Plasma nitriding and plasma nitrocarburizing of electroplated hard chromium to increase the wear and the corrosion properties

We have investigated the effect of plasma nitriding and plasma nitrocarburizing on the microstructure and properties of electroplated chromium. Plasma nitriding and plasma nitrocarburizing are applied to 15-100 mu m thick electroplated hard chromium coatings to increase both the wear and the corrosion resistance. The properties of the plasma-modified hard chromium layers are characterized by measuring the wear resistance with a Taber wear tester and the corrosion resistance with a salt spray fog test. Cyclic voltammetry is performed in a standard electrochemical cell using a 0.5 M H2SO4 solution acidified to pH 0.3. The compound layer after plasma nitriding consists of CrN and Cr2N with a maximum hardness of about 1100 HK0.01. After plasma nitrocarburizing, Cr3C2 and Cr7C3 are formed. After plasma nitrocarburizing, the maximum hardness is increased up to 2200 HK0.01. The wear rate is reduced as compared to an untreated chromium layer. The exposure time in the salt spray fog, before corrosion products are visible can be increased by a factor of 5. The improvement in wear and corrosion resistance after plasma nitriding and plasma nitrocarburizing is discussed considering microstructural changes of the treated layer. (C) 1999 Elsevier Science S.A. All rights reserved.