The standard 7YSZ TBCs were attacked by CMAS in the common way. The reaction scenario includes infiltration of the whole TBC down to the substrate, complete filling of the inter-columnar pores, and disintegration of feather-arms. CMAS infiltration and reaction behavior were independent of annealing atmosphere (vacuum or air) and TBC thickness. All observations on EB-PVD 7YSZ TBCs were in good agreement with literature data available for standard CMAS [6, 8, 30] and modified Fe-CMAS. Therefore, 7YSZ is addressed here only to benchmark the new TBC systems.
Fig. 1 depicts alternative zirconia-based TBC compositions after CMAS attack. All grades showed a reaction scenario similar to 7YSZ with subtle differences only. Infiltration has progressed towards the TBC root area. The CMAS composition in the inter-columnar gaps and larger feather-arms was essentially the same as on top of the TBCs. The high annealing temperature of 1260°C promoted chemical reactions at the column tips (Fig. 1a and e). The columnar structure got dissolved and material was reprecipitated with a slightly different composition than the original TBC incorporating minor amounts of CMAS constituents, similarly as found in for 7YSZ. Some zirconia got dissolved into the CMAS melt close to the reaction regions. Large horizontal cracks were also occasionally observed in the infiltrated TBCs (Fig. 1d) that are usually quite untypical for EB-PVD TBCs not subjected to deposits. For deposit free samples annealed under the same conditions the columnar structure with its feather-arms and pores was still intact. It only underwent the typical morphological changes caused by sintering such as pore coalescence which have been described for 7YSZ in detail elsewhere.
CMAS on 14 YSZ and 29 DySZ on alumina substrates reacted also with the TBC in the root area (Fig. 1b and d). The TBC reprecipitated as globular grains with a composition close to 14 YSZ or 29 DySZ with some dissolved Ca (<2wt%). This reprecipitation was also observed when a small diffusion barrier of platinum (bright grey layer highlighted by the arrows in Fig. 1 (d)) was deposited between the DySZ TBC and the alumina substrate. HfYSZ was evaporated from two sources, namely (i) a 7YSZ ingot and (ii) a hafnia and yttria rich ZrO2 ingot. The rotation through the two vapour clouds over the two crucibles created the distinct striations visible in Fig. 1 (e). The column tips were also heavily attacked and globular Ca-doped fully stabilized zirconia grains are evident with the latter also abundant in inter-columnar gaps and close to the substrate. A similar attack of the TBC with complete infiltration by CMAS was found for CeSZ TBCs with the thickness of the reaction zone depending on the CeO2 content of the top coat. To avoid duplications, pictures of this TBC version are not given.
Fig. 1: SEM cross sections of 14 YSZ (a, b), 29 DySZ (c, d), HfYSZ (e) upon annealing at 1260°C, 2 h under CMAS attack. Top (a, c, e) and close to substrate region (b, d). All images are taken at the same magnification. The dark grey material on top of the TBCs in (a, c, e) is excess CMAS. Arrows indicate inter-columnar gaps.
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