Some indentation and sliding experiments on single crystal and polycrystalline materials

Indentation and sliding experiments were conducted on some fcc, bcc, and hcp structured, single crystal (copper, aluminum, iron, zinc, and cadmium) and polycrystalline (iron, tin, aluminum oxide and magnesium meta-aluminate) materials. Hard indenters (steel, sapphire, and diamond) were used to investigate the anisotropic behavior of these materials in indentation and sliding. A conventional microhardness tester modified to enable sliding of the specimens past the indenter and to incorporate the means for the measurement of frictional force using strain gages was used in this investigation. Optical microscopy (including interference contrast) was used to characterize the specimens after indentation/sliding. An attempt was made to correlate the gross macroscopic behavior in terms of microscopic events. The results of this study show that indentation and sliding on a microscopic scale, such as with a hard indenter can reveal unique characteristics for both single crystal and polycrystalline materials.     

Ultralow Friction Between Steel Surfaces Achieved by Lubricating with Liquid Crystal After a Running-in Process with Acetylacetone

A suitable running-in process is advantageous for reducing friction. The aim of the present work was to study the influence of the running-in with acetylacetone on tribological performance of 4-Cyano-4’-pentylbiphenyl (5CB) liquid crystal. Friction tests were performed between steel surfaces in a ball-on-disk sliding system. After a running-in period of 240 s, the COF of 5CB was measured to be 0.013, which is about a quarter of the value (0.055) without running-in. The reduced contact pressure, caused in running-in process, does not directly lead to a drop in COF. The generation of tris(acetylacetonato) iron(III) induced by the tribochemical reactions between acetylacetone and steel surfaces, and the unique physical properties of liquid crystal are assumed to be reasons for the ultralow COF. Surface analysis was performed to correlate COF with the topography of wear surfaces. An evenly distributed specific grooved structure observed on wear area of the ball may have a beneficial effect on COF as well. We believe our findings can provide an effective and simple solution to reduce COF of liquid crystal between steel surfaces. A better understanding of the tribological behavior is needed for the development of this tribological system and for the possible future applications.     

Contact mechanics of wedge and cone indenters

Several aspects of the mechanics of indentation of a half-space by an elastic indenter which is either conical or wedge-shaped are addressed. These include elucidation of the contact law, the state of stress induced when the indenter is either pressed normally or sliding with Coulomb friction, the strength of the contact, and the influence of shearing forces less than those necessary to cause sliding, including those induced by elastic mismatch.     

MoS2/Ti复合固体润滑膜的力学性能与摩擦特性

采用非平衡磁控溅射法在9Cr18轴承钢基底上制备了厚度约3μm的MoS2/Ti复合固体润滑膜,基于球形压头纳米压痕试验,采用连续刚度法对MoS2/Ti复合固体润滑膜的力学性能进行研究,探究MoS2/Ti复合固体润滑膜力学性能随压痕深度的变化规律;根据压痕试验载荷-位移曲线,采用Hertz接触理论计算MoS2/Ti复合固体润滑膜的弹性模量并与试验结果进行对比;利用CSM摩擦试验机对低速、低载下MoS2/Ti复合固体润滑膜的摩擦特性进行研究;基于压痕试验提出了一种能够更准确计算钢球加载时MoS2/Ti复合固体润滑膜接触应力的方法,并计算了摩擦试验不同载荷下的接触应力。结果表明:MoS2/Ti复合固体润滑膜的力学性能和摩擦特性都会受到表面形貌的影响;除表面初始压入阶段外,MoS2/Ti复合固体润滑膜的弹性模量和接触刚度都随着压痕深度的增大而增大;滑动速度和载荷共同影响MoS2/Ti复合固体润滑膜的摩擦特性。     

Measurement of the friction of paper by the strip-on-drum method

The friction between paper surfaces has been studied in a strip-on-drum geometry, under quasistatic and slow sliding conditions. The method provides a better simulation of some practical applications than other more widely used tests. The mechanics of the test have been analysed for two cases: with constant coefficient of friction; and with the friction force related to normal pressure by a power law. The behaviour of four different types of paper was examined. In all cases, the coefficient of friction of the sample fell with repeated sliding; this was associated with progressive damage to the paper surface, and has implications for the design of standard friction test methods for paper. The coefficient of friction (COF) also depended significantly on the contact pressure, in a way that can be correlated with the relative compressibilities of the different paper structures. The pressure dependence of friction, at low pressures, was accurately modelled by a power-law relationship.     

