive X-ray spectroscopy. The tribological performance for each oil was different, likewise the nature of the tribofilm formed. After a 5 min sliding test, the hardness-depth profile of the worn surfaces was measured; also the cross-sectional microstructure was examined using scanning electron microscopy combined with focused ion beam preparation and transmission electron backscattered diffraction (t-EBSD) techniques. With Oil A, there was a relatively small increase in surface hardness (33% greater than that of the unworn surface), whereas with Oils B and C, the average hardness near the surface was 100% greater than that of the unworn surface. The cross-sectional microstructure using Oil A also differed from Oils B and C, which were quite similar. The result shows that with Oil A refinement of the ferrite grains spreads deeper into the material (>?10 µm), whilst with Oils B and C it was largely limited to 2–3 µm below the surface. It is concluded that the lubricant formulations and their associated tribofilms influenced the extent of deformation in the subsurface layers and consequently influenced the wear performance.     

钙、镁型发动机油复合剂的抗磨性能

用红外光谱、ICP等离子体发射光谱比较了不同类型配方的SJ 15W／40汽油机油在轿车中使用性能的变化，并与SG15W／40的性能进行了比较。结果表明，各种机油对活塞和活塞环的抗磨性能差别不大；对于SI级机油，钙型配方较镁型配方对气缸壁的磨损较小；镁型配方的SI机油对气缸壁磨损大于钙型配方的SG机油。红外光谱分析表明，在所研究的使用期内，各油品的氧化较轻。     

Characterization of Tribofilms Generated from Serpentine and Commercial Oil Using X-ray Absorption Spectroscopy

The chemical and tribological properties of serpentine particles suspended in lubricating oil were investigated using a pin-on-disk high frequency friction machine at 100 °C. The wear scar width of the upper steel pins was measured by an optical microscope. The tribofilm was characterized by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) elemental mapping, and X-ray absorption near-edge structure (XANES) spectroscopy. It was found that the addition of serpentine to commercial engine oil improves its tribological properties. The SEM and EDX elemental mapping shows that a tribofilm formed by the commercial oil with serpentine contains silicon, magnesium, oxygen, phosphorus, sulfur, zinc, calcium, and carbon on the worn surface, which is different from the tribofilm formed by the commercial oil without serpentine. The results of the XANES analysis show that the addition of serpentine to the commercial oil changes the chemical compositions of the tribofilms. This change may account for the better tribological properties of the lubricating oil containing serpentine. The formation mechanism of the tribofilm is discussed.     

X-Ray Photoelectron Spectroscopy Analysis of Antiwear Tribofilms Produced on Boundary-Lubricated Steel Surfaces from Sulfur- and Phosphorus-Containing Additives and Metal Deactivator Additive

X-ray photoelectron spectroscopy (XPS) was performed on AISI 52100 steel surfaces subjected to sliding in the boundary lubrication regime at 32 and 100°C. The specimens were lubricated with base oil blended with individual additives containing sulfur (S), phosphorus (P), or metal deactivator, as well as base oil with all the previous additives in the same amounts as in the single blends. XPS spectra were analyzed to confirm the formation and determine the chemical composition of the antiwear tribofilms produced on the steel surfaces during sliding. The use of S- and P-containing additives on the tested disk surfaces revealed that tribochemical reactions resulted in the formation of antiwear tribofilms containing S- and P-rich components. Results for the multi-additive blend provided evidence for two components in the produced tribofilm, appearing to consist primarily of sulfide and phosphate. This investigation provides new insight into the competing roles of these compounds on the tribological properties of the antiwear tribofilms. The significance of the sulfide components is demonstrated by the more pronounced antiwear effect of the S-containing additive in the multi-additive formulation.     

Tribological Performance and Mechanism of Phosphate Ionic Liquids as Additives in Three Base Oils for Steel-on-aluminum Contact

Butylammonium dibutylphosphate and tetrabutylammonium dibutylphosphate ionic liquids (ILs) were evaluated as antiwear additives for steel-on-aluminum contact in three different base oils, a polyalphaolefin, an ester oil and an IL 1-methy-3-hexylimidazolium hexafluorophosphate, respectively, with similar viscosity and different polarities. The friction experiments were carried out on an Optimal SRV-IV oscillating reciprocating friction and wear tester at room temperature. Results indicate phosphate ILs can effectively improve the tribological properties of the base oil, especially the antiwear property, as additives for steel/aluminum contacts. For the base oils PAO10 and PAO40 with different viscosities, the higher viscosity of PAO40 can be beneficial to reducing the friction coefficient. The worn surface morphologies and chemical compositions of wear scars were analyzed by a JSM-5600LV scanning electron microscope and PHI-5702 multifunctional X-ray photoelectron spectrometer (XPS). The XPS analysis results illustrate that the phosphate IL additives in the base oils with different polarities exhibit the same tribological mechanism. A synergy exists between the adsorbed layers and boundary-lubricating films generated from the tribochemical reaction of IL and the substrate surface, which may reduce the friction coefficient and wear volume of the friction pairs.     

