Effect of bombarding steel with Xe$^+$ ions on the surface nanostructure and on pulsed plasma nitriding process

TL;DR

This study investigates how Xe$^+$ ion bombardment modifies steel surfaces, influencing the microstructure and nitriding behavior, resulting in thicker, more uniform nitrided layers and altered precipitate morphology.

Contribution

It demonstrates that Xe$^+$ ion bombardment prior to plasma nitriding significantly changes surface morphology, strain distribution, and nitrided layer characteristics in steel.

Findings

Pre-bombardment induces nanometric surface patterns and increased roughness.

Strain from bombardment affects depths much larger than projectile range.

Pre-bombarded samples form a single thick nitrided layer instead of a double layer.

Abstract

The modification of steel (AISI 316L and AISI 4140) surface morphology and underlying inter-crystalline grains strain due to Xe$^+$ ion bombardment are reported to affect nitrogen diffusion after a pulsed plasma nitriding process. The ion bombardment induces regular nanometric patterns and increases the roughness of the material surface. The strain induced by the noble gas bombardment is observed in depths which are orders of magnitude larger than the projectiles' stopping distance. The pre-bombarded samples show peculiar microstructures formed in the nitrided layers, modifying the in-depth hardness profile. Unlike the double nitrided layer normally obtained in austenitic stainless steel by pulsed plasma nitriding process, the Xe$^+$ pre-bombardment treatment leads to a single thick compact layer. In nitrided pre-bombarded AISI 4140 steel, the diffusion zone shows long iron nitride needle-shaped precipitates, while in non-pre-bombarded samples finer precipitates are distributed in the material.