Improvement of wear resistance of quenched structural steel by nanostructuring frictional treatment

The paper deals with the study of the structure, deformation hardening, and wear resistance under abrasive and sliding friction of quenched structural steel 50 (0.51% of C), which is subjected to frictional treatment by a hard alloy indenter. The resistance of a steel surface layer hardened by frictional treatment to mechanical effects is estimated using the kinetic indentation method. It is shown that frictional treatment yields a considerable increase in the wear resistance of quenched medium-carbon steel tested in pairs with flint and corundum, as well as under the conditions of adhesive wear and boundary friction, due to the hampering of the processes of microcutting, seizure, and plastic deformation. This is favored by the higher resistance of the nanostructured layer to residual deformation during contact loading.     

Experimental and theoretical evaluation of the deformation component of the coefficient of friction

An analytical model for the calculation of the deformation component of a friction force in sliding of a hard spherical indenter on a viscoelastic material that is modeled as a Kelvin solid is constructed. The influence of mechanical properties of a material and slip velocity of an indenter on the contact characteristics and deformation component of the coefficient of friction is studied. An experimental method for estimation of deformation loss under friction of a high-elasticity material is proposed. A comparison of the results obtained with the use of theoretical model with experimental data is performed.     

Stick–slip friction in dissimilar polymer pairs used in automobile interiors

The static and dynamic friction of dissimilar pairs of plastics used in automotive interiors was measured as a function of normal load, system stiffness, and surface roughness. Glass fiber filled polypropylene (FPP) was slid on polycarbonate (PC) and glass fiber filled styrene–maleic–anhydride copolymer (SMAC) in a single pass, unidirectional sliding test. The friction was characterized by the value of static coefficient of friction (COF) and the number of stick–slip cycles during sliding. It was found that the FPP/PC and FPP/SMAC pairs had fewer instances of stick slip than FPP/FPP, PC/PC, and SMAC/SMAC pairs except for one of the SMAC polymers. The surface texture which had the smallest average radius of peak curvature, had the lowest value of static COF. The decrease in the static COF of polypropylene (PP) caused by the addition of glass fiber was most likely caused by the increase in elastic modulus and hardness.     

Positron Microscopy Studies of Wear Tracks on a Copper Surface

Positron microscopy was used for studies of defects induced by ball indentation or sliding on the surface of well-annealed copper samples. A focused positron beam of 24.5-keV energy was used to scan the deformed region of the indenter impression, and measurements of the annihilation line parameter were performed. In the case of ball indentation, the measured defect distribution correlates well with the von Mises stress distribution of the Hertz contact and the von Mises yield criterion. For the wear track produced by pin on disc dry sliding, an asymmetric defect distribution near the wear track is observed. It indicates the presence of a tangential force that additionally deforms the sample.     

Role of Carbon Nanotube Tribolayer Formation on Low Friction and Adhesion of Aluminum Alloys Sliding against CrN

This work investigates the role of carbon nanotube (CNT) tribolayer formation in reducing friction and adhesion of an Al-alloy engine block material (Al-6.5% Si, 319 Al) sliding against a common piston ring coating, namely, CrN coated steel, when tested under a boundary lubricated condition. Coefficient of friction (COF) values were determined using pin-on-disk type tests as a function of sliding distance using CNT added to ethanol and ethanol without CNT addition. Boundary lubricated tests that used ethanol with 0.14 wt.% CNT resulted in a steady-state COF of 0.16, and reduced Al adhesion to the CrN due to the formation of CNT tribolayers on the Al-alloy contact surfaces. Raman spectroscopy and high resolution SEM suggested the CNT fibers in the tribolayers were damaged and possibly subjected to plastic deformation, and the carbon bonds were possibly passivated by the -H and -OH dissociated from ethanol as suggested by FTIR. The low friction and adhesion observed when ethanol with 0.14 wt.% CNT was used was attributed to the sliding-induced bending and curling of the CNT tribolayers, leading to the formation of rolled sections of tribolayer with a cylindrical morphology (diameter of ~?1 µm) that reduced direct contact between Al-alloy and CrN surfaces.     

Influence of friction on imperfect conical indentation for elastoplastic material

Abstract

The investigation of this paper provides a rationale of the influence of friction in nanoindentation testing for elastoplastic solids. The emphasis is placed on providing a detailed evaluation of the influence of the friction coefficient on the calculated hardness using an imperfect indenter. A new method for calculating the material hardness is derived. The new function can take into account the combined effects of friction and imperfect indenter tip geometry. For further investigations, some numerical simulations are executed. The results show that the new function can provide a good relationship for the hardness calculated in all friction cases. Moreover, the numerical simulations show that the friction coefficient does not significantly affect the curve of load versus indentation depth, whereas it significantly influences the deformations of the specimen surface around the indenter for some materials.     

