H. Zaidi

The influence of the environment on the friction and wear of graphitic carbons: II. Gas coverage of wear debris

Abstract We used various experimental methods to measure the coverage of gases on the wear particles formed during friction. These high specific surface area particles adsorb oxygen, water vapour and atomic hydrogen. Low friction coefficient and low wear rate are clearly associated with high coverage of adsorved gases. It is believed that the common mode of operation of the various gases studied, if any, resides in the high mobility of the adsorbed phase.

The influence of the environment on the friction and wear of graphitic carbons: I. Action of atomic hydrogen

Abstract The friction and wear of graphitic carbons sliding against themselves or against a metal has been investigated as a function of the gaseous environment. Water vapour and oxygen are of prime importance in determining the behaviour of the contact in agreement with previous results. Hydrogen is effective in reducing friction only after it has been dissociated on a tungsten ribbon. The mechanism of reduction of friction by adsorbed gases is discussed in relation with the absorptive capacity of the third body layer. The action of hydrogen explains well the unusually good wear and friction properties of hydrogenated amorphous carbons. The use of modern surface analytical tools in elucidating the mechanisms of friction and wear is discussed.

Influence of electric fields on the tribological behaviour of electrodynamical copper/steel contacts

The crossing of the electric current in rail lines through a copper/steel sliding contact is associated with an electric field between the sliding surfaces. The electric field increases with the current intensity, the sliding speed and the contact resistance, and it decreases with an applied normal load. An electric field enhances the oxidation of surfaces, and the growth of such an oxide layer increases the electric field. Mechanical wear removes the oxide film and then decreases the electric field. The process of wear in an electrical sliding contact is continuously modified by the electric field according to the current intensity. Surfaces can be damaged by abrasive wear, by oxidation, or by an electric arc through the contact. This paper presents the results of an experimental study of the wear behaviour of an electrical sliding contact and its variation in the presence of an electric field. We discuss the effects of the electric field on surfaces, mechanical properties and tribological behaviour.

Damage of surfaces in sliding electrical contact copper/steel

Copper/steel couple is used for transmission of high electrical power under high electrical tension and high electrical current density for good mechanical, thermal and electrical properties of copper and for low junction electrical tension between copper/steel metals [A. Bouchoucha, Thesis INPL, Nancy, France, 1988]. However, high current density in dynamical contact enhances oxidation process of sliding surface and then can induce electric arc and copper damage. Sliding wire copper is damaged by abrasion, by tribooxidation and by electrical arc. We have studied the copper/steel oxidation in sliding electrical contact vs. the mechanical and electrical parameters. The influence of electrical current on surface damage is made in evidence and analysed. We will present the tribological results of sliding contact and we discuss the thermoelectromechanical surface damage process vs. mechanical electrical and thermal parameters.

Friction behaviour of a graphite-graphite dynamic electric contact in the presence of argon

Friction coefficient μ of graphite-graphite at low dynamical load (P=5N, ν<0.04 m s−1) under argon gas at 1.4 × 105 Pa is very low (μ = 0.06). Above the critical value of sliding speed (v=0.04 m s−1) μ is around 0.7. The transition from the low friction state (μ=0.06) to high friction state (μ = 0.7) depends on the historic of friction, particularly on the revolutions number of disc with speed lower than 0.04 m s−1. According to the sliding speed and the sliding time, there appears an hysteresis phenomenon. The aim of this paper is to report those results and to discuss the influence of electrical current on this friction behaviour. The passing of an electrical current through the dynamical contact leads to an increase in the friction coefficient, but only above a critical intensity.

