APS 2ZrO2·Y2O3(ss) coatings were found to resist full penetration by both sand-CMAS and fly ash-CMAS at 1500°C. The flux of Y3+effectively alters the CMAS glass composition to induce crystallization and precipitation of Ca-Y-apatite and Y-depletedc-ZrO2, which block the CMAS penetration pathways. In situ XRD experiments have confirmed that the Ca-Y-apatite crystallizes during the high-temperature interaction between the 2ZrO2·Y2O3(ss) coating and the CMAS, and not while cooling, providing further evidence that the formation of Ca-Y-apatite suppresses CMAS penetration. The formation and the stability of Ca-Y-apatite depend on the volume and the chemistry of the CMAS glass.
Ca-Y-apatite formation is enhanced when the CMAS glass is saturated with Y3+, and the Ca-Y-apatite growth is energetically favorable and kinetically promoted. The Y content of the reprecipitatedc-ZrO2 phase depends on the interaction duration and the location within the interaction zone, making it difficult to use it as a tool for assessing the volume of Ca-Y-apatite formed and the effectiveness of the coating in mitigating CMAS attack. Additional phases, such as YAM and YAG form deep within the interaction zone as a result of Ca-Y-apatite formation. It was also confirmed that as-deposited 2ZrO2·Y2O3(ss) coating are resistant to densification at 1500°C.
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