# High Density RF-DC Plasma Nitriding under Optimized Conditions by Plasma-Diagnosis
## 3 Experimental Results and Discussion
### Low Temperature Plasma Nitriding of AISI304 Stainless Steels
These nitrogen and hardness depth profiles suggest that the high nitrogen atom flux at the surface drives nitrogen supersaturation, together with the nitrogen diffusion process, even at 623 K.  
Let us describe this nitrogen supersaturation and diffusion co-process by EBSD analysis on the cross-section of a nitrided AISI304 substrate at 623 K for 14.4 ks. Figure 9a depicts the IPF mapping of depth. As seen in this map of the fur depth with d \(>\) 70 \(\upmu\)m, the original AISI304 grains with a size of 40 to 50 \(\upmu\)m, are divided into several colored and color-gated zones. In particular, each original AISI304 grain is changed to stripe-colored zones in 30 \(<\) d \(<\) 70 \(\upmu\)m. On the other hand, the original granular microstructure in d \(<\) 30 \(\upmu\)m is completely changed to have a very fine grain structure, with a size less than 0.1 \(\upmu\)m or the resolution limit of EBSD after nitriding at 623 K.  
Figure 9b,c show KAM distribution and phase mapping, respectively. The refined zone in d \(<\) 30 \(\upmu\)m in Figure 9a has the highest misorientation angle and a fine two-phase structure. This KAM in Figure 9b denotes the plastic strain distribution. The transformation from \(\upgamma\)-phase to \(\alpha\)-phase in Figure 9c is induced by plastic straining under the nitrogen supersaturated state. Hence, the grain size of the original AISI304 is refined by high plastic straining everywhere in d \(<\) 30 \(\upmu\)m, so that a huge amount of grain boundaries is generated together with the \(\upgamma\)/\(\alpha\)- and \(\alpha\)/\(\gamma\)-zone boundaries. This refined layer is just equal to the high hardness layer with a high nitrogen content. Figure 9: EBSD analysis on the microstructure at the cross-section of the nitrided AISI304 stainless steel substrate at 623 K for 14.4 ks after polishing and cleansing. (**a**) IPF profile, (**b**) KAM distribution, and (**c**) phase mapping. As stated in [4; 25; 26; 27], the plastic strain in the above is induced by the strain incompatibility between the expanded lattices by nitrogen supersaturation and the original lattices without nitrogen interstitials. Those strain-incompatible zone boundaries form new fine grain and zone boundaries in the layer with d \(<\) 30 \(\upmu\)m. Those new boundaries work as a network of nitrogen solute diffusion paths. The high intensity nitrogen atom flux at the surface penetrates through this network into the depth of substrate, together with nitrogen supersaturation. In the conventional inner nitriding, the high hardness layer was simply coincidental to the high nitrogen content layer. In this nitriding at 623 K, driven by a high intensity nitrogen atom flux at the surface, the nitrogen supersaturation takes place homogeneously with nitrogen diffusion through the network and forms a grain-refined layer with high hardness. Once this nitrogen supersaturation localizes at d \(>\) 30 \(\upmu\)m in Figure 9, the nitrogen diffuses to the depth only through the limited network. Due to this localized nitrogen diffusion, the homogeneous nitriding process terminates intermediately and induces the abrupt change in IPF, KAM and phase maps at d = 30 \(\upmu\)m. Hardness gradually decreases in d \(>\) 30 \(\upmu\)m, since the nitrogen supersaturation advances even by this localized nitrogen diffusion. This reflects the partial microstructure changes in the IPF profile, the locally high plastic strains in Figure 9b, and the localized phase transformation in Figure 9c. As discussed in [26; 27], this intermediate change from a homogeneous to heterogeneous nitriding process is eliminated by using fine-grained stainless-steel substrates. This suggests that the low temperature plasma nitriding by high nitrogen atom flux at the surface is essentially characterized by homogeneous inner nitriding, where the nitrogen supersaturation co-works with the nitrogen diffusion to drive the microstructure refinement, the plastic straining and phase transformation.