Enhancing tribological performance of Ti-6Al-4V by sliding process

The exceptional combination of mechanical, physical and anti-corrosive properties of Titanium alloy Ti-6Al-4?V (Ti64) makes it idle material for the applications like aerospace, automobile, chemical, medical etc. However, Ti64 exhibits poor tribological (friction and wear) properties, which limits its implementation in the intended applications. The tribological performance of the Ti64 can be enhanced by developing a protective layer or coating on its surface. It has been reported in literatures that through rubbing process the oxide layers can be achieved at much lower temperature compared to external heating process. Therefore, an endeavour is made in the present work to achieve a tribo-oxide protective layer on the surface of Ti64 through rubbing process. For this, at first the tribological behaviour of tribo pair: Ti64 pin-alumina disc is studied under dry ambient condition for diverse loading and sliding speed conditions, using pin on disc experimental set-up. The obtained results are compared with literatures. The tribological performance is quantified in terms of coefficient of friction (COF) and wear rate. To investigate the tribological mechanism and behaviour, in-situ analysis was performed on the pin’s surface using (i) scanning electron microscopy, and (ii) energy dispersive analysis of X-ray. The mechanical properties like nano-hardness and elastic modulus of the pins surface were also determined. It was envisaged that the tribological behaviour were extremely transient and depend greatly on what the surface has precisely experienced Based on the experimental observations, the experimental conditions providing (i) Case1: deprived tribological properties and (ii) Case 2: higher oxide layer is selected. Now, to enhance the tribological behaviour of Case 1, the pin with high oxide layer, i.e. Case 2 is used. For this experiment is performed initially for Case 2 conditions for the sliding distance of 1000 m (for developing oxide layer) and the experiment is continued for next 1000 m for Case 1 condition. The experimental results in terms of COF and wear rate are presented and corresponding enhancement in their values are discussed.     

Calculation of tribological characteristics of composite materials

We present the formulas for calculating the friction coefficient when fibrous composite material is rubbed against the homogeneous rigid material. The wear of the composite is taken into consideration in the calculation.     

Molecular dynamics modeling of a single diamond abrasive grain in grinding

In this paper the nano-metric simulation of grinding of copper with diamond abrasive grains, using the molecular dynamics (MD) method, is considered. An MD model of nano-scale grinding, where a single diamond abrasive grain performs cutting of a copper workpiece, is presented. The Morse potential function is used to simulate the interactions between the atoms involved in the procedure. In the proposed model, the abrasive grain follows a curved path with decreasing depth of cut within the workpiece to simulate the actual material removal process. Three different initial depths of cut, namely 4 ?, 8 ? and 12 ?, are tested, and the influence of the depth of cut on chip formation, cutting forces and workpiece temperatures are thoroughly investigated. The simulation results indicate that with the increase of the initial depth of cut, average cutting forces also increase and therefore the temperatures on the machined surface and within the workpiece increase as well. Furthermore, the effects of the different values of the simulation variables on the chip formation mechanism are studied and discussed. With the appropriate modifications, the proposed model can be used for the simulation of various nano-machining processes and operations, in which continuum mechanics cannot be applied or experimental techniques are subjected to limitations.     

Molecular dynamics study of pattern transfer in nanoimprint lithography

A molecular dynamics simulations model of nanoimprint lithography (NIL) is proposed in order to study the pattern transfer and its related phenomena. The proposed model is similar to a real NIL process imprinting an α-quartz stamp with a rectangular line pattern into an amorphous poly-(methylmethacrylate) (PMMA) film. The polymer deformation behavior and the adhesion and friction effects between the stamp and the polymer film are investigated and their dependency on the pattern aspect ratio is discussed. Force fields including bond, angle, torsion, van der Waals, and electrostatic potentials are used to describe intermolecular and intramolecular interacting forces. Nosé-Hoover thermostat is used to control the temperature of the polymer film and cell multipole method is adopted to treat long range interactions. The deformation of the polymer film is observed for two stamps having different aspect ratio patterns. The distributions of density and stress in the polymer film are calculated for the detail analysis of deformation behavior. For a high aspect ratio pattern (aspect ratio = 2.5, imprint depth = 8.0 nm), large amount of springback of the residual polymer film is observed, which is mainly due to the residual compressive stress left in the polymer film. However, for a low aspect ratio pattern (aspect ratio = 1.0, imprint depth = 3.0 nm), the springback is not observed. In addition, adhesion and friction forces are obtained by dividing the polymer film into subregions and calculating the interacting force between each subregion and the stamp. While the adhesion force is nearly constant regardless of the pattern aspect ratio, the friction force increases as the pattern aspect ratio grows, so the friction force becomes larger than the adhesion force when the pattern aspect ratio increases.     

