PSI - Issue 33

Maria Beatrice Abrami et al. / Procedia Structural Integrity 33 (2021) 878–886 / Structural Integrity Procedia 00 (2019) 000–000

885

8

analyses are summarized in Table 3. At room temperature, oxide cracks can be easily detected (Fig. 6b), which promote the oxide layer fracture and the debris formation, as result of the indirect tribo-oxidation (Fig. 6c). For tests at 150 and 200 °C, the oxide delamination can be observed (Fig. 6g-n), contrary to the 100 °C condition (Fig. 6 d-f), where the oxide appears as more uniform. This happens probably because the oxide thickness has not reached the critical thickness to break, as this temperature may not be enough to achieve a tribo-oxidation involving such an oxide layer growth. In support of this assumption, the COF steady value for pin-on-disk test is lower and more stable than those at 150 and 200 °C (Fig. 3). Regarding tests from 100 °C upwards, it can be seen that the oxide is composed of both aluminum and iron (Table 3), unlike at room temperature after 500 m and at each temperature after 100 m, during which only aluminum oxide forms. This suggests that material transfer from the counterpart to the track occurred in these cases. Moreover, iron content detected by the EDS probe significantly increases as temperature increases (from about 12% to 47%), which is due to the stronger chemical interaction between aluminum and iron at high temperature (Vernon et al. 1953). This agrees with a previous study about the high temperature wear behavior of an aluminum alloy (Ferreira et al. 2020) and denotes that the adhesion wear mechanism takes place at high temperatures, which coexists with the tribo oxidative one. As a result of adhesion, the contact area between the sphere and the worn track gets higher, which leads the COF steady value to increase with temperature, as previously pointed out in Fig. 1.

Table 3. EDS analysis (wt%) of areas indicated in Fig. 5. Testing temperature Spectrum O Mg Al

Sc

Fe

1 2 3 4 5 6 7 8 9

-

3.58 2.89 1.84

95.65 71.47 52.76 40.02 50.39 42.20 95.48 48.06 60.93 95.21 41.22 16.43 63.54

0.77

- - -

Room temperature

25.64 45.40 47.84 39.23 45.51

- - - - -

-

12.13

100 °C

1.67

8.71

-

12.29

-

3.62 1.96 2.55 3.95 1.70 2.46 3.78 1.05 -

0.9

-

150 °C

31.83 27.60

- -

18.14

8.92

10 11 12 13 14 15

-

0.84

-

32.52 35.98 33.44

- -

24.56 47.59

200 °C

0.56

- -

-

1.05

35.60

38.71

35.60

-

24.64

Based on what detected with SEM-EDS analysis, the formation of a first oxide layer contributes to the COF decrease at the beginning of the test. However, as the tribo-oxidative wear mechanism is accelerated by the high temperatures, the oxide layer forms and breaks faster, resulting in an increase of the COF value after the transition phase. The COF increasing happens in a more accentuated way with temperature growing, as these conditions can also cause substrate softening and the consequent inability of sustaining the protective oxide layer. Moreover, alumina is not as effective in protecting aluminum alloys as Fe-oxide for steels, as it exhibits no ductility and low adhesion to the substrate (Straffelini 2015). Therefore, this results in fragmentation of the tribo-layer and in a higher COF steady value as temperature increases.