Effect of He Addition on the In-Flight Behavior of ZrSiO4 Powder and Coating Deposition During DC Arc Plasma Spraying

Abstract

Controlling the surface pyrolysis of ceramic powders during high-temperature plasma spraying is a critical challenge for producing phase-pure and structurally integral coatings. Here we show that adding He to the conventional Ar+H₂ plasma working gas effectively suppresses the in-flight surface pyrolysis of ZrSiO₄ particles while maintaining adequate melting for deposition. By combining CFD simulations, optical emission spectroscopy (OES), and coating microstructure analysis, we demonstrate that He addition induces a synergistic “mild cooling and velocity enhancement” effect. As the He volume fraction increases from 0 to 30 vol.%, the electron excitation temperature decreases linearly from 14 202 to 12 412 K, while the maximum jet velocity rises from 940 to 1271 m/s. This shortens the residence time of particles in the high-temperature zone and reduces the surface pyrolysis conversion rate from 42.5% to only 12.3% at the optimal He content of 10 vol.%. Excessive He addition (≥30 vol.%) leads to incomplete melting and formation of monoclinic ZrO₂, degrading coating integrity. This work establishes a quantitative relationship among gas composition, particle in-flight behavior, and coating phase composition and microstructure, providing a surface-engineering strategy for plasma processing of high-temperature ceramic powders.