Yttria-stabilized zirconia (YSZ) is a highly valued ceramic material due to its combination of desirable properties, including toughness, hardness, wear resistance, stability at high temperatures, low thermal conduction, and, in some cases, potential ionic conductivity. It plays a crucial role in many technical industries, such as thermal barrier coatings, solid oxide fuel cells (SOFC), oxygen sensors, optical ferrules, and more recently, biomedical devices and watch parts. Commonly used YSZ, sintered in a conventional way and thus at temperature of 1400°C or higher, exhibit grain size in the range of 0.3 to 1 µm. YSZ ceramics have attractive characteristics like a high mechanical strength, a long-term stability and optimum optical properties. However, there is a trade-off between all these properties, at least for submicronic zirconia. Also, there is a lack of study on these properties in relation to nanoscale microstructure. Perfect control of crystallinity, purity and composition, as well as the smallest possible particles (a few tens of nanometers), are necessary to obtain, after sintering, nanostructured materials with optimized properties. A promising approach for controlling the structural and microstructural characteristics of YSZ is to elaborate nanostructured ceramics using low-temperature sintering processes. Among these processes, classical Spark Plasma Sintering (SPS), High Pressure SPS (HP-SPS) and reactive SPS increase densification while maintaining grain growth to a minimum.

In this presentation, we first propose to explore different routes to determine the most adapted powder for optimizing the nanostructure of this material after sintering. This includes the use of different synthesis routes. We are particularly studying the possibility of achieving reactive sintering between zirconium hydroxide (Zr(OH)₄) and yttrium hydroxide (Y(OH)₃), as these two compounds decompose into zirconia (ZrO₂) and yttria (Y₂O₃) at high temperature. Zr(OH)₄ is commercially available, while Y(OH)3 is not. Some work has been done on the synthesis of the latter and on the study of its thermal decomposition, but this is very scarce in the literature. The very small particle size of this synthesized compound (a few tens of nanometers) is very promising for reactive sintering with Zr(OH)₄. This first part will be supported by a set of characterizations and analyses to understand the properties and behavior of powders. In a second part, the results of using different sintering methods (SPS, HP-SPS and reactive SPS) will be presented, showing the potential of this approach to densify YSZ at low temperature. Indeed, very high densification (> 95 %) with very small grain size (< 200 nm) is then obtained. Thus, to correlate with the optimization of desired properties (mechanical, optical and aging properties), preliminary characterizations in fracture toughness and hardness will be carried out. Transparency and long-term stability will be also discussed.