The toughening of pyrochlore La <sub>2</sub> Zr <sub>2</sub> O <sub>7</sub> by a ferroelastic NdAlO <sub>3</sub> second phase for potential thermal barrier coating applications
Yanfei Wang, Jing Han, Jing-ping Du, Rongjun Liu, Fan Wan
Abstract
Abstract The poor fracture toughness of La 2 Zr 2 O 7 severely limits its application as a high temperature thermal barrier coating topcoat material. To toughen it, a ferroelastic second phase, NdAlO 3 , with a Curie temperature of 1367°C has been introduced to form x NdAlO 3 /(1‐ x ) La 2 Zr 2 O 7 composite ceramics by a spark plasma sintering technique, where x = 10, 20, 30, 40, and 50 mol%. The fracture toughness of sintered composite ceramic compacts is measured at both room temperature and 1200°C, respectively, by a single‐edge‐notch beam test method. The results show that, at room temperature, the residual compressive stress in the La 2 Zr 2 O 7 matrix plays an important role in the toughening of composite ceramics. By eliminating this factor, the remaining toughening effects agree with the measured fracture toughness at 1200°C, suggesting that the other toughening is probably ferroelastic domain switching toughening and that it is still valid at high temperature. Furthermore, the toughening effect arising from ferroelastic domain switching is governed by the overall domain switching zone, which is determined by both individual domain switching zone width and the “concentration” of ferroelastic phases. A relatively high coercive stress of NdAlO 3 and relatively low residual tensile stress in NdAlO 3 second phases contribute to negligible influence of residual tensile stress on domain switching zone width, leading to the continuous increase of fracture toughness of composite ceramics with more NdAlO 3 added at room temperature.