Transformable tetragonal zirconia-based ceramics, such as 3YSZ, offer interesting mechanical properties thanks to the transformation toughening mechanism. In order to achieve the transformation toughening effect, zirconia ceramic must be in the metastable tetragonal phase. This, at the same time, makes the ceramic highly susceptible to unintended tetragonal to monoclinic phase transformation in a humid environment in service. This well-known phenomenon is mostly studied at low temperatures (known as low-temperature degradation, LTD) and is a key challenge for the long-term reliability of zirconia ceramics at low temperatures, in particular when they are used in biomedical applications.

Hydrothermal degradation is often considered to be negligible at temperatures above ≈400°C (above the T₀ (T/M) temperature). Most developed time-temperature-transformation (TTT) diagrams suggest that above the T₀ temperature, the degradation will be extremely slow, nearly zero. In addition, it is well known that the tetragonal phase thermodynamically becomes more stable by increasing the temperature. As a result, there has been less concern about the susceptibility of transformable zirconia ceramics to high temperature degradation (HTD), e.g., when they are used in high-temperature applications such as in solid oxide fuel and electrolysis cells. Nonetheless, we have found that this degradation can occur substantially in solid oxide cells operating at temperatures well above 500°C. The degradation has a detrimental effect on the mechanical strength of the cells, posing a challenge for their long-term robustness.

In this work, we present our recent findings on the HTD of 3YSZ-based ceramics. We have found that HTD can occur within a few years of the operation of solid oxide cells, which is below their desired lifespan of 4-6 years. The work thus provides insight into the susceptibility of metastable (transformable) zirconia ceramics to HTD and the kinetics of phase transformation, and consequently, the challenges facing their application at high temperatures.