Control of local friction of metal/ceramic contacts in aqueous solutions with an electrochemical method

Ball-on-plate sliding friction experiments were designed and performed to show the possibility of local friction control by electrochemical methods. By partitioning a metal plate into charged and uncharged zones, the friction coefficient in these zones can be differential when an external voltage is applied during rubbing. It is also possible to achieve differential friction at different locations of contact by arranging the position of the auxiliary electrode on which the range of electrochemical effects on friction depends. The morphological differences in the worn surfaces between the different zones are given, together with a discussion on the principles of the electrochemical control of friction.     

Tribochemistry of ZDDP in molecular orbital calculations

The molecular orbital parameters of zinc dialkyldithiophosphate (ZDDP) and several metal-atom-cluster models were calculated. The nature and the strength of the interactions between the ZDDP molecules and different metal surfaces are analysed and discussed with the use of frontier orbital theory. By comparing the highest occupied molecular orbital energy (E_HOMO) and the lowest unoccupied molecular orbital energy (E_LUMO) of the ZDDP and the atoms cluster models of Al₆, Cu₆, and Fe₅, it is concluded that ZDDP behaves as an excellent boundary lubricant additive at the interface with iron. The derived molecular orbital parameters illustrate the advantages for tribochemistry studies.     

Tribological process induced conformational transformation of protein may change the friction of cartilage

In-vitro testing procedures have been successfully developed to investigate the effects of tribological process induced transformation of protein-based lubricant on the friction change of articular cartilages. Serum and albumin solutions were the biological lubricants used in this study. The results indicated that the lubricating ability for cartilages deteriorates after the biological lubricants were articulated between polyethylene and stainless steel materials. In addition, the secondary structure change of the albumin molecule has been characterized after the molecules were articulated by the artificial joint materials. We have provided evidence that the conformational change of protein lubricants leads to the friction increase of articular cartilage.     

Relations of counterface materials with stability of tribo-oxide layer and wear behavior of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy

Dry sliding wear tests of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy (TC11 alloy) sliding against AISI 52100 and AISI M2 steels were performed under the load of 50–250 N at 25–600 °C. For two kinds of counterface materials, the titanium alloy presented totally different wear behaviours as the function of temperature. The appreciable variations of the titanium alloy sliding against different counterface materials were attributed the fact that a hard counterface caused unstable existence of tribo-layers by its microcutting action, thus resulting in the increase of wear rate. It is suggested that the hard counterface must be avoided as the counterface for the titanium alloy/steel sliding system.     

Effects of sliding velocity on tribo-oxides and wear behavior of Ti–6Al–4V alloy

Dry sliding wear tests were performed for Ti–6Al–4V alloy on a pin-on-disc wear tester. The wear behavior of Ti–6Al–4V alloy at sliding velocities of 0.5–4 m/s was studied and the tribo-oxides and their function were explored. Ti–6Al–4V alloy presented a marked variation of wear rate as a function of velocity. With the rise and fall of wear rate, Ti–6Al–4V alloy underwent the transitions of wear mechanisms from the combination of delamination wear and oxidative wear at lower speeds to delamination wear at 2.68 m/s, and then to oxidative wear at 4 m/s. These phenomena were attributed to the appearance and disappearance of tribo-oxides. In spite of trace or a small amount, tribo-oxides would change the wear behavior, and even wear mechanism.     

A review of recent developments of friction modifiers for liquid lubricants (2007–present)

Due to the increasing demand of low emission and fuel economy, friction modifiers have been widely used in lubricating compositions to adjust friction and wear properties of lubricants. Recent achievements in the application of friction modifiers for liquid lubricants (2007–present) are reviewed in this paper. There are three types of friction modifiers for liquid lubricants: organomolybdenum compounds, organic friction modifiers, as well as nanoparticles. The tribological properties and lubrication mechanisms of these friction modifiers are discussed. The problems and some suggestions for the future directions of research on friction modifiers are proposed.     

