Influence of microstructure on the durability of gadolinium zirconate thermal barrier coatings using APS & SPPS processes

Abstract

Excellent CMAS resistance in Gadolinium Zirconate, Gd2Zr2O7 (GZO) coatings is attributed to vigorous reactions between the CMAS melt and the GZO that form secondary phases that rapidly crystallize and block further infiltration of CMAS. In this study, Gd2Zr2O7 (GZO) coatings were deposited using air plasma spraying (APS) and solution precursor plasma spraying (SPPS) processes. The relative performance of GZO in furnace thermal cycle tests with and without CMAS was determined. SPPS GZO TBCs showed 8× longer lives in thermal cycling but 10× shorter lives in CMAS testing, as compared to APS GZO TBCs. Superior performance of the SPPS coatings without CMAS can be attributed to characteristic stress relieving vertical cracks in the microstructure, which are absent in APS TBCs. These vertical cracks also act as channels to CMAS melt infiltration, which causes the loss of strain compliance and hence poorer CMAS resistance. X-ray diffraction of the failed SPPS GZO coatings revealed the formation of apatite phases, but the majority of vertical cracks were not sealed. The denser than typical APS coatings on the other hand effectively blocked the CMAS infiltration with CMAS arrest observed at a depth of ~25μm. The SPPS results suggest that the width of vertical cracks is an important parameter for sealing and hence impeding CMAS infiltration Sealing was observed in cracks with <1μm in width. Wider cracks completely defeat the beneficial CMAS blocking behavior of GZO.