For both pyrochlore based-TBCs the reaction depth is higher under AVA attack as compared to CMAS under the same annealing conditions, as shown in Fig. 1. In addition, the corrosive attack of GdZ by AVA (Fig. 1 (a)) is more severe than the reaction of the same coating with CMAS (Fig. 1 (b)). The varying depth of the reaction front implies that inter-columnar gaps are partly filled by both deposits, but for AVA almost the whole column interior has completely reacted while under CMAS attack the reaction is restricted to the outer column rim (compare Fig.1 (a) and (b)). The reaction has proceeded to a depth between 70 and 100µm, leading to a partial loss of Gd in the reaction zone with Si, Ca and Fe being incorporated into the coating. On top of the columns a thin zone of about 10µm thickness is visible that contains large globular grains of 1 to 5µm dimensions rich in zirconia holding substantial amounts of Gd and some Fe. In the former AVA numerous precipitates of varying shape and size occur. The highly complex thermo-chemical interaction of volcanic ash with Gd2Zr2O7 EB-PVD coatings requires more fundamental work including TEM and investigation of powder based mixtures of the reactants, as described in.
Fig. 1: SEM cross section of pyrochlore-based TBCs upon annealing at 1260°C, 2 h. Gd-zirconate under AVA attack (a), for comparison GdZ under CMAS attack (b), and La-zirconate under AVA attack (c). Areas 1 to 3 in (c) designate the former AVA melt upon reaction with LaZ (1), upper (2) and lower (3) reaction zone between AVA and TBC.
In the LaZ coating the reaction has proceeded down as a reaction front to a depth of about 100 µm which is slightly higher than in GdZ. Unlike the CMAS system, preferred reaction within inter-columnar gaps is not observed. Due to inward diffusion of AVA elements and the outward flux of elements from the former top coat into the AVA three different reaction subzones (1-3) can be distinguished (Fig. 1(c)). Reaction zones 1 and 2 basically differ in their ratio of crystalline phases to modified AVA melt and in the grain sizes of phases precipitated, the former one increasing from top to bottom of the bulk reaction zone. Zone 1 consists of the former AVA melt and several precipitated phases of needle-like or facetted morphology while the upper reaction zone 2 is characterized by more numerous Zr-rich precipitates of predominantly globular appearance that are low in their La-content (Fig. 2). The lower reaction zone 3 is the fully reacted former TBC which looks compact. Their columnar coating structure has completely converted into small globular grains in the 1µm range. The content of Zr and La is lower compared to the intact TBC regions, but still higher than in zones 2 and 1. AVA constituents are also confirmed in this zone but at a lower amount than in the upper and outer former AVA zone. The interface between zones 2 and 3 seems to mark the former top coat surface that might has slightly shifted due to corrosive reactions. FIB-assisted TEM investigations were performed from various locations at the boundary between sublayers 1 and 2, and within zone 3 in order to unambiguously identify the phases involved in La2Zr2O7/AVA interdiffusion and reaction.
Fig. 2: SEM cross section of the former AVA melt (zone 1) and upper AVA/LaZ reaction zone 2 upon annealing at 1260°C, 2 h.
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