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Achieving low-friction and robust tribosystems is an open and challenging question to tribologists. In this study, we are considering the diamond films as candidates. Quantification of friction is obtained here using an original technique, so called the oscillating relaxation tribometry. It is based on the study of the oscillating motion of a free-damped 1-DOF mechanical oscillator, the sliding contact acting as a main source of mechanical dissipation (“relaxation”). Therefore, it is possible to determine friction value and its sliding speed dependency, i.e. a kind of “Stribeck curve“ under non-stationary conditions. The interest of such “relaxation tribometer“ is to characterize low friction with an unequalled sensitivity, and to directly obtain the “friction law“ thanks to a robust and rapid experimental test. Polycrystalline diamond films are deposited by the Hot Filament CVD method on SiC substrate, which were partly polished and finished to gradient surface roughness with random-textured asperities. Stainless steel balls of 6.0 mm diameter are used as the rubbing counterface. The normal force is varied between 50 and 300 mN, leading respectively to a maximum Hertzian contact pressure of 0.30 and 0.71 GPa, respectively. Taking into account the experimental setup, the sliding speed is decaying as a damped pseudo-sine motion, the maximum sliding value being set to 0.16 m/s, progressively decaying to 0 when mechanical equilibrium is reached, within a typical duration of 5 s. First results are reported here, using this innovative friction characterization technique. The different contributions to friction of the surface roughness are analyzed and discussed. These results show the major interest of the use of this method to characterize the low-friction behavior of textured diamond films. These results are discussed and the high potential of such textured films in actual tribosystems is presented here.

A new experimental approach is proposed to investigate the friction behaviour and highlight the relationship with the tribofilm formation. It is based on the accumulation of several free dynamic responses of a single-degree-of-freedom damped oscillating system on the same sliding surfaces. By analysing the decreasing amplitude of the damped free dynamic responses and their associated energy decays, it is not only possible to describe the evolution of the tribological behaviour, but also to accurately identify velocity-dependent and velocity-independent friction contributions as the gradually formation of tribofilm. To demonstrate the relevance of this original technique, two tribosystems were studied, each lubricated with either base oil or formulated one, and by comparing the results with those obtained by a conventional linear tribometer.

The aim of this paper is to investigate the tribological conditions required to obtain low friction with a diblock PIB-PEG polymer friction modifier (PFM) blended in base oil (PAO4 + 1% wt PFM) under a severe lubrication regime. Two tribological conditions, rolling/sliding and reciprocating pure sliding, were investigated. A very low friction coefficient (μ ~ 0.035) was obtained at a temperature of 100 °C whatever the tribometer used. ECR measurements, ToF-SIMS characterizations of wear tracks and AFM analysis suggested the presence of an adsorbed polymer film on the rubbing surfaces. ToF-SIMS characterizations showed that the polymer bonds to the steel substrate through polar functions.

The tribological behavior of different linear amines blended to PAO4 alone or in combination with ZDDP was investigated under boundary lubrication regime via the coupling of tribometry and XPS measurements. Using a reciprocating ball-on-flat tribometer, it was evidenced that all tested amines were able to reduce both friction and wear. The combination of primary monoamine (1 wt %) and ZDDP (1 wt %) produced synergistic effect on reduction of both friction and wear in most cases. Experimental investigations suggest that this synergistic effect is due to the fast formation of a zinc-oxide-enriched tribofilm depleted in phosphates. Moreover, our results show that the lubrication mechanism and the composition of the tribofilm are strongly dependent on the amine/ZDDP molar ratio.

Among the possible solutions for achieving low friction, polymer brushes that are grafted onto surfaces are good candidates. The tribological characterization of such layers becomes more difficult when the friction is lower: the signal-to-noise ratio of the friction force that is measured with conventional tribometers impedes the precise quantification. Therefore, we have applied a new technique that has been developed at the Laboratory of Tribology and System Dynamics (LTDS), called the ‘oscillating relaxation tribometer’. The advantage of this original technique is that it characterizes low friction with unequalled sensitivity. The lower the friction, the better the precision, and it permits obtaining the ‘friction law’ directly from robust and rapid experimental tests. In this study, the samples that have been used are the ionic liquid-type polymer brushes (ILPBs) with different thicknesses, which have been grafted onto silicon wafers and steel coupons. The counter-face is a mirror-polished steel ball. We show that (i) a thick ILPB layer on silicon is very resistant to high contact pressure, up to 555 MPa; (ii) the friction behavior that is obtained is close to that of a Newtonian viscous one, even under maximum normal loads; (iii) poorer results are obtained for the thinner sample; and (iv) the repetition, up to 5000 oscillations on the same surface, does not affect the friction damping of the contact, which demonstrates that this film provides a favorable resistance to friction under severe contact conditions. In addition, the feasibility of grafting onto steel surfaces is demonstrated. The results are then discussed, with respect to friction and dissipation.

In situ wear measurements on a hard coating of TiAlN and CrN layers deposited on vitreous carbon have been carried out with synchroton radiation. The results show that wear dynamics can be successfully monitored on a lateral micrometer scale and with a submicrometer depth resolution. The wear process is highly irregular and the local wear rate may vary strongly from one position to another in the same wear track. Most of the ridges and grooves are generated within the first 500 nm and exist over several micrometers.

