D. Paulmier

Influence of adsorbed gases on the surface energy of graphite: consequences on the friction behaviour

Abstract The tribological behaviour of graphite depends on the orientation of the crystallites and on the nature of the environment. The basal planes of graphite are dense and the electrons between carbon atoms in these planes are bonded with covalent character. The friction coefficient parallel to these planes is low. A sliding outside of the basal planes leads to the creation of dangling bonds and to an increase of the Fermi level. Then the wear is abrasive and the friction coefficient increases. Moreover, a correlation between the tribological behaviour of the graphite and the presence of an inert gas environment is observed. An inert gas surrounding the contact favours the reorientation of the crystallites parallel to the sliding direction. That leads to a decrease of the Fermi level and the observed lower friction coefficient is explained. In parallel, the adsorption of atomic oxygen on the basal planes leads to a lower Fermi level by decreasing the conduction electron density and explains the low friction coefficient. Analogous reasoning about the surface energy leads to the same conclusions. The aim of this paper is to correlate the friction behaviour of graphite with its energy level under various environments.

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 the electrical current on wear in a sliding contact copperchrome steel, and connection with the environment

Abstract The couple studied has been chosen because it is used to bring current to railway engines and there are problems with the wear of overhead lines. We have shown the influence of environment with oxygen and of the direction of the electrical current on the wear the copper. The interpretation of results according to the theory predicts a more significant wear of copper when it is the anode. We have calculated contact conditions between the copper wire and the steel rod (area, contact pressure, temperature, …) and we have deduced the speed of growth of the oxidation layer on the copper taking into account the effect of the electrical field. When the steel is the anode, we have shown that copper and its superficial oxide are strongly abraded by the chrome oxide particles formed on the steel rod and this phenomenon is preponderant.

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.

Reduction of tool wear in metal cutting using external electromotive sources

Tool wear is fully recognized as an important factor in materials cutting. The well-established methods for its control are based either on process optimization, application of lubricant or use of tool coatings to provide wear resistance and low friction. Nevertheless, application of external electromotive force (EMF) sources (e.g. magnetic field) during cutting of material may accomplish this role. Summarized in this paper are results of investigations on tool wear evolution and certain surface phenomena (temperature and chip morphology) observed during cutting contact subjected to an external magnetic field. Noteworthy, is its use for reducing tool wear and improving machinability of materials.

Role of transferred layers in friction and wear for magnetized dry frictional applications

Abstract Dry rubbing of the XC48 steel/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 friction tracks of steel/graphite couple and described their friction and wear mechanisms in open air of 20–30% relative humidity at room temperature, in partial vacuum (3×10 −3 Pa) conditions and in argon (purity, 99.95%). From the analyses of experimental data, it is shown that in presence of active gases, the oxide layer growth and the transferred carbon films on the steel track are enhanced by a magnetic field. The carbon layer possesses good adhesion to the steel surface and leads to the best reduction of wear and friction coefficient. However, when the friction test is operated in inert environment and in presence of a magnetic field, the opposed phenomenon is observed. The transfer of a harder steel to a softer graphite surface is responsible for the increase of friction and wear. This abnormal process is due to a magnetization of a ferromagnetic steel which is known to be accompanied by reduction of plasticity and increasing the brittleness.