The Influence of Fuel/Oxygen Ratio in HVOF Spraying on the Deposition of Cr3C2 Coatings

This study was performed to investigate the influence of fuel/oxygen ratio (F/O = 3.2, 3.0, 2.8) on the characteristics of the HVOF sprayed Cr3C2 (7 mass%NiCr) coatings after heat treatment up to 1000°C. Decomposition and oxidation of the Cr3C2 occurred during spraying. The degree of decomposition from Cr3C2 to Cr7C3 was increased with decreasing the F/O ratio. The micro-structural differences of the as-sprayed coatings with F/O ratio cannot be distinguished. However, large pores were diminished and then the coatings became dense by heat treatment. Micro-hardness of the as-sprayed coating which is sprayed with F/O = 3.2 condition was the highest (Hv300=1140) and the hardness was increased to 1660 after heat treatment at 1000°C for 50 h in air. It was supposed that hardness was increased due to the formation of Cr2O3 within the Cr3C2/Cr7C3 matrix and the densification of the coating layer during heat treatment. Mass loss of the as-sprayed coating by erosion varied from 79 to 80.8 mg depending on F/O ratio and varied from 47.2 to 49 mg after heat treatment at 600°C. Erosion resistance of the coating was improved about 40% after heat treatment at 600°C. It is concluded that improvement of erosion resistance of the coatings was due to production of the coatings with a very dense microstructure, high cohesive strength and a very high hardness value by heat treatment.