Recent progress in microtribology

Sliders having very small mass (less than 10 mg) are beginning to be used in magnetic recording devices and very light contacting loads (less than 1 μN) are expected to be used in ultrahigh-density recording devices using the point-recording technique. These newer devices will require wear rates that are virtually zero. The wear on such sliding surfaces is primarily due to the surface interaction forces rather than the load. The ultimate goal of microtribology is to create practical zero-wear devices with very small mass and very light load. Computer simulation of molecular dynamics is used to trace the movement of each atom in sliding surfaces and of each molecule in lubricant films, New tools, such as scanning probe microscopes, are being used to evaluate sliding surfaces and lubricant films. Experimental studies of microwear processes on solid surfaces and of lubrication using very thin lubricant films have recently begun. Microtribology is an important technology for development of new microdevices, and also an important science for understanding the origin of friction and wear. Close cooperation between scientists and engineers is necessary. However, as we have very little of the knowledge needed for microtribology, we must obtain much more data.     

Controlled microtribology of a metal oxide surface

The rational control of the friction and wear (damage) of engineering, as opposed to model, surfaces under practical conditions such as high contact pressures has long been a technological challenge with much fundamental interest. Lubricant fluids and physisorbed surfactant monolayers (boundary lubricants) are effective friction modifiers but often fail at high loads. We show that the chemisorption of a suitably designed single-chained phosphonate surfactant onto crystalline α-alumina surfaces produces robust protective monolayers that significantly reduce the friction forces and wear even at high loads. The mechanisms are explained, which point to some general principles that offer a basis for scale-up in many different engineering systems. This revised version was published online in June 2006 with corrections to the Cover Date.     

The Effect of Different Temperatures on Friction and Wear Properties of CFRPEEK against AISI 431 Steel under Water Lubrication

The tribological behavior of 30 vol% carbon fiber–reinforced polyetheretherketone (CFRPEEK) against AISI 431 steel under different temperatures of water lubrication was investigated. Friction and wear tests were carried out on a disc-on-disc contact test apparatus under different operating conditions. The results reveal that the lubricant temperature has a significant effect on the friction and wear properties of CFRPEEK sliding against AISI 431 steel. The average friction coefficient and wear rate of CFRPEEK increase with increasing lubricant temperature. However, the wear rate of AISI 431 steel did not have a positive correlation with the wear rate of CFRPEEK under different temperatures of water lubrication. Moreover, the original and worn surfaces of CFRPEEK and AISI 431 steel were imaged by environmental scanning electron microscopy and optical microscopy, respectively. The main tribological mechanisms of CFRPEEK sliding against AISI 431 steel were adhesive wear, and increasing the temperature of the lubricant could accelerate wear.     

人工关节润滑系统研究进展

从人工关节润滑机理的理论计算、人工滑液的试验研究及仿生润滑系统设计3个方面,对人工关节润滑系统的研究现状进行了综合评述。指出今后应加强对润滑液的主要组分润滑协同效应、人工润滑剂自身性能和润滑机制、仿生关节润滑系统的研究。     

苦参碱-6、左旋肉碱-柠檬酸离子液体的制备及润滑特性研究

制备苦参碱-6和左旋肉碱-柠檬酸2种离子液体,分析其结构和热稳定性。研究Si_3N_4球与316不锈钢、Al、Cu、Ti 4种金属基体对摩时,2种离子液体作为润滑剂的润滑性能,并与美孚一号润滑油的润滑性能进行比较。结果表明:2种离子液体在不同基体上具有不同的润滑效果,其中左旋肉碱-柠檬酸具有较低的摩擦因数和较高的热稳定性,其润滑下基体表面磨痕较均匀光整且磨损体积小;左旋肉碱-柠檬酸润滑效果较美孚一号润滑油更好,这是因为其在摩擦过程中形成了有效的物理吸附保护膜和摩擦化学反应膜,并生成了更加复杂的含铁化合物润滑涂层,二者共同作用提高了润滑效果。     

Lubrication in steel strip rolling in Japan

Research into, and the state of technology for, lubrication in steel strip rolling in Japan are reviewed. Both cold and hot strip rolling are discussed. Subjects covered include coefficient of friction and oil film thickness, friction pick-up, and roll wear.     

