Julien Fontaine

Tribochemistry between hydrogen and diamond-like carbon films

Abstract The objective of the present work is to propose a model related to the role of hydrogen on the friction mechanism of DLC films. An up-to-date review of the effect of hydrogen on the tribology of DLC films is presented first. Selected experiments performed on two model hydrogenated DLC films are then presented to demonstrate how hydrogen, both as a constituent of the carbonaceous film or as a gaseous species introduced in the surrounding environment during the friction process can influence the intermediate and steady-state friction regimes, in the absence of any oxidating species. For the film with the highest hydrogen content, superlow friction (10−3 range) is reached rapidly in an ultrahigh vacuum. For the film containing the lowest hydrogen content, the combination of a controlled temperature during friction (150°C) with hydrogen diffusion from the bulk of the film towards the sliding activated surfaces of the hydrogen carbon-to-carbon is responsible for an intermediate period with friction in the 10−3 to 10−2 range. Then the steady-state friction coefficient rises up to 0.6, typical for low hydrogenated a-C:H films in vacuum or inert atmospheres. A superlow friction steady-state regime may be controlled over longer periods by introducing a significant pressure of pure hydrogen surrounding the contact during the friction process. Argon at the same pressure does not have any similar lubricating effects. Tribochemistry between hydrogen and the carbonaceous network is thus responsible for the control of the superlow friction regime observed with a-C:H coatings in selected conditions of film composition and atmosphere.

The role of hydrogen on the friction mechanism of diamond-like carbon films

The structure, properties and tribological behavior of DLC films are dependent on the deposition process, the hydrogen concentration and chemical bondings in the films. The present paper reports selected tribological experiments on model DLC films with different hydrogen contents. The experiments were performed in ultrahigh vacuum or in an atmosphere of pure hydrogen or argon in order to elucidate various friction mechanisms. Two typical friction regimes are identified. High steady-state friction in UHV (friction coefficient of 0.6) is observed for the lowest hydrogenated and mostly sp2-bonded DLC film. Superlow steady-state friction (friction coefficient in the millirange) is observed both for the highest hydrogenated film in UHV, and for the lowest hydrogenated film in an atmosphere of hydrogen (10 hPa). The high steady-state friction in UHV, observed for the lowest hydrogenated film with a dominant sp2 carbon hybridization, is associated with a π–π* sub-band overlap responsible for an increased across-the-plane chemical bonding with a high shear strength similar to what is observed with unintercalated graphite in the same UHV conditions. Superlow friction is correlated with a hydrogen saturation across the shearing plane through weak van der Waals interactions between the polymer-like hydrocarbon top layers. This regime is observed during the steady-state period if the film contains enough hydrogen incorporated during deposition. If this condition is not satisfied (i.e., for the film with the lowest hydrogen content), the limited diffusion of hydrogen from the film network towards the sliding surfaces seems to be responsible for a superlow running-in period. The superlow friction level can be reached over longer time periods by suitable combinations of temperature and molecular hydrogen present in the surrounding atmosphere during friction.

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.

SOLID STATE 13C AND 1H NUCLEAR MAGNETIC RESONANCE INVESTIGATIONS OF HYDROGENATED AMORPHOUS CARBON

Various hydrogenated amorphous carbon films have been analyzed by 13C and 1H nuclear magnetic resonance (NMR) spectroscopies. The films have been deposited from acetylene or cyclohexane by dc plasma enhanced chemical vapor deposition, at various dc biases and gas pressures. The total hydrogen content has been measured by forward recoil elastic scattering (FRES). 13C NMR investigations have been performed in various configurations: high power decoupled to determine the sp2:sp3 carbon ratio, cross polarized at magic angle contact spinning with different contact times to provide information on carbon atoms directly bound to hydrogen, and with dipolar dephasing to study the quaternary carbon atoms. By performing the 13C and 1H NMR measurements on the same samples, it was possible to resolve for the first time the seven different forms of unprotonated and protonated CHx carbon, for both sp2(x=0,1,2) and sp3(x=0,1,2,3) carbon hybridizations, as well as the ratio between bound and unbound hydrogen. The results a...