Relations between oxidative wear and Cr content of steels

Dry sliding wear tests at 25-400 °C were performed for 45, 4Cr5MoSiV1 and 3Cr13 steels; the relations between oxidative wear and Cr content of steels were explored. The low and medium-Cr steels had a substantially lower wear rate and increasing tendency than the high-Cr steel at 25-200 °C, but the contrary case occurred at 400 °C. With an increase of ambient temperature, the wear rate of the low and medium-Cr steels first decreased, then increased and reached the lowest value at 200 °C, while the wear rate of the high-Cr steel decreased monotonously with the lowest value at 400 °C. At 25 °C, trace tribo-oxides reduced wear to some extent in adhesive-dominated wear for the low and medium-Cr steels. At 200 °C, a small amount of tribo-oxides formed and reached a thickness of 10 μm on contacting asperities in the low and medium-Cr steels, thus oxidative mild wear prevailed. At 400 °C, a great amount of tribo-oxides appeared in the low and medium-Cr steels; unexpectedly, the high-Cr steel had more tribo-oxides than the low or medium-Cr steels in some cases. Its high wear resistance may be attributed to Cr-strengthened adhesion power of tribo-oxides and matrix.     

润滑剂对Ti（CN）陶瓷摩擦学性能的影响

在销—盘试验机上考察了干摩擦、水润滑及油润滑条件下Ti(CN)/45钢摩擦副的摩擦磨损性能。Ti(CN)陶瓷的磨损主要由粘着剥落和微断裂引起,水对Ti(CN)/45钢摩擦副的摩擦性能无明显改善,但能较明显地减小陶瓷的磨损。油润滑时,摩擦和磨损均得到了明显改善。水和油润滑介质的存在能有效地抑制金属在陶瓷表面的粘着转移,从而降低陶瓷磨损率。采用SEM,XPS,AES等对陶瓷磨痕的分析结果表明,摩擦面上Fe_2O_3的生成对粘着磨损起到了一定的改善作用。油(不含添加剂的液体石蜡)在极压条件下的减磨作用主要是由于其在陶瓷摩擦面上形成了较厚的碳膜(焦质,石墨复合膜)。     

Load-carrying synergism of binary additive systems: dibenzyl disulphide and halogenated hydrocarbons

In this study, the tribological performance of binary additive blends with dibenzyl disulphide and some chlorinated and brominated compounds was investigated. The tribological tests were done on a four-ball and ball-on-disc machines. For the test conditions explored, chlorinated paraffin (40% Cl), benzyl chloride, hexachloroethane, benzyl bromide and two brominated paraffins which contained 29 and 49 mass% of bromine in the presence of dibenzyl disulphide produced strong synergism with regard to their load-carrying capacities. Load-carring synergism increased with the increase of both additive concentrations. Some chlorinated and brominated hydrocarbons produced almost no synergism. Chlorinated hydrocarbons with chlorine atoms on adjacent carbon atoms and when between carbon atoms bonded with chlorine there are one or two no chlorinated carbon atoms did not exhibit antiwear and extreme pressure synergism. Increased chlorine substitution in ethane increased synergism. Amounts of iron sulphide, benzyl thiol and elemental sulphur formed from dibenzyl disulphide as well as sulphur surface concentrations for brominated compounds were strongly reduced in the case of binary additive systems' strongly improved load-carrying properties. The bromine surface concentration after wear test is very low, and independent of additive concentration. A possible explanation for the load-carrying synergism of DBDS and halogenated hydrocarbons binary additive systems is proposed.     

极压抗磨剂的发展现状及作用机理研究

简要概括了润滑油极压抗磨剂的研究动态,对硫型、氯型、磷型、钼型、有机金属型、硼型、稀土化合物型和纳米粒子型等极压抗磨剂的发展现状和磨损机理进行了概述总结.指出高效能、多功能、绿色环保以及各种添加剂的复配使用,是目前和今后摩擦化学领域的主要研究方向.