Stable Friction Coefficient Research Articles

Tribological properties of TiN/diamond and TiAlN/diamond bilayer films sliding against carbon steel

In this study, the tribological properties of the monolayer micro-crystalline diamond and nano-crystalline diamond films, TiN/diamond and TiAlN/diamond bilayer films on cemented tungsten carbide substrates are evaluated by dry sliding against the medium carbon steel counterpart balls, in terms of the coefficient of friction, wear rate, worn surfaces, and chemical transitions in the contacting wear zones. The significant coefficient of friction reducing effect of top-layer TiN and TiAlN coating only happens on the nano-crystalline diamond film, where the stable coefficient of friction of TiN/nano-crystalline diamond or TiAlN/nano-crystalline diamond bilayer coating reduces 9% or 53% compared with the nano-crystalline diamond film. The formed ionic metal oxides such as Fe2O3 or Fe3O4 coming from the chemisorbing of the atmospheric molecular water, oxygen in the air and the delamination of steel ball due to the repeated friction interaction is supposed to be responsible for the coefficient decreasing effect. The TiN or TiAlN film on diamond layer exhibits lower positive wear rate compared with the TiN or TiAlN film itself, due to the load support ability improvement resulted from the high hardness of diamond interlayers. Among all the tested hard films, the TiAlN/nano-crystalline diamond bilayer coating exhibits the valid potential to be the optimized tool coating in carbon steel machining in terms of its low coefficient of friction and wear rate, which may come from the self-lubricated transfer tribolayer formation on the TiAlN layer, as well as the enhanced mechanical supporting capacity of the underneath smooth and hard nano-crystalline diamond interlayer.

Microstructures and mechanical properties of Cu/Ti3SiC2/C/graphene nanocomposites prepared by vacuum hot-pressing sintering and hot isostatic pressing

Abstract Copper-graphite composite is an ideal friction material due to its good heat resistance, wear resistance and friction coefficient stability. The addition of Ti3SiC2 can improve strength, hardness and wear resistance of copper-graphite alloy material without affecting its self-lubricating properties and electrical conductivity. Two-dimensional graphene has become an attractive composite reinforcement owing to their unique electrical, mechanical and thermal properties. Cu/Ti3SiC2/C/Graphene naocomposite materials reinforced with graphene have been fabricated by vacuum hot-pressing sintering and hot isostatic pressing. Microstructures and mechanical properties of Cu/Ti3SiC2/C/Graphene naocomposite materials with different graphene contents have been systematically investigated. Microstructures of the composites were examined by optical microscopy, X-ray diffraction, back scattered electron imaging, scanning electron microscope with energy dispersive spectrometer and transmission electron microscope. The mechanical properties were determined from Brinell hardness, tensile, compressive and shear tests. Results demonstrated that there was an optimum value of graphene content which has an impact on microstructures and mechanical properties of Cu/Ti3SiC2/C/Graphene naocomposite materials. Based on graphene content on microstructure and mechanical properties of Cu/Ti3SiC2/C/Graphene naocomposite materials, strengthening and fracture mechanisms by graphene reinforcement have been analyzed.

Effect of different reinforcement materials on the formation of secondary plateaus and friction properties in friction materials for automobiles

Abstract The effect of combining various reinforcement materials, such as potassium titanate, Cu fiber, and steel fiber on the frictional and morphological properties of brake pads was investigated. The pad types were denoted as PT (with only potassium titanate), PC (with potassium titanate and Cu fiber), and PF (with potassium titanate and steel fiber). The PC pad exhibited the most stable coefficient of friction during repetitive braking across all the test temperatures. The PF pad exhibited a stable coefficient of friction below 250 °C, with unstable behavior at 300 °C. The surfaces of all three pads contained contact plateaus. The PC and PF pads showed primary and secondary contact plateaus, while the PT pad showed only primary contact plateaus.

ZnO nanowires decoration on carbon fiber via hydrothermal synthesis for paper-based friction materials with improved friction and wear properties

Two-step growth of ZnO nanowires (NWs) on carbon fiber (CF) surface via hydrothermal synthesis was studied and their application in the preparation of paper-based friction materials by wet-forming process was also investigated. SEM and EDS results showed a dense and uniform ZnO NWs layer with vertical alignment was well established on surface of CFs. UV–vis spectra and XRD characterization further confirmed the formation of ZnO NWs on CFs surface. In comparison with control sample (paper-based friction material containing pristine CFs), the modified sample (paper-based friction material containing modified CFs) exhibited higher and more stable dynamic friction coefficient and greater wear resistance. It was concluded that the CFs@ZnO NWs had excellent tribological properties and was highly promising for wet paper-based friction material.