Friction is an important phenomenon in various dynamic systems. In order to understand the physical behavior of such systems, it is crucial to choose a friction law that can correctly model the mechanical system. This paper aims to study the dynamic responses of a new tribometer, based on a single degree-of-freedom (sdof) mass-spring system having a sliding contact. We introduce a velocity-dependence friction law in order to understand the dynamic system. The model is studied numerically and experimentally. In this paper, the friction law has been carefully investigated using a linear model (Coulomb and viscous friction) and a quadratic velocity-dependence model, introduced as a power series of the sliding velocity. Results demonstrate that when the decaying envelope is described by both concave and convex forms, a linear friction law could not properly model the system. It is shown that in some cases, a more refined friction law with higher order than quadratic expansion should be introduced to precisely model the dynamic system. In addition, some experimental results are presented using the tribometer, showing a good agreement with the numerical results. The results are then discussed in terms of different lubrication regimes.

The influence of various iron-containing compounds on ZDDP tribofilm formation was investigated. Friction tests were carried out under boundary lubrication using two sliding materials: steel and sapphire. Lubricants were composed of dispersions of zinc metaphosphate in base oil. The tribofilms were characterized using Raman spectroscopy. In the presence of steel, zinc metaphosphate depolymerizes to form a tribofilm mainly composed of zinc orthophosphate. No depolymerization of zinc metaphosphate was observed with sapphire. In subsequent tests with sapphire, iron oxide particles of different natures were added to the dispersion to simulate the iron oxide species present at the surface of the steel substrate. Only the addition of FeOOH (goethite) micro-particles in the dispersion leads to the formation of a depolymerized zinc-iron phosphate tribofilm.

The effect in the tribological performance of WS2 fullerene-like nanoparticles in PAO base oil when adding a ZDDP additive was studied at 100 °C in the boundary lubrication regime. The tribological properties of the dispersion surpass those obtained without one of the two additives. The friction modifier properties of the particles are improved in the presence of ZDDP, while the anti-wear properties of the ZDDP are increased when the particles are added to the dispersion. The composition of the formed tribofilm was investigated. Results show that a 50–60 nm tribofilm is formed on the steel surface composed by WS2 mixed on the ZDDP chemical tribofilm. A WS2-rich layer is observed at the top of the tribofilm. A correlation between the chemical composition of the tribofilm and the tribological properties of the “PAO + WS2 + ZDDP” dispersion was made. Synergy between the two additives was proven.

The friction behavior of steel/steel contacts lubricated with glycerol, 150NS and PAO4 is investigated using a new technique. Then, three additives are used in PAO4 lubricant: oleic, linoleic and stearic acids. The experiments are performed at different temperatures. Moreover, nearly all the systems operate in either the mixed or boundary lubrication regime. The experimental methodology is able, with a high accuracy and without any force transducer, two contributions of friction: μ 0, velocity-independent contribution and μ 1, velocity-dependent one. This study shows that glycerol is the most effective in reducing friction among the three pure lubricants. In addition, only glycerol affects the viscous damping of the system at some operating conditions. Using the additives in PAO4, the friction is reduced. More precisely, the unsaturated fatty acids are more significant in reducing the friction than saturated acids. Moreover, the viscous damping of the apparatus is slightly affected as additives are used in PAO4.

In-situ high pressure X-ray absorption spectroscopy measurements at Zn K-edge were performed on α-Zn3(PO4)2 compound, one of the model materials of the tribochemical film obtained from the ZDDP molecule. The goal of these synchrotron experiments was to investigate pressure-induced effects on the local environment of zinc atoms during a compression/decompression cycle. Modifications of the absorption spectra at Zn K-edge of α-Zn3(PO4)2 show irreversible amorphization around zinc atoms and irreversible increase of the Zn–O distance. These results were identified as the signature of structural changes around the Zn atom in the pressed sample, thus suggesting an irreversible Zn coordination change with the pressure.

Silver–carbon nanocomposite coatings were deposited by plasma-enhanced chemical vapor deposition and d.c. magnetron sputtering of a silver target. Coatings with various metal concentrations were prepared by changing of acetylene and argon gas mixture ratio (C2H2/Ar), and concentrations of more than 40 at.%Ag was achieved in this study. Transmission electron microscope revealed that silver metallic grains with typically 15 nm were dispersed in amorphous carbon host matrix. Size of the grains increased with decrease of the gas mixture ratio due to secondary or triangularly formed metal grains. Tribological behavior of the coatings was investigated using reciprocating tribometer with in-situ electrical contact resistance measurement. Low and stable friction coefficient was achieved in the specimen with relatively low Ag concentration. Elemental mapping results on the ball after the friction tests reveal that tribofilm was formed on the ball when low and stable friction was achieved, and the tribofilm was mainly composed of C and Ag. It can be concluded that formation of the tribofilm is necessary for achieving low and stable friction.

The friction behavior of ta-C and a-C:H as DLC coatings is investigated using a new method. Two green lubricants were used: oleic acid and glycerol. This technique is able to measure with a great accuracy, without any force transducer, two contributions of friction. The first one, μ 0 is the transient friction coefficient at zero speed (which differs from the static and dynamic friction coefficient at constant speed) and the second one, μ 1 is a velocity-dependent friction coefficient. This work aims to investigate the different friction contributions for steel/steel, steel/DLC and DLC/DLC contacts. Moreover, the results are compared with a conventional reciprocating cylinder-on-plane tribometer working at constant sliding speed. Overall, the results show the superiority of the ta-C/ta-C combination in reducing μ 0 .

