The effect of the microstructure on the tribological properties of metal injection molded and hybrid surface treated 17–4PH martensitic stainless steel

• Pores play a positive role during wear, trapping wear debris and reducing friction. • Nitriding creates a surface morphology due to surface stress relaxation. • Nitriding resulted in a high hardness and thickness deviation diffusion layer. • The substrate effect can be reduced by increasing the coating thickness. In this study, 17-4PH martensitic stainless steel was subjected to a hybrid surface treatment consisting of low-temperature plasma nitriding and different mono-, bi, and multilayer coatings deposited by physical vapor deposition methods. The surface and tribological properties of the specimens were evaluated, taking into account the inhomogeneous and porous microstructure inherent to powder metallurgy technology. Results indicate that plasma nitriding of this microstructure resulted in a high hardness and thickness deviation diffusion layer, whose varying properties were reflected in the coatings as well. By increasing the coating thickness, the effect of the substrate was minimized. A microstructure-like surface morphology was observed on the bare-nitrided sample, which was proposed to form due to surface grain expansion, grain shift, and the relaxation of plasma nitriding-induced surface stresses. This stress relaxation also adversely affected the integrity of the applied coatings. In the case of bare-nitrided samples, the surface pores were able to entrap wear debris and reduce the friction coefficient. The application of different coating types, thicknesses, and layer structures produced varying outcomes; however, the application of surface treatments consistently increased the hardness and wear resistance of the material in all cases.