Rippling on Wear Scar Surfaces of Nanocrystalline Diamond Films After Reciprocating Sliding Against Ceramic Balls

The formation of nanoscopic ripple patterns on top of material surfaces has been reported for different materials and processes, such as sliding against polymers, high-force scanning in atomic force microscopy (AFM), and surface treatment by ion beam sputtering. In this work, we show that such periodic ripples can also be obtained in prolonged reciprocating sliding against nanocrystalline diamond (NCD) films. NCD films with a thickness of 0.8 µm were grown on top of silicon wafer substrates by hot-filament chemical vapor deposition using a mixture of methane and hydrogen. The chemical structure, surface morphology, and surface wear were characterized by Raman spectroscopy, scanning electron microscopy (SEM), and AFM. The tribological properties of the NCD films were evaluated by reciprocating sliding tests against Al₂O₃, Si₃N₄, and ZrO₂ counter balls. Independent of the counter body material, clear ripple patterns with typical heights of about 30 nm induced during the sliding test are observed by means of AFM and SEM on the NCD wear scar surfaces. Although the underlying mechanisms of ripple formation are not yet fully understood, these surface corrugations could be attributed to the different wear phenomena, including a stress-induced micro-fracture and plastic deformation, a surface smoothening, and a surface rehybridization from diamond bonding to an sp ² configuration. The similarity between ripples observed in the present study and ripples reported after repeated AFM tip scanning indicates that ripple formation is a rather universal phenomenon occurring in moving tribological contacts of different materials.     

Tribological Evaluation of an Al2O3-SiO2 Ceramic Fiber Candidate for High Temperature Sliding Seals

A test program to determine the relative slitting durability of an alumina-silica candidate ceramic fiber for high temperature sliding seal applications is described. Pin-on-disk tests were used to evaluate the potential seal material by sliding a tow or bundle of the candidate ceramic fiber against a superalloy test disk. Friction was measured during the tests and fiber wear, indicated h the extent of fibers broken in the tow or bundle, was measured at the end of each test. Test variables studied included ambient temperature from 25° to 900°C, loads from 1.3 to 21.2 N, and sliding velocities from 0.025 to 0.25 m/sec. In addition, the effects of fiber diameter and elastic modulus on friction and wear were measured. Thin gold films deposited on the superalloy disk surface were evaluated in an effort to reduce friction and wear of the fibers.

In most cases, wear increased with test temperature. Friction ranged from 0.36 at 500°C and low velocity (0.025 miser) to over 1.1 at 900°C and high velocity (0.25 m/sec). The gold films resulted in satisfactory lubrication of the fibers at 25°C. At elevated temperatures diffusion of substrate elements degraded the films. These results indicate that the alumina-silica (Al₂O₃SiO₂) fiber is a good candidate material system for high temperature sliding seal applications. More work is needed to reduce friction.     

Effect of some material and operating variables on scuffing

This paper summarizes the results of scuffing tests performed on AMS 6260 steel disks, covering three oils (a MIL-L-7808G oil, a MIL-L-23699A oil, and a straight mineral oil), two oil supply temperatures, a variety of sliding and sum velocities, and two modes of operating the test disks such that the potential failure sites on the disk surfaces either do or do not synchronize precisely in repeated cycles of operation. It is shown that, under otherwise comparable situations, (a) different oil-steel combinations allow the operation to penetrate by different degrees into the boundary lubrication regime before scuffing occurs, (b) an increase in the sliding velocity, at constant sum velocity, decreases the scuff failure load and the critical temperature, (c) an increase in the sum velocity, at constant sliding velocity, increases the scuff failure load and the critical temperature, (d) the effect of changing the sliding velocity or sum velocity, at a constant sliding-to-sum velocity ratio, depends on the balance of the opposing effects of sliding ans sum velocities at the particualar velocity ratio of interest, and (e) the scuff failure load and the critical temperature are markedly increased when the potential failure sites on the disk surfaces do not precisely synchronize on repeated cycles of operation.It is further demonstrated that the variations of the oil film thickness at scuffing, the coefficient of friction at scuffing, and the critical temperature with respect to all surface and operating variables correlate satisfactorily with a dimensionless parameter ξ_f.     