Wear-induced roughness in terms of grooves, sharp ridges, and edges leads to scattering of the reflected light and leads unavoidably to a reduction of the optical signals in a standard specular geometry. However, by using a double-layer system consisting of titanium aluminum nitride (TiAlN) on top of a titanium nitride (TiN) layer we obtain an increase in the reflected light as a result of wear. The relative change of reflectance of light from the tribological TiAlN coated surface to the underlying layer of TiN is similar for non-worn surfaces and for surfaces exposed to an abrasive wear process. The induced roughness reduces the signals from worn samples, in a standard specular geometry, by up to 30% compared with unworn samples. Our model system of TiAlN coatings on top of ‘optical’ signal layers of TiN deposited on a 100Cr6 steel substrate, was exposed to a reciprocating wear process with up to 105 repetitive cycles in a linear tribometer. The worn TiAlN layers of thicknesses up to 3 μm, with strongly developed grooves and ridges, were subsequently used for the reflectance measurements. The results show that optical reflectance monitoring is a potential technique for intelligent determination of a residual thickness of realistic tribological coatings prior to complete wear.

Electrically conductive tribo-coatings (i.e. Ir-DLCs) were developed using a hybrid deposition process, combining RF-PECVD with DC magnetron sputtering of Ir. Pin-on-plate tribotests in reciprocating motion were performed in air on Ir, DLC and Ir-DLC coated plates against bearing steel pins under 0.5 N load. The electrical contact resistance was measured during the tribotests, and for Ir-DLC it was below 30 Ω. Neither the pure DLC layer nor the pure Ir layer reached friction coefficients lower than 0.2, while the Ir-DLC went down to 0.05. For Ir-DLCs, however, relatively high and unstable friction was occasionally observed. Possible causes of the instability are discussed by focusing on the characteristics of the tribofilm and the formation of wear debris.

We present here anomalous low friction obtained with highly polished steel on steel hard contact lubricated by glycerol under severe mixed and boundary regimes (λ ratio below 1). We investigated the effects of contact pressure, sliding speed, and temperature on friction coefficient and electrical contact resistance. The mechanism of low friction (typically below 0.02) is thought to have two origins: first a contribution of an ultrathin EHL film of glycerol providing easy shear under pressure, second the chemical degradation of glycerol inside the contact when more severe conditions are attained, generating a nanometer-thick film containing shear-induced water molecules. This new mechanism, called “H-bond Network model”, is completely different from the well-accepted “Monolayer” model working with polar molecules containing long aliphatic chains. Moreover, we show outstanding superlubricity (friction coefficient below 0.01) of steel surfaces directly lubricated by a solution of myo-inositol (also called vitamin Bh) in glycerol at ambient temperature (25 °C) and high contact pressure (0.8 GPa) in the absence of any long chain polar molecules. Mechanism is still unknown but could be associated with friction-induced dissociation of inositol and H-bond interactions network of water-like species with steel surface.

Among all the new carbon forms like fullerenes or nanotubes, carbon onions are of particular interest for tribological applications because of their spherical and tiny shape, and their chemical inertness. Used as additives blended to a synthetic base oil, they present excellent anti-wear and friction-reducing properties. In this paper, we compare tribological performances of two kinds of carbon onions with different structural features. Due to the synthesis method used, carbon onions present or not a residual diamond core inside the graphitic shells. Carbon onions without the diamond core present better anti-wear properties.

Effect of 5 eV atomic oxygen beam exposure on the surface properties of sputter-deposited molybdenum disulfide (MoS2) and diamond-like carbon lubrication films is experimentally evaluated with relevance to space environmental effect in low Earth orbit. X-ray photoelectron spectra indicate that the loss of sulfur and oxidation of molybdenum at the atomic oxygen-exposed MoS2 surface are significant. Depth profiles of sulfur and molybdenum indicate that the oxidation is restricted within 3 nm from the surface. This is due to the fact that Mo oxide plays as a protective layer against further atomic oxygen attack. The surface oxidation affects the friction coefficient, however it is recovered with sliding. Due to the delamination of oxide layer, wear-life of the film is reduced dramatically by certain exposure conditions. On the other hand, no severe oxidation states of carbon atoms are detected at DLC surface from the synchrotron radiation photoemission spectra. In contrast, loss of the DLC film itself is measured by Rutherford backscattering spectroscopy. These analytical results can be explained by the oxidative gasification of carbon atoms at DLC surface.

National audience; Des films de tetrahedral amorphous-Carbon (ta-C) ont été déposés, sous vide poussé, par ablation d'une cible de graphite avec un laser Ti : saphir (durée d'impulsion 170 fs, fréquence de répétition 1 kHz, énergie maximale par impulsion 1,5 mJ, longueur d'onde 800 nm) sur substrats standard et sur biomatériaux (acier AISI 316L, polyéthylène à très haut poids moléculaire). Les propriétés de ces couches (structure, propriétés nanomécaniques et tribologiques) ont été caractérisées, en fonction des conditions d'élaboration, en examinant l'intérêt de l'utilisation d'un laser femtoseconde et leur capacité à satisfaire aux exigences spécifiques du domaine biomédical. Les propriétés d'adhérence des films ont été considérablement améliorées lors du dépôt sur des substrats en acier inoxydable préalablement préparés par décapage ionique in situ sous atmosphère d'argon. La surface hémisphérique d'une tête fémorale, en acier inoxydable, de prothèse de hanche de diamètre 22,2 mm a été revêtue d'un film de DLC adhérent et homogène en épaisseur. La résistance à l'usure de ce revêtement sera quantifiée à l'aide d'un simulateur de marche durant un million de cycles (correspondant à une année d'activité physique d'un être humain).

