T. Le Huu

Transformation of sp3 to sp2 sites of diamond like carbon coatings during friction in vacuum and under water vapour environment

Abstract The development of a deposition system based on DC plasma-assisted chemical vapour deposition (T. Le Huu, H. Nery, H. Zaidi and D. Paulmier, Diamond Relat. Mater. 3 (1994) 1028–1033) has great interest for vapour deposition processes. This process permits the deposition on steel or on glass and other substrates. The advantages of the process are film uniformity on very large surfaces and good adhesion. Diamond and diamond-like carbon were deposited from the plasma with an acetylene-hydrogen mixture (C 2 H 2 H 2 ). The surface properties and the structure of the films were controlled. The friction and wear behaviour of the film was recorded and analysed. Using scanning electron microscopy and electron energy loss spectroscopy we have shown that the transformation of sp 3 to sp 2 carbon sites occurs on the rubbing surface during friction. This transition will be discussed in connection with the flash temperature created at contact asperities. Transition temperature will be calculated as function of Hertzian contact pressure. The aim of this paper is to present the wear and friction behaviour of diamond like carbon films in vacuum or under water vapour and to study the phase transformation of sp 3 to sp 2 bonds in different gaseous environment surrounding the tribo-contact.

Influence of hydrogen contained in hard carbon coatings on their tribological behaviour

Abstract Synthesized diamond-like coatings obtained by hydrocarbons/hydrogen plasma processes contain hydrogen. The ultra-low friction state of diamond coating (friction coefficient μ=0.02) observed in an inert environment is not an intrinsic property of the material. It is induced by the hydrogen contained in the film. The study, under vacuum, of the friction coefficient evolution measured with a pin-on-disc tribometer shows that the transition from a low-friction state to a severe one appears after a critical number of revolutions. This phenomenon shows that abrasive wear during sliding motion slowly leads to the consumption of the hydrogen, and the friction coefficient increases. The aim of this paper is to present the tribological behaviour of diamond-like coatings vs. the amount of hydrogen contained in the coating, and to propose an explanation of the lubricating mechanism of hydrogen contained in the coating on the friction coefficient and its evolution with time and with the environment. A comparison of the tribological behaviours of various hard carbon coatings synthesized with different ratios of carbon/hydrogen/oxygen as was described by Bachmanns equilibrium diagram [1] is presented.

Lubricating properties of diamond-like coating

Abstract The friction of hydrogenated hard carbon coatings against themselves or against a metal depends on the tribocontact environment. A low friction state is associated with an inert environment whereas the physisorption of water vapour induces a transition to a high friction state. A theoretical calculation of the hydrogen bond given in this paper leads to the conclusion that the interaction forces vary with the relative dielectric permittivity of the environment. Small quantities of water vapour inhibit the lubricating properties of the hydrogen contained in the coating. The hydrogen is weakly bonded under vacuum and strongly bonded in a moist environment. The purpose of this paper is to present the tribological behaviour of diamond-like carbon coatings obtained by a chemical vapour deposition process. We present a calculation model of hydrogen-carbon binding under different environments and we discuss the influence on the tribological behaviour.

Tribological behaviour of diamond-like coatings: effect of active gases in atomic and molecular states

Abstract Diamond-like coatings have very good mechanical properties and present a real area of interest in studying their tribological behaviour. However, their friction coefficient is strongly affected by gaseous environments. Under a vacuum, when the coating contains hydrogen, the friction coefficient is lower than that in ambient air. To explain the lubricating mechanism of the gases on the surface, we have experimentally studied the effect of active gaseous species, such as atomic and molecular oxygen, hydrogen, nitrogen and water vapour, on the frictional behaviour in relation to the presence of hydrogen in the substrate. A tribochemical interpretation is proposed in connection with the temperature of the contact asperities.

Tribological behaviour of hard carbon coatings deposited on steel substrates by plasma-assisted chemical vapour deposition

Abstract We have tested the tribological behaviour of steel probes with various diamond-like coatings in a vacuum shell and under various environments against steel. We obtained the coatings by plasma-assisted chemical vapour deposition. Experiments show good adhesion of these coatings to the substrate and good resistance to high shear stresses. This paper presents experimental results for several diamond-like coatings (DLCs), permitting an identification of the tribological behaviour of each coating structure. Coatings with a high electrical conductivity, which indicates the presence of graphite within the DLC, exhibit very good tribological properties: low friction coefficients and very low wear rates. Adsorbable gaseous environments lead to a decrease in the friction coefficient. Diamond-like coatings without the graphite structure show a very different tribological behaviour. The presence of water vapour or other active gases such as oxygen leads to an increase in the friction coefficient.

