Estimation of the Role of Nanosized Stabilizing Powders in Gaining High-Level Crack Growth Resistance of Partially Stabilized Zirconia

Nowadays, zirconium oxide (ZrO2) is widely used for applications in high temperature structural materials. These materials may have applications such as cracking furnaces, catalytic cracking units, fired heaters, hydrogen reformers, utility boilers, ammonia reformers, air heaters, etc. Zirconium oxide (zirconia) is often more useful in its phase stabilized state. In particular, products made of yttria-stabilized zirconia ceramics are characterized by improved strength and crack growth resistance. In this work, ZrO2 ceramics partially stabilized with 3, 4, and 5 mol% Y2O3 (hereinafter: 3YSZ, 4YSZ, and 5YSZ) have been studied. Three sintering temperatures were used to prepare beam specimens of each series: 1450 °C, 1500 °C, and 1550 °C. Vickers microhardness test and single-edge notch beam test were performed and fracture toughness of the material was estimated. It is known that in the case when the sufficient amount of the metastable tetragonal phase is present, an applied stress, magnified by the stress concentration in the crack tip vicinity, can cause the tetragonal phase to convert to monoclinic. This phase transformation associated with the volume expansion causes then put the crack into compression stress at the crack tip, retarding crack growth, and enhancing the fracture toughness. In the case of materials studied in this work, a distinct transformation toughening effect was found for 5YSZ ceramics. Based on the microstructure and fracture surface analyses it was found that nanosized stabilizing powders play a crucial role in gaining high-level crack growth resistance of partially stabilized zirconia.