As shown in Figure 1, the XRD patterns of the raw NiCrBSi powder, as-sprayed and heat treated NiCrBSi coatings are compared. It can be found that the coatings consist of the main phase of γ-Ni (JCPDS No.45-1027) and Ni3B(JCPDS No.98-007-5794), and several kinds of trace phases, including Cr7C3( JCPDS No.11-0550) and M23C6( JCPDS No.35-0783). As to the trace phases, it was reported that the space groups of M23C6 and Mr7C3 are Fm3m(cubic) and Pnma(orthorhombic), respectively. Here,M represents the metal elements including Ni or Cr.
Figure 1. XRD patterns of (a) raw NiCrBSi powder, and(b) post heat-treated and(c) plasma sprayed NiCrBSi coating.
The plasma sprayed NiCrBSi coating exhibited a broad peak at 20 from 40°to 50°, indicating the predominate presence of amorphous phase in the as-sprayed coating. It should be noted that the as-sprayed coating shows certain nanocrystalline phases around 2θ≈ 52° and 76°. During the plasma spraying process, the NiCrBSi feedstock are melted and then solidified between 10-8 and 10-6s, which is measured by a DPV 2000 Diagnostic System(TECNAR Automation LTD., Saint-Bruno, Quebec, Canada). Therefore, large amounts of amorphous phase inevitably forms in the plasma sprayed NiCrBSi coating. Post heat treatment is usually helpful for obtaining a dense coating with high cohesion and adhesion strengths. The broad peak became sharper after heat treatment at 600℃ for 1 h, indicating that the recrystallization of the amorphous phase.
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