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 (

Influence of humidity on microtribology of vertically aligned carbon nanotube film

The aim of this study is to probe the influence of water vapor environment on the microtribological properties of a forestlike vertically aligned carbon nanotube (VACNT) film, deposited on a silicon (001) substrate by chemical vapor deposition. Tribological experiments were performed using a gold tip under relative humidity varying from 0 to 100%. Very low adhesion forces and high friction coefficients of 0.6–1.3 resulted. The adhesion and friction forces were independent of humidity, due probably to the high hydrophobicity of VACNT. These tribological characteristics were compared to those of a diamond like carbon (DLC) sample.

Structure of diamondlike carbon films deposited by femtosecond and nanosecond pulsed laser ablation

The characterization of diamondlike carbon (DLC) films is a challenging subject, considering the diversity of carbon-based nanostructures depending on the deposition process. We propose to combine multiwavelength (MW) Raman spectroscopy and electron energy-loss spectroscopy (EELS) to probe the structural disorder and the carbon hybridizations of DLC films deposited by pulsed laser ablation performed either with a nanosecond laser (film labeled ns-DLC), either with a femtosecond laser (film labeled fs-DLC). Such deposition methods allow to reach a rather high carbon sp3 hybridization but with some significant differences in terms of structural disorder and carbonaceous chain configurations. MW Raman investigations, both in the UV and visible range, is a popular and nondestructive way to probe the structural disorder and the carbon hybridizations. EELS allows the determination of the carbon plasmon energy in the low-loss energy region of the spectra, as well as the fine structure of the ionization threshold...

The role of CN chemical bonding on the tribological behaviour of CNx coatings

Abstract The tribological performance of CN x coatings depends strongly on both the environmental conditions and the nature of the coating, in relation to the deposition process. In this paper, we present and discuss friction results in relation to the nature, crystal structure, chemical composition and hybridization state of CN x coatings prepared by dual ion beam sputtering under various conditions. The films were characterized by infrared spectroscopy, X-ray photoelectron spectroscopy and electron energy loss spectroscopy. By increasing the polarization of the substrate, an increase of the N/C atomic ratio, together with a decrease of the CN/CN and CN/CC bonding ratios are observed. The concentration of CN triple bond is negligible. Reciprocating pin-on-plane friction tests have been carried out in humid ambient air and in ultrahigh vacuum. Steady-state friction in ambient air has been found to be in the range of 0.14–0.25, increasing slightly with the increase of the substrate polarization during deposition. The presence of CN double bond is associated with a lower friction coefficient. The steady-state friction of the film exhibiting the lowest friction in ambient air is near 0.6 in ultrahigh vacuum. The chemical composition of the topmost surfaces both inside and outside the UHV wear tracks of the plane and the pin was investigated by in situ XPS and AES performed at the completion of the friction test in the analytical tribometer. The high friction in UHV is associated with a significant transfer of the iron oxide top layers from the pin as wear particles inside the wear track of the plane, without any CN transfer onto the steel pin.

Nanoindentation and nanofriction on DLC films

Diamond-like carbon (DLC) coatings have been studied for many years as wear-resistant and low friction materials. Their tribological behaviour depends on the nature of the coating, which is determined by the deposition process. It is also strongly affected by environmental conditions. In this study, the nanomechanical properties and nanofriction behaviour of two a-C:H coatings deposited on a silicon substrate and exhibiting different macroscopic friction behaviours were investigated using a three-axial surface force apparatus equipped with a diamond tip. The hardness, Youngs modulus and Poissons ratio of the two coatings were determined. Viscous recovery of the indents was observed a few days after the indentation tests. Low friction coefficients (around 0.06) were measured for the coating that exhibited macroscopic low friction. For the coating that exhibited a higher friction coefficient, dissipative behaviour measured during nanofriction test was enhanced by sliding, compared to the dissipative behaviour measured during normal indentation, suggesting a link between normal viscous dissipation at nanoscale and high friction.