厚度效应对Pb(Zr0.53Ti0.47)O3薄膜微结构、铁电、介电性能的影响

用改进的溶胶-凝胶法在Pt(111)/Ti/SiO₂/Si(100)衬底上制备了不同厚度的高度(111)取向的Pb(Zr_0.53Ti_0.47)O₃薄膜.运用X射线衍射(XRD)和原子力显微镜(AFM)分析了薄膜的微结构,原子力显微镜表明厚度为0.3μm和0.56μm的PZT薄膜的晶粒尺寸和表面粗糙度分别为0.2～0.3μm、2～3μm和0.92nm、34nm.0.3μm和0.56μm PZT薄膜的剩余极化(Pr)和矫顽场(Ec)分别为32.2μC/²、79.9kV/cm, 27.7μC/cm²、54.4kV/cm;在频率100KHz时,薄膜的介电常数和介电损耗分别为539、0.066,821、0.029.     

Nanometre scale mechanical properties of extremely thin diamond-like carbon films

Abstract

Extremely thin diamond-like carbon (DLC) films are deposited by the filtered cathodic vacuum arc (FCVA) and plasma chemical vapour deposition (p-CVD) methods. The target thicknesses of the extremely thin protective DLC films deposited on a Si (100) surface by FCVA and p-CVD are 0·1, 0·4, 0·8, 1·0, 2·0, 5·0 and 100·0 nm. Nanoindentation hardness and nanowear resistance are evaluated by atomic force microscopy (AFM). The nanoindentation hardnesses of 100 nm thick DLC films deposited by FCVA and p-CVD are 57 and 25 GPa respectively. The nanowear test by AFM clarifies the mechanical properties of extremely thin DLC films. The wear depths of 1 and 2 nm thick FCVA-DLC films are extremely shallow. The wear depths of the 1·0 and 2·0 nm thick p-CVD-DLC films exceed the film thicknesses after five sliding cycles. These results reveal differences in the wear resistance of extremely thin DLC films and the superior mechanical properties of FCVA-DLC thin films.     

Tribological Properties of Hard Carbon Films on Zirconia Ceramics

In this study, the authors investigated the tribological properties of hard diamondlike carbon (DLC) films on magnesia-partially stabilized zirconia (MgO-PSZ) substrates over a wide range of bads, speeds, temperatures, and counterface materials. The films were 2 μm thick and produced by ion-beam deposition at room temperature. Tribological tests were conducted on a ball-on-disk machine with MgO-PSZ balls, in open air of 30 to 50% relative humidity under contact loads of 1 to 50 N, at sliding velocities of 0.1 to 6 m/s, and at temperatures of 400°C. Al₂O₃ and Si₃N₄ balls were also rubbed against the DLC-coaled MgO-PSZ disks, primarily to assess their friction and wear performance and to compare it with that of MgO-PSZ balls. A series of long-duration lifetime tests was run at speeds of 1, 2, and 6 m/s under a 5 N load to assess the durability of these DLC films. Results showed that the friction coefficients of MgO-PSZ balls sliding against MgO-PSZ disks were 0.5-0.8, and the average specific wear rates of MgO-PSZ balls ranged from 1 × 10^?5 to 5 × 10^?4 mm³/N·m, depending on sliding velocity, contact load, and ambient temperature. The friction coefficients of MgO-PSZ balls sliding against the DLC-coaled MgO-PSZ disks ranged from 0.03 to 0.1. The average specific wear rates of MgO-PSZ, balls were reduced by three to four orders of magnitude when rubbed against the DLC-coaled disks. These DLC films could last 1.5 to 4 million cycles, depending on sliding velocity. Scanning electron microscopy and micro-laser Raman spectroscopy were used to elucidate the microstructural and chemical nature of the DLC films and worn surfaces.     

