Investigation of changes in surface characteristics of AISI 316L stainless steel induced by different surface activation types in correlation with the surface finishing state

• Revealing dual function of HCN: surface de-passivation and diffusible N/C generation • Martensite transformation induced by preparation affects expanded austenite formation • Uniform expanded austenite formation obstructed by applying only gas-phase activation • Ion sputtering enabled martensite removal and uniform expanded austenite formation Formation of a thick and uniform expanded austenite layer on the austenitic stainless steel AISI 316L by thermochemical diffusion treatment requires the application of an effective surface activation aiming the removal/reduction of the passive oxide film and the interstitials uptake in austenite. This work investigates how two in-situ surface activation types – HCN gas-surface reaction with and without ion sputtering – affect the surface characteristics of AISI 316L before plasma nitrocarburizing on ground and polished surfaces. To this end, the composition, structure, chemical state and the evolving near-surface microstructure are compared before and after applying two types of surface activation using surface-sensitive techniques of transmission electron microscopy, X-ray photoelectron spectroscopy, hard X-ray photoelectron spectroscopy, and time-of-flight secondary ion mass spectrometry. The study shows that applying only the HCN gas-surface reaction as an activation type results in no initial expanded austenite layer forming on a ground surface, due to the formation of a deformed surface layer containing a high amount of fine-grained martensite. In contrast, applying the same activation type to a polished surface, which has a lower amount of near-surface martensite, yields partial surface activation due to the formation of a localized and non-uniform initial expanded austenite layer (0 or ∼ 0.5 µm thick). To effectively eliminate surface martensite and fully activate both surfaces, ion sputtering of the surface is required to be applied aiming in the removal of the passive oxide as well as the near-surface martensite, and consequently forming a relatively thick and uniform initial expanded austenite layer on the ground (∼ 1.4 µm thick) and the polished (∼ 1.2 µm thick) surfaces.