Enhanced Wear Resistance of Gas Nitrided AISI 431 HVOF Coatings at Elevated Temperatures

Abstract

Stainless-steel feedstocks achieve increasing importance as sustainable and cost-efficient alternative regarding thermal spraying. However, the wear resistance is often insufficient for demanding applications. Therefore, an additional surface hardening step by thermochemical processes, in particular by gas nitriding, is promising for enhancing surface functionality. The characteristic porosity of thermally sprayed coatings facilitates deep nitrogen diffusion increasing hardness and wear resistance, due to the formation of precipitates. Because nitrides are thermally stable, applications at elevated temperatures are enabled. The process combination was examined for the ferritic stainless-steel AISI 431 applied on mild steel by high-velocity oxygen fuel spraying (HVOF), followed by subsequent gas nitriding. The influence of the thermochemical treatment with respect to a variation in the nitriding potential has been determined in terms of microstructure, phase formation, hardness distribution as well as reciprocating wear resistance at room and elevated temperature. The increase in hardness over 900 HV0.01 and wear resistance with wear rates consistently lower than 1.3 × 10^-4 mm³ Nm⁻¹ can be attributed to the successful enrichment of nitrogen and the formation of mainly Fe₄N precipitates at the coating’s surface. Even at 350 °C, the nitride surface layer provides better wear protection compared to the as-sprayed condition.

Graphical Abstract