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The tribological behaviour of cobalt–tungsten alloy coatings electrodeposited under direct or pulse current mode from a cobalt sulphate–sodium tungstate bath was investigated. At a low average current density of 05 A dm–2 the coatings contain a low amount of tungsten that leads to a milky appearance of the coatings. These coatings have a high roughness, a low hardness, and exhibit a high coefficient of friction and a high wear in sliding tests against corundum. In contrast, cobalt–tungsten coatings deposited at a higher average current density of 1 A dm–2 also have a milky appearance but exhibit tribological properties comparable to the ones of electrodeposited hard chromium and physical vapour deposited TiN. [ABSTRACT FROM AUTHOR]

Al-Khatib S, Berradja A, Celis J-P, Willems G. To investigate the in vitro coefficient of friction of stainless steel arch wire–bracket combinations under fretting contact test conditions performed in air and in different aqueous solutions, like Ringer solution, Ringer with addition of a buffer, Ringer with addition of glucose, and Coca Cola®. The fretting test set-up used allowed to control on-line the contact configuration and the positioning of the contacting parts. A specific positioning method was used to achieve a parallel alignment of arch wire and bracket slot. The effect of arch wire size, roughness, and test environment were investigated. It was found that the aqueous solutions act as a lubricant compared to air. Friction was affected by the arch wire width while the roughness was found to have a limited effect. Stainless steel 0.018′′ × 0.025′′ arch wires exhibited higher frictional forces than stainless steel 0.017′′ × 0.025′′ arch wires on sliding against stainless steel 0.018′′ × 0.025′′ brackets in the selected test environments when tested under identical fretting test conditions. The wear damage on the arch wire after these in-vitro fretting tests was investigated. It revealed that these in-vitro tests are governed by a competition between oxidational wear and abrasive wear taking place at contact areas between brackets and arch wires. For all aqueous solutions a lower coefficient of friction was found compared to tests performed in ambient air. [ABSTRACT FROM AUTHOR]

The nanotribological properties of magnetron sputtered sulfur deficient MoSx (x=1.3) coatings were investigated by oscillating lateral force microscopy (LFM) in ambient air of 50% relative humidity (RH) and 23 °C. The formation of a transfer film on the Si3N4 cantilever tip originated from the MoSx coating is found. The effect of that transfer film on the adhesive and frictional force between the MoSx coating and the cantilever tip, is reported for this nanoscale friction tests. Wear of the MoSx coatings was found on sliding against a Si3N4 cantilever tip. The sliding behavior of MoSx coatings in oscillating nano- and macro-sliding tests performed at different oscillation frequency is discussed. [Copyright &y& Elsevier]

The wear of materials in sliding contacts is considered as resulting from an energy dissipation due to friction between the contacting first bodies. Up to now, no standard procedure in tribology is available to relate that dissipated energy with wear losses for different sliding wear tests and conditions. In this paper, a procedure is proposed to correlate the volumetric wear loss of one first body with the dissipated energy for unidirectional and bidirectional ball-on-flat tests. The model can be useful to predict the service lifetime of components from a limited number of laboratory tests. The validity and limitation of the wear loss versus dissipated energy model is illustrated for hard coatings like TiN and (Ti, Al)N, and multilayered (Ti, Al)N/TiN coatings. The effect of the applied normal load and the relative humidity (RH) of the ambient air on the wear rate for these different coatings are shown as well. A mild oxidational wear model is used to describe the material loss on these coatings in sliding contacts. [Copyright &y& Elsevier]

