Influence of surface roughness on nitrided-layer growth kinetics during plasma nitriding of SAE 1020 steel

Although previous studies on plasma nitriding of cast irons and steels generally report thinner nitrided layers on rougher surfaces, the physical mechanisms governing this behavior remain insufficiently understood. To address this gap, the influence of initial surface roughness on nitrided-layer growth in SAE 1020 steel was investigated under three plasma nitriding electrical configurations: cathodic, anodic, and floating potential. Four surface finishes — sandblasted, ground with #80 and #220 SiC papers, and polished — were evaluated in both stress-relieved and non-relieved conditions to decouple purely topographical effects from the mechanically affected near-surface state induced by surface preparation. Cross-sectional optical metallography and statistical correlation analyses demonstrated that the sensitivity of layer growth to surface roughness is governed by the electrical potential of the samples. Under cathodic nitriding, where energetic ion and fast-neutral bombardment across the sheath is pronounced, higher surface roughness strongly penalized compound-layer growth: the average compound-layer thickness of stress-relieved specimens increased from 6.7 ± 1.7 µm on the rough sandblasted surface to 12.8 ± 3.3 µm on the polished surface (an increase of approximately 90%). In contrast, under anodic and floating-potential configurations, compound-layer thicknesses remained within 5.0 ± 1 µm and exhibited minimal sensitivity to initial surface finish. Furthermore, while the diffusion-layer depth was strongly dependent on the electrical configuration, reaching 330 ± 20 µm under cathodic conditions compared with 275 ± 15 µm at the anode and 250 ± 15 µm at floating potential, it showed a much weaker dependence on initial surface roughness than the compound layer, as subsurface inward growth is primarily controlled by solid-state bulk diffusion rather than local asperity-sheath interactions. Stress relief reduced material-state scatter without altering these trends. These findings refine previous phenomenological interpretations by demonstrating that surface roughness predominantly affects plasma nitriding kinetics during compound-layer formation when sheath-driven energetic bombardment plays a decisive role in nitrogen mass transfer, whereas diffusion-zone growth remains largely insensitive to surface topography.