Effects of specimen treatment and surface preparation on the isothermal oxidation behaviour of the HVOF-sprayed MCrAlY coatings

Alloys and coatings designed to resist oxidizing environments at high temperatures should be able to develop a surface oxide layer, which is thermodynamically stable, slowly growing and adherent. During oxidation in the temperature range 850–1300 °C, MCrAlY coatings should form Al 2O 3-rich scales, which are reasonably effective for long-term applications under isothermal or thermal cyclic oxidation conditions. This study is concerned with the isothermal oxidation behaviour of high-velocity oxygen fuel (HVOF)-sprayed CoNiCrAlY coatings containing 12 wt.% aluminium. Specimens treated in different ways (ground, polished and EB-remelted) were oxidized at 950 °C in synthetic air for various periods of time. Oxidation studies include thermogravimetric weight gain measurements under isothermal conditions in order to quantify the parabolic oxidation rate constant. The microstructure and morphology of as-sprayed and of oxidized coatings were characterised by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The ground and polished samples show internal oxidation as well as relatively high values of the oxidation rate constants. In the case of EB-remelted coatings, the oxide-scale growth rate is substantially smaller and no internal oxidation was observed.