Evaluation of functionally gradient coatings produced by plasma-spray technique

Functionally gradient coatings are widely used at high temperatures in order to improve performances of materials in automotive, aircraft and aerospace industries; etc. These coating systems contain a steel substrate, bonding layer, cermet layer and ceramic coating, respectively. Furthermore, the composition and morphology of the coatings change gradually from ceramic to metal. In the present work, 304 L stainless steel substrates were coated by functionally gradient coatings consisting of bond coat NiCrAl, 70% NiCrAl + 30% MgZrO3, 50% NiCrAl + 50% MgZrO3, 30% NiCrAl + 70% MgZrO3 as the cermet layer and 100% MgZrO3 as the top coat. These coatings were produced using a plasma-spray technique. The coated samples were analysed using an optical microscope, scanning electron microscope and X-ray diffractometer. The surface roughness was measured via a Perthometer M4P, and the distribution of microhardness was determined on both the coating and the substrate. The results showed that there are no certain transition lines between the bond layer, the cermet layer, and the ceramic layer. The SEM results indicated the presence of porosity, oxide, inclusion and unmelted particles. Surface roughness values decreased with increasing MgZrO3 content. It was also found that the microhardness below a depth of 245 mu m from the coating surface decreased and followed first a linear function in the ceramic, cermet and bond coatings. Tn addition to this, the microhardness of steel substrate showed a parabolic function due to the blasting operation and heat effect during the coating process. (C) 1999 Elsevier Science S.A. All rights reserved.