Molecular Design of Environmentally Adapted Lubricants: Antiwear Additives Derived from Natural Amino Acids

Novel environmentally adapted lubricant additives were synthesized from cystine (Cys ₂ ), an essential amino acid obtained from natural sources. The structural feature of cystine is a dimeric amino acid with a central disulfide bond. The carboxyl groups in Cys ₂ were converted to corresponding esters by reaction with long-chain alcohols. The resultant diesters were soluble in poly-alpha-olefin (PAO) and ester-type synthetic oils. The structural features of the new additives include multifunctional groups on the same molecule, such as disulfide as a tribologically active moiety and polar functional groups as anchors to friction surfaces. The additives consist of hydrogen, carbon, nitrogen, oxygen, and sulfur; they are free of phosphorus, chlorine, and metals. The tribological properties of the additives in a solution of synthetic oil were evaluated by performing laboratory tribotests under boundary conditions. The Cys ₂ -derived additives exhibited comparable antiwear properties to the conventional additive zinc dialkyldithiophosphate (ZnDTP). The additives showed good antiwear properties even at low concentrations of sulfur (160 ppm) in synthetic hydrocarbons such as PAOs. A high concentration of sulfur (640 ppm) was required to obtain an optimized antiwear performance of the synthetic esters as a base oil. The new additives reduced the friction coefficient of PAOs and synthetic esters. A saturated ester of Cys ₂ reduced the friction of PAOs and synthetic esters up to oil temperatures of 150°C. The lubrication mechanism was discussed with respect to the role of functional groups in the additive molecule. The use of amino acids as versatile building blocks for the synthesis of environmentally adapted additives was also pointed out.     

Atomic Force Microscopy and Raman Spectroscopy Investigation of Additive Interactions Responsible for Anti-Wear Film Formation in a Lubricated Contact

Rational formulation of lubricants requires an understanding of additive interactions that impact antiwear film qualities such as thickness, topography, and friction. In an effort to understand the complex additive interactions responsible for formation of anti-wear and friction-reducing films, atomic force microscopy (AFM) in conjunction with Raman microscopy has been used to conduct a nanoscale investigation of the wear tracks formed by a high-frequency reciprocating rig (HFRR) in the presence of various commercial lubricant additives combinations. Of the additives examined, zinc dithiophosphate (ZnDTP)-based additives are found to be solely responsible for the formation of a thick (hundreds of nm) film that exhibits a pitted topography. Addition of a molybdenum-based friction modifier to the lubricant blend reduces the film thickness considerably and reacts to produce MoS ₂ on the surface, suggesting an interaction with the zinc dithiophosphate–based additive that prevents antiwear film formation. Formation of MoS ₂ , found only in the wear track, is consistent with a dramatic reduction of friction coefficient measured in the HFRR. Subsequent addition of borated dispersants to the lubricant reveals a further reduction in friction coefficient and a modest return of anti-wear film. However, addition of detergents to the formulation increases the friction coefficient and also promotes the formation of an anti-wear film. Nanoindentation measurements on the bulk properties of the anti-wear films determined that all of the anti-wear films had similar modulus and hardness measurements which were lower than that of the parent steel material, but did not correlate with the friction measurements obtained from the HFRR. This indicates that nanoscale measurements on material properties of the film are necessary to elucidate friction properties of the interface, and that these properties cannot be determined from macroscale measurements on the bulk film.

     

The friction and wear behavior of 2-(n-alkyldithio)-benzimidazole as additives in liquid paraffin

Three 2-(n-alkyldithio)-benzimidazoles were synthesized. The friction and wear behavior of the synthetic compounds as additives in liquid paraffin were examined with a four-ball machine, with emphasis on revealing the relationship between the chain length of the additive and the friction-reducing ability and lubricating mechanism of the additives. The film formed by the additive during the sliding process was investigated by using X-ray photoelectron spectroscopy. It was found that the synthetic compounds as additives in liquid paraffin had good antiwear performance. The longer the chain length of the synthetic compound, the more stable or less volatile is the compound and the more effective it is in improving wear resistance. This revised version was published online in July 2006 with corrections to the Cover Date.     

Interaction of anti-wear additives with friction surfaces

The effect of esters of phosphonic acids on frictional wear under diverse loads and sliding speeds has been investigated. The efficiency of such phosphonates as anti-wear additives depends on their structure and varies with the frictional modes. Principal groups of phosphonates have been grouped as esters which are ineffective under both high and low loads (esters with no chlorine atoms); esters which are effective under both high and low loads (CCl₃-group containing esters), and esters which are effective under either high or low loads. The interaction of phosphonates with metal (steel) has been studied. Dissimilarity in the efficiency of esters as lubricating oil additives is explained by their different adsorption and chemical actions.     

