ICMCTF1999 Session EP: EP Poster

The interest in advanced solid lubricant coatings is growing steadily since these materials are able to offer unique combinations of low contact friction and an extended lifetime of the friction couple. This is particularly true for space mechanisms applications, where the use of fluid lubricants is often inadvisable due to losses by evaporation and creep, and the consequent possibility of contamination.

Diamond-like carbon films form an important class of advanced solid lubricant coatings. Since the earliest investigations it was shown that this material could exhibit low friction under vacuum conditions, but in other cases, however, high friction in vacuum was observed. This diversity in results is not surprising since diamond-like carbon represents in fact a wide range of carbon based coatings deposited by a variety of techniques. In this work, the tribological properties in vacuum of a-C:H films produced by RF plasma assisted chemical vapour deposition (RF PACVD) were studied. It was demonstrated that such films deposited at low RF bias voltage show lubricating capacity under vacuum conditions. However, the wear resistance in vacuum of currently used sputtered MoS₂ coatings has yet to be achieved.

The evolution of the nickel oxide coating developed on the monocrystalline Ni(111) surface was studied both experimentally and by means of numerical modeling. The NiO/Ni system was submitted to a high temperature creep in the oxygen atmosphere. Using O¹⁸ diffusion method we were able to observe several deformation modes (diffusional creep, gliding at the interfaces,etc.) and periodic cracks generation perpendicular to the charge application direction. The periodicity of the cracks array depends strongly on the external stress applied to the oxide scale. Two steps numerical modeling of the process was performed using first a simplified 1D model and then the 2D finite elements approach. Modeling procedure takes into account epitaxial deformations due to the scale ordering and to the mismatch of the lattice parameters¹, the growth process and the periodic cracks generation. Identification of the visco-plastic material law permits then to obtain the strain (and the stress) distribution in the scale and in the substrate. The comparison of the experimental data with the numerical results allows to formulate several criteria for the oxide scale adherence.

¹ Lahoche L. et al, ICMCTF 98, to be published in Surf. Coat.Technology

Tribological and mechanical properties were evaluated for carbon nitride films deposited by an ionised dc magnetron sputtering system under various conditions. Inductive plasma was generated between a graphite target and substrates.

Friction measurements were carried out on the films in ball-on-rotating disc tests in laboratory air at room temperature. The mean coefficient of friction was obtained after an initial run-in period. Steady-state coefficients of friction observed were between 0.44 and 0.81 with the inductive plasma, and between 0.36 and 0.60 without the inductive plasma. In each case, the values increased with increasing nitrogen partial pressure in the sputtering gas and then approached a constant value.

The films deposited at a pressure of 2.0 Pa tended to be detached from the substrates more easily than those deposited at 5.0 Pa. The stress and hardness of the films were evaluated and discussed together with the structure and adhesive properties of the films.

Carbon nitride (CN_X) films were deposited by dc reactive magnetron sputtering from a C target in N₂/Ar plasma. Films were grown on Si (001), Ni, and steel substrates to thickness of ~1 µm at a constant total pressure of 2.5 mTorr with the N₂ fraction varied from 0 to 1, and the substrate temperature, T_S, varied from ambient to 500 °C. The influences of the N₂/Ar mixture and T_S on the surface roughness, hardness, elasticity, adhesion, residual stresses, wear and friction coefficient of CN_X films of known microstructure and composition are presented.

The mechanical and tribological properties of the coatings were evaluated by nanoindentation, laser-induced ultrasonic surface waves method, scratch testing, substrate curvature technique, and pin-on-disk test.

We have shown earlier that T_S < 200 °C results in an amorphous phase with properties that are practically not affected by the N concentration. For T_S > 200 °C, a transition from a graphite-like to a fullerene-like phase was observed with N incorporated in the film.

