  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.
