An investigation of friction and wear mechanisms of carbon-carbon composites in nitrogen and air at elevated temperatures

This paper presents the general results of an investigation of the friction and wear of a carbon-carbon (C/C) composite material in dry nitrogen and ambient air using a constant speed type of test apparatus. With brake applications in mind, much of the work has been carried out at experimental conditions that allow the temperature of the specimen to reach 700 °C by frictional heating. The steady-state coefficient of friction in dry nitrogen is high (0.35–0.5) because nitrogen does not have any lubricative effect on C/C composites. In ambient air, however, C/C composites undergo an abrupt transition from a low-friction regime (μ = 0.1–0.2) to a high-friction regime (μ = 0.25–0.5) at 150–200 °C. The friction in ambient air is generally lower than that in dry nitrogen at elevated temperatures, which suggests that oxygen in air could have a limited lubricative effect on C/C composites. The existence of temperature-dependent friction and wear regimes in ambient air implies that the performance of brakes made with C/C composites is markedly affected by the thermal properties of the composites, the humidity content in the atmosphere, and operating conditions that can influence the operational temperature of these brakes.