For CN_X (0X (0S = 350 °C, where a transition from graphite-like to fullerene-like occurs, a dramatic increase in hardness, elasticity and stresses is observed. Furthermore, the rms roughness decreases from 15.0 to 0.4 nm and the film exhibits anisotropic properties. For CN_X (0.15S = 350 °C, the fullerene-like phase exhibits high stress levels up to -5 GPa, hardness and elasticity, the latter reaching 90% recovery; the friction coefficient is ~0.2 and the film shows a smooth surface.

Under the influence of high contact pressure the tribological performances of the carbon-carbon composite materials essentially depend on the carbon fibres texture and on the properties of the transferred layers.

In this study, employing a combination of scanning electron microscopy and energy dispersive X-ray spectroscopy, the authors explored the friction tracks properties and described the role of the transferred layers in friction and wear of copper/carbon-carbon composite couples in open air of 20-30% relative humidity at room temperature.

Tow multidirectional carbon fibre reinforced carbonaceous matrix material were investigated.

One of the tested composites is based on three directional (3-D) construction consisting of multiple yarn bundles located within the structure with a mail texture, the other is a 2-D array with a taffeta texture.

The results of the tribological tests indicated that the friction coefficient and wear rate decrease when the carbon fibres exhibits a mail texture.

For a 2-D carbon-carbon composite, the contact surface has a rough appearance and fibre surfaces are marked with wear grooves.

Patch of scattered wear debris and worn fibres with longitudinally oriented structure are clearly visible on the contact surface.

The matrix carbon appear to be covered by a very granular layer constituted by a mixture of copper oxide and copper particles as revealed by E.D.S analysis.

The friction track of 3-D carbon-carbon composite is covered with a layer of debris film-like structure

Again, the wear grooves on the carbon fibres are observed.

However, these wear grooves are relatively narrow and shallow compared with the grooves observed on the surface fibre of a carbon-carbon composite with a taffeta texture, which means that the wear rate is lower in this case.

Plasma Electrolytic Deposition (PED) is based on the deposition and modification effects of an electric spark or arc discharge on the surface of an electrode immersed in an appropriate electrolyte. Although the discharge phenomena associated with electrolysis were discovered a century ago, studies of potential industrial applications of PED have been carried out only in the last 20 years, In particular, Micro-arc Discharge Oxidation (MDO) deposition onto aluminium and titanium alloys and Micro-arc Plasma Diffusion (Nitride/Carbide/Boride) treatment of steels have been reported. The results of recent scientific studies are reviewed in this paper, including the physical and chemical fundamentals of plasma electrolysis, the equipment and deposition procedures for coating production as well as the effects of the electrolyte. The metallurgical and tribomechanical characteristics of coatings or layers formed by PED techniques are also described.

Micro-arc discharge oxidation operates at a potential above the breakdown voltage of the oxide film growing on the surface of an aluminium (or other light alloy) workpiece, which forms the anode of the electrolytic system. Complex compounds are synthesised inside high voltage breakthrough channels across the growing oxide layer and the coating growth proceeds in both directions from the substrate surface (i.e into the substrate and out from the surface). Micro-arc oxidation of aluminium allows the formation of thick (up to 500µm) and hard (up to 23GPa) surface layers with excellent adhesion to the substrate, by the use of pulsed DC (or AC) power supplies and a suitable aqueous electrolyte. Micro-arc Plasma Diffusion (MPD) is carried out at a potential above the breakdown voltage of the gas bubbles formed on the surface of the workpiece, which in this case forms the cathode of the system. The localised plasma thus created provides reactive and diffusive elements for the substrate and provides intense localised heating of the workpiece. This paper describes 10-20µmthick surface compound layers (1200HV) and 200-300µmdiffusion layers can be produced on steel substrates with excellent mechanical and chemical properties in only 3-5 minutes of treatment by use of MPD nitriding/carburising treatments.

Biomedical materials generally require good wear resistance and good high cycle fatigue properties. Cobalt-chromium alloys have good corrosion resistance, high strength and surface hardness. However like-against-like sliding contacts under severe sliding wear conditions are prone to adhesive and abrasive wear (hard surface particles are broken off and act as a third-body abrasive.

