PSI - Issue 33

Pietro Tonolini et al. / Procedia Structural Integrity 33 (2021) 1152–1161 / Structural Integrity Procedia 00 (2019) 000–000

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2. Material and methods

The material considered in this study is a pearlitic EN-GJL-250 cast iron, with graphite I A 4/5 – ISO 945 and a mean hardness of about 20 HRC. Two sets of samples were machined from GCI as-cast industrial disc brake in form of rings and discs accordingly to the geometry prescribed respectively for BoR and PoD tests, as shown in Fig. 1a and 1b. One of the two-sample set was laser-cladded by using two different powders: a martensitic stainless steel powder (SS) and the same powder reinforced with the addition of tungsten carbides (WC) spherical micro-particles (R-SS) and then machined to an average roughness Ra of about 0.25. For comparison, discs for PoD tests were also machined from a commercial HVOF coated brake disc. Table 1 shows the list of the samples and the corresponding tests that were carried out. Blocks and pins used in the tests as counterparts were machined from commercial low-metallic passenger car brake pads without Cu (Fig.1c and 1d).

Table 1 Summary of tested samples. Sample BoR

PoD

GCI

yes yes yes

yes yes yes yes

GCI + SS

GCI + R-SS GCI + HVOF

no

Fig. 1 Sliding wear test samples dimensions.

Coatings morphological observation were carried out on polished cross-sections by means of a Zeiss-LEO EVO 40 scanning electron microscope (SEM) equipped with an Oxford energy-dispersive spectroscopy (EDS) probe for chemical analysis. Micro-Vickers hardness coatings measurements were performed by means of a Vickers Mitutoyo HM-200, applying a load of 0.5 Kgf for 15 s. Regarding the tribological tests, PoD and BoR tests setup were chosen in order to evaluate the coefficient of friction (CoF) and the mass loss under different contact conditions.In particular, PoD tests were performed with a commercial THT tribometer (CSM Instruments) where the disc is mounted horizontally and rotates under the pressure exerted by the vertically mounted pin. In this configuration, the wear debris are not removed from the contact region. A linear velocity of 1 m/s and a normal pressure of 0.7 MPa were applied for a total sliding distance of 35 km. The radius of the wear track was 16 mm. On the other hand, BoR machine is a tribometer realized at the University of Brescia according to the ASTM G77 standard. In this case, the ring is coupled on the electrical motor shaft and during its rotation into a vertical plane the block is pressed against the outer cylindrical surface of the ring. In this case, the type of contact is non-conformal at