Figure 1 shows the DSC curves of the cold-sprayed Ni–Al and Ni–Al–CuO coatings. For comparison, the DSC curve of the cold-sprayed Al–CuO coating is also plotted in Fig. 1. Three separate peaks in the temperature range of 450–700 ℃ can be observed for both the Ni–Al and Ni–Al–CuO coatings. For the Ni–Al–CuO coating, a fourth peak marked D in Fig. 1(b) appeared at 740–780 ℃. The two broad peaks A and B lie below the eutectic temperature of 640 ℃ based on the Ni–Al phase diagram , indicating that these reactions are controlled by solid-state diffusion. The third peak C identified for the Ni–Al coating is a sharp exothermic solid–liquid reaction peak at 640 ℃, also being observed for the Ni–Al–CuO coating. Peak D in the DSC curve of the Ni–Al–CuO coating indicates that CuO took part in the reaction at 740 ℃. As for the cold- sprayed Al–CuO coating, only one endothermic peak appeared, at 660 ℃, corresponding to the melting point of Al.
Fig. 1 (a) DSC curves of cold-sprayed Ni–Al, Ni–Al–CuO, and Al–CuO composites (the binary Al–CuO bulk material was formed by cold
spraying and had an Al/CuO atomic ratio of 2:3) and (b) magnification of dashed frame area in (a)To identify the details of the reaction associated with each exothermic peaks in the DSC curves of the Ni–Al–CuO sample, it was heated to the corresponding peak temperatures (480, 580, 650, and 780 ℃ for peaks A, B, C,and D, respectively) at a rate of 10 ℃/min under argon atmosphere then quenched. The x-ray diffraction (XRD) spectrum and SEM image results are shown in Fig. 2. CuO and Cu species could not be detected due to the low content of CuO. For the solid-state reaction corresponding to peak A, it was found that Al3Ni was the first reaction product, as shown in Fig. 2(b). Al3Ni2intermetallic compound was formed at 580 and 650 ℃ (peaks B and C), respectively. These results are consistent with reported data regarding the Ni–Al reaction under the same annealing condition. The EDS results indicated that the molar ratio of Cu to O in the CuO particle shown in Fig. 2(c) was about 53:47. The elemental distribution in Fig. 2(d) confirms that the boundary of CuO could be clearly distinguished from the surrounding Al after heat treatment at 650 ℃. These results suggest that the CuO particle did not take part in the reaction at 650 ℃. For the sample heat treated at 780 ℃, the size of the CuO particle was significantly reduced and the concentration of O atoms was quite low, as shown in Fig. 2(e) and (f).
Fig. 2 (a) XRD patterns of Ni–Al–CuO coating after heating; SEM images of sample heated at (b) 480 ℃, (c) 650℃, and (e) 780 ℃; (d) and(f) show elemental distributions corresponding to the region in (c) and (e), respectively
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