Enhancing braking systems performance via phosphor thermometry

In many manufacturing processes as well as in tribological applications, it is desirable and often necessary to measure the surface temperature. Actually, the behavior of materials at their interface under frictional conditions is strongly influenced by temperature, whose accurate determination at the interface becomes of paramount importance. It's the case of braking systems, in which the temperature at the contact surface of the disc and the brake pad during braking impacts significantly on brake performance, due to the heat generation during the process that modifies the geometry of the so-called friction layer which forms between the disc and the pad. Common practise is to infer surface temperature by using thermocouples inserted in the brake pad itself. The limits of this contact technique are well known and are essentially related to the poor thermal contact of the sensor and to the heat flux along the probe. In order to ensure a higher reliability of these measurements a validation by using a different independent method should be performed. Unfortunately, non-contact infrared thermometry cannot be used for this validation because it is severely hampered by the issues associated with the unknown emissivity of the surface, which value strongly changes during the braking process. An alternative approach, based on phosphor thermometry, is proposed. At INRiM a phosphor-based luminescence decay thermometer has been developed. The phosphor selected as a temperature sensor is Chromium-doped YAlO3 (Cr:YAP), a phosphor which exhibits a wide dynamic range, a high temperature sensitivity and a long luminescence lifetime. A dedicated system for simulating the operative thermal conditions of a brake testing rig has also been constructed. The core of this system consists in a special tool, i.e.an instrumented brake pad, which houses both the termocouples and the phosphor sensors. This brake pad has been used for a laboratory comparison of phosphor temperature measurements against thermocouple measurements in the temperature range from ambient to 450 °C. A hot plate calibration system has been employed to reproduce the comparison reference temperatures. A complete uncertainty budget has also been established. This tool has been further exploited as a transfer standard for on site measurements of surface temperature in the brake testing rig at ITT's industrial facilities to circumvent the critical issues mentioned above. The activity presented here has been performed in the framework of the EMPIR 17IND04 EMPRESS 2 Project "Enhancing process efficiency through improved temperature measurement 2".