Among diamond-like carbon (DLC) coatings, hydrogenated amorphous carbon (a-C:H) films are of special interest, since some of them may exhibit coefficient of friction in the millirange, known as “superlow friction”. This paper will review some key requirements for achieving this specific friction regime. At first, the formation of a carbonaceous transfer film on the steel counterface seems necessary for friction reduction. Oxide layers apparently slow down this build-up, as a-C:H surface reacts preferentially with metallic iron. Secondly, the formation and evolution of the transfer film on the sliding counterface seems to control the frictional behavior. By performing tribological experiments under different controlled environment, it is possible to have some clues on the evolution of the transfer film and on its relation with the friction level observed. Phenomena of loss and restoration of superlow friction are for instance very helpful. Finally, all hydrogenated amorphous carbon films do not lead to superlow friction under vacuum. The role of hydrogen content is known to be critical, but it appears to be strongly dependent on deposition process. Some mechanical properties of the films can also be correlated with friction reduction, like viscoplasticity. All these results suggest that surface rheological properties are of paramount importance in the achievement of superlow friction with a-C:H films.

We report here on the friction behavior of fine- and coarse-grained Ti3SiC2 against steel and Si3N4 balls. Two successive friction regimes have been identified for both grain sizes and both counterparts. First, Type I regime is characterized by a relatively low (0.1–0.15) friction coefficient, and very little wear. Sliding occurs between a tribofilm on the ball and the Ti3SiC2 plane when against steel. Then, a Type II regime often follows, with increased friction coefficients (0.4–0.5) and significant wear. Compacted wear debris seems to act as a third body resulting in abrasion of the ball, even in the case of Si3N4. The transition between the two regimes occurs at different times, depending on various factors such as grain size, type of pin, and normal load applied. Some experiments under vacuum showed that the atmosphere plays also a major role. The reason for this evolution is not fully clear at that time, but its understanding is of major technological importance given the unusual good properties of this material.

Ti–C–N thin films were synthesized at near room temperature by reactive magnetron sputtering of titanium and graphite targets in Ar or Ar/N 2 mixtures. The microstructure and chemical composition of the coatings were studied by transmission electron microscopy, atomic force microscopy (AFM), electron diffraction, X-ray diffraction, X-ray photoelectron spectroscopy and electron energy loss spectroscopy. For a pure Ar atmosphere, a microstructure constituted by small grains (10–20 nm) of a TiC phase encapsulated into an amorphous matrix is observed. AFM lateral force mapping shows a strong frictional contrast between the two phases. When nitrogen is introduced in the gas phase during preparation, the granular microstructure is not seen and the chemical composition is enriched in amorphous nonstoichiometric CN x phases (a-CN x ) with different content of nitrogen (0.5≤ x ≤0.7). The type of structure and chemical bonding of the Ti–C–N films are correlated with the tribological properties at the microscopic level in order to establish the synthesis conditions leading to the nanocomposite formation.

Among diamond-like carbon coatings, hydrogenated amorphous carbon films are of peculiar interest, since some of them exhibit under vacuum coefficient of friction in the millirange, so-called “superlow friction”. This paper will review some key requirements for achieving this peculiar friction regime. Firstly, not all hydrogenated amorphous carbon films lead to superlow friction. Hydrogen content is known to be critical, but it appears to be strongly dependent on deposition process. Some mechanical properties of the films can also be correlated with friction reduction, like viscoplasticity. Secondly, formation and evolution of a transfer film on sliding counterface seems to control frictional behavior. By performing tribological experiments under controlled environment, combined with the use of “Triboscopy”, it is possible to have some clues on the evolution of the transfer film and on its relation with the friction level observed.

Compared to conventional nanosecond laser ablation, femtosecond pulsed laser deposition (PLD) allows the production of higher energy ions (up to a few keV) in the plasma plume, which strongly affect the structure and properties of the deposited films. In this work, tetrahedral diamond-like carbon (ta-C) films have been deposited by femtosecond PLD on various substrates, including 316L stainless steel, in order to extend the wear resistance of materials used for hip joint replacement. The deposition process has been optimized to obtain smooth and wear-resistant carbon films by ablating a graphite target in ultra-high vacuum conditions at room temperature, with a laser fluence (energy density) ranging from 1 to 6 J cm −2 . In situ sputter cleaning of the substrates in an argon atmosphere prior to carbon deposition has been widely investigated: tensile tests show that the adhesion of the films on stainless steel substrates is remarkably enhanced by removing the contamination and oxidized top-coats. The films exhibit high wear resistance (in the 10 −8 to 10 −9 mm 3 N −1 m −1 range) with moderate hardness (in the 20–30 GPa range), which may be favorable for the accommodation motion between contacting surfaces in a hip joint. The ability of these films to satisfy the biomedical requirements is discussed and the possibility to deposit homogeneous ta-C films on a 22.2-mm diameter hemispherical surface is examined. Finally, homogeneous films have been deposited on a 316L stainless steel femoral head, whose wear behavior will be quantified using a walking simulator during several millions of cycles (corresponding to the human activity during several years).

Among diamond-like carbon coatings, hydrogenated amorphous carbon films have the characteristic of exhibiting superlow friction under ultra-high vacuum. However, most of the films reach this superlow friction regime only for a very short duration. The role of hydrogen on such phenomena is paramount. This paper focuses on the loss of superlow friction and on its restoration through the introduction of hydrogen gas in the friction (or sliding) environment. “Triboscopy”, which is a special way of plotting friction data, enabled the understanding of the degradation of the friction coefficient and the healing mechanisms by hydrogen.

