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In this paper, we study the wear resistance of multi-layered composites of Cu/SiC + Gr hybrid composites prepared by layer compaction and pressure sintering. The tribological behavior and wear resistance of the composites were evaluated at a range of sliding speeds (5, 10, 30 and 35m/s) in a laboratory scale inertia brake dynamometer for brake friction material applications. The wear surface morphology and mechanisms were studied using scanning electron microscopy (SEM), XRD, and stereoscopy. The microstructure of the composites was also characterized using SEM and optical microscopy and the mechanical response in compression and flexure was evaluated. The results of these tests indicate that the density, wear resistance, braking behavior and mechanical response can be significantly improved by the presence of a layer of copper away from the sliding surface. The presence of the layer also improved friction and wear resistance significantly. The formation of mechanically mixed tribolayer and oxides (Fe3O4) reduced the wear rate and stabilized the friction coefficient at 30 and 35m/s. Finally, crack deflection and branching at the interface between the composite and Cu layers improved the flexural strength of the layered composites. The fractography analysis indicates a quasi-cleavage intergranular fracture in the composite layer and a purely ductile fracture in the Cu layer. � 2014 Elsevier B.V.

The main objective of the present work is to investigate the dry sliding wear behaviour of a magnesium matrix composite reinforced with zinc oxide nano-particles. Magnesium matrix composites have many applications, especially in the automotive and aerospace industries, due to their superior specific properties. A magnesium matrix composite with 0.5. vol.% ZnO nano-reinforcement was prepared using powder metallurgy and was hot extruded to eliminate pores. The wear behaviour of the Mg/ZnO nano-composite was investigated by conducting dry sliding tests as a function of wear with an oil-hardened non-shrinking (OHNS) steel disc as the counterpart on a pin-on-disc apparatus. Wear tests were conducted for normal loads of 5, 7.5 and 10. N at sliding velocities of 0.6, 0.9 and 1.2. m/s at room temperature. The variations of the friction coefficient and wear rate with the sliding distances (500. m, 1000. m and 1600. m) for different normal loads and sliding velocities were plotted and analysed. To study the dominant sliding wear mechanism for various test conditions, the worn surfaces were analysed using scanning electron microscopy. The wear rate was found to increase with the load and sliding velocity. � 2014 Elsevier Ltd.

A two-phase nanocomposite coating that consists of inclusions of silver in a vanadium nitride matrix (VN/Ag) was investigated as a potential adaptive coating with a reduced friction coefficient from 25 to 1000 °C. This nanocomposite structure was selected based on the premise that silver and silver vanadate phases would form on the surface of these coatings, reducing their friction coefficient in the (i) room to mid-range and (ii) mid-range to high temperatures, respectively. Silver and vanadium were expected to react with oxygen at high temperatures and create a lubricious silver vanadate film on the coating. The VN/Ag coatings were deposited using unbalanced magnetron sputtering and their elemental composition was evaluated using X-ray photoelectron spectroscopy. The tribological properties of the materials against Si 3N4 balls were investigated at different temperatures. The lowest friction coefficients recorded for samples with identical compositions were 0.35, 0.30, 0.10 and 0.20 at 25, 350, 700 and 1000 °C, respectively. Post-wear testing Raman spectroscopy and X-ray diffraction (XRD) measurements revealed the formation of silver vanadate compounds on the surface of these coatings. In addition, real time Raman spectroscopy and high temperature XRD revealed that silver vanadate, vanadium oxide and elemental silver formed on the surface of these coatings upon heating to 1000 °C. Upon cooling, silver and vanadium oxide were found to combine at about 400 °C, leading predominantly to the formation of silver vanadate phases on the surface of these materials. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. 58 5326 5331 5326-5331

Suitability of different multi-axial parameters in predicting fretting fatigue life of Ti-6Al-4V specimens has been investigated. Ameliorating effect of surface treatments on fretting fatigue has been studied. In simple uni-axial/multi-axial fatigue tests, nucleation as well as propagation of cracks occur under the influence of identical stresses. Hence nucleation accounts for most of the total life. Fretting fatigue crack nucleation occurs due to very large contact stresses, effect of which is felt only close to the surface (due to steep gradients). Propagation mostly occurs due to lower stresses in the bulk of the material (negligible influence of contact tractions) and forms a significant portion of total life. Total life has to be taken as sum of initiation life calculated from different multi-axial fatigue parameters and propagation life from conventional fracture mechanics approach. Steep stress gradients necessitate the adoption of a statistics based approach to predict the crack initiation life, based on an assumed distribution of flaws. The quality of comparison between predicted and experimentally observed failure lives provides confidence in the notion that conventional fatigue life prediction tools can be used to assess fretting fatigue failure. Effect of surface treatments like shot-peening with or without additional surface coatings on total life of the specimen and on friction coefficient has been studied. � 2009 Elsevier Ltd. All rights reserved.

