The work reports for the first time galvanic current measurements for the friction material – backplate couple. In particular, two configurations including different friction materials and a steel-based backplate are considered. Measurements are performed using Linear Sweep Voltammetry (LSV) and Zero Resistance Ammeter (ZRA)–based techniques and allow to: 1) investigate the interplay between galvanic currents and localized corrosion events at the interface between each investigated friction material and the corresponding backplate; 2) correlate the corrosion potential of stand-alone materials with that of multilayered specimens; and 3) identify friction materials which show a sacrificial anode behavior with respect to the backplate.
Brake System and Related Corrosion Phenomena
The brake system is a core component of cars, motorbikes, bikes, airplanes etc.. Its main task is to modulate the speed of moving vehicles by converting the kinetic energy into heat.1,2 In the case of modern cars or motorbikes, the speed modulation can be performed by using the so-called disc-brake system.1,2,3,4,5,6 This generates the braking torque by forcing two brake pads against a disc by the means of a caliper.2,3,5,6 In the case of cars, the disc and the caliper are enclosed within each wheel and, as a consequence, can be exposed to corrosion phenomena, mostly related with atmospheric or environmental conditions.1,3,7,8
Brake Pads and Electrochemical Methods
Brake pads are key elements of disc-brake systems for car and motorbike applications.2,4 At a first approximation, brake pads are realized by supporting a friction material (FM) onto a steel-based backplate (BP) (See Figure 1A). Nevertheless, a third component named underlayer (UL) is usually comprised between the FM and BP. The UL is necessary for at least the following motivations: a) achieve a certain comfort level during braking (damping of vibration and noise generated by the friction of the FM against the disc); b) guarantee the proper adhesion of the FM to the BP; and c) provide a thermal barrier between the disc and the caliper. For sake of completeness, it is important to mention that a further adhesive layer can be present at the interface between UL and BP. Nevertheless, in the present work the adhesive layer is considered as a part of the UL. Due to their composition, both the FM and UL can be considered as composite materials and typically include abrasives (carbides, oxides), metallic moieties (Fe, Cu, Zn,…), fillers and lubricants.9,10,11