Molecular dynamics simulations of atomic-scale tribology between amorphous DLC and Si-DLC films

Silicon-doped diamond-like carbon (Si-DLC) films possess the potential to improve wear performance of DLC films in humid atmospheres and at higher temperatures. But many experimental results of Si-DLC films show that their structures and tribological properties changed greatly with silicon content. Therefore, molecular dynamics (MD) simulations were used to study the sliding friction process between DLC and Si-DLC films on un-lubricated boundary condition. The results show that a part of sp² bonding of the Si-DLC films is converted into sp³ bonding with the addition of silicon atoms, and the sp³/sp² ratio increases with the increase in silicon content. A transfer film between the DLC and Si-DLC films is formed and the friction force changes with the silicon content. Moreover, the simulations have showed that the silicon addition promotes the bonding of interfilms being formed.     

Comparison of tribological properties of various carbon plastics under water-lubricated sliding friction

Carbon plastics based on polyphenylene sulfide matrix and carbon fabric have been synthesized using various technological procedures. Comparative triboengineering test results are cited for pure polyphenylene sulfide, carbon plastics on its base, and carbon plastics of grade FUT and UGET. Carbon plastics based on polyphenylene sulfide show higher wear resistance as compared to FUT and UGET. Carbon plastics based on polyphenylene sulfide with slight addition of C₆₀ fullerene show even higher wear resistance.     

Effect of pin specimen contact length in the sliding direction on tribological results of pin-on-disc tests

Tribological properties of a material depend on a wide range of parameters/conditions. While some of them are obvious, some are quite subtle. One of these subtle parameters has been identified and reported here. Under sliding conditions, whenever surface layers are formed due to the interaction at the sliding interface (e.g., tribochemical interaction), the resultant tribological response has been conjectured to depend upon the relative length of the pin, parallel to the sliding direction, with respect to the corresponding length of the disc track. The results obtained from the experiments reported in this paper as well as those reported by different researchers in the past, furnish evidence to support this conjecture. It can be considered as a general parameter because of this wide spectrum of applications. It is shown in this paper that this parameter helps in selecting the optimum contact geometry of the pin under various experimental conditions. This might reduce the observed scatter in tribological test results. Such a study leads to the conclusion that, for tribological testing, a square/rectangular contact configuration of pin is preferable to a circular one. This revised version was published online in June 2006 with corrections to the Cover Date.     

Improvement in tribological performances of magnesium alloy using amide compounds as lubricating additives during sliding

The tribological characteristics of a magnesium alloy, AZ91D (die-casting), are investigated in a sliding lubricating system using various amide compounds as lubricating additives on a Timken type tester against a bearing steel (AISI52100) ring. Results indicate that a significant improvement in the tribological performance exists using the amide compounds as additives. The number of amido group (–CONH₂) in additive molecules and the molecular structure of amide compounds have significant effect on the tribological characteristics of magnesium alloy. Electromicroscopy reveals that the mild abrasive wear is a predominant wear mechanism of magnesium alloy using an amide additive while the dominated wear mechanism is a severe abrasive wear with severe material deformation using only base oil. Observation shows the formation of boundary film on the magnesium alloy. XPS analysis suggests the occurrence of tribo-chemical reactions between Mg and amide compounds with the formation of chemically stable compound (or complex) of magnesium and amide, as well as the formation of friction polymer.     

Applications of ceramics for tribological components

Fine ceramics are expected to be applied as new materials to many tribological components because of their superior heat resistance and corrosion resistance and their high hardness. In this paper, wear and frictional test results from applications of these components in machines are given and the capabilities of fine ceramics are described.     

Investigation of the effect of additives to lubricant Litol-24 on abrasive wearing of steel under sliding friction

The structure of a three-ball friction machine developed for testing lubricants is described and its advantages over the four-ball one in tests with sliding friction are shown. Using the developed machine and the testing procedure, a lubricant grease Litol-24 was subjected to testing with additions of solid lubricants (graphite powder and MoS₂) and organic dopes containing P, Cl, S, and O. According to results of investigation of their lubricity in conditions of boundary friction with abrasives, the optimal formulations of lubricants have been determined.     

