Multi-scale mechanical properties of Fe-based amorphous/nanocrystalline composite coating synthesized by HVOF spraying

Abstract

Fe-based (Fe–Cr–B–P–C) amorphous/nanocrystalline composite coatings were synthesized by high velocity oxy-fuel (HVOF) thermal spray method with varying powder feed rates and the multi-scale wear behaviour of the coatings’ is reported here. Microstructural characterization of the composite coating envisaged the presence of embedded nanocrystalline phases in the amorphous matrix. The porosity content decreased, whereas the amorphicity of the coating increased gradually with increment in the feed rate. The combined effect of (i) splat morphology and (ii) extent of devitrification on the mechanical and tribological properties of the various coatings was investigated. Increasing the powder feed rate resulted in higher hardness of the coating, which was attributed to better inter-splat bonding and reduction in α-Fe content in the amorphous matrix of the coating due to lower extent of devitrification. Nanotribology test on a single-splat revealed increment in wear resistance at elevated feed rate due to the reduction in volume fraction of softer nanocrystalline α-Fe phases. Besides, dry sliding wear investigated the coatings’ wear behaviour on a global basis and revealed decreasing trends for both wear rate and coefficient of friction with increasing feed rate. Most importantly, the Fe-based composite coatings exhibited low wear volume during nanotribology and lower friction coefficient, low wear rate of dry sliding wear study, compared to an SS316L coating, prepared using industrially optimized parameters. The enhanced wear resistance of the composite coating compared to that of the stainless steel coating makes it an effective method of surface protection for metallic substrates.