Tribological roles of nanometre thick Ag layers on Hertzian macroscopic contacts

Abstract

Tribological performance of subnano to nanometre thick Ag layers deposited on Si(111) has been examined under ultra high vacuum conditions to understand effect of surface thin layers on the wear and friction characteristics. The slider was made of a diamond sphere 3 mm in radius. As a result, a minimum of the coefficient of friction 0·007 was observed over a film thickness range of 1·5–10 nm. The sliding planes were observed by Auger electron spectroscopy, reflection high energy electron diffraction (RHEED), synchrotron orbital radiated X-ray diffraction (SOR-XD) and scanning tunnelling microscopy (STM). No worn particles were found after 100 reciprocal sliding cycles, and the very low friction coefficient lasted for at least 1000 sliding cycles. Observations using STM on the sliding surfaces confirmed that the stacking Ag(111) planes slid. The SOR-XD and RHEED verified that a tribo-induced orientation of polycrystal film occurs as Ag(111) sliding planes are oriented parallel to the sliding direction on the track. The friction force of as deposited epitaxial Ag films as a function of the load was constant. On the other hand, in the 5 nm thick Ag films annealed to form complete single crystals, the friction coefficient showed a strong load dependency. At a load of 250 mN or more, the annealed films showed a low and static friction coefficient. These results suggested that the shearing resistance of nanometre thick Ag layers exhibits a strong anisotropic performance within the thickness range of nanometres, along with an orientation of Ag during sliding. Experimental results of sliding tests were discussed on the contribution of surface atoms to the friction, an extraordinarily low wear rate of the Ag layers, and the relationship between the nanoscopic structure and macroscopic tribological performance.     

Direct surface temperature measurement by infrared radiation in elastohydrodynamic contacts and the correlation with the blok flash temperature theory

An infrared microdetector was employed to measure surface temperatures in elastohydrodynamic contacts with Hertz pressures up to 2 GPa, sliding velocities up to 6.0 m s^?1 and rolling velocities up to 1.25 m s^?1 with slide/roll ratios from ?2 to +2. Different surface roughnesses were also employed. The lambda ratio (mean film thickness to composite surface roughness) was varied from 20 to considerably less than 1. The surfaces employed were AISI 52100 steel against Al₂O₃ and the lubricant was a typical naphthenic hydrocarbon. High maximum surface temperatures have been observed (to 300 °C). Analysis of the data shows very good correlation with the Blok flash temperature theory for simple sliding (∑ = ± 2). An extension of this theory to include two moving surfaces at unequal temperatures predicts the ball surface temperature quite accurately.     

Tribological properties of ultra-thin ionic liquid films on single-crystal silicon wafers with functionalized surfaces

Two kinds of room temperature ionic liquid (RTIL) films carrying vinyl and hydroxyl functional groups were prepared on single-crystal Si wafers by spin coating. The tribological properties of the RTIL films sliding against AISI-52100 steel ball and Si₃N₄ ball in a ball-on-plate configuration were investigated on a dynamic–static friction coefficient measurement apparatus, using perfluoropolyether (PFPE) film as a comparison. The tribological behaviors of the ionic liquid films sliding against the same counterparts at extended test durations were also evaluated using a universal UMT-2MT test rig. The morphologies of the wear tracks of the RTIL films and the counterparts were examined using a scanning electron microscope equipped with an energy-dispersive X-ray analyzer attachment. It was found that the tribological performances of the ionic liquid films were closely related to the chemical structures of the RTILs and the chemical characteristics of the substrate surfaces. The films of vinyl group functionalized ionic liquids on hydroxylated substrate and vinyl group modified substrate exhibited very good friction-reduction and wear-resistant properties. It was assumed that there were enough strong forces between the films and substrate in these cases, and the ionic liquid molecules maintained good flexibility simultaneously. The films on hydrogen-terminated and methyl-terminated substrate showed poor tribological performance, which could be related to the relatively weak forces between the films and substrates. Moreover, the films on hydroxylated substrate showed lower friction at higher sliding velocities, which was assumed to be governed by the more rapid adsorption of the ionic liquid molecules on the steel ball at a higher sliding velocity. In addition, the ionic liquid films also had excellent tribological properties as they slid against silicon nitride ball. Therefore, it was supposed that the ionic liquid films could be used as a kind of universal lubricant for various combinations of the frictional pair.     

