Some recent progress in microtribology in Japan

The micro in the word of microtribology is defined as science and engineering supported by modern physics and chemistry. In the engineering area, the ultimate goal of microtribology is to create practical zero-wear devices. The importance of microtribology was recognized by Japanese scientists and engineers in the latter half of the 1980s, and much progress has been made in microtribological studies of mechanisms of friction, lubrication with very thin liquid films, and microwear processes.     

Friction and wear performance of MoDTC‐containing and ester‐containing lubricants over steel surfaces under reciprocating conditions

Lubricant oil can be regarded as a complex mixture of base oils and additives, each one with its specific functions and behaviour. In this paper, the interaction of a molybdenum dialkyldithiocarbamate (MoDTC)‐based additive and combinations of a polyalphaolefin and a synthetic ester is investigated. A reciprocating ball‐on‐disc configuration was used for tribological tests. The effect of MoDTC is seen as a sharp drop in the coefficient of friction. This friction reduction is affected by the base fluid: the effect is more intense and lasts longer when the ester content is decreased. The applied normal force also affects the MoDTC effect, which is not sustainable at higher loads. Copyright © 2014 John Wiley & Sons, Ltd.     

Tribological Properties of Langmuir–Blodgett Films on Silicon Surface in Microscale Sliding Contact

Microtribological properties of Langmuir–Blodgett (LB) films transferred from behenic acid (BehA), 2,4-heneicosanedione (HD) and its copper complex ((HD)₂Cu) onto silicon surface were examined. To better understand the wear resistance performance of these LB monolayers, a comparison was made with a chemically grafted octadecyltrichlorosilane (OTS) monolayer. Auger electron spectroscopy (AES) was used for identification of the chemical composition of the monolayers, worn areas and counterpart surfaces. We observed that the studied LB films in microscale sliding contact exhibited stable friction properties comparable to OTS, and better wear performance than OTS at high contact pressure. The tribological properties of these LB monolayers were explained in terms of molecular packing density and molecular transfer to the counterpart surface. The relationship between the wear resistance of the studied LB films and the degree of molecular packing of the surfactants indicated that the wear properties of the LB films are strongly associated with the degree of molecular packing. We suggest that the steady low friction and high wear resistance of the BehA monolayer may partly be attributed to the transfer of the amphiphilic molecules to the counterpart surface in the contact region.     

Effect of surface chemistry on the tribological performance of a MEMS electrostatic lateral output motor

The effect of surface chemistry on the tribological performance and reliability of a MEMS lateral output motor is reported. Relative humidity (RH) and octadecyltrichlorosilane (OTS) self-assembled monolayer (SAM) coatings were used to change surface chemistry. Electrical and tribological performance of uncoated and OTS-coated motors were found to be dependent on RH. For uncoated motors, excessive wear of sliding contacts and welding (permanent adhesion) of static contacts were observed at 0.1% RH. Degradation of electrostatic force and high static friction (stiction) forces limited dynamic performance and reliability and caused device sticking at and above 70% RH. Around 50% RH, uncoated motors exhibited negligible wear, low adhesion, and a wear life at least three orders of magnitude longer than in the dry environment (experiments were stopped without failure after about one billion cycles). Water vapor behaved as a gas phase replenishable lubricant by providing a protective adsorbed film. The OTS coating broadened the operating envelope to 30–50% RH and reduced stiction, which allowed better dynamic performance at high RH. The OTS coating improved durability at 0.1% RH, but it was still poor. At high RH, stiction problems reoccurred when the OTS coating was worn away. By controlling and balancing surface chemistry (adsorbed water and OTS), excellent performance, low friction and wear, and excellent durability were attained with the lateral output motor.     

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。     

Tribology of hot metal forming

Many aspects of the tribology of hot metal forming are poorly understood. This leads to difficulties when modelling the forming operations to develop process improvements. It also handicaps the development of new tool materials, since the operating conditions with which they will have to cope are not fully described. Several key issues in friction, heat transfer, lubrication, wear and fatigue are discussed. This is an area of tribology with many opportunities for research. It is concluded that progress will be made through a combination of techniques, including laboratory simulation, computer modelling and industrial trials.     

