Multiscale Microscopic Insights into Low-Temperature Degradation in Zirconia: Enhancing Ceramic Stability

Zirconia, a ceramic material known for its high fracture toughness, strength, and stability at elevated temperatures and pressures, is utilized in various applications such as solid oxide fuel cells, thermal barrier coatings, and sensors. Tetragonal zirconia, in particular, is highly desirable due to its unique properties, such as high fracture toughness, low thermal conductivity, and high melting temperature. Zirconia doped with 3-mol% Yttria helps stabilize the tetragonal phase and prevents its transformation to the monoclinic (t-m) phase at room temperature. However, extended exposure to extreme conditions, a process known as aging, has been observed to induce spontaneous phase transformations, leading to the development of microcracks and mechanical failure, referred to as low-temperature degradation (LTD). Our study hypothesizes that aging results in the preferential segregation of elements near the grain boundaries, initiating local phase transformations and altering the grain environment.<br/>To investigate this hypothesis, a multi-scale microscopy approach, specifically employing Atomic Force Microscopy (AFM) and Atom Probe Tomography (APT) was utilized to examine the yttria-stabilized zirconia (YSZ) to provide fundamental insights into the degradation mechanisms. The investigation includes a comprehensive study of the material's response to artificial aging, including structural, crystallographic, and chemical analyses. Firstly, the Design of Experiments approach was employed to predict the process parameters necessary for artificially aging samples, relying on information collected from existing literature. AFM allowed us to examine surface topography and changes in grain size, providing detailed morphological changes that occur at the material’s surface with aging. APT assisted in acquiring elemental distribution and segregation, offering 3-D compositional mapping with sub-nanometer resolution. The initial results show variation in grain size distribution and grain curvature at all stages of aging, i.e., among unaged, partially aged, and fully aged samples. We also observed preferential segregation of Aluminum and Silicon at certain grain boundaries. Additionally, a difference in Yttria concentration at the grain boundaries between the aged and unaged samples was observed, leading to a possible initiation of local phase transformation. These fundamental investigations enabled to generate novel insights for LTD mechanisms, which is a critical need for designing robust and durable ceramics for practical applications.<br/><br/>This work is supported by NSF award number DMR2114595. APT research was supported by the Center for Nanophase Materials Sciences (CNMS), which is a US Department of Energy, Office of Science User Facility at Oak Ridge National Laboratory. The authors thank Dr. Jonathan Poplawsky and Mr. James Burns for APT specimen preparations and conducting the APT experiments.