纳米碳球作为合成酯抗磨剂的摩擦学性能研究

采用表面化学改性的方法得到硬脂酸修饰的纳米碳球,在四球摩擦试验机上考察纳米碳球在合成酯类润滑油中的抗摩擦性能,探讨纳米碳球的抗磨与润滑机制。结果表明,通过表面改性,显著提高纳米碳球在润滑油中的分散稳定性;纳米碳球作为合成酯润滑油抗磨剂表现出优良的抗磨减摩性能;在转速1.200 r/min,载荷150 N的实验条件下,质量分数0.07%的改性纳米碳球可使三羟甲基丙烷混酸酯平均摩擦因数减小幅度达到30%左右,磨损率降低33%,质量分数0.15%的改性纳米碳球可使季戊四醇油酸酯的平均摩擦因数降低幅度达到50%以上,磨损率降低14.6%     

Effect of novel phosphazene-type additives on the tribological properties of Z-DOL in a steel-on-steel contact

The effect of hexakis(1,1,5-trihydroperfluoropentoxy)cyclotriphosphazene (X-100) and a novel synthetic bridged alkoxycyclotriphosphazene (denoted LA-2P) as additives in Z-DOL, a perfluoropolyether-type lubricant, on the friction and wear behavior of a steel-on-steel system was investigated. Thus, the friction and wear test of a steel disc sliding against the same steel counterpart ball was carried out on an SRV oscillating friction and wear tester. The chemical features of the worn steel surfaces were analyzed by means of X-ray photoelectron spectroscopy. The results indicate that both additives are effective in improving the tribological performance of Z-DOL and in preventing its catalytic decomposition. Z-DOL/LA-2P shows the best anti-wear ability. Moreover, both additives also increase the thermal stability of Z-DOL in the presence of AlCl₃ as a catalyst. In particular, the Z-DOL/LA-2P mixture shows the highest complete thermal degradation temperature.     

Tribological Characteristics of Combined Layered Phosphate and Silicate Additives in Mineral Oil

A mixture containing α-Zr(HPO₄)₂·H₂O, a small particle layered-phosphate, and α-Na₂Si₂O₅, a microparticle-layered silicate, is investigated for its potential as an additive in mineral oil. A four-ball tester is employed to measure the maximum non-seizure load, wear scar diameter, and friction coefficient of the combined additives. Friction and wear tests indicate that synergistic behavior exists between the two base materials; the load capacity, anti-wear, and friction reducing ability of the base oil are markedly increased when the combined materials are added. More important is the combined materials exhibit considerable advantage over typical lubricant additives, such as MoS₂ and graphite, presenting good application prospects as lubricant additives. The X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy results illustrate that the synergistic behavior of the combined material stems from the small α-ZrP particles confined in the valleys of the contacting surfaces that polish the wear surface, as well as the layered α-Na₂Si₂O₅ microparticles that effectively fill voids in the wear surface.     

Correlation Studies of WS 2 Fullerene-Like Nanoparticles Enhanced Tribofilms: A Scanning Electron Microscopy Analysis

The beneficiary effects of tungsten disulphide (WS₂) inorganic fullerene-like nanoparticles (IFLNPs) in the lubrication industry were shown in recent years. However, their successful incorporation into lubricants (oils, greases) is not straightforward. In practice, the lubricant contains several components for different purposes, e.g. reducing the oxidization of the oil (antioxidant), minimizing the wear rate (anti-wear additive), dispersants, etc. These additives can contain chemically active compounds, which under the lubrication process (where locally extreme conditions can develop: high pressure and flash temperatures) can change the chemistry in the contact zone and block the beneficial effects of the inorganic nanoparticles. In this investigation, poly-alpha-olefin (PAO) is being used as base oil in which the WS₂ nanoparticles and different additives are mixed. A ball-on-disc sliding test revealed that certain additives inhibit the nanoparticles to reduce friction (less than 5 % decrease in friction coefficient), while in other cases, the friction reduction was above 50 %. The comparison is being made between PAO + additive and PAO + additive + IFLNPs. Scanning electron microscope and energy dispersive X-ray spectroscopy were used to investigate the elemental composition of the tribofilms formed on the wear marks. Further analysis was made in order to reveal correlations between elemental compositions of the tribofilms and external parameters such as the friction coefficient and wear rate. For instance, a strong correlation between tungsten content of the tribofilm and the friction coefficient was found.     

水溶性磺酸盐在难燃液压液中的性能研究

为提高环境友好型润滑油极压抗磨添加剂三嗪衍生物的水溶性,将水溶性和防锈性良好的官能团磺酸盐引入三嗪衍生物,合成2种含磺酸盐的三嗪衍生物STB和STC,并评价其在水-乙二醇难燃液压液(HFC)中的理化性能和摩擦学性能。结果表明:STB和STC水基润滑添加剂在HFC中能够起到很好的极压抗磨减摩作用,其中质量分数2%STB和1%STC可使HFC的极压值提高到696 N,相比基础油液的极压值提高了109%。SEM和XPS分析结果表明,添加剂在摩擦过程中发生了化学反应,生成了无机硫化物和有机氮等组成的复杂边界润滑膜,从而起到了良好的抗磨减摩作用。     

Effects of anti-wear additives on friction and wear properties of Cr2O3 coating

The effects of some anti-wear additives on the friction and wear behaviour of plasma-sprayed Cr₂O₃ coating were investigated using a block-on-ring tester at ambient conditions. The results show that zinc dialkyldithiophosphate (ZDDP), tricesyl phosphate (TCP) and tributyl phosphate (TBP) significantly reduce the wear of Cr₂O₃ coating lubricated by paraffin oil. Additive concentrations as well as sliding time have great influence on the wear. The friction coefficient varies slightly with test conditions. The analysis by XPS of worn surfaces indicates that the wear resistance of these additives is due to the formation of tribochemical reaction films by reacting with Cr₂O₃ coatings.