An Investigation of Property Changes of Copper-Free Brake Pads During Wear Testing: Pressure and Temperature Dependence of Pad Modulus, and the Correlation Between Modulus and Friction Coefficient 2021-01-1276

Earlier publications have demonstrated that pad and disc properties change during storage and also during the SAE J2522 Brake Effectiveness Test Procedure. The current investigation was undertaken to find out how the properties change under milder braking conditions, using the SAE J2707 Wear Test Procedure. A copper-free formulation was selected for the investigation and tested on an inertia dynamometer using a front caliper designed for a passenger car. The pad dynamic modulus changed up or down throughout the test, depending on the test conditions. The pad dynamic modulus, the pad natural frequencies and the disc natural frequencies all decreased by the end of the test. Under high-speed, high-deceleration and high-temperature braking conditions, the pad surface region permanently expands, which results in reduced dynamic modulus and also leads to reduced pad thickness loss as compared with pad weight loss. A good correlation is established between the friction coefficient and the pad dynamic modulus: the friction coefficient increases with decreasing pad dynamic modulus and this relationship holds for epoxy-carbon nanoparticle composites as well as for soft metals sliding on cast iron or steel. This relationship explains the pressure sensitivity as well as the speed-sensitivity of the Mu. With decreasing dynamic modulus of the pad during testing, the Mu increases, and the temperature rise of the disc declines. Three competing processes are taking place simultaneously in the pad during storage or during brake testing, affecting the pad thickness - i.e., resin cross-linking, spring-back due to internal stresses and compaction under the braking pressure. Changing pad dynamic modulus and changing lubricity of the friction film layer affect disc temperature rise during braking. As pad and disc properties are changing continuously throughout a test, a question arises regarding what properties to measure and when to measure if any correlation is to be established between pad properties and performance - i.e., friction, wear and NVH. In addition, the inboard pad and the outboard pad change their properties at different rates, thus creating a challenging situation for establishing property-performance relationships.