Tetrahedral diamond-like carbon films (ta-C) were deposited by femtosecond pulsed laser deposition (PLD) on various substrates, including 316L stainless steel and ultra high molecular weight polyethylene, in perspective to extend the wear resistance of materials used in hip joints. Usually films are deposited by plasma enhanced chemical vapor deposition or by PLD, principally performed with laser pulse duration in the nanosecond range. In the present study, DLC films were deposited by femtosecond laser ablation in high vacuum conditions at room temperature. The 800 nm, 150 fs laser pulses are generated by a mode-locked Ti:Sapphire laser, including an amplification stage. The fluence ranges from 1 to 6 J/cm 2 . Sputter cleaning of the substrates in an argon atmosphere prior carbon deposition was widely investigated; the adhesion of the films onto the stainless steel substrates was then considerably enhanced by removing the contamination and oxidized top-coats. The adhesion properties of films deposited in various conditions on 316L stainless steel substrates were studied by tensile tests. The films exhibit high wear resistance (in the 10 −8 mm 3 N −1 m −1 range) with moderate hardness (in the 20–30 GPa range), which may be favorable for the accommodation motion between contacting surfaces in a hip joint. The capacity of these films to satisfy the biomedical demand is discussed.

Des films de Diamond-Like Carbon (DLC) ont ete deposes, sous vide pousses par ablation d'une cible de graphite avec un laser Ti : saphir (duree d'impulsion 150 fs, frequence de repetition 1 kHz, energie par impulsion 1,5 mJ, longueur d'onde 800 nm) sur substrat standard. Les proprietes de ces couches (structure, adherence, proprietes nanomecaniques) ont ete caracterisees, en fonction des conditions d'elaboration. La caracterisation par spectroscopie Raman de la structure des depots obtenus a revele la presence de diamant nanocristallin au sein d'une matrice amorphe. Le taux d'hybridation sp 3 des couches, determine a partir des spectres obtenus par XANES (X-ray Absorption Near Edge Spectroscopy), est voisin de 70 %. Ces differents resultats sont correles avec la caracterisation par imagerie resolue en temps du panache plasma cree par ablation laser femtoseconde.

Diamond-like carbon (DLC) films were deposited in high vacuum conditions at room temperature, by ablating graphite targets with femtosecond laser pulses. The structure of the films deposited onto silicon substrates were characterized by Raman spectroscopy and X-ray absorption near edge spectroscopy (XANES). Films exhibit unusual structure, Raman spectra showing the presence of nanocrystalline diamond in amorphous matrix. Plasma plume was imaged by a gated ICCD camera in the UV-Visible range. The behavior of the plume shape as well as the kinetic energy of the particles are investigated. The behavior of the expansion dynamics of the plume and the properties of thin films are studied in order to determine the optimal growth conditions for femtosecond pulsed laser deposition of DLC films.

Pulsed laser ablation is a well-known technique used for thin film deposition of hard and wear resistant diamond-like carbon (DLC) films. Most of the previous studies were carried out by using pulse duration in the nanosecond range. Compared to conventional nanosecond laser ablation, femtosecond laser allows the production of high energy (up to a few keV) ions in the plasma, which may strongly affect the structure and properties of the deposited films. The present study was achieved by ablating graphite targets with femtosecond (10−15 s range) laser pulses. DLC films were deposited under vacuum onto (1 0 0) p-type silicon substrates at room temperature. The laser pulse energy was 1.5 mJ at a repetition rate of 1 kHz. The fluence (or energy density) range was between 1 and 6 J/cm2. The nature and mechanical properties of the films are characterized by X-ray absorption near-edge spectroscopy and nanoindentation techniques. The tribological behavior of the films are also investigated in a pin-on-plate configuration. Correlations between the structure of the films and some of their properties are highlighted, depending on the deposition conditions. Discussion is focused on the comparison between present results obtained using the femtosecond mode, with previously published results related to DLC films deposited using the nanosecond mode.

In this work, a carbon nitride film prepared by ion-beam-assisted deposition onto silicon was investigated using reciprocating sliding tests under different surrounding atmospheres (ambient air, dry air, dry nitrogen (DN), and ultra-high vacuum) and testing conditions (contact pressure, nature of the counterface). The carbon–nitrogen (CNx) coating provided a ‘superlow’ friction coefficient of 0.007–0.008 in DN atmosphere independently of the mating material (steel or sapphire balls). The build-up of a smooth transfer film onto the ball surface was detected by optical microscopy. The change in the friction coefficient was spatially recorded along the track as sliding progressed. This technique, called ‘triboscopy’, combined with variable amplitude tests performed on the same wear track, revealed that the nature of both counterfaces (ball and film) has to be modified during a short running-in period before achievement of the superlow friction regime. When other surrounding atmospheres are used, the superlow friction coefficient is not attained even if the counterfaces are previously rubbed in DN. This assembly of tools allowed the characterization of environmental influence on the tribological behavior of CNx coatings in terms of friction coefficient, material transfer, and chemistry of the sliding interface, and the comparison with that observed for diamond-like coatings.

