Influence of Surface Treatment of Bondcoat and Yttrium in Bondcoat on Oxidation Behavior of Thermal Barrier Coatings

This paper focuses on the oxide layer formed at bondcoat/topcoat interface of thermal barrier coatings (TBCs), which grows faster in comparison with the oxide layer formed on overlay coatings, although both bondcoat and overlay coating are made of MCrAlY alloy (M is Ni or Co, or combination of Ni and Co). The authors clarify the influence of ① the surface treatment (shot peening) of bondcoat and ② yttrium in bondcoat on the oxidation behavior of the thermal barrier coatings. Oxidation tests were carried out at 950°C in air by means of TBC specimens. The specimen consists of Ni-base superalloy (Inconel738LC), CoNiCrAlY bondcoat and ZrO2-Y2O3 topcoat. In the TBC specimen with as-sprayed bondcoat, the oxide layer consisted of “wart-like oxide” and “continuous alumina layer”. The wart-like oxide is caused due to un-melted particles of CoNiCrAlY alloy that remained at the bondcoat/topcoat interface in spraying process. The surface treatment of bondcoat prevented the formation of wart-like oxide because the unmelted particles were removed. In the TBC specimen with as-sprayed bondcoat, yttrium-rich oxide particles were included in the continuous alumina layer. The surface treatment of bondcoat and/or the removal of yttrium from the chemical composition of bondcoat in TBC specimen prevented the formation of the yttrium-rich oxide particles in the continuous oxide layer, and as a result, the growth of continuous alumina layer was decreased. There is a possibility that the application of the bondcoat without yttrium restrains the growth of the continuous oxide layer in TBC.