Microstructure and mechanical properties of Tribaloy coatings deposited by high-velocity oxygen fuel

Tribaloys are Cobalt-based alloys with general composition M-Mo-Cr-Si (M = Co), with a microstructure of Laves intermetallic phases dispersed in a Co solid solution. The chemical composition plays a major role in volume fractions of the strengthening phases in these alloys, which in turn dictate the mechanical properties and tribological performance of these materials. In this study, two Co-based Tribaloy powders, i.e., Co-28.5Mo-17.5Cr-3.4Si and Co-28.5Mo-8.5Cr-2.6Si, were coated on mild steel substrate by high-velocity oxygen fuel (HVOF) spraying. Microstructure and distribution of Laves phases were studied using scanning electron microscopy (SEM) equipped with energy-dispersive X-ray (EDX) analysis as well as electron channeling contrast imaging (ECCI). Detailed phase analysis of powders and coatings was performed using X-ray diffraction (XRD). Electron backscatter diffraction (EBSD) was conducted on sprayed coatings to reveal the distribution of phases in coatings. Co3Mo2Si and Co3Mo were found to be the main constituent of intermetallic Laves phases in HVOF-sprayed coatings. A significantly lower volume fraction of Laves phases was identified in both coatings as compared to similar alloys in the literature, due to an extremely high cooling rate and solidification in HVOF. The Laves-free solid solution featured nano-crystalline microstructure with a similar local hardness to those of Laves phases. Highly flattened splats that contained nano-crystalline supersaturated solid solution displayed weak interfaces within the microstructure where cracks initiated and propagated along boundaries during splat cohesion testing.

Automated summary

In this study, two Co-based Tribaloy powders were coated on mild steel substrate using high-velocity oxygen fuel (HVOF) spraying. The microstructure and distribution of Laves phases in the coatings were investigated. It was found that Co3Mo2Si and Co3Mo were the main constituents of the intermetallic Laves phases in the sprayed coatings. The volume fraction of Laves phases in the coatings was significantly lower compared to similar alloys due to high cooling rate and solidification in HVOF. The Laves-free solid solution exhibited a nano-crystalline microstructure with similar local hardness to the Laves phases.