Dr. ing. Dora Magheţ, Dr. ing. Gabriela Mărginean, Prof.dr.ing. Ion Mitelea, Prof. dr.ing. Arjana Davidescu, Prof.dr.ing. Waltraut Brandl

 

Abstract

Coatings are commonly used in order to protect high temperature alloys, to prevent the surface degradation and to insulate the material against the hot environment. The protection against oxidation is assured through the formation of a compact, stable and adherent oxide layer (usually Al2O3) that provides a barrier between underlying metal and the hot gases. A type of coatings that have been extensively used in the last years are MCrAlY coatings (M=Fe, Co, Ni). A well adherent and protective alumina scale is formed on the MCrAlY's during the oxidation in the temperature range 850-1300 grade C. Although these coatings have an excellent oxidation resistence, their life time can be limited by different environmental factors. During the high temperature exposure of the coatings the aluminum concentration could drop below a certain critical value. The coating cannot longer form the protective alumina scale and it should be replaced. The oxidation behaviour of MCrAlY's may be influenced basides the spraying tehniques or subsequent treatment of the deposited coatings, by varying the chemical composition of the sprayed power. In this research work the oxidation behaviour of various High-Velocity Oxygen Fuel (HVOF) - sprayed MCrAlY coatings, obtained from three types of powerders: FeCrAlY (6 wt.% Al), CoNiCrAlY (8 wt.% Al) and modified CoNiCrAlY powder (doped with 2wt.% Al2O3-nanopower) were investigated. The coatings were isothermally oxidized at 950 grade C for different periods of time using thermogravimetric weight gain measurements in order to quantify the oxidation rate constant. The kinetics of the oxidation reactions showed that the coating obtained from the doped MCrAlY-powder exhibited the best oxidation behaviour.