The Difference in Running-In Period and Friction Coefficient Between Self-Mated Si3N4 and SiC Under Water Lubrication

Running-in periods and friction coefficients of SiC and Si₃N₄ sliding against themselves under water lubrication were investigated with a pin-on-disk apparatus at sliding speed of 120 mm/s and a normal load of 5 N under ambient conditions. It was found that the running-in period of self-mated Si₃N₄ is much shorter than that of self-mated SiC, and also that the steady-state friction coefficient of self-mated Si₃N₄ was lower (0.0035) than that of self-mated SiC (0.01). The difference in mechanism was analyzed from the point of view of electronic structure and surface chemistry.     

Enhanced Mechanical and Tribological Properties of Graphene-Reinforced TiAl Matrix Composites

This study investigated the mechanical properties and dry-sliding friction and wear behaviors of graphene-reinforced TiAl matrix composites in expectation of providing valuable information for the application of graphene. The results suggested that the incorporation of graphene apparently improved the microhardness, fracture toughness, and tribological properties of the composites. For the composite with 3?wt% graphene, the microhardness increased by 129%, the fracture toughness increased by 149%, the friction coefficient decreased by 37% and the wear rate decreased by 78%. Also, the microstructural analyses of the worn surfaces indicated that three types of graphene-rich films, with different percentages of coverage, were generated on the worn surfaces under various wear conditions. An evolution mechanism of the films as a function of wear conditions was proposed, and the corresponding variation of friction and wear behavior was also discussed.     

Tribochemical effects on the friction and wear behaviors of diamond-like carbon film under high relative humidity condition

Friction and wear behaviors of diamond-like carbon (DLC) film in humid N₂ (RH-100%) sliding against different counterpart ball (Si₃N₄ ball, Al₂O₃ ball and steel ball) were investigated. It was found that the friction and wear behaviors of DLC film were dependent on the friction-induced tribochemical interactions in the presence of the DLC film, water molecules and counterpart balls. When sliding against Si₃N₄ ball, a tribochemical film that mainly consisted of silica gel was formed on the worn surface due to the oxidation and hydrolysis of the Si₃N₄ ball, and resulted in the lowest friction coefficient and wear rate of the DLC film. The degradation of the DLC film catalyzed by Al₂O₃ ball caused the highest wear rate of DLC film when sliding against Al₂O₃ ball, while the tribochemical reactions between DLC film and steel ball led to the highest friction coefficient when sliding against steel ball.     

不同碳原子数及羟基数醇中Ti3SiC2/Si3N4摩擦副的摩擦学性能

Ti_3SiC_2和Si_3N_4等陶瓷材料是一种潜在的生物燃料发动机及功能性运动部件材料,为研究其在醇类生物燃料及润滑剂中的摩擦学性能,利用往复式摩擦试验机研究Ti_3SiC_2/Si_3N_4摩擦副在不同碳原子数直链醇(乙醇、丁醇、辛醇和十二醇)和不同羟基数醇(乙二醇和丙三醇)液体介质中的摩擦学性能。结果表明:Ti_3SiC_2/Si_3N_4在醇介质中的摩擦因数随碳原子数和羟基数的增加而减小,总体平均摩擦因数在0.06～0.11范围内变化,但丁醇中摩擦因数最高,为0.25;Ti_3SiC_2/Si_3N_4配副的磨损率均随碳原子数和羟基数的增加而减小,Ti_3SiC_2的磨损率在4.48×10~(-7)～9.33×10~(-9) mm~3/(N·m)范围内变化,Si_3N_4的磨损率在4.05×10~(-6)～2.91×10~(-7)mm~3/(N·m)之间变化,其中在辛醇和十二醇中几乎没有磨损。研究表明:在醇介质中Ti_3SiC_2/Si_3N_4摩擦副的摩擦状态属于边界润滑状态,摩擦界面微凸体和磨屑的犁沟效应是造成高摩擦的主要原因;摩擦化学反应是Ti_3SiC_2/Si_3N_4在醇介质中的摩擦行为的一个特点,摩擦化学磨损和磨粒磨损是材料磨损的主要机制;链长越长、羟基越多,醇的黏度越大,承载能力越强,犁沟效应和磨粒磨损降低,摩擦因数和磨损率也降低。     