Abstract: Nanostructured alumina (Al2O3) and nanostructured cermet coatings containing alumina dispersed in a FeCu or FeCuAl matrix, were deposited by atmospheric plasma spraying (APS) from nanostructured powders. These coatings were characterized by SEM, EDAX, TEM, XRD and nanoindentation. Friction and wear behaviour were investigated by sliding and abrasion tests. TEM and XRD revealed that a nanostructuring was retained in the APS deposited coatings. The nanostructured ceramic and cermet coatings were compared in terms of coefficient of friction and wear resistance. Nanostructured cermet coatings appeared to offer a better wear resistance under sliding and abrasion tests than nanostructured Al2O3 coatings. The role of Fe, Cu, and Al additions to the Al2O3 coatings on friction and wear behaviour, was investigated. In the case of FeCu- and FeCuAl-based cermet coatings containing alumina, though the starting material consist of only two compounds, the coatings contain up to four different phases after plasma spraying. The mechanical properties of these different phases namely crack sensitivity and elasto-plastic deformation was determined by nanoindentation. The failure mechanisms were investigated and an attempt was made to establish a ‘structure–property’ relationship. It was shown that an appropriate balance between hard and soft phases results in optimum tribological properties of the nanostructured cermet coatings. [Copyright &y& Elsevier]

Gluconate plating baths were successfully used for the synthesis of homogenous, crack-free nanostructured gradient Co-Sn coatings. The grain size of electrodeposited Co-Sn sub-layers was found by X-ray Diffraction to range from 10 up to 143 nm, whereas the increase of Sn content into the Co-Sn sub-layers led to an increase of average grain size. The average hardness of the gradient coating was found to be significantly higher than that of the pure Sn electrocoating, due to the presence of intermetallic phases. In addition, the frictional properties of the Co-Sn gradient coatings strongly depend on the thickness of the pure Sn top layer, as the thin top layer can shear when sliding against the counterface material. On the other hand, thick Sn layers as in the case of pure Sn electrodeposits result in a higher coefficient of friction of the tribosystem due to adhesive wear phenomena, brought about by the formation of debris at the sliding contact. Furthermore, nanostructured gradient Co-Sn coatings are found to have significantly higher wear resistance when compared to pure Sn coatings. The main wear mechanisms observed were adhesive and abrasive wear of the Sn top layers and subsequent deformation and cracking of the intermetallic based Co-Sn sub-layers. [ABSTRACT FROM AUTHOR]

Abstract: A nanostructured cermet coating consisting of alumina dispersed in a Fe–Cu–Al matrix was deposited by supersonic spraying. The experiments revealed a strong effect of deposition parameters and chemical composition of the powders on the structural characteristics of the Fe–Cu–Al+Al2O3 sprayed cermet. This cermet is made up of complex metallurgical phases as revealed by electron microscopy and X-ray diffraction. The mechanical properties of the different phases detected were determined by nanoindentation. Finally, the friction and wear behavior of this nanostructured sprayed cermet were compared to the ones of benchmark materials. It was found that the Fe–Cu–Al+Al2O3 cermet coating exhibit better tribological properties than the benchmark materials thanks to an appropriate balance of hard and soft phases, and a nanostructuring. The wear mechanism was investigated to establish a ‘structure–property’ relationship for this type of nanostructured cermet coatings. [Copyright &y& Elsevier]

Abstract: The degradation of titanium-based oral rehabilitation systems can occur by corrosion and wear processes taking place simultaneously (tribocorrosion) and influenced by the presence of fluorides. In this study, the tribocorrosion behavior of titanium in artificial saliva solutions containing fluorides is investigated. Before sliding tests were started up, electrochemical measurements such as open circuit potential (OCP) and impedance spectroscopy (EIS) were performed to get information on the corrosion behavior of titanium in artificial saliva solutions. Then, sliding wear tests were carried out at the OCP taken by the test sample. Scanning electron microscopy and weight loss measurements were performed after the tribocorrosion tests. The presence of a compact passive surface film on titanium immersed was demonstrated in artificial saliva free of fluorides and in presence of up to 227ppm F−. However, a progressive degradation of titanium was observed at a F− concentration of 12,300ppm. Additionally, the corrosion and wear resistance of the titanium oxide film formed at a F− concentration of 12,300ppm differ from the ones obtained up to 227ppm F− reflected by a decrease of the coefficient of friction although the material loss increased. The synergism between wear and corrosion processes on titanium needs thus to be further investigated to reach a reliable prediction of the long-term behavior of titanium-based prostheses and implants in the oral cavity. [Copyright &y& Elsevier]