Tribological Characteristics of Rubber-Based Friction Materials

This article deals with the rubber-based friction materials (RBFMs) which can be used in brake system. The physico-mechanical and tribological properties of a series of fiber filled RBFMs containing steel wool and aramid pulp at different concentrations along with a fiber-free reference material were characterized. Rubber–glass transition induced at higher sliding velocities was identified based on the friction fade behavior of the RBFMs. The rubber–glass transition which is inherently originated by viscoelastic response of polymeric binder was found to be influential on the tribological properties of the RBFMs. It was revealed that steel wool increased coefficient of friction (COF) and improved friction recovery behavior at low volume percent (7.5 vol.%) but it aggravated the COF at high concentration of steel wool (15 vol.%) and severe sliding conditions because of harsh abrasive mechanism. Aramid pulp improved the fade behavior at high sliding velocities and increased COF due to formation of sticky contact patches. It was revealed that steel wool increased the wear rate while aramid pulp did not affect the wear rate significantly, contrary to the increase in the friction coefficient of RBFM. SEM analysis was proved to be useful in correlating the wear rates of composites to the topographical changes on the worn surfaces.     

The Effect of Time,Temperature, and Environment on the Sliding Behavior of Polytetrafluoroethylene

In sliding tests, a weighted sled on PTFE runner, was pulled over a PTFE track by a stationary friction force. It was found that the sliding speed was influenced by rest periods between tests and by the nature of the preceding experiment. These time effects were investigated and are described.

Other sled tests were carried out with curved contacting surfaces such as a surfaces such as a cross-cylinder and a sphere-on-flat geometry. Sliding speeds of PTFE sliding against steel and against itself were measured for several stationary friction forces. Time effects were checked.

Friction tests were made with a scratch tester by dragging fine diamond styli across a flat PTFE surface and the results compared with free-rolling friction.

Experiments were also conducted in dry nitrogen, in air of various humidities, and in pure helium and oxygen. The results are described.     

Tribological Properties and Electrical Signal Transmission of Copper–Graphite Composites

The correlation between tribological properties of resin bonded copper–graphite composites and electrical signal transmission is investigated. Particular attention is given to the loss of electrical signal at the sliding interface and the amount of signal noise as a function of the composition in the composite. A custom designed slip-ring type tribotester with a copper ring has been used for this study. Results show that the tribological properties of the sliding couple, that is closely associated with morphology of sliding surfaces and the presence of transfer films, determine the amount of voltage drop at the interface and electrical signal noise. The change of the applied load during sliding experiment changes the coefficient of friction (COF) and energy loss at the sliding interface, suggesting that the effective contact area at the rubbing surface of the composite strongly affects the amount of voltage drop. It is also found that the electrical signal noise is directly related with the fluctuation of the friction coefficient caused by the variation of contacts at the sliding interface and by the transient patches of transfer films on the surface of the ring.     

Evaluation of the friction of WC/DLC solid lubricating films in vacuum

The accuracy of nanopositioning is to a large extent limited by the friction-caused errors, particularly in vacuum environments. An investigation of the friction behaviour of sp²-bonds dominating diamond like carbon (DLC) coatings and WC_1−x/DLC, WC(N)/DLC multilayer coatings, which are considered to be used in nanopositioning in vacuum, have been performed by a vacuum microtribometer. By using an atomically smooth Si sphere as a counterface, the reciprocating sliding friction was measured at a normal load <5 mN, and running speed at a 1–100 μm/s in ambient air and in ultra high vacuum (UHV) at 10⁻⁷ Pa, and correlated with microstructures and properties of the coatings. When tested in UHV, the coefficient of friction (COF) for pure DLC coatings (thickness: 700 nm) changes significantly between 0.2 and 0.4. Once the thickness of DLC layers is limited to 5 nm by formation of multilayer coatings, the COF in UHV decreases by nearly one order to 0.02–0.05. We suggest that the deformation of DLC films and the transfer films determines COF. Thick DLC coatings can induce more plastic deformation and consumes more energy in sliding resulting in a high COF. Thickening of the transfer film in running leads to a continuous decrease of COF since the deformation of the transfer films turns easier. The low COF of multilayer coatings is mainly due to their confinement of the thickness of DLC films. A consistent velocity-strengthening frictional behaviour of both WC_1−x/DLC and WC(N)/DLC coatings in UHV indicates that the transfer films acting as a thin layer of granular material. Further study of the friction behaviour with the presence of such granular materials might be interesting for the further development of tribological coatings for vacuum applications.     