Plasma immersion ion implantation (PI³) provides a non-directional method of implanting ions into materials using a low temperature plasma. A cathode sheath forms around the workpiece and, by superposition of high voltage pulses, ions from the plasma are accelerated towards the workpiece surface and implanted to shallow depths. Implantation occurs across the whole of the target geometry, alleviating the need to manipulate as in conventional ion beam processes.

In this study triode plasmas were employed with a hot negatively biased tungsten filament providing thermionic-electron plasma support. Low pressure plasmas (5-10mTorr) containing nitrogen and carbon were produced, and pulsed negative bias voltages of between 5 and 20kV (with a varying HV pulse frequency of between 0.1 and 1kHz) were applied to the alloy. Evaluation of the implanted samples by XRD, microhardness testing and dry sliding wear (pin-on-disc) was performed to assess the influence of gas pressure, bias voltage and pulse frequency on treatment effectiveness.

The mechanical and tribological properties of plasma thermochemical heat treatments have been compared with PVD TiN, CrN coatings and nitrogen ion implantation of the titanium alloy Ti-6Al-4V using a rubber-wheel-type abrasion tester and a pneumatic impact tester. Using a thermionic triode plasma assisted system, a hot filament supported glow discharge was produced with different compositions of nitrogen, carbon and hydrogen for the plasma treatments (substrate temperature 700-750°C). The duplex system consisted of plasma nitriding followed by plasma-assisted physical vapour deposition (PA) PVD of TiN ("Nitron"). Two groups of nitrogen implanting regimes were chosen for the study. An increased nitrogen ion dose and higher pre-implant temperature was used, to produce a larger diffused layer of nitrogen ions into the substrate to enhance load support.

At elevated temperatures and increased process times the surface roughness of the alloy was found to increase significantly, although this roughening effect was reduced by depositing a thin titanium layer by (PA) PVD, prior to the plasma treatment. In this study we describe how it is possible to increase the substrate/surface load support with surface microhardnesses of >1500Hk and investigate this effect on the abrasion and impact resistance of the titanium alloy. The plasma diffusion treatments all increased the load support provided by the substrate but provided limited improvement in the abrasive wear resistance compared to the (PA) PVD CrN coating and the "Nitron" duplex system.

The application of magnetic field alters significantly the plastic properties of crystal and surface oxidation.

The action of magnetic field on sliding surfaces: ferromagnetic steel XC48/aluminium may cause a surface modifications and then the friction and wear behaviour modifications.

Aluminium and steel surface modifications induced by the magnetic field during sliding test are analysed and performed by Vickers Microhardness, Optical Microscopy, Scanning Electron Microscopy, EDS analysis and by the oxidation ratio measurement.

This paper presents a study of the surfaces modification, the wear and friction behaviour of aluminium in non feroomagnetic, aluminium/ferromagnetic XC48 steel sliding contact in magnetic field.

The presence of pre-existing crack in a layer structure is expected to modify the subsurface contact stress field owing to elastic contact. In case of brittle failure the presence of such cracks is likely to lower the critical stress required to trigger crack propagation and/or nucleate new cracks. In the present study boundary element method is retained to model such effects. This numerical technique has been shown to be especially well adapted for modelling of contact-induced elastic subsurface stress fields. In addition, this technique is very economic in terms of required computer performance and can be easily implemented on a standard P.C..

In a first part we consider a homogeneous, isotropic solid half-space placing the crack at various depths with a_H/10 < t < 3a_H (a_H being the hertzian contact radius). We show that for the upper limit the presence the presence of a crack has practically no effect anymore.

In a second part we consider layered composites with a coating to substrate modulus ratio of roughly 2 and the inverse situation. Here again the position of the crack is modified : within the coating, at the interface and in the substrate. The object is to show the effect of crack in relation with the unperturbed stress distribution.