Controlling plasma-based surface modifications of an austenitic alloy by thermochemical and athermal diffusions.

Under nitrogen plasma immersion ion implantation, the energy density per pulse E Pulse /A was found to be proportional to the amount of the N-expanded austenite phase (γ N) in a superaustenitic stainless steel, as previously seen for an austenitic-ferritic alloy. Since γ N occurs in a range of stoichiometries, the parameter was also contrasted with the ion fluence Γ, which affects the retained dose. Γ varies inversely with E Pulse /A and influences oppositely the properties of the modified surfaces. The temperature was the same among the treatments (320 °C) to rule out thermal diffusion as an extra variable. The increase of E Pulse /A (for applied voltages 6.2–10.4 kV) produces thicker layers (1.4–2.2 µm), as an increase in lattice defects and stresses enhances N-diffusion, while high Γ values favor the N-saturation at the top surface for the opposite reason. The lowest E pulse /A (6.2–9.5 kV) result in brittle cases with a possible thin nitrides layer on top, whereas γ N prevails in the ductile layer produced by the highest one (10.4 kV). The N-concentration governs the strength against plastic deformation: hardness varies up to 32% from the highest (6.2 kV) to the lowest Γ treatment (10.4 kV). In summary, the correlation of E pulse /A with Γ is indispensable for controlling properties of the modified surfaces for performance purposes. [Display omitted] ● The energy density per pulse E Pulse /A controls the γ N amount in superaustenitic SS. ● At the same PIII temperature, the layer thickness grows with E Pulse / A (1.4-2.2 µm). ● The role of implantation defects and lattice stresses in the N diffusion is evidenced. ● E Pulse /A and ion fluence Γ vary inversely and rule synergistically the surface properties. ● The N-concentration in the layer increases with Γ and governs its mechanical strength. [ABSTRACT FROM AUTHOR]