On the Use of Calcium Fluoride as an Infrared-Transparent First Body for In Situ Temperature Measurements in Sliding Contact

This article presents an investigation of the temperature changes in dry sliding contact for braking applications. An original metrology method was developed using a special pad with a calcium fluoride ‘window’ in its centre. As calcium fluoride is transparent to infrared radiation (up to 92% in 1–5 μm spectra), it allows access to the disc surface during friction for estimating its temperature using a two-colour pyrometer. Using this set-up, the surface temperature of the disc was successfully determined during friction. In addition, the temperature in the contact area was compared to that measured immediately outside the contact area; the difference between them proved to be very minimal (<5%). The effect of introducing a calcium fluoride ‘window’ on friction was also studied, and the results show that its use does not affect the friction coefficient. Finally, the wear mechanisms of the calcium fluoride were studied through the characterisation of the worn surfaces using several techniques.