TiAlBN films were produced by simultaneously sputtering from TiAl and TiB2 targets in Ar/N2 mixtures at a substrate temperature of 150 °C. The goal of this investigation was to study the influence of the nitrogen content (0-40 at.%) on two sets of four coatings with B/Al ratios of approximately 1 and 5. All coatings showed very dense structures and were mainly found to be only partially crystalline. Evidence of Al, AlN, Ti, TiB2, TiN and BN bonding was observed in the XPS spectra, and correlation of the compositional and mechanical data showed that film hardness and elastic modulus decreased with the presence of a 'soft' phase (Al, Ti or amorphous BN). Hardness and elastic modulus values of up to 25 and 250 GPa, respectively, were found for TiAl0.3B1.7 coatings. Coatings with a similar elastic modulus to that of the steel substrate showed the best adhesion. Friction coefficient values between approximately 0.5 and 0.7 were recorded in sliding wear experiments against SAE52100 and WC/6%Co balls. Although friction coefficients could not be correlated to the amorphous BN contents, it was noted that low combined ball and disc wear rates were found for coatings containing large amounts of BN. Coatings with B/Al 5 and elastic moduli of ≤290 GPa showed two orders of magnitude better sliding wear resistance against WC/6%Co balls compared to commercially available TiN. TiAlBN films were produced by simultaneously sputtering from TiAl and TiB2 targets in Ar/N2 mixtures at a substrate temperature of 150 °C. The goal of this investigation was to study the influence of the nitrogen content (0-40 at.%) on two sets of four coatings with B/Al ratios of approximately 1 and 5. All coatings showed very dense structures and were mainly found to be only partially crystalline. Evidence of Al, AlN, Ti, TiB2, TiN and BN bonding was observed in the XPS spectra, and correlation of the compositional and mechanical data showed that film hardness and elastic modulus decreased with the presence of a `soft' phase (Al, Ti or amorphous BN). Hardness and elastic modulus values of up to 25 and 250 GPa, respectively, were found for TiAl0.3B1.7 coatings. Coatings with a similar elastic modulus to that of the steel substrate showed the best adhesion. Friction coefficient values between approximately 0.5 and 0.7 were recorded in sliding wear experiments against SAE52100 and WC/6%Co balls. Although friction coefficients could not be correlated to the amorphous BN contents, it was noted that low combined ball and disc wear rates were found for coatings containing large amounts of BN. Coatings with B/Al approximately 5 and elastic moduli of ≤290 GPa showed two orders of magnitude better sliding wear resistance against WC/6%Co balls compared to commercially available TiN. 113 126 133 126-133

There is presently considerable interest in wear resistant coatings produced using closed field unbalanced magnetron sputtering technology. For example, layered films of diamond-like carbon (DLC) with tungsten or titanium additions have been widely reported. The benefit is that the mechanical properties are enhanced (e.g. giving greater toughness) also it is possible to control the stress state and enhance adhesion. Here we report the further development of this concept by the addition of TiN, TiCN and TiC layers in DLC-based composites, utilizing an additional source of electrons in the vicinity of substrate to enhance ionisation of the plasma and increase coating density. Composite coatings of ceramics TiN, TiCxNy, TiC, CrN, TiCrN, TiCrCN, TiCrC, metal doped Tix%-DLC and their combinations were deposited on 316 stainless steel substrates. The mass flow of reactive gases into the chamber was controlled using plasma optical emission monitoring to achieve the desired coating composition. The morphology of the coatings was investigated and correlated with Knoop microhardness, scratch adhesion, pin-on-disc and wet abrasive wheel tests. Dense T-type structures were found for most of the coatings and a high toughness of Ti30%-DLC coating with a TiC interlayer was observed. Low friction coefficients of 0.15-0.18 for coatings with Tix%-DLC layers confirmed their benefit in sliding wear applications, while TiCN coatings were found to be the best in abrasive wear conditions. © 1995. 73 185 197 185-197