Influence of Organic Friction Modifier on Liquid Slip: A New Mechanism of Organic Friction Modifier Action

Recent work using a number of experimental techniques has shown that simple Newtonian liquids can slip against very smooth, lyophobic surfaces. Most previous work has examined lyophobic surfaces produced by forming self-assembled monolayers on smooth substrates. However it has also been shown that octadecanoic (stearic) acid solution in hydrocarbon can promote slip of the liquid against smooth surfaces. This raises the intriguing possibility that one mechanism of action of organic friction modifier additives may be to promote slip in hydrodynamic contacts and thus reduce friction. To test this conjecture, experiments have been carried using a low-load, hydrodynamic friction tester in which a glass cylinder is loaded against a very smooth sapphire flat which has been previously immersed in stearic acid solution. The contact is supplied with a solution of stearic acid in hexadecane and the glass cylinder is rotated at a speed sufficient to generate a full hydrodynamic film. It is found that this system gives significantly lower friction than when using hexadecane alone or when the sapphire is slightly roughened. The results are consistent with the occurrence of liquid slip at the sapphire surface.     

锂基复合脂润滑下HIP—Si3N4／Mo50钢系统磨损试验研究

采用Timken摩擦试验机,在不同合成润滑脂润滑下,对热等静压氮化硅工陶瓷与Mo50钢系统进行了定量试验研究,并采用SEM扫描电镜对磨痕表面形貌进行了定性分析。     

Influence of protein conformation on frictional properties of poly (vinyl alcohol) hydrogel for artificial cartilage

Poly (vinyl alcohol) (PVA) hydrogel is one of the anticipated materials for artificial cartilage. In our previous studies, wear of PVA hydrogel depended on content of proteins in lubricants. The secondary structures of bovine serum albumin (BSA) and human gamma globulin (HGG) were investigated in circular dichroism spectroscopy to clarify the influence of the proteins on frictional properties. BSA and HGG were mainly composed of the α-helix and the β-sheet, respectively. BSA containing the α-helix structure showed low friction compared to HGG composed of the β-sheet structure in mixed or boundary lubrication mode. The α-helix structure forms low shear layer because the α-helix structure is easily released from surfaces and low cohesive strength. HGG forms uniform adsorption layer, but showed higher friction than BSA in the rubbing with single protein. In the repeated rubbing with changing of lubricants from HGG to BSA, however, the final friction was reduced, because an optimum layered structure of proteins was formed. Hence, layered structure of proteins appears to play an important role to protect rubbing surfaces and to reduce friction. In heat treatment tests, heat-induced BSA showed very low friction because of reduction of the α-helix structure. Heat-induced HGG did not show large differences from native HGG, but could not bring low friction with heat-induced BSA. Thus it was shown that the protein conformation has effective influences on friction.     

Enhancing the machining performance by cutting tool surface modifications: a focused review

In machining, cutting tools suffer from severe surface wear, especially in the cutting of difficult-to-cut materials. A major cause of tool wear is the friction generated at the tool-work and tool-chip interfaces, which produces a great deal of frictional heat and abrasion. In order to extend tool life and improve the quality of machined components, a host of techniques have been applied to modify the rake and flank faces of cutting tools. These techniques aim at providing cutting tools with improved resistance to external loading, better tribological performance and/or better chemical stability. This article presents a review of the fundamentals behind which the friction and wear in machining are reduced by modifying the cutting tool surface with the commonly used techniques, such as surface coating, high energy beam treatment, and surface texturing. The effects of these surface modifications on improving the cutting performance are also analyzed. Future research directions are finally discussed.     

Using FIB-processed AFM cantilevers to determine microtribology characteristics

Microtribology characteristics were determined by using a combination of single asperities and three types of FIB (focused ion beam)-processed cantilevers for AFM (atomic force microscope). First, single gold asperities were rubbed with single and parallel leaf springs. For the parallel leaf spring, the pull-off force was proportional to the worn area of the gold asperity peak. The total volume of the gold asperity only slightly changed with rubbing. Second, the friction force on a worn asperity was measured by using a double parallel leaf spring, and the results showed that the friction force was proportional to the sum of the normal load and the pull-off force.     

