Effect of MoS2 on transfer film formation and friction coefficients in NAO friction material

Purpose The purpose of this paper is to investigate surroundings for transfer film formation and removal, the effect of the transfer film formation on friction coefficients, the effect of four different abrasive components, ZrO2, ZrSiO4, Al2O3 and Fe3O4, on transfer film formation and the effect of lubricating component MoS2 on transfer film formation and friction coefficients. Design/methodology/approach Two different MoS2 contents of 5.5 and 8.5 per cent were added to friction materials with no MoS2 content, which have four different abrasive components, ZrO2, ZrSiO4, Al2O3, Fe3O4. Friction tests composed of three different stages were conducted for those materials, and the friction surfaces of the counterpart disks were examined by scanning electron microscopy (SEM) to access the formation of transfer film at each stage. Findings For the transfer film formation, high temperature was a prerequisite, but the magnitude of deceleration rate was not important. The effect of the transfer film formation was to reduce the friction coefficients for most friction materials. Friction coefficients of materials which contain lubricating component MoS2 were higher than those which contain no MoS2 for most friction materials. The effect of the lubricating component MoS2 was to suppress the formation of transfer film, thus resulting in increase in friction coefficients. Research limitations/implications The transfer film was rather thin, with thickness of 1-2 µm for most friction materials. That hindered the examination of mechanical properties of the transfer film, such as hardness. Practical implications This research explained the surroundings for transfer film formation, and its effect on friction coefficients. The research suggests to suppress the formation of transfer film to make friction materials with high friction coefficient, and the lubricating component MoS2 can be used for the purpose. Social implications Development of high-friction-brake materials conventionally depends on the use of strong abrasive components, which may induce attacking of counterpart disks. The enhancement of friction coefficients with addition of MoS2 content is expected to open a new prospect in development of high-performance friction materials, which can be applicable to brake pads for racing cars. Originality/value The study is in pursuit of the transfer film formation in successive friction stages, which revealed the conditions for transfer film generation and removal. Specimen