B₂O₃ fluxing-assisted bonding mechanisms enabling plasma spraying of gas-tight corrosion-resistant NiCrB coating

Atmospheric plasma spraying (APS) is arguably the most powerful and versatile route for preparing thick metal coatings on large-scale engineering parts for protection against corrosion. However, APS processing has long suffered from a bottleneck drawback, namely, the inadequate bonding at the boundaries of individual splats as building blocks, allowing corrosive agents (especially gaseous molecules) to permeate through. A key culprit hampering the wetting and bonding is the thin oxide scale that inevitably forms on the splat surfaces. In this paper, we found that by adding B into the feedstock powder, a thin B₂O₃ scale preferentially forms on the splat surfaces, which offers self-fluxing throughout the entire APS process to bond the intersplat interfaces completely. The mechanisms to form bonding are clarified by experimentally examining into intersplat bonding by focused ion beam-prepared interface samples assisted with numerical simulation of melt/substrate interface temperature during spreading. The coating microstructure, its barrier effect to halt the penetration of corrosive agents, and tensile adhesive strength are characterized. Results reveal that the low-melting-temperature B₂O₃ on splat surfaces gets liquidized upon the impact of the ensuing impinging molten droplet, which then fully wets the already-deposited splat from its center to periphery. This achieves complete metallurgical bonding between the consecutive splats. The tensile tests reveal an exceptional tensile adhesive strength of 260 MPa. Moreover, the consequent build-up of dense metallic coatings presents an exceptionally resistant barrier to the infiltration of gaseous corrosive media. The hardness higher than 900 HV of the B-alloyed coatings endows them with high wear resistance and a novel effective alternative to significantly hazardous hard Cr plating. The present full-process fluxing strategy is expected to work satisfactorily for many coating recipes, not only for Ni-based, but also Cu-based, Fe-based, and Co-based alloy coatings. Therefore, the present strategy sets up APS as the “protection of choice” for anti-corrosion and anti-wear needs.