Friction and wear behavior of SiC particles deposited onto paper-based friction material via electrophoretic deposition

Abstract In order to improve the friction and wear properties of the paper-based friction materials, SiC particles (SiCp) were uniformly deposited onto the paper-based friction material via electrophoretic deposition with 300 V, 400 V and 500 V deposition voltages, obtaining the SiCp-content of 10.8, 15.9 and 22.9 wt%. The microstructure and thermal stability of the paper-based friction materials were characterized by scanning electron microscope (SEM) and thermogravimetry (TG). And the porosity, mechanical and tribological properties of samples were tested by mercury injection apparatus, electromechanical universal testing machine and friction tester. The results show that the uniform distribution of SiCp in the paper-based friction materials decreases the porosity and obviously improves the thermal stability and mechanical properties. Meanwhile, the incorporation of SiCp increases the stability of friction coefficient and the wear resistance of paper-based friction materials. Especially, the wear rate of paper-based friction material with 400 V deposition voltage treatment (CP400) for optimal SiCp-content of 15.9% decreases by 51.2% from 5.8 × 10−14 to 2.8 × 10−14 m3/(N·m) compared with that of the bare paper-based friction material (CP0). The electrophoretic deposition techniques provide the possibility of uniform addition of micro-nano particles to the paper-based friction material, which is beneficial to improve the friction and wear properties.

Effect of Atmosphere and Temperature on the Tribological Behavior of the Ti Containing MoS2 Coatings Against Aluminum

MoS2 coatings exhibit low coefficient of friction (COF) when sliding against aluminum; however, the magnitudes of their COF show high sensitivity to environmental conditions. Ti could reduce the sensitivity of the frictional behavior of MoS2 coatings to moisture. This study examines the tribological properties of Ti containing MoS2 coating (Ti–MoS2) tested against an aluminum alloy (Al-6.5% Si) in ambient air (58% relative humidity, RH), dry oxygen, dry air and dry N2 (< 4% RH) atmospheres. The Ti–MoS2 coating exhibited similar COF values under an ambient (0.14), dry oxygen (0.15) and dry air (0.16) atmospheres. It was found that oxidation of MoS2 to MoO3 was responsible for high COF under these testing conditions as revealed by Energy-dispersive X-ray Spectroscopy (EDS) and micro-Raman spectroscopy. However, a low and stable COF of 0.07 was observed under a dry N2 condition. This work further showed that the tests performed at elevated temperatures, up to 400 °C in a dry N2 atmosphere sustained the low and stable COF of the Ti–MoS2 coatings. The sliding tests performed under a dry N2 atmosphere prevented the formation of MoO3 and as a result, the Ti–MoS2 coatings maintained low COF values. Low COF values were also attributed to the formation of MoS2 transfer layers.

Preparation and properties of MNSiO2/CN40/PF nanocomposites

MNSiO2/CN40/PF nanocomposites have been fabricated in this study. Nano‐SiO2 was initially surface‐modified with γ‐methacryloxypropyl trimethoxysilane (MPS). The resulting nano‐SiO2 was denoted as MNSiO2. Core‐shell composites, MNSiO2/CN40, was fabricated by a solution blending method, in which MNSiO2 served as the core and liquid acrylonitrile butadiene rubber (CN40) served as shell. Phenol formaldehyde resins (PF) were prepared by means of condensation polymerization method. MNSiO2/CN40/PF hybrid with a fixed weight ratio (2:98) of MNSiO2/CN40 composites to PF were prepared by a modified routine of preparation of pure PF. MNSiO2/CN40/PF nanocomposites were prepared by incorporating filler particles with MNSiO2/CN40/PF hybrid via melt blending, followed by compression molding method. The impact of MNSiO2 content on the properties of the MNSiO2/CN40/PF nanocomposites was systematically investigated by varying the weight ratio of MNSiO2 to CN40 in MNSiO2/CN40 core‐shell composites. The results revealed that MNSiO2/CN40 significantly enhanced the mechanical and frictional properties of the as‐prepared MNSiO2/CN40/PF nanocomposites. Compared to pure PF, the impact strength of MNSiO2/CN40/PF hybrid containing 1.0 wt% of MNSiO2 in MNSiO2/CN40 composites increased by 29.2%, and the initial storage modulus was enhanced dramatically. In addition, MNSiO2/CN40/PF hybrid demonstrated stable friction coefficient, while the wear mechanism of pure PF was adhesive and fatigue wear. The MNSiO2/CN40/PF nanocomposites exhibited a combination of adhesive, abrasive, and fatigue wear. POLYM. COMPOS., 40:179–186, 2019. © 2017 Society of Plastics Engineers