One of the major problems of CN x -based hard coatings for mechanical applications is their poor adhesion to steel substrates. In the present case, a TiN/Ti gradient underlayer has been deposited between the carbon nitride films and a high-speed steel substrate by means of magnetron sputtering. The same method was used to grow different CN x top layers in which various deposition parameters (Ar/N 2 ratio, DC bias) were varied. A silicon-doped CN x film was also prepared by dosing a certain amount of the gaseous precursor Si(CH 3 ) 3 Cl in the reaction chamber. Scratch tests have shown that the introduction of the underlayer leads to an improvement of the adhesion in all cases. Tribological measurements clearly indicate that a good adhesion between the substrate and the coating is always needed in order to obtain low and stable friction coefficients. The wear rate and nanohardness of the samples have been also studied in relation with the different growing conditions. The results show an improvement of the adhesion, hardness and wear resistance without an increase of the friction coefficient for the samples synthesised using Ar/N 2 mixtures and DC-biasing of the substrate during deposition.

In the present paper, we compare the tribological behaviour of Si-free and Si-containing carbon nitride films grown on high speed steel substrates against steel counterfaces. The CN x coatings have been prepared by magnetron sputtering of a carbon target in a N 2 atmosphere while the SiCN x films were obtained by the same method, but adding a vapour pressure of Si(CH 3 ) 3 Cl. In the case of pure CN x , the presence of water molecules in the gas phase produces a negative effect in the tribological response while the Si-containing film is able to maintain a low friction value (0.12) even under humid atmosphere similarly to Si-DLC coatings. To achieve a better understanding of the friction mechanism, both Si-free and Si-containing films were characterised by Energy Filtered Transmission Electron Microscopy (EFTEM), X-Ray Photoelectron Spectroscopy (XPS) and Infrared Spectroscopy (IR). The buffer effect of silicon-doped CN x , decreasing the moisture sensitivity of the friction coefficient, is attributed to the adsorption of water molecules on SiO 2 domains formed in the Si-containing films. This adsorbed water may lubricate the contact in humid atmosphere allowing the shear strength to diminish.

Reducing the electrical contact resistance (ECR) and the coefficient of friction (CoF) are major technological requirements for sliding contacts. This report provides electrical-tribological characteristics of a Copper containing Diamond-like Carbon (Cu-DLC) nanocomposite coating deposited on a brass plate, investigated by using a ball/plate reciprocating tribometer. The counterpart was a brass ball. While the initial ECR was hundreds of milliohms, it gradually decreased with cycles and reached approximately 2 milliohms after 600 cycles. CoF started below 0.35 and decreased progressively, and stabilized around 0.25 after 600 cycles. The good electrical-tribological characteristics lasted up to approximately 7500 cycles and CoF increased rapidly after. Sliding surfaces at different cycles were analyzed by optical microscope, laser-scanning microscope, EDX, TEM and nanoindentation. The results of analyses revealed that a copper-rich tribofilm was built up on the ball, which grew with cycles. The Cu-DLC wore gradually and its substrate was exposed after less than 1000 cycles. Nevertheless, detrimental effects were not observed either on ECR or on CoF up to 7500 cycles. The unique structures and properties of the tribofilm and the worn Cu-DLC surface are discussed, attempting to account for their key roles on the good characteristics.

International audience; Inorganic fullerene-like molybdenum disulfide (IF-MoS2) nanoparticles are known to exhibit great friction and wear-reducing abilities in severe boundary lubrication regimes, when added to a base oil alone. Their use in fully formulated lubricants was investigated in this study, and the tribological benefits attributed to the IF-MoS2 nanoparticles were found to be lost in the presence of dispersants. Various experimental techniques were used on three reference oils (base oil containing only IF-MoS2, only dispersants and both IF-MoS2 and dispersants) in order to understand the effect of succinimide-based dispersants on the three phases needed for effective nanoparticle-based lubrication, namely (1) the passing of the nanoparticles through the contact (2) the exfoliation of the IF-MoS2 inside the contact and (3) the adhesion of the released MoS2 platelets on the friction surfaces. The dispersants were shown to improve the dispersion of the nanoparticles in the oil by reducing their agglomeration, but prevented the adhesion of a low-friction MoS2 tribofilm on the steel surfaces. In-situ contact visualization revealed that the well-dispersed nanoparticles passed through the contact and exfoliated nanoparticles were observed after tribological testing. These results imply that nanoparticle dispersion itself does not seem to be an issue concerning nanoparticle effectiveness, even though the reduced agglomerate size and inertia may have affected nanoparticle flow near the contact, as well as entrapment and exfoliation conditions inside the contact. The use of succinimide-based dispersants may, however, have affected the tribochemistry of the contact, by an excessive adsorption on the steel surfaces and/or by encapsulating the released MoS2 platelets, preventing tribofilm adhesion. A balance was finally found between nanoparticle dispersion and friction reduction, but for very low dispersant concentrations and after a running-in period. The role of succinimide-based dispersants and their effect on nanoparticle lubrication were discussed in the light of these results.

Seed-assisted solution synthesis of hollow IF-MoS2 nanoparticles allows independent control of particles size and MoS2 slabs crystallinity. Variations of the reaction mixture composition influence the particle size in the range 50–150 nm. As demonstrated by Rietvelt refinement of the X-ray diffraction patterns, the sulfide crystallinity depends only on the post-treatment temperature (350–750 °C) and not on the particle size. The tribological properties of new MoS2 nanoparticles prepared by seed-assisted solution technique were investigated and showed a strong decrease in the friction coefficient and wear compared with base oil. Small particles of 50–60-nm size showed the best results. The particle size above 100 nm is deleterious for the lubrication properties since it hinders particles penetration into the contact zone. MoS2 slabs crystallinity had lesser influence on the lubrication efficiency. However, less-crystallized samples treated at 350 °C showed better lubrication, apparently because of easier exfoliation of the individual MoS2 slabs, leading to more efficient formation of tribofilm.