纳米润滑油添加剂抗磨减摩分子动力学模拟研究

研究分子动力学模拟的基本原理,建立摩擦副的分子动力学模型。通过计算对磨材料原子与被磨面材料原子之间作用力,建立运动方程,通过数值方法求解运动方程,模拟出不同时刻原子运动轨迹,根据运动轨迹和原子间作用力分析摩擦磨损过程。对Fe/Fe和Fe/Cu 2种摩擦副的抗磨减摩性能进行模拟计算,分析磨损量与模拟时间和载荷曲线的变化趋势。结果表明有中间纳米Cu润滑层的Fe/Cu摩擦副具有良好的摩擦性能。     

Molecular dynamics simulation of the contact process in AFM surface observations

In the present work, several molecular dynamics simulations have been performed to clarify dynamically the contact mechanism between the specimen surface and probe tip in surface observations by an atomic force microscope (SFM) or friction force microscope (FFM). In the simulation, a three‐dimensional model is proposed where the specimen and the probe are assumed to consist of monocrystalline copper and rigid diamond or a carbon atom, respectively. The effect of the cantilever stiffness of the AFM/FFM is also taken into consideration. The surface observation process is simulated on a well‐defined Cu{100} surface. From the simulation results it has been verified that the surface images and the two‐dimensional atomic‐scale stick‐slip phenomenon, just as is the case for real AFM/FFM surface observations, can be detected from the spring force acting on the cantilever. From the evaluation of the behaviour of specimen surface atoms, the importance of the specimen stiffness in deciding the cantilever properties can also be understood. The influence of the probe tip shape on the force images is also evaluated. From the results it can be verified that the behaviour of the specimen surface atoms as well as the solid surface images in AFM/FFM surface observations can be understood using the molecular dynamics simulation of the model presented.     

Improvement in the tribological properties of imidazolium‐derived ionic liquids by additive technology

Molecular design of wear‐preventing and friction‐reducing additives for ionic liquids is described. The tribological properties of carboxylic acid‐derived additives were evaluated by a ball‐on‐flat type tribotest under reciprocating motion. Tetraalkylammonium and tetraalkylphosphonium salts of N‐benzyl‐protected aspartic acid were dissolved in 1‐alkyl‐3‐methylimidazolium‐derived ionic liquids. They prevented wear remarkably and reduced friction fairly. Influences of alkyl group in imidazolium molecule on additive response were observed. In tetraalkylammonium‐derived ionic liquids, the additive reduced wear but did not reduce friction under these conditions. The salt of N‐acetyl‐protected glutamic acid prevented wear, but did not reduce friction. Copyright © 2008 John Wiley & Sons, Ltd.     

Tribological studies of coated pistons sliding against cylinder liners under laboratory test conditions

The presence of coatings and surface topography play an important role in the tribological performance of sliding components. Depending on the coating used, it is possible to reduce friction and/or reduce wear. However, although there may be low friction and wear‐resistant coatings suitable for use in pistons, some coatings may hinder the tribological performance by changing the lubrication regime or by preventing additives from their intended function through chemical mechanisms. In this work, piston skirt segments extracted from a commercial aluminium alloy piston were coated with a diamond‐like carbon (DLC) coating, a graphite–resin coating or a nickel–polytetrafluoroethylene (Ni–PTFE) coating and were tribologically tested using a reciprocating laboratory test rig against commercial grey cast iron liner segments. The tribological tests used commercial synthetic motor oil at a temperature of 120 °C with a 20 mm stroke length at a reciprocating frequency of 2 Hz. Results showed that the graphite–resin coating, although it may serve as a good break‐in coating, wears rapidly. The Ni–PTFE coating showed friction reduction, whereas the DLC coating wore off quickly due to its small thickness. Furthermore, the higher hardness of the DLC coating relative to the cast iron liner surface led to pronounced changes on the liner counterface by polishing. In contrast with the uncoated piston skirt segments, all of the coatings prevented the formation of a visible tribochemical film on the cast iron surface. Copyright © 2012 John Wiley & Sons, Ltd.     

