Carbon and nitrogen S-phases for solid-solution hardening of thermally-sprayed 316L stainless-steel coatings

S-phase formation enables a precipitation-free hardening by solution of carbon or nitrogen at the interstices of the austenitic crystal lattice. However, the correlations of thermochemical post-treatment and the structural properties of thermally-sprayed coatings have not yet been understood. Solid-solution hardening of high-velocity oxy-fuel and atmospheric plasma sprayed AISI 316L coatings was conducted by carbon and nitrogen enrichment. The resulting crystal structures and the lattice parameters were determined by X-ray diffraction. A different expansion of the lattice parameters was found for carbon and nitrogen solid solutions. Supersaturation of the matrix cause a volume increase. Diffusion enrichment at the interface between coating and substrate can cause delamination. The identified phases were assigned to the different domains in the microstructure using color etchant Beraha-II. Differences in the resulting microstructures were examined for both coating technologies in comparison to the bulk-material reference. Strong increases in nanoindention hardness and wear resistance were found for thermochemically post-treated coatings. These coatings are suitable for enhanced performance in applications with superimposed corrosive load and wear.