Study of magnetized or electrical sliding contact of a steel XC48/graphite couple

Abstract The effects of a d.c. magnetic field or an electric current on the frictional behaviour of steel XC48/graphite were investigated. Experimental results are as follows. 1. (1) At ambient atmosphere, both a magnetic field present in a tribocontact and an electric current through the sliding contact decrease the mean value of friction coefficient and decrease its fluctuations also. Electric current enhances sliding surface oxidation according to surface polarity. A magnetic field may increase the pressure of molecular oxygen around the tribocontact and increase the sliding surface oxidation. An oxide layer on the sliding surface leads to a low friction state. 2. (2) Under an inert gas such as argon the application of a magnetic field or the passage of an electric current through the contact leads to an increase of the friction coefficient. This is due to a transfer of fine particles of iron from the steel to the graphite which leads to the formation of metallic junctions during friction.

Influence of a magnetic field on the wear and friction behaviour of a nickel/XC 48 steel couple

Abstract The friction and wear behaviour of a nickel/XC 48 steel couple was studied and analysed in the presence and absence of a d.c. magnetic field on a pin-on-disc tribometer. A magnetic field was applied to the nickel pin and remained constant during each test. Experiments were conducted in ambient atmosphere at different applied normal loads, sliding velocities and magnetic field intensities. The experimental results showed that, at ambient temperature, the application of a magnetic field increased the friction coefficient and microhardness of the sliding surface and decreased the wear rate. The results were interpreted by observation and analysis of the surface and wear particles. Scanning electron microscopy (SEM) showed that, when a magnetic field was applied, the sliding surface was filled with thin, black particles. The sliding contact wear mechanism was evaluated.

Stability of lubricating properties of graphite by orientation of the crystallites in the presence of water vapour

Abstract The friction of graphite is characterized by different stable regimes which are qualified by a friction coefficient μ ranging in the interval 0.1-0.6 and by a wear rate varying similarly. It changes from one stable regime to another according to experimental conditions. The passivation of the dangling bonds of superficial carbon atoms created during the wear process by chemisorption of adsorbable gases (atomic hydrogen, atomic or molecular oxygen, dissociation of water vapour) reduces surface energy, contact adhesion and thus friction (its coefficient falls from μ = 0.6 to 0.3); it reduces the wear rate by a factor of about 100. A high orientation of superficial crystallites obtained in the presence of water vapour during sliding, causes the second transition from μ = 0.3 to μ = 0.1. The wear rate is again reduced by a factor of 10 approximately. An X-ray analysis of crystallites of a surface with a friction coefficient reduced to about 0.1, has shown a high orientation of the (001) planes within a 5° angular dispersion. The film composed of these highly oriented crystallites is very stable. It is well resistant to friction under high vacuum and at high temperature. It is only altered when arching through the contact.

Steel surface modifications in magnetised sliding contact

Abstract Modifications in the mechanical properties of a ferromagnetic steel surface in sliding contact under the influence of a d.c. magnetic field were investigated. A magnetic field was applied to the steel pin, remaining constant during each test. Experiments were conducted at ambient temperature under different applied normal loads, sliding velocities and magnetic field intensities. Experimental results show that at ambient temperature the application of a magnetic field decreases the fluctuations in the friction coefficient and wear rate and increases the microhardness of the sliding surfaces. The dislocation density increases in the thin coating of the magnetised sliding contact interface. A simple model for the agglomeration of dislocation on the sliding contact is proposed. The results were interpreted by observation and analysis of the surface. Optical microscopy shows that when a magnetic field was applied the sliding surface was covered with thin black particles. The magnetic field promoted the oxidation of the surface.

Correlation between the tribological behaviour of graphite and its mechanical properties versus the interplan distance

Abstract Lamellar materials as graphite are used in several technological fields. Because of its anisotropic properties, graphite is a good material of friction. Its covalent properties prevent jamming in a sliding contact and its lamellar aspect reduces shear strain. However, depending on the environment around the contact, its friction coefficient values range between 0.02 and 0.6 and specific wear varies in a ratio of 1000. This behaviour can be correlated to the crystal parameters variation (distance between basal plans) of the material. From a compilation of our experimental results, obtained on graphite under different environments, we suggest a correlation between the elastic constants of the material considered like a composite material and its tribological behaviour.