Hot corrosion and high-temperature deformation of yttria -stabilized zirconia thermal barrier coatings
Abstract
The effect of V2O5 melt infiltration on the phase stability, thermal and mechanical properties of plasma-sprayed 7 wt% Y 2O3-ZrO2 (YSZ) was assessed by infiltration experiments at temperatures of 750°-1200°. Based on SEM and TEM observations combined with EDS, reactions between molten V2O'5 and YSZ were found to occur concomitantly with the infiltration process at all temperatures investigated. Near the surface, the reaction front proceeds in a planar fashion presumably via dissolution and precipitation, while the V2O5 melt infiltrates along the interconnected pores and reacts with the YSZ deep inside the coating. Typically, two different morphologies could be distinguished in the cross-section of the infiltrated coating that corresponds to the planar reacted zone (PRZ) and the melt infiltrated reaction zone (MIRZ). PRZ is a fine-grained surface reacted region where the vanadium concentration is higher or comparable to the yttrium concentration and MIRZ is a dense infiltrated region which displays abrupt vanadium concentration changes along the thickness direction. Temperature as well as morphological parameters, such as pore size and connectivity, was found to influence the dissolution/infiltration kinetics. In-situ XRD was employed to clarify the reaction mechanism between V 2O5 melt and YSZ. Concurrent formation of zirconium vanadate and yttrium vanadate were observed at 700°C and 750°C, suggesting a similar reactivity of yttrium and zirconia with vanadium oxide. The zirconium vanadate phase decomposed subsequently after 150 and 60 minutes at 700° and 750°C, respectively. The t'-ZrO2 phase transformed to a monoclinic phase through the formation of ZrV2O7, without gradually losing yttrium in t'-ZrO2 or partitioning into a cubic zirconia phase as previously thought. Therefore, zirconia-based ceramics have very limited resistance to vanadium oxide because of the zirconia. The Johnson-Mehl-Avrami (JMA) analysis based on the in-situ X-ray results at temperatures from 700°C to 900°C demonstrated a change in the JMA exponent n. This change is believed to be linked to a change in the reaction mechanism. Finally, the effects of the reaction between V2O5 and YSZ on thermal conductivity and creep behavior of the stand-alone coatings were evaluated. Due to limited infiltration, up to 1 wt% V2O 5 did not change these properties significantly.
Degree
Ph.D.
Advisors
Trice, Purdue University.
Subject Area
Materials science
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