Grafting methyl acrylic onto carbon fiber via Diels-Alder reaction for excellent mechanical and tribological properties of phenolic composites

Abstract Carbon fibers (CFs) were grafted with methyl acrylic via Diels-Alder reaction at the different oil bath temperature effectively creating a carboxyl functionalized surface. The effect of grafting temperature on the surface morphology and functional groups of carbon fibers were investigated by FTIR, Raman spectroscopy, XPS and SEM respectively. The results showed that the optimal grafting temperature was 80 °C, and the relative surface coverage by carboxylic acid groups increased from an initial 5.16% up to 19.30% significantly improved the chemical activity without damaging the skin and core region of the carbon fibers. Mechanical property tests indicated that the shear and tensile strength of the sample with the grafting temperature of 80 °C (CFRP-3) increased obviously by 90.3% and 78.7%, respectively, compared with the pristine carbon fibers reinforced composite. Further, the sample CFRP-3 exhibited higher and more stable friction coefficient and improved wear resistance, while the wear rate decreased 52.7%, from 10.8 × 10 −6 to 5.1 × 10 −6 mm 3 /N m. The present work shows that grafting methyl acrylic via Diels-Alder reaction could be a highly efficient and facile method to functionalize carbon fibers for advanced composites.

Low friction of graphene nanocrystallite embedded carbon nitride coatings prepared with MCECR plasma sputtering

Graphene nanocrystallite embedded carbon nitride (GNECN) coatings were fabricated with the mirror confinement electron cyclotron resonance (MCECR) plasma sputtering system under low energy electron irradiation at various N2/Ar ratios. N2/Ar ratio was clarified to be an effective deposition parameter for tailoring the composition and structure of the GNECN coatings. It was observed that the deposition rate, N/C atomic ratio, internal stress, surface roughness, and scratch depth of the prepared GNECN coatings change greatly with the increasing N2/Ar ratio from 1/11 to 1/3. Graphene nanocrystallite was clearly identified in the amorphous carbon nitride structure from the analyses of TEM, XPS, and Raman spectroscopy. Moreover, the friction behavior of the GNECN coatings sliding against Si3N4 balls in both ambient air and N2 gas stream showed less dependency on the N2/Ar ratio. Specifically, high friction coefficients ranging from 0.10 to 0.15 were obtained in ambient air, whereas, stable and low friction coefficients of <0.05 were achieved in N2 gas stream. Optical images and Raman spectra of the worn surfaces on the Si3N4 balls and GNECN coatings suggested that a homogeneous tribofilm on the mating ball surface together with the generation and evolution of the nano-size graphene structure in the GNECN coating are the key points in achieving stable and low friction coefficients of <0.05 of the GNECN coatings in N2 gas stream. Finally, it was argued that the combination of low energy electron irradiation and nitrogen atom incorporation in the MCECR plasma sputtering system results in the embedding of soft graphene nanocrystallites into hard carbon nitride matrix, providing a beneficial architecture for achieving stable and low friction coefficients of GNECN coatings in N2 gas stream.

Investigation on tribological behaviour of boron doped diamond coated cemented tungsten carbide for cutting tool applications

In this paper, tribological performance of boron doped diamond (BDD) films and boron doped graded layer diamond thin films (BDD/transition layer/NCD) were studied in detail. The widely used cemented tungsten carbide (WC-6wt%Co) was selected as a substrate material for diamond coating. Diamond films were deposited on WC-Co by Hot-filament CVD reactor (HFCVD) setup. Tribology experiment was conducted using a reciprocating tribometer with a normal load of 30N and a sliding velocity of 10mm/s for a constant stroke length of 3mm. Silicon nitride (Si3N4) ball was used as a counterpart to study the friction and wear behaviour of diamond films. The surface morphology, topography & roughness of the diamond films were analysed using scanning electron microscope and atomic force microscope respectively. The hardness of the thin diamond films was measured by using Berkovich nano indentation test method. In MCD coating (sp3 C-C) the wear rate increases with increase in hardness. On the contrary for NCD (sp2 graphite), wear rate decreases with decrease in hardness because of the corresponding phases. The test results found that BDD and boron doped graded layer shows a stable lowest friction coefficient values of 0.004 and 0.003 compared with conventional microcrystalline diamond films (0.007). On the other hand, the wear diameter of the silicon nitride ball for BDD and boron doped graded layer are found to be 620μm and 785μm, relatively lower in comparison with microcrystalline diamond films (897μm). The wear track width was measured by scanning electron microscope and shows that BDD and boron doped graded layer indicates lower wear track width 564μm and 596μm compared with microcrystalline diamond films (712μm). Raman mapping was conducted on the wear track of the diamond films to know about the phase pure diamond (sp3) and partly graphite phases (sp2) which in turn contributes for the distinct residual stresses in thin films. The obtained lower friction coefficient for BDD on WC-Co cutting tool can be suitable for machining of aluminium based metal matrix composites effectively.

