The aim of the present work is to study the microstructure of high temperature oxidation and corrosion resistant MCrAlY coatings prepared by high-velocity oxy-fuel (HVOF) spraying and to compare them to vacuum plasma sprayed (VPS) MCrAlY coatings. In this work, CoNiCrAlY, NiCoCrAlYTaReSiHf, and NiCrAlY coatings were prepared on nickel based superalloys by VPS spraying and by two types of HVOF spray process (DJ Hybrid 2600 and JP-5000). The influence of spray parameters, i.e. oxygen flow rate and spray distance, in one of the HVOF processes (DJH 2600) on coating structures and properties were studied also in more detail. The coatings sprayed with different spraying processes were studied in their as-sprayed state and after heat-treatment by optical microscope and scanning electron microscopy (SEM). The results show that selection of appropriate particle size of the spray powder has marked importance in the HVOF process; presence of fine powder particles produce oxides in the coatings and ease clogging of the nozzle in the HVOF gun. The use of appropriate size distribution of the spray powder enables preparation of HVOF MCrAlY coatings with low oxide content and dense structure. The oxide content of HVOF sprayed coatings should be kept as low as possible in order to make the post heat-treatment active in producing a dense microstructure with correct phase structure. The oxide content can be controlled e.g. by flame temperature control by means of oxygen flow rate and by appropriate spray distance selected individually for each powder. Unacceptable high oxide contents in the as-sprayed structures tend to remain unchanged after heat-treatment, whereas well-developed coating microstructure and phase structure are obtained with HVOF sprayed coatings with low oxide content. Microstructures developed in heat-treated VPS and HVOF sprayed coatings are compared.

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