Tribo-mechanical features of nitride coatings and diffusion layers produced by cathodic cage technique on martensitic and supermartensitic stainless steels

The mechanical and tribological properties of martensitic AISI 420 (MSS) and supermartensitic HP13Cr (SMSS) stainless steels plasma nitrided using the cathodic cage design were investigated. Nitride compounds were produced in the plasma from the cage´s sputtered ions, which eventually deposited on the electrically isolated sample. Prior to nitriding, samples were austenitized and oil quenched in order to attain a fully martensitc microstructure. Nitriding was carried out at different temperatures (350-450 °C) for 6 h. Mechanical properties and scratch resistance were characterized by instrumented indentation and nanoscratch, and were correlated with morphological and microstructural changes by using atomic force microscopy, scanning electron microscopy, X-ray diffraction and profilometry. Both nitrided MSS and SMSS comprised two distinct regions: (i) an outermost nitride layer (300-800 nm thick) containing deposited ε-Fe2 -3N and γ´-Fe4N nitrides and (ii) a diffusion zone composed of N-expanded martensite phase αN. At 450 °C, diffusion zones thicknesses were 4 μm (MSS) and 6 μm (SMSS). Nitride layers presented intense brittleness under normal and tangential loadings. Critical loads for nitride layer detachment were 49 mN and 127 mN for MSS and SMSS, respectively. However, after removal of the thin nitride layers, the toughened diffusion zones were revealed, being uniformly distributed through the entire sample area. Its hardness was 11 GPa. Such value corresponds to a twofold and threefold increase from the hardness of MSS and SMSS substrates, respectively, which are comparable with other plasma nitrided steels at similar temperature conditions.