Pin-on-disc and dynamometer investigation of low-metallic and NAO friction materials sliding against cast iron: performances and wear mechanisms

Mattia Alemani¹, Matteo Federici ², Cinzia Menapace², Stefano Gialanella², Guido Perricone¹, Giovanni Straffelini²

¹Brembo S.p.A. Stezzano, Italy; ²Dept. Industrial Engineering, University of Trento, Italy,

INTRODUCTION: During braking, the kinetic energy of the vehicle is dissipated and turned into heat by the friction forces between pads and rotating disc. Commercial discs are typically made of pearlitic cast iron, which exhibits several advantages, including good sliding behavior against different counterface materials, and high thermal conductivity that is helpful for reducing the contact temperature. Friction materials for automotive applications are composites containing, even more than 30 ingredients and usually classified as: low-metallic (low-met) and Non Asbestos Organic (NAO) materials. Actually, nowadays, all of them do not contain asbestos. Low-met materials contain a total amount of Fe and Cu, in the form of powder of fibers, in the range 10-50 wt.-% percent, whereas NAO materials contain a lower amount of metallic ingredients, usually below 10 wt.%. In addition, low-met materials typically display high values of the friction coefficient (usually in the range 0.4 – 0.6), whereas NAO materials display lower values of the friction coefficient (around 0.3-0.4).¹

During sliding, friction layers form on the contacting surfaces. These layers are made of primary and secondary plateaus. Primary plateaus feature hard constituents, such as metallic fibers or ceramic particles, which protrude from the pad surfaces. Secondary plateaus are made of wear debris trapped in between the pad-disc mating surfaces, that pile-up and are compacted against the primary plateaus.² In general, friction and wear behavior of braking systems is determined by the characteristics and properties of the friction layers that dynamically form and are disrupted during sliding. Friction and wear tests on brake materials are conducted using Pin-on-Disc (PoD) tests or Dynamometer tests (dyno tests). Dyno tests are accelerated bench tests carried out on real parts and thus better reproduce the real braking conditions with respect to PoD tests that are simplified tests, typically carried out in drag conditions, i.e., under constant sliding velocity and applied contact pressure. Notwithstanding these simplified aspects, PoD tests feature several advantages, including shorter testing times, lower cost, and the possibility to better relate the friction and wear behavior to the operating conditions. Of course, data obtained from PoD tests must be interpreted with caution in order to transfer safely the obtained information to the real brakes. This is really not a simple task, and a correct approach should be based on the identification of the dominating wear mechanism responsible for the damage.

In the present investigation, the dry sliding behavior of two commercial friction materials, a low-met and a NAO, was investigated comparatively, by means of PoD and dyno tests. In all cases, friction and wear data were obtained and the wear mechanisms were analyzed with the aim of achieving further insight into the possibilities afforded by PoD tests in the investigation of brake materials.

METHODS:  Two commercial friction materials were investigated, a low-met material, codenamed M1, and a NAO material, codenamed M6. M1 contains quite a large amount of abrasives, such as Al₂O₃ and MgO, whereas M6 contains a low amount of abrasive and a high amount of barite. Cylindrical pins with a diameter of 6.0±0.1 mm and a height of 10.0±0.1 mm were machined from commercial pads and used for the PoD tests. The tests were carried out with an average nominal contact pressure of 1 MPa and a sliding velocity of 1.57 m/s, corresponding to mild wear conditions. The tests were carried out at room temperature (25°C) and at a disc temperature of 300°C to explore the high-temperature

behavior of the friction materials, affected by the thermal decomposition of the polymeric binder, i.e., the phenolic resin. The dyno tests were run following a cycle based on the modified SAE J2707 Method B. The characterization of the worn surfaces was conducted in planar and cross sectional modes, using a scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectroscopy (EDXS).

RESULTS AND DISCUSSION:  In Table 1, the average values of the coefficient of fiction (COF) and specific wear coefficients, K_a, obtained from the PoD tests are listed. As expected, the low-met M1 material displays a higher COF and higher wear rate with respect to material M6. At room temperature, wear is rather mild in both cases, whereas it becomes severe at 300°C, because of the softening of the polymeric binder.

Table 1 – Results of the PoD tests.

The worn out pins, made of material M1, display the typical friction layers comprising primary and secondary plateaus. The friction layer is quite different in material M6, and it is characterized by the presence of a mechanically-mixed layer, as shown by the cross-section displayed in Figure 1. The presence of a large amount of small barite particles promote the formation of vortex structures by shear instability. This results in the formation of a rather dense and compact friction layer that induces low friction and low wear.³

Figure 1 – SEM micrograph of the cross section of material M6.

In the dyno tests, the COF varies during each stop and also depends on the test block. From all the recorded data, average values have been: 0.42 for M1 and 0.34 for M6 respectively. An integral evaluation of the wear can be also obtained. The integral wear coefficients were 1.3 10^-14 m²/N and 4 10^-15 m²/N for M1 and M2 respectively. The results are very similar to those obtained from the PoD tests at room temperature and this is in agreement with the (disc) temperature measurements that for both materials were well below 300°C. The damage mechanisms were also similar. In particular, material M6 is characterized by the presence of the particular friction layer detected after the PoD tests (figure 1). To summarize, the tribological data obtained with PoD and dyno tests are quite similar in terms of wearing phenomena. Of course, these results have to be confirmed further, by investigating different materials within the same classification class. 

REFERENCES:  1. Bijve, Poly. Comp. (1997), 2. Eriksson, Wear (2002), 3. Menapace, Wear (2018).