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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the beginning of the test and the wear debris are self-removed from the contact region. A linear velocity of 0.4 m/s and a load of 200 N were applied for a sliding distance of 27 Km. In this way the contact pressure was varied indicatively from 100 MPa (non-conformal contact) to 2 MPa at the end of the test. During the tests, the friction force was continuously measured and the CoF was then plotted against the sliding distance. The mass loss of test specimens was evaluated by weighing the samples before and after the test by using a laboratory scale (Gibertini E42-B). The wear rate (W) was obtained dividing the mass loss by load and sliding distance and a minimum of three repetitions were carried out for each sample in both test configurations. The wear mechanisms were then studied observing the worn surfaces of coupled materials with the scanning electron microscope. 3. Results and discussion Table 1 summarizes the chemical compositions and the thickness of different coatings as measured by SEM-EDS analyses performed on samples cross-sections (Fig.2). The average chemical composition of the friction material used for the tests is shown in table 2.

Table 2 Characteristics of coatings. Coating Interlayer

Total thickness

Average chemical composition [wt%]

SS

NO ̴̴̴̴̴̴̴̴ NO ̴̴̴̴̴̴̴̴

700 µm 700 µm 100 µm

13% Cr-87% Fe

R-SS

13% Cr-87% Fe (+ 10% WC) 16% Cr- 18% Ni- 66%WC

HVOF

10-13 µm pure-Ni ̴̴̴̴̴̴̴̴

Table 3 Average chemical composition of the brake pad material. Elements [wt%] C O Mg Al Si S

Ti

Cr

Fe

Ni

Zn

Sn

Ba

Other

Brake pad

N/A

16.8

7.26

2.94

0.91

3.31

0.13

0.80 20.41

0.23

0.76

4.19

4.00

Bal.

The low adhesion of the laser-cladding coatings to the GCI substrates can be argued by looking at the interfaces in figures 2a and 2b, where graphite lamellas have promoted the formation of large gaseous voids e during the initial stage of depositions (Li et al., 2019). Furthermore, the WC micro-particles added to the R-SS coating (Fig.2b) exhibit an irregular distribution in the coating, along with signs of slight diffusion into the metal matrix. Some pores are also present within the clad. Differently, the commercial HVOF coating shows the presence of a pure Ni interlayer (with a thickness in the order of 13 microns) that should improve the adhesion between the substrate and the WC-NiCr coating and no porosity is evident at the interface. In addition, the pores into the coating, which are typical of this deposition process, are quite small. (Fig.2c).

Fig. 2 Cross section microstructure of coatings: a) SS; b) R-SS; c) Commercial HVOF.

The average micro-HV measurements carried out on the coatings are presented in Figure 3. The higher quantity