Influence of the atmospheric plasma spraying parameters on the coating structure and the deposition efficiency of silicon powder

Abstract

Silicon coatings are usually produced by atmospheric plasma spraying (APS) and used as bond coats in environmental barrier coatings. The deposition efficiency (DE) of silicon powders is generally at a low level in APS processes. The reasons for the low DE values of silicon powders have not been sufficiently investigated in the literature. The aim of this study was to investigate in detail the influence of process parameters on the coating structure and deposition efficiency of a silicon powder processed with APS. A silicon powder with a size distribution of f = –53 + 15 µm was sprayed using a three-cathode plasma generator to produce coatings. The parameters such as plasma gas type, plasma gas flow rate and current intensity were varied widely. Accordingly, the power of the plasma generator increased from P = 19.4 to 51.3 kW, which allowed different melting and evaporation degrees of the powder. Particle velocities and temperatures were measured using a particle diagnostic method. The coatings were investigated in terms of their surfaces and structures using electron scanning microscopy (SEM). The porosities of the coatings were measured using an image analysis system. The deposition efficiency of the processed powder was determined. The results show that the used parameters led to high particle velocities in a range of about vp = 270–360 m/s. High particle temperatures of Tp = 2,650–3,390 °C were determined. The coating porosity varied from Φ = 2% to Φ = 15%. The porosity value of Φ = 2% is significantly lower than the values reported in the literature. The deposition efficiency of the powder changed from DE = 1.5% to DE = 28%. The value of DE = 28% is about 40% higher than the values reported in the literature. The strong grit-blasting effect was the main reason for the lowest DE value of DE = 1.5%. The strong evaporation effect was the main reason for the second lowest DE value of DE = 11.1%. Numerous melted particles and semi-melted particles splashed upon impact with the substrate, resulting in silicon melt loss. In addition, solid cores of semi-molten particles could bounce off, which also resulted in silicon loss. Splashing and bouncing were the main factors affecting DE for the parameter sets with DE values ranging from 18.7% to 28%.