Indentation Evaluation of Elastic–Plastic Response and Surface Modifications in Stainless Steel

This study applies a depth-based description of indentation response to AISI 321 stainless steel and a detonation-sprayed FeNiCoMoW coating. Martens hardness HM was evaluated from the maximum indentation depth, while a residual-depth-based quantity Hpl was calculated using the retained depth. The corresponding hardness ratio Hpl/HM was examined in relation to the retained-depth fraction. Instrumented Vickers indentation was performed on untreated AISI 321 stainless steel and the FeNiCoMoW coating. Maximum and residual indentation depths were obtained from load–displacement curves. Coating morphology was examined by optical microscopy and phase composition by X-ray diffraction. The FeNiCoMoW coating formed a continuous layer with a 145 ± 8 μm thickness. Compared with the substrate, the coating reduced the retained-depth fraction from 0.906 to 0.788 and increased the hardness ratio from 1.22 to 1.61. Indentation hardness increased from 2.3 ± 0.17 GPa for the substrate to 8.65 ± 1 GPa for the coating. X-ray diffraction identified the (Mo,W)₂C carbide crystalline phase together with an fcc (Co,Ni,Fe) solid solution and σ- and μ-type intermetallic compounds. The proposed descriptors provide a compact characterization of indentation response and distinguish retained-depth behavior from conventional contact-depth-based hardness measurements. For the investigated coating, the lower retained-depth fraction and higher hardness indicate a greater recovered-depth contribution during unloading.