Synthesis of In Situ ZrB₂-SiC-ZrC Coating on ZrC-SiC Substrate by Reactive Plasma Spraying

In situ synthesis feasibility of ZrB₂-SiC-ZrC composite coatings on ZrC ceramics by reactive plasma spraying (RPS) was investigated. To help to understand the phase evolution during plasma spraying process, reaction behavior in the ZrH₂-Si-B₄C system was explored carefully by differential scanning calorimetry. The results indicated that the phase transformation sequence in the ZrH₂-Si-B₄C system could be described as ZrH_1.66, Zr₃O, ZrC, ZrB₂, Zr₂Si, ZrSi, and SiC. The prior formation of ZrC was due to high diffusion rate of C atoms from B₄C. ZrB₂ was produced above 1100 °C. As the temperature increased, SiC were finally formed by the reaction of ZrC with ZrSi and B₄C. The RPS composite coatings mainly consisted of ZrB₂, SiC, and ZrC phases, except for a small fraction of ZrO₂ phase. The microstructural characterization exhibited more dense melted splats, which appears to increase gradually with the increase in spraying currents and distances. The coatings had typical lamellar structure and adhered to the substrate well. The microhardness values were higher than 1000 HV₁, but there were few variations with varying spraying currents and distances.