Characterization and mechanical/tribological properties of nano Au-TiO2 composite thin films prepared by a sol-gel process

Nano Au-TiO₂ composite thin films on Si(1 0 0) and glass substrates were successfully prepared with a facile sol-gel process followed by sintering. The morphology and mircostructure of the films were investigated via X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and scanning electron microscopy (SEM). The Au particles, of diameter 14-22 nm depending on the sintering temperatures used, were found to be well dispersed in the TiO₂ matrix, with a small amount of the particles escaped from the film. The surfaces of the films were uniform, compact and crack-free. Hardness and elastic modulus of the films were measured by using the nanoindentation technique. Friction and wear properties were investigated by using a one-way reciprocating tribometer. It was found that the highest hardness and elastic modulus values were obtained for the films prepared with 500 °C sintering temperature. The films displayed superior antiwear and friction reduction performances in sliding against an AISI 52100 steel ball. With 5.0 mol% Au, the friction coefficient was only 0.09-0.10 and the wear life was more than 2000 sliding cycles. The friction coefficient and wear life decreased with increasing sliding speed and load. The failure mechanism of the Au-TiO₂ films was identified to be light scuffing and abrasion. Those films can be potentially applied as ultra-thin lubricating coatings.     

The role of transfer film and back transfer behavior on the tribological performance of polyoxymethylene in sliding

The role of transfer films formed during the sliding of polymer composites against steel counterfaces was studied in terms of the tribological behaviors of the composites. The composites were prepared by compression molding and sliding tests were run in pin-on-disk sliding configuration. The counterface was made of tool steel hardened to 55–60 HRC and finished to a surface roughness of 0.09–0.10 μm Ra. Wear tests were run for 6 hrs at the sliding speed of 1.0 m/s and contact pressure of 0.65 MPa. Transfer films formed on the counterfaces during sliding were investigated using Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). The results showed that as the transfer film became smooth and uniform, the wear rate decreased. The examination of worn surfaces using Energy Dispersive Spectroscopy (EDS: dot mapping mode) showed the back-transfer of the steel counterface material to the polymer pin surface. This behavior is believed to strengthen the polymer pin surface during sliding thereby contributing to the decrease in wear rate. This paper was recommended for publication in revised form by Associate Editor Jae Cheon Lee Minhaeng Cho received his B.S. and M.S. degrees in Mechanical Engineering from Chung-Ang University, Seoul in 1993 and 1995, respectively. He received his M.S. degree in Materials Science and Engineering from Oregon State University in 2000, and his Ph.D. in Mechanical Engineering from Iowa State University in 2004. Dr. Cho is currently an Assistant Professor at the School of Mechanical Engineering at Chung-Ang University in Seoul, Korea. His research interests are in the area of tribology, surface phenomena, and functional surfaces such as laser surface texturing and ultra-thin coatings.     

Nano/Microtribological Properties of Ultrathin Functionalized Imidazolium Wear-Resistant Ionic Liquid Films on Single Crystal Silicon

Ionic liquids (ILs) are considered as a new kind of lubricant for micro/nanoelectromechanical system (M/NEMS) due to their excellent thermal and electrical conductivity. However, so far, only few reports have investigated the tribological behavior of molecular thin films of various ILs. Evaluating the nanoscale tribological performance of ILs when applied as a few nanometers-thick film on a substrate is a critical step for their application in MEMS/NEMS devices. To this end, four kinds of ionic liquid carrying methyl, hydroxyl, nitrile, and carboxyl group were synthesized and these molecular thin films were prepared on single crystal silicon wafer by dip-coating method. Film thickness was determined by ellipsometric method. The chemical composition and morphology were characterized by the means of multi-technique X-ray photoelectron spectrometric analysis, and atomic force microscopic (AFM) analysis, respectively. The nano- and microtribological properties of the ionic liquid films were investigated. The morphologies of wear tracks of IL films were examined using a 3D non-contact interferometric microscope. The influence of temperature on friction and adhesion behavior at nanoscale, and the effect of sliding frequency and load on friction coefficient, load bearing capacity, and anti-wear durability at microscale were studied. Corresponding tribological mechanisms of IL films were investigated by AFM and ball-on-plane microtribotester. Friction reduction, adhesion resistance, and durability of IL films were dependent on their cation chemical structures, wettability, and ambient environment.     

