Friction and Wear of Graphite in Dry Helium at 25, 400, and 800°C

Four high-strength, fine-grain commercial-graphite friction couples were rubbed with oscillating motion for a distance of 6250 or 12 000 cm in dry helium (∼ 2 ppm H₂O) at 25, 400, and 800°C. Loads of 2 or 8 kg were used to produce nominal contact pressures of between 4.2 and 35.0 kg/cm². The friction coefficient for all the materials was a complex function of sliding distance, temperature, and degree of outgassing prior to sliding. When the graphites were outgassed at 400°C, the friction during initial sliding (25 to 250 cm) at 25, 400, and 800°C first increased by factors of 1.1 to 4 and then decreased rapidly to values similar to that at the onset of sliding. The maximum friction transients of outgassed graphite were observed at 400°C, and minimums were observed at 800°C. The transient was not produced when outgassing and sliding were conducted at 25°C. The friction coefficient for most of the sliding distance varied between 0.35 and 0.40 at 25 and 400°C, while at 800°C the typical coefficient was 0.30. Wear properties of the graphites appeared similar. During sliding under equal loads, the wear at 25°C was about ten times greater than at 400 or 800°C. The wear rate apparently decreased rapidly with sliding distance. Wear factors ranging from 3 × 10⁻⁵ to 5 × 10⁻⁴ cm³/kg load were measured at 400 and 800°C for both 6250 and 12 500 cm sliding distance.