C. Donnet

Recent progress on the tribology of doped diamond-like and carbon alloy coatings: a review

Abstract Diamond-like carbon (DLC) coatings have been widely recognized as being a wear-resistant solid lubricant with a low friction coefficient. Its tribological behavior strongly depends both on the tribotesting conditions and the nature of the coating, which in turn depends on the technique used for film deposition. Recently, there have been several attempts to improve the tribological behavior of DLC coatings by the addition of elements, such as silicon, nitrogen, fluorine and various metals. The paper will present an updated review of the tribological properties of doped DLC, in comparison with the conventional hydrogenated and non-hydrogenated carbonaceous films.

Friction control of diamond-like carbon coatings

The friction reduction of contacting surfaces in relative motion may be achieved through the use of solid lubricant coatings. The control of friction and wear through diamond-like carbon (DLC) coatings strongly depends on both the environmental conditions and the nature of the coating, as determined by the deposition process. The paper presents and discusses friction results linking the structure and composition of DLC coatings prepared by PACVD and varying precursor and bias, with physical and mechanical properties and tribological behavior in controlled environments. The wide range of the friction coefficients observed, from less than 0.01 to more than 0.5, and the different mechanisms involved are explained by the effects of the deposition process and tribological parameters.

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.

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.

Bonding structure in amorphous carbon nitride: A spectroscopic and nuclear magnetic resonance study

Since the prediction of Liu and Cohen [Science 245, 841 (1989)] of the potential extraordinary mechanical properties of crystalline β-C3N4, many authors have attempted its synthesis. However, in most cases, the obtained materials are amorphous phases with a complex bonding structure. Their characterization is complicated due to the absence of a reference compound, the lack of long-range order, and the poor knowledge about their bonding structure. In this article, we present 1H, 13C, and 15N solid-state nuclear magnetic resonance (NMR) measurements for the determination of the bonding types in amorphous CNx films. NMR measurements do not require long-range order and are able to clearly identify the signals from the sp2- and sp3-bonded phases. The analysis of the data obtained by other characterization techniques, such as infrared spectroscopy, x-ray photoelectron spectroscopy, electron energy-loss spectroscopy, and x-ray absorption near-edge spectroscopy on the same sample, based on the information acquired by NMR, enables the description of a structure model for the studied amorphous-CNx phase prepared by dc-magnetron sputtering and to revise the interpretation found in the literature.Since the prediction of Liu and Cohen [Science 245, 841 (1989)] of the potential extraordinary mechanical properties of crystalline β-C3N4, many authors have attempted its synthesis. However, in most cases, the obtained materials are amorphous phases with a complex bonding structure. Their characterization is complicated due to the absence of a reference compound, the lack of long-range order, and the poor knowledge about their bonding structure. In this article, we present 1H, 13C, and 15N solid-state nuclear magnetic resonance (NMR) measurements for the determination of the bonding types in amorphous CNx films. NMR measurements do not require long-range order and are able to clearly identify the signals from the sp2- and sp3-bonded phases. The analysis of the data obtained by other characterization techniques, such as infrared spectroscopy, x-ray photoelectron spectroscopy, electron energy-loss spectroscopy, and x-ray absorption near-edge spectroscopy on the same sample, based on the information acquire...

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.

The respective role of oxygen and water vapor on the tribology of hydrogenated diamond-like carbon coatings

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.

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.

Advanced solid lubricant coatings for high vacuum environments

Abstract Solid lubricant coatings for vacuum applications have seen considerable developments for many years, because of the use of advanced coating techniques, such as physical or chemical vapor deposition processes. The need for fully understanding the relationships between the nature of the lubricant coatings and their tribological performances in relation to the nature of the environment during sliding has become more pressing. The present paper discusses and compares the friction behavior of two kinds of thin film solid lubricant (pure MoS 2 and hydrogenated diamond-like carbon (DLC)) from a pressure range less than 5×10 −8 hPa to ambient air. Friction coefficient values less than 0.15 in ambient air and less than 10 −2 in ultrahigh vacuum have been recorded using pin-on-fiat tribometers. Present results are also compared with previous work published by others. The potentiality of DLC coatings used as solid lubricant for space applications is thus highlighted, in comparison with the more extensively used MoS 2 coatings.