Thermal levels and subsurface damage induced by the occurrence of hot spots during high-energy braking

Thermal levels of hot spots formed during high-energy braking with an organic matrix composite pad facing a steel disc have been studied using an infrared camera and a two-colour pyrometer. The coupling of these two devices allowed us to follow the friction areas and measure the disc surface temperature by taking into account the non-uniformity and variation of the emissivity. It also provided information about thermal gradients on the rubbing surfaces. Hot spot evolutions have been compared to friction contact variations. Material analyses revealed the occurrence of severe damage beneath the hot spots, particularly in the form of plastic deformation and microcracks propagating perpendicularly and parallel to the rubbing surface. In addition, solid–solid phase transformations taking place beneath the hot spots were observed in the vicinity of the rubbed surface. Finally, frictional force variations and subsurface damage were correlated with the surface temperature.