High velocity oxy-fuel thermal spraying of a NiCoCrAlY alloy

During the last few years third-generation high velocity oxy-fuel (HVOF) spray systems have gained interest concerning the deposition of sensitive metal alloys such as MCrAlYs. This work deals with the HVOF thermal spraying of a NiCoCrAlY alloy and its isothermal oxidation behaviour. Two HVOF systems were investigated, DJ 2600 and JP 5000. For both spray systems, the influence of the spray parameters on the oxidation during spraying as well as on the coating porosity was investigated. The coatings with the lowest oxygen content and the lowest porosity were subjected to heat-treatment in vacuum and oxidation at 1100 °C in air. The microstructure of the coatings was studied as-sprayed as well as following heat-treatment. Moreover, the morphology and thickness of the oxide scales were characterized by metallography, scanning electron microscopy, energy dispersive X-ray spectrometry and XRD. The oxidation kinetics were determined by use of thermogravimetry. The DJ system led to the lowest coating porosities after thermal treatment as well as to the lowest oxidation rates. The oxide scale on the NiCoCrAlY, formed at 1100 °C, consisted mainly of α-Al2O3, which was also seen after short oxidation times (2 h).