Microstructural Analysis of Nickel-Based Composite Coatings and Their Effect on Micro-hardness and Nano-indentation Behavior

The coating reliability and failure are inextricably linked to its microstructure, from which all the other characteristics are derived. Based on the properties obtained from multiscale assessments, after a thermal flame spraying processing applied on X18 carbon steel, this investigation aims at expounding the contribution of the prior grit blasting treatment and the presence of the Ni-based bonding layer in determining the coating characteristics. For that, a NiCrBSi self-fluxing alloy reinforced with tungsten carbides was selected to realize the deposits in the open atmosphere, using the acetylene as fuel. The characterization of the coating was carried out through optical microscopy, micro-hardness measurements and nano-indentation tests. Quantitative data were estimated by processing the obtained optical micrographs. The results show that the coating contains fine precipitates and has dendritic microstructure surrounded by a eutectic phase. The micro-hardness profile proved that the prior grit blasting treatment had induced a substrate hardening near the interface where the hardness roughly achieves 500HV. The cooling in the air environment has activated the initiation of cracks. The nano-indentation results reveal that the elastoplastic response of the matrix differs depending on the nature and the proportion of phases.