Microhardness and corrosion behavior in CrN / electroless Ni / mild steel complex coating

Abstract

The electroless nickel-phosphorous (Ni-P) coating was chosen as an interlayer to improve the properties of the CrN/mild steel (MS) composite. A hypophosphite-reduced acid solution was used to first deposit electroless Ni-P onto MS substrates, and then the CrN overlayer was deposited by reactive r.f. magnetron sputtering onto the electroless Ni-P modified substrate. The electroless Ni-P layer crystallizes with the precipitation of a Ni3P phase during r.f. sputtering, and thus a coating-substrate composite of CrN/Ni-Ni3P/MS is formed. The electroless Ni-P coating increases the surface hardness of the steel substrate to more than three times. The surface hardness of the CrN coating modified by an electroless Ni-P interlayer exhibits a hardness higher than 2000 HK0.015. The usual substrate effect on the microhardness of the coatings is nearly eliminated with the complex coating feature, and a significant enhancement of surface hardness in the coating assembly is achieved. The corrosion tests indicate that the Ni-Ni3P/MS configuration exhibits a more positive Ecorr value (i.e. less electronegative) than CrN/MS and the corresponding potential curve is shifted toward the low-current side, indicating a better anti-corrosion performance. Through comparison of the Ecorr values and the polarization curves, it is demonstrated that the CrN/Ni-Ni3P/MS composite exhibits significantly higher corrosion resistance than the Ni-Ni3P/MS and CrN/MS coating configurations. © 2002 Elsevier Science B.V. All rights reserved.