The oxidation behaviour of HVOF thermal-sprayed MCrA1Y coatings

A new high velocity oxygen fuel (HVOF) spraying process to produce MCrAlY coatings was developed and optimized. The HVOF-sprayed MCrAlY coatings were isothermally oxidized at 950 degrees C and 1050 degrees C in air as well as in different oxidizing atmospheres (synthetic air, He-10% O-2 and He-10% synthetic air). The oxidation behaviour of HVOF-sprayed coatings is compared with that of VPS coatings. Under the chosen oxidation conditions, which assure a high oxygen partial pressure, the oxidation kinetics of the two coatings are very different, i.e. the oxidation rate of the HVOF sprayed coating is considerably lower than that of the VPS coating. In the present paper, this observation is explained by the presence of finely divided alpha-Al2O3 particles in the HVOF-sprayed coating, which are formed during spraying. The Al2O3 probably hinders the grain boundary diffusion of the elements. As a consequence, the oxide scale growth is very low. This effect is more evident at high temperature. The as-sprayed coating as well as the oxidized coatings were examined by optical microscopy, X-ray diffraction, scanning electron microscopy and transmission electron microscopy to study the appearance and the composition of the oxide scales and the phase transformations in the MCrAlY coating. (C) 1997 Elsevier Science S.A.