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Abstract: Nanostructured WC–Co and WC–Co–Al coatings, with about 300-μm as-deposited coating thickness, were deposited by high velocity oxy-fuel (HVOF) spraying. Agglomerated nanostructured cermet powders produced by the Mechanomade® process was used for HVOF spraying. Dense and well-adherent coatings with crystal sizes below 30nm were deposited on stainless steel 304 substrate. Porosity was less than 5% and the bond strength with the substrate was around 60MPa. Experimental data on friction, wear, and abrasion resistance revealed that nanostructured WC–Co based coatings containing some Al as alloying element, exhibit improved tribological characteristics in comparison to nanostructured and micron-sized WC–Co coatings. This was attributed to a carbide particle distribution within the coating revealed by SEM, the absence of brittle W2C-like phases revealed by XRD, and the presence of Al at particle/matrix boundaries revealed by TEM. [Copyright &y& Elsevier]

Biofilm formation is widely reported in the literature as a problem in the healthcare, environmental, and industrial sectors. However, the role of biofilms in sliding contacts remains unclear. Friction during sliding was analyzed for titanium covered with mixed biofilms consisting of Streptococcus mutans and Candida albicans. The morphology of biofilms on titanium surfaces was evaluated before, during, and after sliding tests. Very low friction was recorded on titanium immersed in artificial saliva and sliding against alumina in the presence of biofilms. The complex structure of biofilms, which consist of microbial cells and their hydrated exopolymeric matrix, acts like a lubricant. A low friction in sliding contacts may have major significance in the medical field. The composition and structure of biofilms are shown to be key factors for an understanding of friction behavior of dental implant connections and prosthetic joints. For instance, a loss of mechanical integrity of dental implant internal connections may occur as a consequence of the decrease in friction caused by biofilm formation. Consequently, the study of the exopolymeric matrix can be important for the development of high-performance novel joint-based systems for medical and other engineering applications. [ABSTRACT FROM PUBLISHER]

Abstract: Friction at different force, length, and time scales is of great interest in tribology. The mechanical, chemical, and physical (atomic) interactions, each operating at their own force and length scale, make friction a highly scale dependent event. This work is an attempt to trace important mechanisms of friction on commonly used engineering materials over a normal force scale ranging from nN up to N, and thereby altering the contact size from nm2 up to mm2. The relevance of existing theories on friction is verified on different engineering surfaces taking into account mechanical (hard/hard, hard/soft), chemical, and physical aspects of the sliding surfaces. The applicability of Amontons’ law is experimentally investigated. For rough surfaces it was found that the existence of a constant coefficient of friction over a wide force and length scales is only a special case. For a hard/hard tribosystem (like DLC/Si3N4), a linear dependence of friction on normal force was observed, whereas a non-linear relationship was more evident on hydrophilic surfaces and hard/soft couples. Irrespective of the material system considered, the dependence of friction on normal force can be altered by modifying the surface roughness or texture of the material couple. In all, changes in the force and length scales bring about significant changes in the governing mechanisms of friction. The experimental findings were in good agreement with recent elasto-plastic and fractal contact mechanical theories on friction. [Copyright &y& Elsevier]

Abstract: Electrochemical and friction measurements during unidirectional sliding tests are used to monitor and to evaluate the tribocorrosion behavior of a passivating material. Tribocorrosion is the interplay between chemical, electrochemical, and mechanical processes leading to a degradation of materials immersed in a corrosive environment. The tribocorrosion of AISI 316 stainless steel immersed in a diluted sulfuric acid solution at 25°C was investigated. A modification of the surface characteristics and material loss result from the combined periodic mechanical removal of the surface film during sliding, and the subsequent (electro)chemical re-growth of that surface film in-between two successive sliding contact events. This phenomenon is addressed in this paper as a latency time effect, and its influence on the wear volume and the coefficient of friction is reported. Furthermore, the effect of galvanic coupling between areas without and covered with surface films, on the open-circuit potential is analyzed in terms of Hertzian contact area. Finally a new approach of the concept of synergism in tribocorrosion is presented and validated for AISI 316 immersed in 0.5M H2SO4 under sliding against zirconia. [Copyright &y& Elsevier]

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 investigation of the tribological performance of MoS2-based coatings in air of high humidity is critical for the future use of such low-friction and high-wear-resistant coatings in ambient air. Sulfur-deficient MoS x coatings with a basal plane ( x = 1.3) and a random ( x = 1.8) crystallographic orientation were produced by planar magnetron sputtering. The coefficient of friction and the wear loss of MoSx coatings in comparison with TiN and amorphous TiB2 coatings were investigated in bi-directional sliding fretting tests performed in ambient air of different relative humidity. The wear rate expressed as a volumetric loss per unit of dissipated energy was determined. From these results, the best friction and wear performance was achieved with basal-plane-oriented MoS x coatings tested at a relative humidity in the range of 10-50%. A coefficent of friction of 0.06-0.08 and a wear rate of 4 × 103 μm3J-1, at a normal load of 1 N and a fretting frequency of 10 Hz, were recorded for that type of MoS x coatings. [ABSTRACT FROM AUTHOR]

Fretting tests (mode I) were performed on TiN coatings at test temperatures of 23–500 °C. The evolution of the coefficient of friction with the number of fretting cycles for tests performed in that temperature range was recorded. Differences in that evolution were analysed based on the frictional energy dissipated in the sliding contact during the fretting tests. That analysis demonstrated the role of dissipated frictional energy and thermal energy in initiating a structural modification of the debris. The structural modification of the debris coincides with the transition from high friction conditions taking place in the presence of amorphous debris to low friction conditions in the presence of nano-crystalline debris. [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]