Tribological Properties of AlSi12-Al&#8322;O&#8323; Interpenetrating Composite Layers in Comparison with Unreinforced Matrix Alloy.

Alumina–Aluminum composites with interpenetrating network structures are a new class of advanced materials with potentially better properties than composites reinforced by particles or fibers. Local casting reinforcement was proposed to take into account problems with the machinability of this type of materials and the shaping of the finished products. The centrifugal infiltration process fabricated composite castings in the form of locally reinforced shafts. The main objective of the research presented in this work was to compare the tribological properties (friction coefficient, wear resistance) of AlSi12/Al2O3 interpenetrating composite layers with unreinforced AlSi12 matrix areas. Profilometric tests enabled both quantitative and qualitative analyses of the wear trace that formed on investigated surfaces. It has been shown that interpenetrating composite layers are characterized by lower and more stable coefficients of friction (μ), as well as higher wear resistance than unreinforced matrix areas. At the present stage, the study confirmed that the tribological properties of the composite layers depend on the spatial structure of the ceramic reinforcement, and primarily the volume and size of alumina foam cells.

Tribological Behavior of Tool Steel Under Press Hardening Conditions Using Simulative Tests

Press hardening is employed in the automotive industry to produce advanced high-strength steel components for safety and structural applications. This hot forming process depends on friction as it controls the deformation of the sheet. However, friction is also associated with wear of the forming tools. Tool wear is a critical issue when it comes to the dimensional accuracy of the produced components and it reduces the service life of the tool. It is therefore desirable to enhance the durability of the tools by studying the influence of high contact pressures, cyclic thermal loading, and repetitive mechanical loading on tool wear. This is difficult to achieve in conventional tribological testing devices. Therefore, the tribological behavior of tool–workpiece material pairs at elevated temperatures was studied in a newly developed experimental setup simulating the conditions prevalent during interaction of the hot sheet with the tool surface. Uncoated 22MnB5 steel and aluminum–silicon (Al–Si)-coated 22MnB5 steel were tested at 750 °C and 920 °C, respectively. It was found that higher loads led to lower and more stable friction coefficients independent of sliding velocity or surface material. The influence of sliding velocity on the coefficient of friction was only marginal. In the case of Al–Si-coated 22MnB5, the friction coefficient was generally higher and unstable due to transfer of Al–Si coating material to the tool. Adhesion was the main wear mechanism in the case of uncoated 22MnB5.

Synergistic Interactions of a Synthetic Lubricin-Mimetic with Fibronectin for Enhanced Wear Protection.

Lubricin (LUB), a major mucinous glycoprotein of mammalian synovial fluids, is believed to provide excellent lubrication to cartilage surfaces. Consequently, when joint disease or replacement leads to increased friction and surface damage in the joint, robust synthetic LUB alternatives that could be used therapeutically to improve lubrication and surface protection are needed. Here, we report the characterization of a lubricating multiblock bottlebrush polymer whose architecture was inspired by LUB, and we investigate the role of fibronectin (FN), a glycoprotein found in the superficial zone of cartilage, in mediating the tribological properties of the polymer upon shear between mica surfaces. Our surface forces apparatus (SFA) normal force measurements indicate that the lubricin-mimetic (mimLUB) could be kept anchored between mica surfaces, even under high contact pressures, when an intermediate layer of FN was present. Additional SFA friction measurements show that FN would also extend the wearless friction regime of the polymer up to pressures of 3.4 MPa while ensuring stable friction coefficients (μ ≈ 0.28). These results demonstrate synergistic interactions between mimLUB and FN in assisting the lubrication and wear protection of ideal (mica) substrates upon shear. Collectively, these findings suggest that our proposed mimLUB might be a promising alternative to LUB, as similar mechanisms could potentially facilitate the interaction between the polymer and cartilage surfaces in articular joints and prosthetic implants in vivo.

