Hardening of HVOF-Sprayed Austenitic Stainless-Steel Coatings by Gas Nitriding

Austenitic stainless steel exhibits an excellent corrosion behavior. The relatively poor wear resistance can be improved by surface hardening, whereby thermochemical processes offer an economic option. The successful diffusion enrichment of bulk material requires a decomposition of the passive layer. A gas nitriding of high velocity oxygen fuel spraying (HVOF)-sprayed AISI 316L coatings without an additional activation step was studied with a variation of the process temperature depending on the heat-treatment state of the coating. A successful nitrogen enrichment was found in as-sprayed condition, whereas passivation prevents diffusion after solution heat treatment. The phase composition and microstructure formation were examined. The crystal structure and lattice parameters were determined using X-ray diffraction analysis. The identified phases were assigned to the different microstructural elements using the color etchant Beraha II. In as-sprayed condition, the phase formation in the coating is related to the process temperature. The formation of the S-phase with interstitial solvation of nitrogen is achieved by a process temperature of 420 °C. Precipitation occurs during the heat treatment at 520 °C. In both cases, a significant increase in wear resistance was found. The correlation of the thermochemical process parameters and the microstructural properties contributes to a better understanding of the requirements for the process combination of thermal spraying and diffusion.