Friction and wear properties of hard carbon coatings at high sliding speed

Abstract The friction and wear behaviours of hard carbon coatings have been studied mainly at low sliding speed. This paper presents a study at high sliding speed (30–35 m s −1 ). Various types of hard carbon coatings were deposited on stainless steel (304L) pins. These coatings are produced by plasma-assisted chemical vapour deposition processes under different deposition conditions. The surface properties and morphology of the coatings have been characterised by surface analysis methods such as scanning electron microscopy and Raman spectrometry. The coated pins slide against a steel XC30 rotating disc. The hardness of disc after heat treatment is 5 GPa. The experimental results show that these coatings exhibit very low friction coefficients and high wear resistances. This behaviour of hard carbon coatings can be explained by the simultaneous presence of the sp 3 and sp 2 bonds hybridisation, by the carbon-hydrogen clusters on the surface of these films, and by hydrogen desorbing from the hard carbon films during friction inducing their reconstruction or creating on them double (π, sp 2 ) bonds.

Influence of diamond crystal orientation on their tribological behaviour under various environments

Abstract Tribological properties of diamond coatings are very complex; if the wear rates remain very low, the friction coefficient is in a large range of values. This behaviour can be explained by: (a) the deposition method: the way the dangling bonds are saturated is not the same depending on whether diamond is obtained by chemical vapour deposition (hydrogen saturation) or combustion flame method (oxygen saturation); (b) by the intrinsic properties of diamond crystals which also take a predominant part in the tribological behaviour. In particular, a change in the growth direction induces different physico-chemical properties for the coatings, and consequently modifies their friction coefficient and its evolution with time. The aim of this paper is to highlight the influence of two particular crystal orientations, 111 and 100, on the friction of the diamond-coated pin/high-speed steel disc couple and on the existence and the importance of the transferred layers. The role of two of the experimental parameters was also studied by realising the tribological tests under various environments (vacuum, oxygen and water vapour) and by applying different normal loads (0.2 N and 1.4 N): the water vapour exposition under low load induces different results for the 111 and 100 oriented coatings. The sliding surfaces were also observed by SEM and analysed by EDS to study the sight and the composition of the transferred layers.

Growth and orientation of diamond crystal thin films obtained by the combustion-flame method

Abstract The dependence of the texture formation and crystal-growth mechanism of diamond films on the deposition parameters have been studied. The samples were prepared by the combustion-flame method under various growth conditions. Results show that the structure and surface morphology of the diamond films depend on the following deposition parameters: temperature distribution on the substrate, C 2 H 2 O 2 gas ratio, and particularly the incident substrate angle α, which describes the angle between the incident acetylene feather and the substrate surface. Scanning electron microscopy (SEM) observations and Raman spectrometry (RS) analysis of a variety of samples show that epitaxial, textured diamond films with {111} facets were formed with a 90° incident angle. When the incident angle is 60°, the films consist of {100} facets. The quality of the diamond films increases as the incident angle decreases. In the present paper, we discuss the growth ratio and the quality of diamond films as a function of the incident angle.

Autolubrication of diamond coatings at high sliding speed

Abstract We investigated the friction and wear properties of diamond coatings at high sliding speed ( V =30 m/s). Various types of diamond coatings were deposited on tungsten carbide pins. These coatings were produced by a combustion flame method under different deposition conditions. The surface properties and morphology of the coatings have been characterised by surface analysis methods, such as scanning electron microscopy (SEM) and Raman spectrometry (RS). The coated pins were tested by sliding against a rotating steel XC30 disc in ambient atmosphere. The hardness of the coated pin and the disc after heat treatment is 70 and 5 GPa, respectively. The experimental results show that at low sliding speed ( V =1 m/s) the diamond coatings exhibit a very high friction coefficient of about 0.9–1.1. At high sliding speed ( V =30 m/s) the friction coefficient of these films decreases to 0.09–0.15, the wear of the diamond films also decreases. This tribological behaviour of diamond coatings can be explained by the transformation of sp 3 to sp 2 carbon sites on the rubbing surface during friction at high sliding speed. A model of the friction is proposed to explain this phenomenon.

Influence of water vapour and oxygen on the tribological behaviour of diamond coatings/steel couple

Abstract Tribological properties of diamond coatings depend on many parameters, especially on the elaboration process, the coating surface topography, the nature of the counterface and the environment. This paper presents a study concerning diamond coatings, deposited by combustion flame process, rubbing on a HSS steel disc under different environments: air, vacuum, water vapour, oxygen and argon. We applied in our experiments two different normal loads: a high one (1.6 N) to observe the transferred debris, and a lower one (0.37 N) to avoid deep indentation. With a 1.6 N normal load, the friction coefficient increased under oxygen (μ: 0.15–0.18) and water vapour (μ: 0.15–0.20): that is due to the thickening of the oxide layer. When the normal load is decreased to 0.37 N, the obtained results were similar to the last case, except under water vapour where the friction coefficient increased to 0.25; the influence of the superficial tension forces will be therefore studied. To confirm our conclusions concerning the interactions between diamond and steel, and to highlight the phenomena occurring during the contact, we also present surfaces observations of both pin and disc.