Evaluation of niobium nitride coatings produced on AISI ۴۳۰ steel by niobizing and plasma nitriding

In this work, AISI ۴۳۰ stainless steel was duplex treated via niobizing followed by plasma nitriding and vice versa. Niobizing was performed through pack cementation in a powder mixture of Al۲O۳, NH۴Cl, and FeNb at ۹۵۰ ̊C for ۲h. Then one group of samples (Nb-PN) were plasma nitrided at ۴۵۰ °C for ۶ h at an atmosphere of ۷۵% N۲ and ۲۵% H۲, and the other group were nitrided and then niobized in a similar condition (PN-Nb). The cross sections and samples surface were investigated by optical and scanning electron microscope (SEM). Phase identification was performed using XRD. The surface hardness and electrochemical behavior of the treated samples were assessed. Results revealed that an approximately ۱۵ μm thick layer that contained NbN, NbCr۲, and Fe۲Nb was formed after duplex treatment. The highest surface hardness value achieved was ۷۵۳ HV۰.۰۹۸ in the Nb-PN sample, which was ۳.۵ times higher than that of the bare stainless steel. Plasma nitriding of the niobized layer caused a significant improvement in corrosion resistance; i.e., the Nb-PN sample reduced the Icorr to a value (۰.۷۲ μA/cm۲) ten times lower than that of the bare stainless steel (۹.۵ μA/cm۲).

Keywords:

Niobium nitride , Duplex treatment , Pack cementation , Plasma nitriding , Stainless steel