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CHAPTER 2 LITERATURE REVIEW 2.3.3 New Top Coat Materials Recently, rare earth zirconate has been considered as the future materials for TBC top coat. Compared to traditional YSZ top coat, rare earth zirconate provides higher sintering resistance and lower thermal conductivity [52,56-58]. Moreover, rare earth zirconates stay in the stable single crystal phases during thermal cycling [59,60] which is beneficial for the future engine design aiming at higher operating temperatures [1]. Today, Gd2Zr2O7 (GZO) has been widely tested and it provides good property for CMAS resistance [41,60,61]. To observe its interaction with CMAS, the degradation mechanism is really like alumina protective coatings. For CMAS, it only penetrates into the 30 μm of the depth and attracted by top coat. The chemical reaction could be written as below. CMAS + Gd2Zr2O7 C0.15Gd0.42Zr0.05Si0.38O1.64 (apatite) + Zr0.77Gd0.2Ca0.03O1.87 (fluorite) + Mg0.33Al0.63Zr0.02Gd0.02 (spinel) + residual elements In this reaction, apatite plays an important role as a key product [62,63] because its melting temperature is 1930 °C [64] and the place it deposited highly affects the degradation of CMAS penetration. The interaction mechanism between CMAS and Gd2Zr2O7 is really similar to the case between Al2O3 and CMAS. As illustrated in Figure 2.12, in the early stage, CMAS and Gd2Zr2O7 top coat interact in the columnar gaps and nucleate the products within the columnar gaps. Once the gaps are blocked by apatite and other elements, CMAS starts to Page 39PDF Image | Volcanic Ash Degradation on Thermal Barrier Coatings
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