ICMCTF1998 Session EP: E - Posters

Oxide ceramic coatings produced by micro-arc discharge treatment can provide a considerable increase in the wear life of components made from aluminium alloys ¹. To use these coatings in tribological applications, it is necessary to understand their friction behaviour in contact with different materials under a wide range of loading conditions. In this paper the friction characteristics of Al-Si-O coatings formed on A16.3Cu and A14.3Cu1.5Mg0.6Mn substrates are studied under continuous and reciprocating sliding contacts against steel and WC balls. The tests are performed under ambient atmosphere conditions with the loading varied from 0.2 to 20 N. Relations between the friction behaviour and mechanical properties, structure and phase composition of the coatings are discussed to explain the results obtained.

It is found that at least two factors (the relationship between the ball and coating mechanical properties and their ability for adhesive interaction) are responsible for the tribological behaviour of the friction pair. A minimum friction coefficient is reached when the hardness of the coating Al₂O₃ matrix corresponds to the hardness of the ball material. The friction coefficients here are caused by the adhesive interaction and amount to 0.10-0.16 and 0.20-0.28 sliding against WC and steel respectively. For the softer coatings the friction coefficient rises to 0.8-1.0 with increases in the loading, and for the harder ones it rises to 0.5-0.7 independently of the loading. Both the raising of the abrasive component of the wear and changing of the contact area due to substrate strain are discussed as possible reasons for the friction variation.

¹ A A Voevodin, A L Yerokhin, V V Lyubimov, M S Donley and J S Zabinski, Surface and Coatings Technology, 86-87 (1996) 516-520.

Conventional dimethylsiloxane polymers (silicones) have a high erosion rate in atomic oxygen which make them unsuitable for long term exposure in the Low Earth Orbit (LEO) environment. Advanced silicon-oxygen backbone polymers can exhibit much lower atomic oxygen with vacuum ultraviolet (AO/VUV) erosion rates. However, there is currently a lack of space-qualified polymer coating systems available that can withstand the requirements of 30-35 years exposure in LEO. There is a need for flexible coatings stable in the LEO environment.

This presentation describes the performance of proprietary non-dimethylsiloxane inorganic polymers as a coating system resistant to AO/VUV. These coatings may be suitable for the protection of organic materials and composites in LEO. The inorganic polymer is soluble in common organic solvents and can be applied onto a variety of substrates by inexpensive spinning, spraying, or dip coating techniques. Kapton was the primary substrate used in this program, although other materials such as graphite epoxy composite can also be coated.

Performance data of the proprietary coating will be presented including the erosion rate in terrestrial AO/VUV tests, coating adhesion before and after thermal cycling and surface morphology by SEM before and after AO/VUV exposure. The potential application of this coating as an AO/VUV resistant varnish as well as a binder for flexible thermal control paints will be discussed.

In order to prevent the damage or the spallation of an oxide thin film during an oxidation process, it is important to delineate the effects of the different origins of stresses¹ which usually take place at the metal/oxide interface undergoing transformation. We have developped a general model which allows us to study the influence of the metallic ion ordering in a three-dimensional interface, which is strongly related to the epitaxial relationship. We have shown that a phase transition can lead to a mechanism of low-temperature relaxation of microstresses due to modification of the interface. The model is applied to the case of both simple and more complex epitaxial systems such as (001)Fe//(001)FeO, (001)Ni//(001)NiO and (111)Ni//(001)NiO. The numerical calculation of the growth and the residual stresses in the metal/oxide system has been performed taking into account the microstructural morphology of the interface and also the growth deformation due to the interfacial ordering that has been detected by LEED analysis².

¹C. Liu, A.M. Huntz and J.L. Lebrun, Materials Science and Engineering, A160, 113 (193) ²T.M. Christiansen, C. Raoul and J.M. Blakely, Appl. Surf. Science, 26, 408 (1986)

This paper describes an investigation on the combined influence of environment and applied magnetic field on the friction and wear processes of aluminium. Experiments are carried in ambiant air on aluminium/steel couple. Applied magnetic field modifies the size and the behaviour of third body in sliding contact and increases the rubbing surface microhardness.

The aim of this paper is to present the experimental study of friction and wear behaviour of aluminium and their variations induced by applied magnetic field.

Plasma nitriding is usually used for ferrous materials to improve the surface properties of these materials. Knowledge about properties of thin surface layers is essential for designing engineering components with optimal wear performance.

In our study, we have investigated and compared the microstructural, mechanical and tribological properties of plasma and pulse plasma nitrided AISI 4140 steel. The influence of nitriding case depth as well as the presence of compound layer on tribological behaviour was also examined. Plasma and pulse plasm nitriding was carried out using commercial nitriding processes. Nitrided samples were fully characterised, using metallographic, SEM microscopy, microhardness and profilometry techniques, before and after wear testing. Wear tests were performed on pin-on-disc wear testing machine in which nitrided pins were mated to hardened ball bearing steel discs. The wear tests were carried out under dry conditions where hardened samples were used as a reference. The resulting wear loss as well as the coefficient of friction was monitored as a function of load, sliding speed and test time. Several microscopy techniques were used to analyse the worn surfaces and wear debris in order to determine the dominant friction and wear characteristics.

Dry rubbing of the steel XC48/graphite couple has been studied in the presence of a magnetic field and the results are compared with those obtained from similar experiments without magnetic field.

In this study, employing a combination of scanning electron microscopy and energy dispersive X-ray spectroscopy, the authors explored the properties of the wear particles and the friction tracks of steel/graphite couple and described their friction and wear mechanisms in open air of 20-30% relative humidity at room temperature, under reduced pressure (3.10^-3 Pa) conditions and in a pure argon (purity, 99.95%).

The results of the tribological tests indicated that the friction coefficient decreased about 30% with the presence of a magnetic field applied through the contact in open air. However, in vacuum or in argon, magnetic field increased the friction coefficient respectively about 33% and 70%.

The wear rates of graphite disc were lower in air than under inert environment.

The wear is very low when the transferred carbon films present a good adhesion on metal surface of the counterface. This adhesion depends on the cohesion of solid particles within the film, itself depending on the chemical properties of the transfer layer. Then, thin graphite films have a longer life than thick mixed films.

The results of the surface and the wear debris analytical studies, obtained on graphite/steel magnetized sliding contact, have suggested a correlation between the transferred layers and its tribological behaviour