Tribological Behavior of TiAl Metal Matrix Composite Brake Disk with TiC Reinforcement Under Dry Sliding Conditions

In this investigation, the effect of TiC particulate reinforcement and sintering parameters on tribological behavior of TiAl metal matrix composite (TMMC) has been studied and compared with commercially conventional gray cast iron to evaluate the use of TMMC as brake disk material in an automobile. Three sample disks of TMMC containing TiC particulate reinforcement (D1-5 wt.%, D2 and D3-10 wt.%) were produced by the spark plasma sintering process. D3 compared with D2 was sintered at a higher temperature to evaluate the effect of SPS parameters on the wear characteristics of TMMC. All experiments were performed on pin-on-disk tribotester under a dry sliding condition with different loads (10-11.5 N) and sliding velocities (0.2-0.9 m/s). It is found that higher content of TiC increased TMMC hardness and density. XRD technique has been used to analyze the phase composition. Owing to the high sintering temperature, α-2 Ti3Al phase was formed which further enhanced the matrix anti-wear capability. Scanning electron microscope (SEM) was used to capture the wear track and observe wear mechanism. Energy-dispersive spectroscopy (EDS) has been used to analyze the tribofilm and wear debris. The results showed that the tribofilm for TMMC was mainly composed of metal oxides. Oxidation of Al and Ti due to frictional heat provides wear-resistant protective layer. Under almost all sliding conditions, TMMC, especially disk D3, exhibited minimum wear rate and stable friction coefficient, whereas gray cast iron exhibited lower and unstable friction coefficient as well as higher wear rate. TMMC has shown superior tribological characteristics over gray cast iron in terms of low wear rate along with stable and adequate friction coefficient which is necessary for braking operation and life of brake disk. However, further investigation on full-scale automobile conditions is needed for its practical application.

The self-sense monitoring of coating wear by in situ SiO2-coated SrAl2O4:Eu2+,Dy3+ luminescent particles

Monitoring of coating health was achieved by embedding sensor materials SrAl2O4:Eu2+,Dy3+ in tribological coatings, and luminescence spectra of sensor materials were used for damage sensing by monitoring the intensity of the spectra. When the light emitting from coating disappeared, it indicated the coating wearied off and the substrate exposed. But emission intensity of the SrAl2O4:Eu2+,Dy3+ in tribological coating was quenched in preparation process which will weaken the effect of coating instructions. In this work, SiO2-coated SrAl2O4:Eu2+,Dy3+ were coupled with Cu-14Al-X alloy powders to form sensor composite coating by supersonic plasma spraying. The composite coatings before and after SiO2-coating were compared of phase composition, microstructure, luminescence properties and tribological performance. The composite coating shows not only good resistance to wear but also exhibits strong green light emitting under UV light. The SiO2-coating could prevent phosphors quenching due to high temperature in the supersonic plasma spraying process, and could repair the wear surface. The composite coating with SiO2-coated SrAl2O4:Eu2+,Dy3+ particles had very stable friction coefficient, When the coating ratio of SiO2 was 10% for SrAl2O4:Eu2+,Dy3+ particles, the luminescent properties and friction property of composite was optimal.

Composites materials for friction and braking application

The brake pads are an important component in the braking system of automotive. Materials used for brake pads should have stable and reliable frictional and wear properties under varying conditions of load, velocity, temperature and high durability. These factors must be satisfied simultaneously which makes it difficult to select effective brake pads material. The paper presents the results of the study for characterisation of the friction product used for automotive brake pads. In the study it was developed four frictional composites by using different percentages of coconut fibres (0%, 5%, 10%, 15%) reinforcement in aluminium matrix. The new composites tested in the laboratory, modelling appropriate percentage ratio between matrix and reinforcement volume and can be obtained with low density, high hardness properties, good thermal stability, higher ability to hold the compressive force and have a stable friction coefficient. These characteristics make them useful in automotive industry.

