Inhibition of the formation of the B metastable phase in yttrium oxide plasma-spray coatings by the addition of zirconia

Yttrium oxide plasma-spray deposits show a metastable B phase together with the cubic stable form. This B form is prejudicial for thermal applications of these coatings (i.e. weakens mechanical properties, etc.). Attempts have been made to avoid the formation of this metastable phase during the plasma-spraying operation by use of an inhibitor, such as ZrO₂. A large solid solution with cubic C phase was found to form between ZrO₂ and Y₂O₃ (Y_1–xZr_xO_{1.5 + x/2 0.5 – /2}). A method is described for preparing powders suitable for plasma spraying. Several compositions were tested. B phase is no longer formed for ZrO₂ additions as low as x = 0.03. The influence of ZrO₂ addition on Y₂O₃ coatings properties is discussed.