Finite element analysis of spherical indentation to study pile-up/sink-in phenomena in steels and experimental validation

The ball indentation technique based on deforming a material with a spherical indenter is an useful non-destructive tool for evaluating mechanical properties from a very small volume of material. In this work, the indentation test carried out using a 1.0 mm diameter tungsten carbide ball to penetration depths of around 100–200 μm is modeled using finite element (FE) method and analyzed for three steels having different yield stress and strain hardening exponent. The FE generated load–depth curve is compared and verified with the experimental load–depth data for the three materials. The role of the contact friction at the indenter–specimen interface on both the load—depth plot and indentation profile are examined. The development of pile-up/sink-in during indentation and its dependence on strain hardening characteristics of the material, contact friction and indentation depth are analyzed using the FE model. The indentation profiles obtained from simulation are compared with experimental profiles and the implication of pile-up phenomenon on accurate evaluation of stress–strain values from the experimental indentation load–depth data is discussed.     

Wear properties of Saffil/Al,Saffil/Al2O3/Al and Saffil/SiC/Al hybrid metal matrix composites

The purpose of this study is to investigate the wear properties of Saffil/Al, Saffil/Al₂O₃/Al and Saffil/SiC/Al hybrid metal matrix composites (MMCs) fabricated by squeeze casting method. Wear tests were done on a pin-on-disk friction and wear tester under both dry and lubricated conditions. The wear properties of the three composites were evaluated in many respects. The effects of Saffil fibers, Al₂O₃ particles and SiC particles on the wear behavior of the composites were elucidated. Wear mechanisms were analyzed by observing the worn surfaces of the composites. The variation of coefficient of friction (COF) during the wear process was recorded by using a computer. Under dry sliding condition, Saffil/SiC/Al showed the best wear resistance under high temperature and high load, while the wear resistances of Saffil/Al and Saffil/Al₂O₃/Al were very similar. Under dry sliding condition, the dominant wear mechanism was abrasive wear under mild load and room temperature, and the dominant wear mechanism changed to adhesive wear as load or temperature increased. Molten wear occurred at high temperature. Compared with the dry sliding condition, all three composites showed excellent wear resistance when lubricated by liquid paraffin. Under lubricated condition, Saffil/Al showed the best wear resistance among them, and its COF value was the smallest. The dominant wear mechanism of the composites under lubricated condition was microploughing, but microcracking also occurred to them to different extents.     

Wear Characteristics of Traditional Manganese Phosphate and Composite hBN Coatings

In this study, the tribological properties of traditional manganese phosphate coatings and composite hBN coatings

composed of nano-hexagonal boron nitride (hBN) in layered manganese phosphate crystals on AISI 1040 steel were investigated. Wear tests were carried out under controlled temperature and humidity using ball-on-disc tribometers for samples that were either submerged in oil or retaining oil on their surfaces at a sliding speed of 2.5 cm/s with loads of 1, 3, 5, and 10 N and sliding distances of 40, 80, 100, and 120 m. The surface profiles before and after the tests were used to characterize the wear. The surfaces of the coated samples were examined using scanning electron microscopy (SEM). The coefficients of friction and wear rates of the coated samples were also measured. The average wear rates of the composite hBN-coated samples were significantly lower than those of the traditional manganese phosphate–coated samples for each of the loading conditions for the oil submersion and retained oil tests. The coefficient of friction (COF) values for the traditional manganese phosphate–coated samples did not change significantly with increasing load. The COF values for the composite hBN coated–samples decreased with increasing load in the oil submersion test.     

Scratch Behavior of ZrO<Subscript>2</Subscript> Thin Film Prepared by Atomic Layer Deposition Method on Silicon Wafer

To evaluate friction behaviour and adhesion strength of 130 nm-thick zirconia (ZrO₂) film produceded by atomic layer deposition (ALD) on Si substrate, scratch tests were performed at two different scales; micro- and macro-scales. Surface morphology, roughness, crack propagations and interations between the surface and sliding indenter were also investigated. Scratch test was also conducted with Si substrate as a reference. The test results showed that friction coefficient (COF) was influenced by the scale of scratch test regardless of the tested materials. In microscale test, the 130 nm-thick zirconia film failed at the critical load of 96 mN and direct relationship between the generation of micro-cracks and friction coefficient was observed. Based on the Hertzian contact theory and experimental results, the macroscratch width was much greater than that in microscratch test although the maximum contact pressure were comparable in both cases. Further discussion was made with regard to the influence of the contact pressure on COF, crack generation and film removal. Various types of failure mode were identified through analyzing the mechanical response of scratch tracks both at micro- and macroscale tests. This study suggested that 130 nm-thick ALD-ZrO₂ film showed better tribological and adhesion properties at microscale contact than macroscale contact.