Tribological characteristics of silicon nitride at elevated temperatures

A sliding friction-and-wear test for silicon nitride (Si₃N₄) was conducted using a ball-on-disk specimen configuration. The material used in this study was HIPed silicon nitride. The tests were carried out from room temperature to 1000°C using self-mated silicon nitride couples in laboratory air. The worn surfaces were observed by SEM and the debris particles from the worn surfaces were analyzed for oxidation by XPS. The normal load was found to have a more significant influence on the friction coefficient of the silicon nitride than an elevated temperature. The specific wear rate was found to decrease along with the sliding distance. The specific wear rate at 29.4 N and 1000°C was 292 times larger than that at room temperature. The main wear mechanism from room temperature to 750°C was caused by brittle fracture, whereas from 750 to 1000°C the wear mechanism was mainly influenced by the oxidation of silicon nitride due to the increased temperature. The oxidation of silicon nitride at a high temperature was a significant factor in the wear increase.     

Tribochemical Effects on Tribological Properties of Self-Mated Zirconia Ceramic in HFC-134a Gas

Tribochemical effects on the tribological properties of self-mated zirconia ceramic in CF₃CH₂F (HFC-134a) were investigated using a ball-on-disk type environmental tribometer. The friction chamber of the tribometer was attached to a micro-spot X-ray Photoelectron Spectrometer (XPS) for ensuring that surface analysis be conducted without exposuring the frictional surfaces to air. It was found that HFC-134a gas was an effective lubricant for zirconia ceramic, especially at a pressure higher than 10³ Pa. The products of tribochemical reactions between zirconia and HFC-134a molecules were detected. The amount and chemical state of the tribochemical products seemed to control the tribological behaviors. Thus, the role of tribochemical products on the tribological properties of zirconia in HFC-134a gas at 10⁴ Pa was studied in detail under applied loads of 0.6–5.0 N and sliding speed of 0.04–0.35 m/s. It was found that severe tribochemical reactions occurred at low speeds and high loads. The formation of ZrF₄ accelerated the chemical wear of zirconia, and raised the friction. Zirconia ceramic is suitable for use at moderate load and sliding speed under a reactive environment.     

Tribological Characteristics and Tribochemical Reactions of Various Ceramics Lubricated with HFC-134a Gas

The role of tribochemical products in the friction and wear reduction of ceramics with different fractional ionic character in CF₃CH₂F (HFC-134a) gas was investigated using a ball-on-disk type tribometer. Without exposure to air, the wear tracks on the disks were characterized with the aid of a micro-spot X-ray Photoelectron Spectroscope (XPS) whose analytical chamber was connected to the friction chamber of the tribometer. Further, the adsorption and desorption behaviors of HFC-134a molecules on the nascent surfaces of the ceramics were studied using an adsorption test apparatus in high vacuum. It was found that the lubricating effect of HFC-134a gas was closely related to the fractional ionic or covalent characters of the ceramics. HFC-134a gas was more effective in lubricating ionic ceramics than the covalent ceramics. XPS analysis revealed that metal fluorides were mainly formed on the frictional surfaces of the ionic ceramics, whereas the composition of the tribochemical products on the frictional surfaces of the other ceramics was complicated. The adsorption tests proved that HFC-134a was decomposed to an olefin CF₂=CHF on the nascent surfaces of the ionic ceramic Al₂O₃ and the covalent ceramics. However, the formation of organic fluorine-containing compounds was not detected on the frictional surfaces of the ionic ceramics by XPS. This result implies that the mechanism of tribochemical reactions is strongly dependent on the bond type of ceramics. It is concluded that the low friction and wear of the ionic ceramics in HFC-134a gas result from the metal fluorides formed with high surface concentration on the sliding surfaces.     

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.     

