The microstructures of the NiCr matrix in different regions as schematically shown in Fig. 1 were examined by TEM. It is clearly found that the microstructural features of the NiCr binder phase changed after the spraying (Fig. 2). At the area adjacent to the coating/substrate interface as indicated by mark A in Fig. 1, NiCr matrix showed an amorphous structure as indicated by the halation diffraction pattern shown in Fig. 2b. The splat deposited directly on the substrate surface can achieve a cooling rate from 106 to 108 K/s. Therefore, it is reasonable for the binder phase to solidify in the form of an amorphous phase.
Fig. 1. Schematic of the area for TEM observation.
Fig. 2. Morphology of NiCr matrix in different regions.
In the region close to the interface (mark B in Fig. 1), NiCr alloy matrix exhibited nano-crystalline structure (Fig. 2c), which was clearly confirmed by the diffuse ring diffraction pattern (Fig. 2d). In the region relatively far from the interface as indicated by mark C in Fig. 1, NiCr binder appeared to be nano-crystalline with relatively large grain sizes of about 50 nm as shown in Fig. 2e and its diffraction pattern appeared as disperse spots or intermitted circle as shown in Fig. 2f and g.Those results clearly revealed that NiCr binder matrix presented a fine grain structure. Moreover, the large grain size was observed in the region far away from substrate/coating interface compared with the region near to the interface. Nevertheless, there is no doubt that the cooling rate may decrease and consequently the grain size is increased.
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