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.     

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.     

润滑剂减缓微动磨损研究进展

综述了油、脂润滑减缓微动磨损的条件、作用机制,以及油、脂性能对微动磨损的影响等基础研究的进展.介绍了油、脂润滑减缓微动磨损的工业应用研究现状,以期为工业正确使用润滑油、脂降低微动磨损指明方向.     

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。     

有机无灰减摩剂对流体动压润滑减摩性能影响机制

有机无灰减摩剂能减低流体动压润滑摩擦因数,对提升流体动压润滑性能提供了新思路。采用单油酸甘油酯、油酸、季戊四醇四异硬脂酸酯、司盘80作为有机无灰减摩剂,以质量分数5%与PAO 5基础油进行调和,制备4种不同润滑油样,采用流体动压润滑试验机对比其摩擦学性能。试验结果表明,有机无灰减摩剂在边界润滑和混合润滑中均具有一定减摩效果,在流体动压润滑下减摩效果更明显。其中,含质量分数5%季戊四醇四异硬脂酸酯的油样具有最佳的减摩作用,其在流体动压润滑状态下可使基础油的摩擦因数降低19.6%。分子动力学模拟结果显示,有机无灰减摩剂在摩擦表面产生滑移现象,减少了分子间碳链的相互缠绕,进而降低流体动压润滑下的摩擦因数。     

Tribological characteristics of self-assembled monolayer with siloxane bonding to Si surface

The friction-reducing performance of adsorbed organic molecules on a solid surface is well known. To understand the tribological performance of organic molecules adsorbed on the surface, a well-defined molecular film on a very smooth Si wafer was used. This study examined the effect of the number of expected siloxane bonds of self-assembled monolayers (SAMs), as well as the effect of SAM alkyl chain length, on the tribological performance of SAMs. The performance of the SAMs used in this study corresponded with our observations of their molecular density on the surface and the degree of molecular orientation. Slight differences in these characteristics resulted in no significant differences in the SAMs’ tribological characteristics under a mild loading condition. However, the friction experiment under a severe condition apparently produced significant differences in tribological behavior. To confirm the effect of SAM molecular orientation on the tribological characteristics, the effect of temperature on OTS-SAM behavior was studied. The effect of molecular orientation on the tribological characteristics by changing temperature under the severe condition was also significant.     

润滑对机械密封性能的影响

分析了相同配对材料在清水和干摩擦系数以及介质润滑性能对摩擦系数和磨损的影响,分析结果表明：润滑剂不仅对减少摩擦和磨损有着重要的作用,同时还与机械密封的配对材料有密切关系。     

纤维取向、转速、载荷对飞龙材料的摩擦磨损行为影响

在MRH-03型环-块摩擦试验机上,在不同转速、载荷下对不同纤维取向的“飞龙”材料以及饱和吸水后的材料在纯水和人工海水下的摩擦性能进行研究。结果表明：不同纤维取向的“飞龙”材料具有不同的摩擦特性,摩擦方向垂直于单张纤维布时摩擦因数最大,而磨损率最低,摩擦位于单张纤维布上时摩擦因数最小而磨损率最大;随着转速增加,摩擦因数和磨损率均有所降低;随着载荷增加,纯水润滑时磨损率增大,海水润滑时磨损率无明显变化;饱和吸水后,材料的摩擦因数和磨损率,纯水润滑条件下降低,海水润滑条件下增大。SEM形貌分析表明,飞龙材料的磨损行为受到纤维对基体材料的支撑作用和基体材料对纤维的保护作用协同影响。     

铝材轧制加工过程中的摩擦特点及其影响

摩擦在铝材轧制生产过程中影响产品质量、轧辊的磨损和轧制过程的力能消耗。在分析铝材轧制过程摩擦特点、摩擦磨损机理的基础上,着重分析了摩擦对轧制过程、轧制压力、变形区、最小可轧厚度和产品质量的影响。釆取合适的工艺润滑,是减少摩擦与磨损,降低轧制压力,提高生产效率和产品质量的有效途径。