Synthesizing Yttria-Stabilized Zirconia
2YSZ provides a promising alternative for structural ceramic applications.
Zirconia is a versatile material with interesting physical and chemical properties. When stabilized with yttria, it is useful across a range of industries, especially for physically demanding structural ceramic applications that require high strength and wear, as well as fracture resistance.
3 mol% yttria-stabilized zirconia (3YSZ) has been widely utilized in structural ceramic applications for many years. However, 2 mol% YSZ (2YSZ*) offers all the desired properties of 3YSZ, as well as the added benefit of improved fracture toughness that is inherent in lower-yttria-content zirconias. As a result of a proprietary synthesis process, this 2YSZ also maintains good flexural strength and ageing resistance. 2YSZ provides a promising alternative for structural ceramic applications, either as a ready-to-press powder, the zirconia additive in zirconia-toughened alumina (ZTA) or the matrix in alumina-toughened zirconia (ATZ), and also cermets.
*Developed and manufactured by Innovnano using emulsion detonation synthesis (EDS), its proprietary synthesis process.
Structural Zirconia Ceramics
The interesting properties of zirconia come from its complex and temperature-dependent phase transitions. At room temperature, pure zirconia exists in its monoclinic phase, which has characteristics such as good thermal mechanical strength that make it suitable for refractory applications.
At temperatures higher than 1,175°C, zirconia transforms to a tetragonal phase. This transformation corresponds to altered properties that provide exceptional wear resistance, high component and flexural strength, and excellent durability. Because of these desirable properties, the tetragonal phase is used for structural ceramics in physically demanding applications. If the temperature increases beyond 2,370°C, zirconia transforms into its cubic state, which is commonly used in jewelry to mimic diamonds.
While zirconia in its tetragonal phase is highly sought after, it is required at temperatures much lower than the 1,175°C that marks its transition into this phase. For a number of applications, a stabilizing dopant (e.g., yttria) is therefore added to monoclinic zirconia to cause a phase transition to tetragonal without increasing the temperature. The process of doping zirconia with yttria produces yttria-stabilized zirconia (YSZ), in which some of the Zr4+ ions are substituted in the crystal lattice for the slightly larger Y3+ ions. YSZ exhibits all of the desirable properties of the zirconia tetragonal phase (e.g., high strength and wear and fracture resistance) at room temperature, making it suitable for industrial and engineering applications under normal operating conditions.
YSZ has many uses across a range of widespread industries. It is especially suitable for use as a structural ceramic, able to withstand physically demanding conditions and applications. Recent market reports have suggested that the advanced structural ceramics market is set to see healthy growth over the upcoming years.1 Looking at North America specifically, the technical and advanced structural ceramics market is expected to reach nearly $6.7 billion by 2020 (equating to a compound annual growth rate of 6.7% from 2015).2 The advent of new materials, such as 2YSZ, helps to meet this increasing growth and demand for both existing and emerging applications.
2YSZ vs. 3YSZ
The amount of yttria added to stabilize the zirconia can be varied, depending on the zirconia phase and the required properties of the pressed ceramic piece. Generally, lowering the amount of yttria stabilizer increases fracture toughness; however, there is a trade-off with reduced mechanical strength and age resistance. 2YSZ provides an alternative to conventional 3YSZ, combining excellent fracture toughness with good stability and ageing resistance while maintaining high flexural strength.
Tests have shown that the mechanical strength of 2YSZ remains above 1,000 MPa, while fracture toughness is significantly increased from 5 to up to 14 MPa.m0.5 when compared to 3YSZ. The stability and ageing of the new powder has also been tested at an independent laboratory to ensure the lower yttria content does not adversely affect important structural ceramic properties. Cyclic stress-strain ageing tests in saline solution (on 2YSZ bars that have undergone cold isostatic pressing and conventional sintering) were carried out, with the test pieces all passing the ISO 13356 standard methodology of 1 million cycles at 320 MPa (maximum) and 20 Hz frequency without failure.
The four-point bending strength was determined for 2YSZ bars before and after the cyclic stress-strain experiment. After 106 cycles under the described conditions, the test showed only a 13% loss in flexural strength. In addition, further cyclic stress-strain ageing experiments were successfully performed (106 cycles at 20 Hz) using 1,100 MPa as maximum amplitude of stress load, which corresponds to almost double the values achieved with a standard 3YSZ.
New Synthesis Approach
The prospect of a high fracture toughness structural ceramic alternative to the widely used 3YSZ is achieved through a synthesis approach termed emulsion detonation synthesis (EDS). EDS is a defined cycle of high temperatures, pressures and rapid quenching in a fully automated system, based on the detonation of two water-in-oil emulsions in a single-step reaction. The energetic nature of EDS contributes to the stabilization of the zirconia, a process that has been extensively tested. The resultant powders have a nanostructure (with increased specific surface area due to smaller grain sizes) to which the improved structural properties of hardness, fracture toughness, flexural strength and resistance to thermal shock are attributed.
Using EDS, the purity, chemical structure, morphology and final properties of the synthesized ceramic powders can be controlled. This improves quality and consistency, as well as production capacity, to produce enhanced powders in an efficient and cost-effective way.
The EDS process has opened the door to innovative ceramic powder possibilities. For example, 2YSZ can also be incorporated in zirconia-alumina (ZTA/ATZ). These composite ceramic materials consist of zirconia (in the form of YSZ) particles dispersed in an alumina matrix (ZTA), or alumina particles dispersed in a YSZ matrix (ATZ). The result is a material that benefits from the best properties of both materials, combining, and in some cases improving on, these characteristics. ATZ/ZTA is often harder and tougher than YSZ alone; as such, it is sometimes more suitable, depending on the type of application.
With EDS, there is also the potential to produce ATZ/ZTA through a single step co-detonation. This allows all properties and benefits to be realized in one material—whether from YSZ itself, EDS and the resultant nanostructure, or the combined nature of a composite material. In addition to ATZ/ZTA, 2YSZ is also suitable as the zirconia phase of other ceramic composites and cermets.
Nanostructured 2YSZ strikes a balance between two key structural ceramics needs, combining the fracture toughness of a low yttria-containing YSZ with the phase stability of a high yttria-containing YSZ. The combination of properties from this new structural ceramic material offers a new development for manufacturers of ceramic components. This is particularly true where these ceramics are used in advanced engineering and industrial applications.
1. “Advanced Structural Ceramics Market Analysis, Trends, Market Application, Regional Outlook, Competitive Strategies and Forecast, 2016 To 2024,” Grand View Research, www.grandviewresearch.com.
2. “Technical and Advanced Structural Ceramics: North American Markets,” BCC Research, www.bccresearch.com.
3. Kelly, J. Robert and Denry, Isabelle, “Stabilized Zirconia as a Structural Ceramic: An Overview,” Dental Materials, 24, 2008, pp. 289-298.