Synthesis of ploy(p-methoxyphenol) and evaluation of its antioxidation behavior as an antioxidant in several ester oils

Ploy(p-methoxyphenol), denoted as PMOP, was synthesized via an enzymatic polymerization method with p-methoxyphenol as the monomer. The corrosion resistance, tribological properties and thermal stability of as-synthesized PMOP were examined. The antioxidation behavior of as-synthesized PMOP as an antioxidant in ester oils was evaluated by rotary oxygen bomb test and pressurized differential scanning calorimetry. It was found that PMOP exhibits excellent antioxidation ability at 150 °C and 210 °C in various base oils. Besides, PMOP has much better antioxidation ability in synthetic ester oil like di-iso-octyl sebacate than several commonly used commercial hindered phenolic antioxidants.     

Influence of temperature and ZDDP concentration on tribochemistry of surface-capped molybdenum sulfide nanoparticles studied by XANES spectroscopy

The tribological behavior of surface-capped MoS₃ nanoparticles (nano-MoS₃) in hydrocarbon oils was studied in combination with ZDDP at test temperatures in the range of 100–160 °C and at ZDDP content of 0–1.0 wt% in oil. It was demonstrated that this combination of additives demonstrates high antiwear and antifriction efficiency, especially at high temperatures and low ZDDP content. X-ray Absorption Near Edge Structure (XANES) spectroscopy at the sulfur, molybdenum, and phosphorus edges was used to identify the chemical species in the tribochemical films. It was established that the tribofilms formed by combination of ZDDP and nano-MoS₃ contain phosphate-based layers incorporating MoS₂-type fragments. An increase in temperature and ZDDP content results in an increase in tribofilm thickness, while the relative Mo content in tribofilm decreases. Under the tested conditions, the best tribological properties are demonstrated by the composition comprising 500 ppm Mo and 0.1 wt% ZDDP in oil.     

Ionic Liquids as Lubricants of Titanium–Steel Contact

The friction and wear behavior of grade 3 titanium have been studied against AISI 52100 steel at room temperature and at 100 °C, in the presence of six ionic liquid (IL) lubricants, four imidazolium ILs, 1-ethyl-3-methylimidazolium tetrafluoroborate (L102), 1-octyl,-3-methylimidazolium tetrafluoroborate (L108), 1-hexyl, 3-methylimidazolium hexafluorophosphate (L-P106) and 1-benzyl,3-methylimidazolium chloride (ClB), and two quaternary ammonium salts, the chloride derivative AMMOENG™ 101 (AM-101) and the dihydrogenphosphate AMMOENG™ 112 (AM-112), and compared with that of a mineral base oil. At room temperature, all ILs, except L102, give similar mean friction values, below 0.20, with a 60% reduction with respect to the mineral oil. All ILs, except L102, also reduce titanium wear rates. The poor performance of the short alkyl chain tetrafluoroborate L102 is due to tribocorrosion. The best antiwear performance at room temperature is found for the imidazolium chloride (ClB), although corrosion of the AISI 52100 steel ball is observed. At 100 °C, L-P106 maintains the room temperature friction values and shows a 80% wear rate reduction with respect to room temperature. L-108 fails at 100 °C after a sliding distance of 200 m due to decomposition and tribocorrosion. The friction and wear mechanisms and surface interactions are discussed from friction–sliding distance curves, SEM, EDS and XPS analysis, and XRD data.     

Synergistic lubricating effect of several ashless dithiocarbamates with Mo-donor additives

Several kinds of alkylenebis(dialkyldithiocarbamates) were synthesized and the tribological properties as additives in base oil (100N) and Li-soap greases were evaluated using 4-ball tester and SRV tester (Schwingreibverschleissprüfung für Schmierstoffe). The results show that all additives reduce wear. Especially, when Mo(Zn)-donor additives such as Mo-DTC, Zn-DTP, and a vegetable oil modified organomolybdenum complex were blended in base oil containing methylenebisdibutyldithiocarbamate as sulfur donor, synergism was seen both in terms of oxidation stability through thermo gravimetric analysis (TGA) and wear properties using a 4-ball tester. TGA showed that oils containing sulfur-donor and Mo(Zn)-donor additive mixtures possess improved oxidation stabilities. Also, these oils have good antiwear properties under mild pressure conditions. The elemental composition of the antiwear films generated on steel counterfaces were investigated with scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX). It was found that sulfur-donor and Mo-donor additives reacted with the counterface metal and generated a surface protective film consisting of FeS and MoS₂.     

Comparative study of the tribological properties of ionic liquids as additives of the attapulgite and bentone greases

Three kinds of ionic liquids (1‐butyl‐3‐methylimidazolium hexafluorophosphate (L‐P104), 1‐hexyl‐3‐methyl imidazolium hexafluorophosphate (L‐P106) and 1‐octyl‐3‐methylimidazolium tetrafluoroborate (LB108)) were added to the attapulgite base grease and the bentone base grease to investigate and compare the tribological behaviours of the ionic liquids with the two base greases at room temperature and 150°C. Tribological tests were performed using a ball‐on‐plate reciprocating tribometer. The attapulgite base grease showed better wear resistance properties than that of bentone base grease by adding ionic liquids as additives. At same time, the attapulgite base grease showed excellent friction‐reducing and wear resistance properties at high temperature (150°C). Also, we discussed the tribological mechanism of the attapulgite base grease at both room temperature and 150°C from the aspect of the structure of the grease thicker. Copyright © 2012 John Wiley & Sons, Ltd.