Investigation Of The Mechanism Of Voltage－Controlled Friction Of Metal/Ceramic Sliding Couples In Aqueous Solutions

It has been found in recent years that the friction coefficient of a metal/ceramic sliding couple in an aqueous solution increases in the presence of an external voltage. The objective of this thesis is to clear up the causes of the phenomenon. A survey of the previous work on effects of external electric fields on tribological behaviors of materials was presented at first. Then an approach combining double-electrode and triple-electrode techniques was adopted and the test rigs were set up. A series of experiments were undertaken, focusing on the key factors including electrode parameters, lubricants and the interface of metal/solution. Based on a detailed analysis of the obtained experimental results, an electrochemical model on the mechanism of the voltage-controlled friction (VCF) phenomenon was proposed. In this study it has been shown that the electrode potential of the metal side of a metal/ceramic sliding couple determines whether friction coefficient increases or not. The VCF takes place only in a defined electrode potential zone, the specific span of which depends on the material of the metal work piece. The morphology of worn surface and wear rate are different under different electrode potential zones. Wear rate is lower relatively in the potential zone where VCF appears. The location of auxiliary electrode relative to the contact point is also an important factor. If it is too far away from the contact point, the VCF phenomenon will disappear. The surface area of auxiliary electrode is one of factors influencing the VCF. Both of graphite and platinum can be workable as auxiliary electrode instead of active metals like iron.Several different types of solutions, including neutral, alkaline, acid one and a non-aqueous one were selected to explore their effect on the VCF phenomenon. The results have indicated that the presence of a proper external voltage can lead to a substantial increase in friction coefficient of ceramic/metal couples when neutral solutions containing long-chain or short-chain anions are used, but a slight decrease in friction coefficient when an alkaline or acid solution is tested. The neutral solutions containing long-chain anions show a better performance with regard to the magnitude of change and reversibility of friction coefficient. The cyclic voltammetry and chronoamperometry results have revealed that the potential zone where VCF phenomenon appears accords with that of the water electrolysis. Taking all of the experimental findings into account, an electrochemical model, described as following, of the VCF phenomenon has been proposed. The root cause of the increase in friction coefficient is the formation of an insoluble film on the rubbing metal surface under the conditions of extremely negative electrode potential and high pH value of the solution around the contact area due to the electrolysis of water. The film is presumed as hydroxide. This study has deepened our understanding of the VCF phenomenon, widened the selection ranges of lubricant and auxiliary electrode suitable for the VCF, primarily clarified the electrochemicalmechanism of the VCF and thus provided the base for the future applications of the VCF in industry.