Analysis of simulated scanning of atomic-scale silicon surface by atomic force microscopy

This study constructs a contact-mode atomic force microscopy (AFM) simulation measurement model with constant force mode to simulate and analyze the outline scanning measurement by AFM. The simulation method is that when the probe passes the surface of sample, the action force of the atom of sample received by the atom of the probe can be calculated by using Morse potential. Through calculation, the equivalent force on the cantilever of probe can be acquired. By using the deflection angle equation for the cantilever of probe developed and inferred by this study, the deflection angle of receiving action force can be calculated. On the measurement point, as the deflection angle reaches a fixed deflection angle, the scan height of this simulation model can be acquired. By scanning in the right order, the scan curve of the simulation model can be obtained. By using this simulation measurement model, this study simulates and analyzes the scanning of atomic-scale surface outline. Meanwhile, focusing on the tip radii of different probes, the concept of sensitivity analysis is employed to investigate the effects of the tip radius of probe on the atomic-scale surface outline. As a result, it is found from the simulation on the atomic-scale surface that within the simulation scope of this study, when the tip radius of probe is greater than 12 nm, the effects of single atom on the scan curve of AFM can be better decreased or eliminated.     

Dry sliding tribological behavior of nanocrystalline and conventional polycrystalline copper

Dry sliding tribological behavior of an electro-deposited nanocrystalline Cu (nc Cu) and a conventional coarse-grained Cu (cg Cu) has been investigated using a ball-on-disc tribometer with cemented tungsten carbide ball as the counterface. Experimental results showed that the wear resistance of copper with the nanocrystalline microstructure was enhanced relative to the coarse-grained form. The steady-state friction coefficient of the nc Cu was obviously lower than that of the cg Cu when the load is below 20 N. The wear volume of the nc Cu was always lower than that of the cg Cu for the applied load range from 5 to 40 N. With increase of the load, the difference in wear resistance between the nc and the cg Cu decreased. The enhancement of the wear properties of the nc Cu was associated with the high hardness and the low work-hardening rate of the nanocrystalline structure, and easily being oxidized of wear debris, which was attributed to grain refinement.     

Molecular dynamics simulation of intrusion of a C60 molecule ball into sliding contact space

The intrusion process of a C60 ball into a sliding contact space with an included angle made up by two silicon substrates (100) was simulated using the molecular dynamics approach. The simulation was carried out using Tersoff potential of C and Si atoms at room temperature of 300 K. The included angle was defined as initial entry angle changing from 20° to 90° in the simulation for studying the effect of the initial entry angle during the intrusion process. The dependence of the initial entry angle on the number of sticking Si atoms of upper substrate was calculated. The results showed that the number of sticking atoms increased with the increasing of initial entry angle, and the number of sticking atoms was divided into three regions with different slopes, which could be used to evaluate the intrusion performance of a C60 ball into the sliding contact space. Copyright © 2011 John Wiley & Sons, Ltd.     

复合准周期势下的纳米摩擦分子动力学模拟

采用Frenkel-Kontorova模型和涨落-耗散机制相结合的方法模拟一维原子链在复合准周期势能下纳米摩擦过程,考察了在复合准周期势能下各种因素对摩擦的调制作用.结果表明在复合势能形式中对摩擦起到主要调制作用的为构成复合势能中周期较小的余弦势能,此势能的周期和振幅的大小决定了调制作用的强弱.     

Transfer film bonding and wear studies on CuS-nylon composite sliding against steel

The tribological behaviour of nylon 11 reinforced with lead sulphide filler was studied. The composite specimens with different filler proportions were made by compression moulding. The friction and wear experiments were run under ambient conditions in a pin-on-disk machine with the composite pin riding on the flat surface of a steel disk. It was found that 35 vol.% PbS-nylon composite had the highest wear resistance. The friction and wear tests were run with this composite at different loads, speeds and counterface roughnesses. The wear rate increased considerably when the load was increased from 19.6 N to 39.2 N and the sliding speed from 1 m/s to 2 m/s, but the effect of these increases on the coefficient of friction was very small. The wear rate also increased abruptly when the surface roughness was increased from 0.11 to 0.3 μm but the coefficient of friction was not affected. It was found that the wear process was dominated by the transfer film that formed on the counterface. The transfer film and the worn surfaces were studied by scanning electron microscopy. XPS analysis indicated chemical bonding between the polymer composite transfer film and the steel counterface.     

Theoretical and practical problems of thermal dynamics and simulation of the friction and wear of tribocouples

The main postulates of the thermal dynamics and simulation of friction and wear of tribocontacts are considered. Practical problems are considered of the application of multipurpose friction machines to estimation of the friction heat resistance of materials and thermoimpulse friction. The example of braking devices and clutches is used to evaluate the application of the thermal dynamics method and friction and wear simulation for prediction of the working characteristics of tribodevices at the design stage.