Tribology performance, airborne particle emissions and brake squeal noise of copper-free friction materials

Copper and its alloy have been widely used in the formulation of non-asbestos organic (NAO) friction materials as reinforcements and/or additives. But U.S.A has raised a legislation to reduce the copper content in friction materials due to the toxicity of copper in wear debris. Thus, there is a need to develop copper-free friction materials which also owns the desired tribology performance. In the present study, two new copper-free non-asbestos organic (NAO) friction materials were developed by replacing the copper fiber with steel fiber (named SFM) or ceramic fiber (named CFM). Friction and wear behaviors, airborne wear particle emissions and brake squeal noise of these two copper-free friction materials were evaluated and compared with a reference copper containing friction material (RFM) by using a pin-on-disc test rig. Results show that all friction materials have comparable friction coefficients, and the specific wear rates of SFM and CFM are larger than that of RFM. The mean fractal dimension of RFM surface is larger than those of SFM and CFM surfaces. For the airborne particle emissions, RFM presents the lowest particle number emissions and particle mass concentrations (PMCs) of PM₁₀ (particles smaller than 10 μm) and SFM presents the highest values of these two results. For brake squeal noise, the brake squeal noise of all friction materials has comparable A-weighted sound pressure levels (SPLs). It can be concluded that the replacement of copper fiber with steel and ceramic fibers can obtain comparable tribology performance and brake squeal noise, but it will increase the particle emissions.