Chemical Structure and Micro-Mechanical Properties of Ultra-Thin Films of Boron Carbide Prepared by Pulsed-Laser Deposition

Ultra-thin boron carbide films with a thickness of about 40 nm were deposited on silicon substrates by means of pulsed-laser ablation of a sintered B₄C target in vacuum. Together with the determination of the film composition by X-ray photoelectron spectroscopy (XPS) and the observation of the surface topography by atomic force microscopy (AFM), the chemical structure of the films was studied by Fourier transform infrared (FTIR) spectroscopy. Mechanical characterization of the films was performed on the micron and sub-micron scales by means of nano-indentation and micro-scratch tests, from which the hardness, Young's modulus and micro mar resistance of the films were determined. The optimal values were obtained for the films prepared at elevated temperature of 600 °C, with hardness of 39 GPa, Young's modulus of 348 GPa and micro mar resistance (MMR) of 5.0 × 10³ GPa, in comparison with those of 23, 252, and 7.1 × 10² GPa, respectively, for the films prepared at room temperature.     

Effect of capillary formation on friction and pull-off forces measured on submicron-size asperities

Asperities with hemispherical peaks were fabricated on a silicon substrate using a focused ion beam. Pull-off and friction forces were measured on each asperity using atomic force microscopy (AFM) in high vacuum (HV) of 2 × 10^–5 Pa. The probe of the AFM cantilever had a flat square tip, approximately 1 × 1 m² in area. The radius of curvature of the asperity peaks ranged from 70 to 610 nm. The results showed that the pull-off force was roughly proportional to this radius. The friction force was proportional to the pull-off force. Effects of the substrate temperature on pull-off force on a plane (the flat substrate) and friction force on an asperity were also examined. The pull-off force on the flat substrate increased with increasing contact time at a substrate temperature of 100 °C or lower, but was independent of contact time at 190 °C or higher. This suggests that the capillary cannot form at a substrate temperature of 190 °C or higher. The friction force increased with lower sliding velocities at 100 °C or lower, suggesting the capillary has a lubricating effect that prevents direct solid contact.     

Effect of nanoparticles on the tribological behaviour of short carbon fibre reinforced poly(etherimide) composites

The tribological behaviour of nano-TiO₂ particle filled polyetherimide (PEI) composites, reinforced additionally with short carbon fibre (SCF) and lubricated internally with graphite flakes, was investigated. The wear tests were conducted on a pin-on-disc apparatus, using composite pins against polished steel counterparts under dry sliding conditions, different contact pressures and various sliding velocities. It was found that the conventional fillers, i.e. SCF and graphite flakes, could remarkably improve both the wear resistance and the load-carrying capacity. With the addition of nano-TiO₂, the frictional coefficient and the contact temperature of the composite were further reduced, especially under high pv (the product of the normal pressure, p, and the sliding velocity, v) conditions. Based on microscopic observations of worn surfaces and transfer films on the counterparts, possible wear mechanisms were discussed.     

Carbon Nanotube Reinforced Polyimide Thin-film for High Wear Durability

In this paper, the influence of single walled carbon nano tubes (SWCNTs) addition on the tribological properties of the polyimide (PI) films on silicon substrate was studied. PI films, with and without SWCNTs, were spin coated onto the Si surface. Coefficient of friction and wear durability were characterized using a ball-on-disk tribometer by employing a 4 mm diameter Si₃N₄ ball sliding against the film, at a contact pressure of ∼370 MPa, and a sliding velocity of 0.042 ms⁻¹. Water contact angle, AFM topography, and nano-indentation tests were conducted to study the physical and mechanical properties of the films. SWCNTs marginally increased the water contact angle of PI film. The addition of SWCNTs to PI has increased the hardness and elastic modulus of pristine PI films by 60–70%. The coefficient of friction of PI films increased slightly (∼20%) after the addition of SWCNTs, whereas, there was at least two-fold increase in the wear life of the film based on the film failure condition of coefficient of friction higher than 0.3. However, the film did not show any sign of wear even after 100,000 cycles of rotation indicating its robustness. This increase in the wear durability due to the addition of the SWCNTs is believed to be because of the improvement in the load-bearing capacity of the composite film and sliding induced microstructural changes of the composite film.     

