Thermal shock resistance of double-layer thermal barrier coatings
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To reveal the thermal shock resistance of double-layer thermal barrier coatings (TBCs), two types of TBCs were prepared via atmospheric plasma spraying, i.e., Gd2Zr2O7/yttria-stabilized zirconia (GZ/YSZ) TBCs and La2Zr2O7 (LZ)/YSZ TBCs, respectively. Subsequently, thermal cycling tests of the two TBCs were conducted at 1100 °C and their thermal shock resistance and failure mechanism were comparatively investigated through experiments and the finite element method. The results showed that the thermal shock failure of the two TBCs occurred inside the top ceramic coating. However, the GZ/YSZ TBCs had longer thermal cycling life. It was the mechanical properties of the top ceramic coating, and the thermal stresses arising from the thermal mismatch between the top ceramic coating and the substrate that determined the thermal cycling life of the two TBCs together. Compared with the LZ layer in the LZ/YSZ TBCs, the GZ layer in the GZ/YSZ TBCs had smaller elastic modulus, larger fracture toughness, and smaller thermal stresses, which led to the higher crack propagation resistance and less spallation tendency of the GZ/YSZ TBCs. Therefore, the GZ/YSZ TBCs exhibited superior thermal shock resistance to the LZ/YSZ TBCs.
Introduction Thermal barrier coatings (TBCs) have been widely used in hot components such as turbine blades [1, 2, 3]. According to the structure of ceramic layer, TBCs can be divided into singlelayer TBCs, double-layer TBCs, and gradient TBCs. At present, the most widely used single-layer TBCs is yttria-stabilized zirconia (YSZ) TBCs. But the YSZ will occur to phase transformation when the service temperature exceeds 1250 °C or it is in long-term service at 1200 °C. Moreover, the YSZ has weak ability to isolate external oxygen and is prone to sintering at high temperature [4, 5, 6]. Thus, the service life of the YSZ TBCs is reduced. Some studies have shown that the thermal cycling life of the double-layer TBCs is longer than that of the single-layer TBCs [7, 8]. Due to pyrochlore (P) structure compounds have high phase stability and low sintering rate, and they have strong ability to isolate external oxygen; therefore, the P-structure compounds are suitable for the top ceramic coating material in double-layer TBCs [9, 10]. La2Zr2O7 (LZ) and Gd2Zr2O7 (GZ) are a typical P-structure material. However, the difference of the thermal expansion coefficient between the LZ and the substrate is significant (αLZ = 9.1 × 10−6 K−1
and αsubstrate = (18–19) × 10−6 K−1 [11]), which is adverse to the thermal shock resistance of the LZ/YSZ TBCs. Recently, researchers have found that GZ has not only lower thermal conductivity (λYSZ = 2.12 W/m K, λLZ = 1.56 W/m K, and λGZ = 1.16 W/m K [11, 12]) but also a larger thermal expansion coefficient (αGZ = 10.4 × 10−6 K−1 [13, 14]) than that of the LZ. Therefore, the GZ has attracted the attention of TBCs researchers. Nevertheless, the studies of thermal shock resistance of TBCs were mostly focused on the comparison between singlelayer TBCs and double-layer TBCs, but the comparisons b
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