Zirconia is a versatile material that has interesting chemical and physical properties. When zirconia is stabilized with yttria, it is useful across a wide range of industries, especially for physically demanding industrial applications where high strength, wear, and fracture-resistance are required.
For many years, 3 mol % yttria-stabilized zirconia (3YSZ) has been used extensively in structural ceramic applications, but now there is a practical alternative from the yttria-zirconia family.
Innovnano’s 2 mol % YSZ (2YSZ) provides all of the required properties of 3YSZ as well as the additional benefit of enhanced fracture toughness that is inherent in lower yttria content zirconias. As a result of the proprietary synthesis process from Innovnano, 2YSZ maintains excellent ageing resistance and flexural strength.
With this new development for the ceramic industry, Innovnano’s 2YSZ serves as a good alternative for structural ceramic applications, either as the zirconia component in zirconia-toughened alumina/alumina-toughened zirconia (ZTA/ATZ) and cermets or as a ready-to-press powder.
Structural Zirconia Ceramics
As shown in Figure 1, zirconia's interesting properties originate from temperature-dependent and complex phase transitions. Pure zirconia exists in its monoclinic phase at room temperature and has specific characteristics including excellent thermal mechanical strength that make this material suitable for refractory applications.
Zirconia transforms to a tetragonal phase at temperatures higher than 1,175 °C. This transformation corresponds to altered properties that offer high component and flexural strength, excellent wear resistance, and exceptional durability.
These highly desirable properties make the tetragonal phase suitable for structural ceramics in physically demanding applications. At more than 2,370 °C, zirconia can be transformed into its cubic state, which is generally used in jewelry to mimic diamonds.
Figure 1. Zirconia undergoes phase transformations
Although zirconia is highly sought after in its tetragonal phase, it requires temperatures much higher than 1,175 °C to transform into this phase. Therefore, in several applications, a stabilizing dopant (e.g. yttria) is added to monoclinic zirconia to trigger a phase transition to tetragonal, without increasing the temperature.
Yttria-stabilized zirconia (YSZ) is produced by doping zirconia with yttria. In YSZ, some of the Zr4+ ions are replaced in the crystal lattice with the slightly larger Y3+ ions. At room temperature, YSZ exhibits all of the desirable properties of the zirconia tetragonal phase (e.g. high strength, and fracture- and wear-resistance), making it suitable for engineering and industrial applications under standard operating conditions.
2YSZ vs. 3YSZ
Depending on the required properties of the pressed ceramic piece and the zirconia phase, the amount of yttria added can be varied. Generally, reducing the quantity of yttria stabilizer increases fracture toughness, but there is a trade-off with reduced age resistance and mechanical strength.
Innovnano’s 2YSZ provides manufacturers with an alternative to traditional 3YSZ. 2YSZ combines superior fracture toughness with ageing resistance and stability and at the same time maintains high flexural strength.
Although the 2YSZ material is not new to the ceramics market, industrial quantities are hard to come by. However, Innovnano have recently added this material to the range of products manufactured using its unique Emulsion Detonation Synthesis (EDS) manufacturing method.
At an independent laboratory in Germany, the ageing and stability of the new powder has been examined to ensure that the lower yttria content does not negatively affect these significant structural ceramic properties.
Cyclic stress-strain ageing tests were performed in saline solution (on 2YSZ bars that have undergone cold isostatic pressing (CIP)), and all test pieces passed the ISO 13356 standard methodology of 1 million cycles at 20 Hz without failure.
EDS – A Unique Synthesis Approach
The EDS method is a defined cycle of high pressures, temperatures and quick quenching in a fully automatic system, based on the detonation of two water-in-oil emulsions in a single step reaction. The active nature of EDS contributes to zirconia stabilization, a process that has been widely tested by Innovnano’s team.
The resulting powders have a nanostructure - with increased specific surface area as a result of smaller grain sizes - which is attributed to the enhanced structural properties of hardness, flexural strength, fracture toughness, and resistance to thermal shock.
Innovnano can control the chemical structure, morphology, purity, and final properties of the synthesized ceramic powders using EDS. This process has significantly enhanced quality, consistency, and production capacity to produce improved powders in a cost-effective and efficient manner.
Therefore, the nanostructured 2YSZ powder from Innovnano is a high fracture toughness structural ceramic alternative to current 3 mol % equivalents. This powder is offered in a ready-to-press form for uniaxial pressing and isostatic pressing (HIP/CIP) to produce coatings and components that are highly stable, strong, and durable.
Innovnano’s EDS process presents more possibilities for innovative ceramic powders. For instance, 2YSZ can also be integrated as a component of zirconia-alumina (ZTA/ATZ). These composite ceramic materials contain alumina particles dispersed in an YSZ matrix (ATZ), or zirconia (in the form of YSZ) particles dispersed in an alumina matrix (ZTA).
The resulting material gains from the best properties of both materials, combining, and in some cases enhancing on, these characteristics. Most often, ATZ/ZTA is harder and tougher than that of YSZ alone and sometimes, it is more suitable based on the application type. At Innovnano, the EDS process could be used to produce ATZ/ZTA through a single step co-detonation at Innovnano.
This makes it possible to realize all properties and benefits in a single material – whether from EDS, the resulting nanostructure, and YSZ itself, or the combination of a composite material. In addition to ATZ/ZTA, 2YSZ is also suitable as the zirconia phase for other cermets and ceramic composites.
Innovnano’s proprietary EDS method provides nanostructured 2YSZ with the stability of a high yttria-containing YSZ and the fracture toughness of a low yttria-containing YSZ, striking a balance between two main structural ceramics needs. This unique combination of properties provides an exciting new development for coating and ceramic component manufacturers, particularly in advanced engineering and industrial applications.
This information has been sourced, reviewed and adapted from materials provided by Innovnano.
For more information on this source, please visit Innovnano.