Effects of moisture adsorption on laboratory wear measurements of brake friction materials

As is the case for tribological systems in general, the wear of brake friction materials, sliding against brake discs or drums, is influenced by many factors. This dependency requires very complicated test matrices in order to characterize the range of lining wear properties. Furthermore, the wear rate is very low under normal operating conditions, and thus a substantial amount of test time is needed before the wear progresses to reach a measurable amount. One way to reduce the test time is to use small brake material samples and to measure their weight change with an accurate electronic balance. A laboratory-scale, block-on-disc brake testing machine was used to measure the wear rate of two brake friction materials against cast iron. While conducting these tests, the lining specimen weight changes caused by moisture adsorption were found to be similar in magnitude to the weight change caused by wear. These effects were studied using weight change experiments with both desiccants and heating. Gravimetric methods for wear measurement in porous materials like brake linings are problematic. Several alternatives are offered for mitigating this problem.