1.Raman Spectroscopy Analysis
To identify the compositional change after wear tests at RT and 600 ◦C, Raman spectra were collected from the wear scars and compared with those from non-abraded areas. Figure 1a shows the Raman spectra of the as-sprayed and the annealed coatings after wear tests at RT. The as-sprayed coating shows no Raman signal both on the wear scar and non-abraded area. For the annealed coating, the wear scar area shows weak peaks corresponding to Cr2O3 and graphitic carbon. Compared to the non-abraded area, the peak intensities are significantly reduced due to partial damage of the oxide scale. After wear tests at HT, as shown in Figure 1b, the wear scar areas of the as-sprayed and the annealed coatings not only retain the mean peaks of Cr2O3 and carbon, but also present an additional,broad peak at 700 cm−1 corresponding to a poorly crystalline spinel oxide NiCr2O4. During the sliding wear test, the original oxide scale was removed to expose the underlying fresh surface. Ni in the binder phase was rapidly oxidized to NiO, and subsequently reacted with Cr2O3 to form NiCr2O4.Combing the results from Figure 8a,b, the following information can be drawn: (1) The as-sprayed and the annealed Cr3C2-NiCr coatings have different wear behaviors at RT and at HT. Test temperature is critical to the wear mechanism; (2) At HT, new oxide (NiCr2O4) is formed on the wear scar region, suggesting a frictional heat at the contact surface is also crucial to the tribo-oxidation during HT wear.
Figure1. Raman spectra of the wear scar and the non-abraded regions of the as-sprayed and the annealed coatings after wear tests at (a) RT and (b) 600 ◦C. The annealed coatings with different holding times show similar features before and after wear tests, therefore only the 192 h-annealed coating is presented here as a representative.
2.Surface Morphology of the Wear Scar
Figure 2 shows the morphology and the corresponding element mapping of the worn surface of the as-sprayed coating after RT wear test. During friction and wear tests, the softer binder phase is preferentially worn out, and the hard carbides protrude from the coating surface. Cracks (orange arrows in Figure 2c) and pull-out of carbides (ellipse in Figure 2c) caused by normal pressure and friction, causing some Cr3C2 debris (circles in Figure 2b). Under the applied load in the abrasion test, hard particles (Cr3C2) slide along the contact surfaces and thus leave a large number of grooves (as pointed by white arrows in the high-magnification image in Figure 2b,c). These features indicate that the RT wear behavior is dominated by abrasive wear. EDS element mapping in Figure 2c shows Si-rich and O-rich regions with a certain amount of Ni and Cr, resulting from the localized friction heat. Raman analysis of area d (Figure 2d) shows two typical peaks for graphite without signals for Cr, Ni or Si oxides. Figure 3 shows the morphology of the worn surface of the 192 h-annealed coating. Abrasion only occurs at the center of the wear scar (Figure 3c), but grooves are not as severe as those in Figure 2c. Other areas are covered with broken and deformed oxide scales without abrasion features (Figure 3b), indicating that the broken oxide scales can still protect the coating from abrasive wear.
Figure 2. (a) Surface morphology of the wear scar of the as-sprayed coating after wear test at RT; (b,c), high-magnification images of the selected regions marked by 1,2 in (a); (d) Raman Spectrum of the selected area in (c); (e) EDS elemental mapping of Ni, Cr and Si of (a); (f) EDS elemental mapping of Ni, Cr, O and Si of (c)
Figure 3. (a) Surface morphology of the 192 h-annealed coating after wear test at RT; (b,c), high-magnification images of the selected regions marked by 3,4 in (a); (d) EDS elemental mapping of Ni, Cr, Si, C and O of (a).
Figure 4 shows the morphology and the corresponding element mapping of the worn surface of the as-sprayed coating after wear test at 600 °C. Compared with coatings after RT wear test, the surface morphology after HT wear test shows a smaller number of slight grooves and a smoother worn surface. And no particles fracture or cracks are observed (Figure 4b). EDS mapping (Figure 4c) shows homogenous distribution of Cr, Ni and Si on the wear scar despite of a higher oxygen concentration in the wear scar than the non-abraded region. This confirms a severe tribo-oxidation due to a high flash temperature at the contact surface. The change from abrasive wear at RT to oxidation-dominated wear at HT results to the higher wear rates.
Figure 4. (a) Surface morphology of the wear scar (the region between two dash lines) of the as-sprayed coating after wear test at 600 ◦C; (b) high-magnification image of the selected region marked by the dashed rectangle in (a); (c) EDS elemental mapping of Cr, Ni, O and Si of (a).
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