The element and morphology evolution of GEZ/YSZ coating are detected and investigated in detail after thermal cycling test. To further investigate the failure of TBCs systems, the cross-section and TGO (mainly Al2O3) morphologies of TBCs systems, GEZ, YSZ and NiCoCrAlYHf coatings are compared in Fig. 1a. The GEZ and YSZ coatings still show a similar columnar microstructure. The cross-section images also show that GEZ coating is well combined with YSZ. Furthermore, the ceramic interface also exhibits smooth combination and no obvious lamination after thermal cycling test. As shown in Fig. 1b, the thickness of TGO layer reaches about 10 μm. At the end of thermal cycling test, the massive cracks have spread over the TGO layer. That might be relevant with the reduction of thermal stability and strain tolerance. Thus, the TGO broken is generally attributed to the TGO growth and cracks evolution within TGO layer, leading to the spallation and delamination failure. Fig. 1c shows the EDS line scanning of the cross-section morphology after thermal cycling test. The presence of Gd, Er, Zr and O four elements have been detected in the top GEZ coating. The presence of Zr, Y and O three elements have been detected in the YSZ coating. The massive Al has been detected due to the TGO layer growth. After thermal cycling test, there is no obvious concentration of these elements. It indicates that the element composition of GEZ and YSZ coatings is relatively stable under high temperature. Fig. 1d shows the EDS line scanning of TGO layer. The presence of Al and O have been detected in TGO. The massive Al has been detected due to the TGO layer growth.
Fig. 1. (a) Cross-section morphology of TBCs, (b) Cross-section morphology of TGO layer, (c) EDS line scanning of TBCs and (d) EDS line scanning of the TGO layer after thermal cycling test.
Furthermore, the massive cracks have spread over the TGO layer at low Al distribution position. To further elucidate the interface stability and failure critical factor of TBCs systems, TGO morphology and elemental mappings of GEZ/YSZ coating are detected and investigated after thermal cycling test. As shown in Fig. 2a, some bright particles have been formed in TGO layer. Fig. 2b–f show the element area distribution EPMA maps correspond to the Al, Ni, Cr, Co and Y five elements. The Al has been distributed in the TGO layer (Fig. 2b). The vertical and horizontal cracks have spread over the TGO layer. As shown in Fig. 2c–f, the Ni, Cr, Co and Y four elements have diffused from NiCoCrAlYHf coating to TGO layer. The bright particles are the Ni-Al-Cr-Co-Y oxides. Evidently the element diffusion from substrate or NiCoCrAlYHf coating to YSZ/NiCoCrAlYHf interface might bring some volume change in thermal cycling test. Furthermore, the expansion coefficient mismatch among NiCoCrAlYHf, TGO and YSZ or GEZ would lead to a high residual stress accumulation between TBCs systems and substrate. These reasons might lead to the high cracks formation rate and further lead to the failure of TBCs systems. In particular, the diffusion of Ni, Cr, Co, Al and Y five elements also lead to the formation of Ni-Al-Cr-Co-Y oxides. Thus, the mismatch between Ni-Al-Cr-Co-Y oxides and Al2O3 also might lead to the formation of interface instability. As a result, the element diffusion might be critical factor for cracks extension leading to the failure of TBCs. This will provide more insight into the effect of elements and cracks evolution on the failure of TBCs systems.
Fig. 2. (a) Cross-sectional morphology of TGO layer and corresponding element area distribution EPMA maps: (b) GAl, (c) Ni, (d) Cr, (e) Co and (f) Y.
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