Abstract: As-plated and heat-treated electrodeposited NiP and composite NiP–SiC coatings were investigated in bi-directional ball-on-disc sliding tests. All tests were performed under gross slip conditions. Heat treatment decreases the wear volume loss during fretting in ambient air for all coatings investigated. Heat-treated NiP coating has a lower wear volume loss compared to composite NiP–SiC coatings for all sliding tests. The wear rate at the bi-directional sliding test was found to be lower relative to the wear rate at uni-directional sliding test. [Copyright &y& Elsevier]

A methodology is proposed to examine the tribological behavior of materials subjected to successive ascents and descents in temperature. The experimental approach is based on performing two types of high temperature reciprocating sliding tests and subsequently correlating the evolution of the coefficient of friction of the tribosystem with test temperature. In the first series, the sliding tests are conducted at increasing temperatures with the aim to identify the temperature regions where changes in friction might occur. These changes can be either due to a surface modification (e.g. a change in the oxidation and/or wear mechanism) or structural one (e.g. phase transformation). In the second series, tests are performed under isothermal conditions selected based on the outcome of the elevated temperature results, with the aim to evaluate the influence of oxidation on the frictional behavior of the thermally stable system. As a case study, electroplated intermetallic Ni-Sn coatings containing either 16 or 64 wt% Sn of interest on jet engine bearings, were investigated. These coatings were tested between 25 and 400 °C in ambient air relevant for that application. [ABSTRACT FROM AUTHOR]

The porous structure of anodic aluminium oxide (AAO) can be used in versatile applications such as a lubricant reservoir in self-lubricating systems. Such systems are subjected to biaxial loading, which can induce crack formation and propagation, ultimately leading to catastrophic mechanical failure. In this study, the mechanical and tribological behaviour of AAO, prepared from two different types of electrolytes (sulphuric and oxalic acids), are studied in detail. The electrolytic conditions are adjusted to render highly tuneable average pore diameters (between 16 and 75 nm), with porosity levels ranging from 9% to 65%. Well-ordered porous AAO are produced by two-step anodization at rather low temperatures. Mechanical properties, mainly hardness and Young's modulus, are investigated using nanoindentation. Both the porosity degree and the composition of the electrolytic baths used to prepare the AAO have an influence on the mechanical properties. Ball-on-flat configuration was used to estimate the tribological behaviour under dry conditions. No major cracks were observed by scanning electron microscopy, neither after indentation or fretting tests. In the running-in period of tribology experiments the pores were filled with debris. This was followed by the formation of a highly adherent tribolayer – a third body consisting of fine worn particles originated from both the sample and the counterbody. Pore diameter and porosity percentage are found to strongly affect hardness and Young's modulus, but they do not have a major effect on the frictional behaviour. [ABSTRACT FROM AUTHOR]

Abstract: Contact between macroscopic surfaces occurs on asperities and local nano-/micro-scale asperity interactions govern macroscopic tribological behavior. Tribological phenomena like friction and wear are highly scale dependent. Friction measurement carried out in the range of micro-/milli-Newton loads are gaining prominence in the tribological research as they are bridging the gap between classical macro-scale tests and high precision nano-scale experiments. In this paper, recent advancements in micro-scale domain are discussed and compared with the existing macro- and nano-scale friction research. There exists a striking similarity between micro- and nano-scale frictional behavior of various materials and dependence of friction on capillary forces, surface topography, and phase composition of mated materials. Micro-tribological experiments not only offer precision and sensitivity of nano-scale (or LFM) measurements, but also give reliable information due to its multiple asperity contact configuration. [Copyright &y& Elsevier]

Abstract: The friction of diamond and diamond-like carbon (DLC) materials was evaluated in reciprocating sliding wear testing under controlled relative humidity. The testing conditions were a displacement stroke of 100μm, an oscillatory frequency of 8Hz and a normal load of 2N. The coefficient of friction of diamond and hydrogen-free DLC (a-C) coatings against a corundum sphere in the steady regime decreased with an increase in relative humidity. A water layer physisorbed at the interface between the mating surfaces played two major roles: acting as a lubricant and increasing the true area of contact. However, it was noticed that the friction coefficient of the hydrogenated DLC (a-C:H) coatings first increased and then decreased with increasing relative humidity in the steady state. There appeared to be a critical relative humidity for the a-C:H coatings, at which the steady-state friction reached the maximum value. The frictional behaviour of the a-C:H coatings also showed dependence on the wear test duration. The interaction between hydrogen and oxygen at the interface between the a-C:H coating and water layer was mainly responsible for such behaviour. [Copyright &y& Elsevier]

A Mo–N surface modified layer on Ti–6Al–4V alloy was obtained using a plasma surface alloying technique. A comparison of the tribological behavior of untreated and plasma Mo–N modified Ti–6Al–4V alloy has been made in this study. Results show that the Mo–N modified layer with a single Mo2N phase significantly improves the tribological behavior of the treated Ti–6Al–4V alloy under oscillation sliding wear in ambient air. The Mo–N surface modified layer not only reduces the coefficient of friction, but also decreases the wear of Ti–6Al–4V alloy and prevents the first bodies from sticking at the contacting points. [Copyright &y& Elsevier]

Transfer films on corundum balls from sulfur deficient molybdenum disulfide (MoSx) coatings with different crystallographic orientations were investigated after fretting tests performed in ambient air of different humidity levels. The morphology of wear tracks on MoSx coatings and of transfer films on corundum balls were investigated by light optical microscopy with Normarski contrast. The thickness of transfer films was measured by scanning white light and optical phase-shifting interferometry, and their composition was analyzed by X-ray photoelectron spectroscopy. The effect of relative humidity in fretting tests on the composition of the transfer films as well as the effect of the transfer film on the tribological performance of MoSx coatings in fretting wear tests is discussed. [Copyright &y& Elsevier]