Measurements of tribological properties of poly(pyrrole) thin film bearings

We report the results of a recent study on the tribological properties of electropolymerised thin films at light loads and low speeds. Poly(pyrrole) films incorporating different counter-ions have been electrochemically deposited onto gold electrodes on the plano-convex glass substrates and studied extensively. The measuring apparatus has been greatly improved from that reported earlier and now provides simultaneous monitoring of frictional force and wear. High precision capacitive gauging is employed to provide high resolutions of frictional force of better than 100 μN and height variation (wear) of 2 nm. A large number of specimens of poly(pyrrole) grown from five different counter-ions were prepared and their performances evaluated. The film morphology of each type of film was examined by atomic force microscopy (AFM) for control of the variability of film formation. Results are presented for the friction coefficients and wear rates observed for the films typically at a load of 2 N and a sliding speed of 5 mm s⁻¹. The effects of normal loading force and sliding speed on the friction coefficient are also discussed with a load range of 0.2–5 N and a sliding speed up to 30 mm s⁻¹.     

Nanowear Mapping: A Novel Atomic Force Microscopy Based Approach for Studying Nanoscale Wear at High Sliding Velocities

Most micro/nanoelectromechanical system (MEMS/NEMS) devices and components such as microgears and micromotors operate at very high sliding velocities (of the order of tens of mm/s to few m/s). Nanoscale tribology and mechanics of these devices is crucial for evaluating reliability and failure issues, including those stemming from high wear. We have developed a novel AFM based approach for studying nanoscale wear at sliding velocities up to 10 mm/s. The technique is demonstrated by mapping wear of silicon resulting from two- and three-body abrasions, and that of diamondlike carbon (DLC) resulting from phase transformation of DLC to a graphite-like phase. The novel AFM based approach for nanowear mapping provides a reliable as well as a fast means for investigating wear on the nanoscale as a function of normal load and sliding velocity.     

High temperature tribology of ceramics

The friction and wear behaviour of self-mated couples of MgO---ZrO₂, Al₂O₃ and two types of SiSiC were studied under dry sliding conditions in a special pin-on-disc high temperature tribometer. The temperature was varied between 25 and 1000°C, and the sliding speed from 0.03 m s⁻¹ to 3 m s⁻¹. The morphology of the worn surfaces was studied by means of SEM, and their phase distribution by X-ray diffraction and TEM analyses. The results show that the wear coefficients of all couples mostly increase with increasing temperature and sliding velocity. The wear of MgO---ZrO₂ is influenced by tribo-induced phase transformations while α-Al₂O₃ retains its original structure for all test conditions. For SiSiC delamination and fatigue of the interface Si/ß-SiC predominate. At higher temperatures and sliding velocities tribo-oxidation is effective. The friction coefficients lie between 0.5 and 1.0 under steady-state conditions but for short test durations lower values can occur. The couple SiSiC/SiSiC has low friction coefficients at low sliding velocities and temperatures, even if the steady-state region is reached.     

In situ thermal measurements of sliding contacts

This study examines frictional heating and the associated temperature rise for a sliding circular contact using an in situ thermal micro-tribometer. Observation of the contact temperature used a radiometric approach to measure local temperature at the sliding interface with an emphasis on full field imaging and thermal accuracy. Filled natural rubber samples were slid against optically smooth CaF₂ counter-samples. Temperature rise was measured for externally applied normal forces ranging from∼100 to 1000 mN and sliding velocities ranging from∼250 to 1000 mm/s, producing temperature rises between ∼3 and 26 °C. Measured temperature rise was compared to the analytical models of Jaeger, Archard, and Tian and Kennedy for the average temperature rise in sliding contacts.