纳米铜润滑油添加剂在工程摩擦学中的研究进展

纳米铜具有低剪切强度和晶界滑移效应,与减摩剂、抗磨剂、抗氧剂等润滑油添加剂共同发挥协同减摩抗磨和自修复效用,具有较强的工程应用潜力。综述近年来纳米铜作为润滑油添加剂的工程摩擦学研究进展,讨论纳米铜在润滑体系中的润滑机制,总结分散稳定性、粒径及含量等因素对纳米铜颗粒摩擦学性能的影响规律,阐述增强纳米铜颗粒分散稳定性的方法。指出目前对纳米铜添加剂的摩擦学研究和润滑机制的认知仍缺乏系统性和统一性,且由于纳米铜表面较高的自由能,导致润滑油体系稳定性和润滑有效性不确定等问题,制约了纳米铜作为润滑油添加剂的工业应用和推广。最后展望纳米铜添加剂的发展方向。     

The ep performance of certain S-benzylisoformamidinothiocarbamides in the four-ball test

Load carrying capacities of certain 1-formamidino-2-S-benzyliso-3-arylthiocarbamides were assessed using a four-ball extreme-pressure (ep) lubricant testing machine. All the compounds tested were found to be effective at high loads. 1-formamidino-2-S-benzyliso-3-phenylthiocarbamide was potentially useful at high loads; m-chlorophenyl- and p-tolyl derivatives were effective at low loads as well. For a comparative study a reference additive was used. Scanning electron microscopy was used to study the tribological properties.     

A study of 2-(n-alkyldithio)-benzoxazoles as novel additives

The syntheses of 2-(n-alkyldithio)-benzoxazoles are described. Their wear and friction performances are examined in liquid paraffin using a four-ball machine. The synthesized products have been found to exhibit appreciable antiwear capability. The effect of additive chain length on wear scar diameter and friction coefficient is investigated under different test conditions. The film formed by the additive during the sliding process was investigated by using X-ray photoelectron spectroscopy. This revised version was published online in August 2006 with corrections to the Cover Date.     

Tribological Behaviors of 52100 Steel in Carbon Dioxide Atmosphere

The tribological behavior of 52100 steel in a carbon dioxide (CO₂) atmosphere was investigated using a reciprocating ball-on-disk tribometer. X-ray photoelectron spectroscopy (XPS) was used to identify the adsorbed surface layers and tribochemical products. We found that CO₂can substantially reduce friction and wear of the steel. Adsorbed and reacted surface layers containing iron carbonate and/or bicarbonate play an important role in reducing friction. A disk, exposed once to CO₂atmosphere, also shows a low friction for a long time even in a vacuum environment. An optimum CO₂pressure exists for effectively reducing friction and wear. A low-pressure CO₂atmosphere is insufficient to produce iron carbonate. In contrast, high pressure engenders serious chemical wear.     

半金属基粉末摩擦片摩擦磨损性能研究

在不同工况下研究半金属基粉末摩擦片与淬火45#钢配副时,载荷和转速对其摩擦磨损性能的影响,并分析其磨损机制。结果表明,在油润滑和水润滑下,半金属基摩擦片高速下的磨损量要明显低于低速下的磨损量,而干摩擦下其高载高速下的磨损要高于高载低速时的磨损量。油润滑下随载荷的增大,半金属基摩擦片的摩擦因数逐渐升高;水润滑下随载荷的增大,高速时摩擦因数先增大后减小,低速时则逐渐降低;干摩擦下随载荷的增大,高速时摩擦因数呈现出先升高后降低再升高的趋势,低速时则先升高后降低。干摩擦时摩擦面十分粗糙,有比较明显的沟状磨痕和硬质颗粒脱落后残留的凹坑;而水润滑和油润滑时摩擦面较为光滑。     

碳基薄膜水润滑性能的研究进展

评述了碳基薄膜如类金刚石薄膜(DLC)和非晶氮化碳(a-CNx)薄膜水润滑的研究现状和进展。分析了第2元素加入和摩擦副材料对碳基薄膜在水中摩擦磨损特性的影响,探讨了碳基薄膜在水中的磨损机制。指出:氢化或氮化碳基薄膜的磨损率与摩擦副材料的水合反应有关,若摩擦副材料易于摩擦水合反应,碳基薄膜的磨损率很低;3种DLC薄膜在水中的磨损率与DLC的种类和对磨钢球材料无关,都在10-8mm3/(N.m)的数量级上变动;a-CNx/Si基非氧化物陶瓷摩擦副显示很低的摩擦因数和低的磨损率;在相同条件下,a-CNx薄膜比a-C薄膜更能显示优异的水润滑性能。