1.XRD and Raman Spectra Analysis
XRD patterns collected from the top surfaces of the as-sprayed and the annealed coatings are presented in Figure 1a. The as-sprayed coating mainly consists of crystalline Cr3C2 and supersaturated Ni(C,Cr) solid solution. The broad hump in the 2θ range between 42.5◦ and 44.5◦ (marked by the asterisk) is related to the amorphous or nanocrystalline phase, which is also observed in a previous study. The peaks for Cr3C2 have a higher intensity than the strongest peak for Ni(C,Cr) at 2θ ~43◦. After annealing at 600 ◦C, the characteristic peaks for Cr3C2 and NiCr show a reduction and an enhancement in their intensity , respectively . In particular, the strong, broad humps at 42.5◦–44.5◦ observed in the as-sprayed coating is replaced by a weak, asymmetric peak after annealing. Peaks for Cr2O3 can be observed and they increase in intensity with increasing annealing time.
Figure 1. (a) X-ray diffraction patterns and (b) Raman spectra collected from top surfaces of as-sprayed and annealed coatings.
Raman spectra collected from the top surfaces of the annealed coatings are presented in Figure 1b. Vibrational modes of Cr2O3 at 298, 353, 554 and 615 cm−1 and two additional bands of graphitic carbon at ~1350 and ~1600 cm−1 can be observed after annealing due to oxidation of carbides as described by the chemical reaction Cr3C2 + O2 → Cr2O3 + CO/CO2 with some free carbon as residue. With increasing annealing time, Raman peaks for both Cr2O3 and carbon show enhanced intensity . The combined information from XRD and Raman spectroscopy suggests that Cr2O3 is the dominant oxidation product. Other oxides, i.e., NiO and NiCr2O4, are not detected.
2.Microstructure Analysis
Figure 2a shows the cross-sectional BSE image of the as-sprayed coating before surface polish. The coating is dense and ~370 µm in thickness. Porosity calculated from image analysis is 2.2% ± 0.4%. An expanded view of the as-sprayed coating is also shown in Figure 2a, where two phases can beclearly identified by the image contrast. The light gray areas marked by black arrows are the Ni-rich binder phase, as later revealed by EDS analysis. Variation of greyscale of the Ni-rich phase can be observed, which may be attributed to a small amount of carbide dissolution. The dark gray particles surrounded by the NiCr binder phase (indicated by the white arrows) are carbides. Some pores are also spotted (black). No internal oxides are detected in the as-sprayed coating.
Figure 2. (a) Cross-sectional microstructure of the as-sprayed coating; (b–f) cross-sectional SEM images of the coatings after annealing at 600 ◦C for 24, 48, 96, 192 and 384 h, respectively . The white arrows in (b–f) indicate the precipitation of secondary carbide particles.
spallation was observed on the surface of the annealed coating. In Figure 3a,b, the oxide-scale thickness (h) is plotted as a function of annealing time (t) and square root of time (t0.5), respectively . The oxide-scale growth kinetics obeys the parabolic law that a linear relationship can be observed on the h-t0.5 relationship, suggesting a diffusion-controlled growth mechanism. The growth rate, kp, calculated from the linear fitting of h-t0.5 relationship, is 1.17 × 10−12 cm2·s−1.
Figure 3. Oxides scale thickness as a function of (a) annealing time and (b) square root of annealing time.
Figure 4 shows the surface morphology of the oxide scale (before depositing the Ni protection layer), where an increase in the surface roughness can be observed with increasing annealing time. For example, after annealing for 6 h, the surface is smooth and a slight contrast difference can be observed, which suggests that the oxide-scale is thin to present the two-phase structure of the underlying coating. Under secondary electron (SE) mode, the brightness contrast indicates a difference in height. The bright regions exhibit a higher altitude than the dark regions, suggesting two different ways of oxides growth, namely the outward diffusion of Cr for the NiCr binder phase and the inward diffusion of oxygen for the carbides. However, with increasing annealing time, e.g., 192 and 384 h, the surface shows homogeneous and dense structure with presence of large particles, indicating the formation of a dense and continuous oxide-scale with certain thickness. EDS point analysis (not shown) reveals that the lager particles on the coating surface after 384 h annealing are rich in Cr and O.
Figure 4. Surface morphology of the oxide scale after annealing at 600 °C for 6, 24, 48, 96, 192 and 384
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