Diamond-like carbon films (DLC) have been deposited by plasma-enhanced chemical vapor deposition using a commercial r.f. high-density plasma (HDP). The hardness and elastic modulus have been determined from force–displacement curves using a nanoindentation instrument. The friction coefficients and wear rates have been measured using a pin-on-flat tribometer in ambient air and UHV at maximum Hertzian contact pressure of 1.09 GPa. It was found that the hardness, Young's modulus, and wear resistance of the films increased with decreasing hydrogen content of the films. A viscoplastic behavior has been observed for films with hydrogen content higher than 30% indicating that these films have a softer ‘polymer-like’ structure. The unbound hydrogen present in the HDP films contributes to the enhancement of the viscoplastic behavior of the film and to the decrease of the friction in UHV. The correlation between viscoplastic properties, film structure and tribological behavior of the HDP films is presented and compared to those obtained with samples prepared by d.c. PECVD.

A double-layer model system, consisting of a thin layer of tribological titanium aluminum nitride (TiAlN) on top of titanium nitride (TiN), was deposited on polished 100Cr6 steel substrates. The TiAlN top-coatings were exposed to abrasive wear by a reciprocating wear process in a linear tribo-meter with up to 10 5 repetitive cycles, eventually leaving the embedded TiN signal layer uncovered at the bottom of the wear scar. The worn surface was characterized by subsequent image processing. A color detection of the wear scar with the exposed TiN layer by a simple optical imaging system showed a significant increase up to a factor of 2 of the relative color values from the TiAlN top layers to the embedded TiN signal layers. This behavior agrees well with the results of optical reflectance detection experiments with a red laser on the same system. Thus we have demonstrated that image processing by color detection is a potential technique for early warning or determination of residual thickness of tribological tool coatings prior to complete wear.

CN x thin coatings have been deposited by dc magnetron sputtering using a graphite target in nitrogen atmosphere under different experimental conditions. X-Ray photoelectron spectroscopy, electron-energy loss spectroscopy, nuclear magnetic resonance and Fourier-transformed infrared spectroscopy were used to elucidate the structural chemistry of each film. The tribological behavior has been investigated using a reciprocating pin-on-flat tribometer in a wide range of environmental conditions: ambient air, dry air and nitrogen. Tribochemical effects in relation to the nature of the surrounding atmosphere during friction tests are presented. Strong relationships between the N/C ratio, the nature of C–N bonds and the friction behavior are highlighted and discussed. An optimum in the friction and wear properties was found for a maximum in the CN/CC ratio for the studied set of CN x samples. The XPS/AES analysis of the sliding counterfaces support a destabilization of the CN x network under friction and build-up of a carbon rich tribolayer under steady-state.

This paper will present physical and tribological properties of diamond-like carbon (DLC) films deposited by plasma-enhanced chemical vapor deposition using a commercial RF high density plasma (HDP). The films have been prepared from acetylene or acetylene+hydrogen mixtures using a range of HDP conditions. The composition and optical properties of the DLC films have been characterized by forward recoil elastic scattering (FRES) and Fourier transform infrared spectroscopy (FTIR). The tribological properties have been measured in ambient air and in dry nitrogen using a pin-on-flat tribometer. While the friction coefficients in air (

Solid lubricant coatings for vacuum and space mechanisms are widely used when conventional liquid lubrication is prohibited, either when the operating conditions become too severe (extreme temperatures, ultrahigh vacuum) or when a clean environment is required. While the well-known MoS2 lamellar solid lubricant is the most extensively used material today, diamond-like carbon (DLC) coatings are studied as potential candidates for a wear resistant material with low friction in vacuum conditions. Diamond-like carbon-based functionally gradient Ti/a-CH(Ti) films have been deposited by the hybrid technique of magnetron sputtering and d.c. plasma-enhanced chemical vapor deposition, in various conditions. Analytical characterization coupled with tribological tests in ultrahigh vacuum and ambient humid air have been performed to identify relationships between the deposition conditions, the composition and the properties (stress, friction) of the films. Depending on the properties of the DLC which are in turn dependent on the deposition procedure, the investigated films present a wide range of tribological behavior, including friction coefficients in UHV below 0.02. Typical DLC structures and compositions allowing the achievement of extremely low friction in vacuum and good behavior under air are identified and discussed.

The tribological behavior of diamond-like carbon coatings (DLC) strongly depends on the chemical nature of the test environment. The present study proposes to explore the influence of water vapor and oxygen on the friction behavior of a hydrogenated DLC coating exhibiting ultralow friction in ultrahigh vacuum (friction coefficient below 0.01). Using a UHV tribometer, reciprocating pin-on-flat friction tests were performed in progressively increasing or decreasing partial pressures of pure oxygen and pure water vapor. The maximum gaseous pressures of oxygen and water vapor were 60 hPa and 25 hPa (1 hPa = 100 Pa), respectively, the second value corresponding to a relative humidity (RH) of 100% at room temperature. It was found that, for the pressure range explored, oxygen does not change the ultralow friction behavior of DLC observed in UHV. Conversely, water vapor drastically changes the friction coefficient at pressures above 0.5 hPa (RH = 2%), from about 0.01 to more than 0.1. Electron energy loss spectroscopy and in situ Auger electron spectroscopy have been performed to elucidate the friction mechanisms responsible for the tribological behaviors observed with the two different gaseous environments. In all cases no significant oxidation has been observed either inside the wear scars or in the wear debris particles. Ultralow friction is systematically associated with a homogeneous carbon-based transfer film. The higher friction observed at partial pressure of water vapor higher than 0.5 hPa, is associated with a thinner transfer film. Consequently friction seems to be controlled by the transfer film whose kinetics of formation strongly depends on the partial pressure of water vapor.

