M. Belin

Super-low friction of MoS2 coatings in various environments

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.

Tribochemistry of diamond-like carbon coatings in various environments

Abstract 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.

Diamond-like carbon-based functionally gradient coatings for space tribology

Abstract 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.

Wear-resistant fluorinated diamondlike carbon films

Fluorinated diamondlike carbon (FDLC) films have been deposited on Si wafers by rf plasma-assisted chemical vapor deposition, under a variety of conditions. The films have been characterized by FTIR and index of refraction measurements, RBS and FRES analysis for determination of film composition, and stress measurements from the bending of the wafers by the deposited films. Friction and wear measurements have been performed using pin-on-flat and pin-on-disk testers in ambient air, at maximum Hertzian contact pressures ranging from 320 to 1100 MPa. By adjusting the deposition parameters, the properties of the FDLC films could be changed from soft films, with no significant wear resistance, to films containing more than 20% F and having wear resistance comparable to unfluorinated DLC. The tribological properties of the FDLC films are discussed in relation to their physical properties, as determined by the deposition conditions.

Friction mechanisms of amorphous carbon nitride films under variable environments: a triboscopic study

Abstract 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.

Tribological and mechanical properties of diamond-like carbon prepared by high-density plasma

Abstract 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, Youngs 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.

Diamond-like carbon prepared by high density plasma

Abstract 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 (

Study of plasma expansion induced by femtosecond pulsed laser ablation and deposition of diamond-like carbon films

Abstract 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.

Tribochemical effects on CNx films

Abstract 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.

Tribological behaviour and chemical characterisation of Si-free and Si- containing carbon nitride coatings

Abstract 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.