Formation mechanism of gas phase in supersonic atmospheric plasma sprayed NiCr-Cr3C2 cermet coatings

A typical plasma sprayed NiCr-Cr3C2 cermet coating usually consists of carbides, metal binder and gas phase. Among these phases, the gas phase has a great influence on the mechanical properties of as-sprayed coating. However, an in-depth understanding of the formation mechanism of gas phase is still lacking. Therefore, in the present work, the NiCr-Cr3C2 cermet coatings were deposited by supersonic atmospheric plasma spraying method and the formation of gas phase was discussed by the collection of flattened particles. Results showed that the flattened particles showed characteristics of multi-layer solidification. The bubbles inside or on the surface of flattened particles were mainly formed by the gas porosity and gas shrinkage, and then some pores were formed under non-equilibrium solidification. The number and size of pores gradually decreased with the increase of gas solubility and cooling rate, which resulted in the formation of NiCr-Cr3C2 coating with lower porosity and higher microhardness. A theoretical model based on experiments was also proposed to elaborate the formation mechanism of gas phase inside cermet coatings. [Display omitted] •A theoretical model was proposed to elaborate the formation mechanism of gas phase.•Multi-layer solidification was found in a single flattened particle during SAPS.•Content and size of pores can be controlled by particle's velocity and temperature.•High velocity remarkably reduced porosity of flattened particles as well as coatings.•High cooling rate effectively prevented the coalescence of small-sized bubbles.