An original dynamic tribotest was used to study the friction response of two kinds of tribological surface: zinc-dialkyl-dithio-phosphate (ZDDP) thermal film and ZDDP tribofilm. This apparatus allows friction to be determined with no need for measuring tangential force. A velocity dependent friction contribution was found in each case in addition to a velocity independent one, showing the viscous character of such films. Furthermore, during test on ZDDP thermal film, the viscous damping represents 18% of the total dissipated energy as it is 16% for ZDDP tribofilm.

The ultra-low friction coefficient (typically in the 10−2 range) of MoS2-based coatings is generally associated with the friction-induced orientation of ‘easy-shear’ planes of the lamellar structure parallel to the sliding direction, particularly in the absence of environmental reactive gases and with moderate normal loads. We used and AESXPS ultra-high vacuum tribometer coupled to a preparation chamber, thus allowing the deposition of oxygen-free MoS2 PVD coatings and the performance of friction tests in various controlled atmospheres. Friction of oxygen-free stoichiometric MoS2 coatings deposited on AISI 52100 steel was studied in ultra-high vacuum (UHV: 5 × 10−8 Pa), high vacuum (HV: 10−3 Pa), dry nitrogen (105 Pa) and ambient air (105 Pa). ‘Super-low’ friction coefficients below 0.004 were recorded in UHV and dry nitrogen, corresponding to a calculated interfacial shear strength in the range of 1 MPa, about ten times lower than for standard coatings. Low friction coefficients of about 0.013–0.015 were recorded in HV, with interfacial shear strength in the range of 5 MPa. Friction in ambient air leads to higher friction coefficients in the range of 0.2. Surface analysis performed inside the wear scars by Auger electron spectroscopy shows no trace of contaminant, except after friction in ambient air where oxygen and carbon contaminants are observed. In the light of already published results, the ‘super-low’ friction behaviour (10−3 range) can be attributed to superlubricity, obtained for a particular combination of cystallographic orientation and the absence of contaminants, leading to a considerable decrease in the interfacial shear strength.

Organized and dense monolayers were produced by chemical grafting of molecules and by Langmuir Blodgett (L-B) deposition. Friction, wear and indentation of these films on a microscale have been studied using atomic force microscopy friction force microscopy (AFM/FFM). For comparison, macroscale friction and wear measurements have also been made. Mechanisms of friction, wear and lubrication of chemical grafted and L-B films are discussed. These studies show that coefficient of friction, wear resistance, and nanoindentation properties of C18 double grafted films are superior to L-B films. It is concluded that chemical grafted films may be suitable for lubrication of microcomponents.

In this paper, we study a kinematic friction model of a single degree-of-freedom oscillating system. This model has the following dimensionless equation of motion: Display Formulay″+2ζy′+y=-μky′ in which the kinematic friction is described by a quadratic function of the velocity as the following: Display Formulaμky′=μ0+μ2y′2sgny′+μ1y′ The sign function preserves the asymmetry of the friction force. The model captures most of the friction behavior that has been observed experimentally using a recent apparatus known as “dynamic oscillating tribometer”. This experimental setup is based on the non-linear free dynamic response of this kind of oscillator. However, this technique is able to carefully determine, with no need for any force transducer, the velocity-independent and velocity-dependent friction coefficients, μ0 and μ1 respectively, for a linear description of the kinematic friction, μk, around y′ = 0 where: Display Formulaμky′=μ0sgny′+μ1y′ In this context, the principal aim is to investigate numerically and analytically the effect of the additional quadratic term. To analyze the free dynamic response and their corresponding envelops, different numerical methods are performed. New characteristics of the envelops are discussed in detail with respect to the form of the kinematic friction coefficient. This allows a better comprehension of the results observed experimentally.Copyright © 2012 by ASME

Diamond-like carbon films deposited on silicon wafers by r.f.-plasma-assisted chemical vapour deposition were friction tested in controlled atmospheres in a reciprocating pin-on-plate configuration using a steel sphere. Friction experiments were carried out in a vacuum range from 10 -7 to 50 Pa, in dry nitrogen and in ambient air. Analytical investigations of the wear process were peformed using transmission electron microscopy-electron energy loss spectroscopy and secondary ion mass spectroscopy. In all cases a transfer film was observed to form on the steel pin during the first 100 cycles, associated with relatively high values of the friction coefficient (0.2–0.3) at this stage. Beyond N =100 cycles the friction coefficient decreased to 0.006–0.008 in a vacuum below 10 -1 Pa and to 0.01–0.07 in a vacuum of 10–50 Pa and in dry nitrogen. The shearing ability of the interfacial film depends strongly on the nature of the atmosphere during friction, which affects the surface composition of the sliding counterfaces. A high vacuum is associated with ultralow friction and low wear. A poor vacuum and an inert atmosphere are associated with low friction and moderate wear. Ambient air is associated with relatively high friction and severe wear, coupled with the formation of roll-shaped debris of amorphous carbon containing iron oxide precipitates.