The effects of spray parameters on the microstructure and thermal stability of thermal barrier coatings formed by solution precursor flame spray (spfs)

Solution precursor spraying is a new method of applying thermal barrier coatings (TBCs) on metallic substrates. In this work, the injection of a solution precursor has been made axially into an oxy-acetylene flame to deposit 7 wt.% yttria stabilized zirconia (7 wt.% YSZ) coatings. For this purpose a home-made atomizer has been inserted into the nozzle of a flame spray torch. The precursor used was an aqueous solution containing zirconium and yttrium salts to make a solid solution of 93 wt.% ZrO2 and 7 wt.% Y2O3 (7YSZ) in the coating. A martensitic stainless steel plate has been used as substrate on which a bond coat of NiCrAlY alloy was applied by atmospheric plasma spray process. The effects of spray distance and atomizing gas (oxygen and hydrogen) on the microstructure, phase composition, thermal shock and thermal exposure resistance of the coatings have been examined. The results show that the use of H2 as the precursor atomizing gas instead of oxygen increases the enthalpy of the flame the result of which is coatings with the least non decomposed precursor and highest crystalline YSZ. This effect causes the severe cracking at thermal shock and thermal exposure tests in coatings made by oxygen atomizing due to higher amount of non-decomposed precursor. By contrast, coatings made by hydrogen atomizing showed very little cracking because of increased enthalpy of the flame and a small quantity of non-decomposed precursor.