Friction and wear performance of functionally graded ductile iron for brake pads

Due to its wear resistance, functionally graded ductile iron (FGDI) is a potential material for automotive brake pads. In this investigation it was studied (i) under well controlled sliding conditions in a typical tribological model-test to show in detail the mechanism behind improved wear resistance, namely the “property on-demand” functionality in the sub-surface zone and (ii) in a validation experiment on a disc-brake machine that mimic realistic brake-contact conditions, at various elevated temperatures against a carbon-reinforced ceramic disc in order to understand its tribological behaviour in a real automotive brake application. The results revealed a positive effect of the functional gradient zone (FGZ) on the wear performance of the functionally graded ductile-iron pads. Most notably, improved wear resistance was observed under sliding model-test conditions as well as a more-stable wear and wear that is less sensitive to the contact temperature during a disc-brake test. In addition, the sliding model-tests revealed that FGZ leads to faster running in and a more stable coefficient of friction. The disc-brake tests also show that FGZ ensures a relatively constant coefficient of friction for all contact temperatures. Surface analysis of the pads further revealed that oxidized, smoothened plateaus containing graphite nodules were formed on the pads. On the other hand, a patchy and layered transfer film resulting from the pads was formed on the disc's surface.

Three-body abrasive wear by (silica) sand of advanced polymeric coatings for tilting pad bearings

Aromatic Thermosetting Polyester (ATSP)-based coatings exhibit extremely low wear rates (4.15×10-8mm3/Nm) under boundary lubrication simulating tilting pad bearings in harsh conditions. It is acknowledged that coatings’ resistance to abrasive particles encountered in the bearing is also critical; the abrasive wear resistance of ATSP-based coatings and polyether ether ketone(PEEK)-based coatings are studied in this work using an accelerated wear experimental configuration with submerged lubrication and 2% weight of silica sand. The coatings’ performance is compared with bare substrate (C182000 chromium copper) material. The results demonstrate that ATSP-based coatings have good abrasive wear resistance and very stable coefficient of friction (COF) because of their elastic deformation and a black tribo-layer that forms on the surface. ATSP-based coatings exhibit higher wear rate after adding the silica sand, while the wear rate of PEEK-based coating decreased with silica sand compared with no silica sand condition due to silica sand working as solid lubricant. The abrasive wear mechanism of ATSP was studied by varying the duration of experiments between 3420m and 13,680m, different silica sand size particles and different pin materials; scanning electron microscopy (SEM), micro-scratch testing and X-ray photoelectron spectroscopy (XPS) were used to verify the wear mechanisms.

Tribology and sliding electrical contact resistance of e-beam hard Au: Effects of annealing

Nanocomposite Au-ZnO thin films in the dilute oxide (<5.0vol%) regime were synthesized by electron beam (e-beam) evaporation, as alternatives to electroplated Au hardened with Ni. Tribological measurements of e-beam hard Au were made while passing current through sliding contacts; electrical contact resistance (ECR) and friction data were simultaneously acquired during the test. The friction, wear and ECR behaviour were studied for the as-deposited film condition, and after annealing at 250°C and 350°C in air. The study revealed that the 250°C annealed Au-2 vol% ZnO film exhibited the lowest, stable friction coefficient s (µ~0.25) and ECR (~35 mΩ) during sliding. Furthermore, the wear rate of this 250°C annealed ZnO hardened Au nanocomposite film was an order of magnitude lower at 1.5×10−5mm3/Nm than for a typical Ni hardened, electroplated Au film at 1.3×10−4mm3/Nm. Cross-sectional transmission electron microscopy studies inside the wear surfaces revealed that the extremely stable, low friction coefficients and wear rate of annealed Au-2 vol% ZnO film was due to partial coverage of the wear surface with a ZnO tribofilm that reduced the adhesive contact contribution to wear with minimal impact on ECR. The potential implications of this study in the search for an environmentally friendly alternative to widely used electroplated hard Au are discussed.

Microstructures and Properties of Nano-structural WC-12Co Coatings Deposited by AC-HVAF

WC-12Co powder with nano WC grains was used to deposit wear resistant coatings by AC-HVAF (acukote high velocity air-fuel) spray system. The phase compositions and microstructures of the coatings were examined. The microhardness, fracture toughness and wear resistance were investigated. According to the X-ray diffraction (XRD) analysis, the principal phase is WC, and its decomposition products are not found. The sprayed coating has a porosity lower than 1% and the grain sizes of coatings are among 80∼100 nm which contribute to an average microhardness (HV 0.3 ) as high as 19 403 MPa on the surface and 17 410 MPa on the cross-section. At the load of 15 N and rotational speed of 1198 r/min of WC counter body, the nano-sized coatings show a decreased average mass loss by 40% compared with micro-sized coatings, and a stable friction coefficient of 0.26∼0.28 (micron-sized coatings: 0.25∼0.4) in the dry wear conditions. It is concluded that the sprayed nano structural WC-12Co coatings have a better wear resistance.