Improved Damage Tolerance of Structural Ceramic Materials
Desirable properties position the tetragonal phase of zirconia as an optimum structural ceramic material for physically demanding applications.
Zirconia is a versatile material with fascinating physical and chemical properties. When stabilized with yttria, it is useful across a range of industries, especially for demanding industrial applications that require high strength, as well as wear and fracture resistance. 3 mol% yttria-stabilized zirconia (3YSZ) has been widely used in structural ceramic applications for many years due to its excellent mechanical properties, including high flexural strength and good fracture toughness when compared with other technical ceramics.
It is now possible to use a different material—2 mol% yttria-stabilized zirconia (2YSZ)—that provides all the benefits of 3YSZ, but with higher fracture toughness values.* At the same time, damage tolerance and resistance to aging are improved in this alternative option for any application that requires strong and long-lasting materials.
*Developed by Innovnano
Zirconia Structural Ceramics
At room temperature, zirconia exists in its monoclinic phase but transforms to a tetragonal phase at temperatures higher than 1,175°C. This transformation alters the zirconia’s properties, providing exceptional wear resistance, high component and flexural strength, and excellent durability. These desirable properties position the tetragonal phase of zirconia as an optimum structural ceramic material for physically demanding applications.
Maintaining zirconia in its tetragonal state at a range of useful temperatures is therefore key. A process called doping that involves the addition of oxides to the zirconia crystalline structure makes this possible. While different oxides can be used to stabilize the higher temperature tetragonal phase (e.g., CaO, MgO and CeO2), yttria (Y2O3) is the most commonly used due to its high solubility in the zirconia lattice.1 During the doping process, some of the Zr4+ ions are substituted in the crystal lattice for the slightly larger Y3+ ions to form yttria-stabilized zirconia (YSZ).2,3 YSZ exhibits all of the desirable properties of the zirconia tetragonal phase at room temperature, making it suitable for use at normal operating conditions.
Minimizing Yttria to Maximize Fracture Toughness
The amount of yttria dopant used to stabilize the zirconia can be varied to produce different crystalline structures, depending on the required properties of the ceramic end product. Lowering the amount of stabilizing yttria results in a marked increase in fracture toughness, which is highly sought after to improve component performance. However, it is generally the case that zirconia stabilized with low amounts of yttria is less resistant to aging and has reduced flexural strength. Breaking this rule, 2YSZ manufactured using a proprietary process** maintains all of the desirable mechanical properties of higher yttria-content YSZs.
The synthesis technology of the proprietary process involves 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. (For additional details on this process, see “Synthesizing Yttria-Stabilized Zirconia,” Ceramic Industry, June 2017)
The energetic nature of the process contributes to the stabilization of the zirconia, and the resultant powders have a nanostructure (with increased specific surface area) to which the improved structural properties are attributed. These properties include hardness, fracture toughness, flexural strength, the ability to sinter at lower temperatures, and resistance to thermal shock. The process produces 2YSZ ceramics that combine excellent fracture toughness with good stability and aging resistance while maintaining high flexural strength (see Table 1).
When synthesized via the proprietary process, 2YSZ provides an alternative to conventional 3YSZ. It offers a flexural strength above 1,000 MPa, with fracture toughness significantly increased from 5 to higher than 14 MPa.m0.5 when compared with benchmark 3YSZ. It therefore provides an alternative for structural ceramic applications, either as a ready-to-press powder or as the zirconia component in zirconia-toughened alumina/alumina-toughened zirconia (ZTA/ATZ) and cermets.
**Emulsion Detonation Synthesis (EDS), proprietary to Innovnano
Testing Aging Resistance
Independent stability and aging tests have been carried out to ensure the lower yttria content of this 2YSZ does not adversely affect important structural ceramic properties. Cyclic stress-strain aging tests in saline solution were carried out using proprietarily synthesized 2YSZ bars that have undergone cold isostatic pressing (CIP) and conventional sintering. All pieces passed the ISO 13356 standard methodology (106 cycles, 320 MPa, 20 Hz frequency) without failure. The four-point bending strength was also determined before and after the experiment; results showed that only 13% of flexural strength was lost.
Further cyclic stress-strain aging experiments were successfully performed (20 Hz, 106 cycles) using 1,100 MPa as maximum pressure, highlighting resistance to mechanical aging. Studies using 3YSZ in the literature report that, of 13 specimens tested at a maximum pressure of 650 MPa, none achieved 106 cycles.4
Hydrothermal aging investigations under ISO 13356:2015 (5 h, 134°C, 0.2 MPa) used 2YSZ pellets produced by uniaxial pressing and sintering. Results showed that 2YSZ produced with the proprietary process exceeds the standard to retain 85% of its flexural strength. Additionally, these 2YSZ pellets were aged according to more demanding conditions for 96 hrs and retained 80% of the initial mechanical properties.
Improved Damage Tolerance
Being able to withstand demanding conditions when in service, and therefore having good damage tolerance, is important for structural ceramic components. The intrinsic critical defect sizes and flexural strength have been investigated both theoretically and practically.
According to the Griffith criteria,5 the flexural strength of a ceramic part is both positively affected by the material fracture toughness and negatively affected by the intrinsic critical defect size. Based on this theory, two materials with the same flexural strength but distinct fracture toughness values can react differently to defects.
In the case of 2YSZ and 3YSZ, investigations showed that both parts have the same flexural strength but different fracture toughness values of 14 and 5 MPa.m0.5, respectively (see Table 1). Using the Griffith criteria equation, this translates to a critical flaw size of 1.76 µm for 3YSZ and 13.83 µm for 2YSZ. This means that 2YSZ material processed through the proprietary synthesis can support larger defects with better mechanical properties when compared with 3YSZ.
To assess these theoretical predictions, a practical assessment of the flaw tolerance behavior of the proprietarily synthesized 2YSZ was performed in comparison with benchmark 3YSZ. Uniaxial pressed disk samples were prepared at 100 MPa in a 20 mm die and sintered at 1,450ºC (3YSZ) and 1,350ºC (2YSZ) for 2 hrs. After sintering, samples were mirror polished and different size defects were purposely created using a micro-indentation system with a Vickers tip (see Figure 1A, p. 22) and different applied loads for 15 sec. The biaxial flexural strength was assessed before and after defect insertion using a piston-on-three ball system according to ISO 6872. The introduced defect is placed face down (see Figure 1B, p. 22) to ensure that the pre-crack effect is taken into consideration in the mechanical test.
The biaxial flexural strengths of benchmark 3YSZ and proprietarily synthesized 2YSZ products before and after defect creation are compared in Figure 2A. The undamaged parts of both products have similar flexural strengths that are in line with the properties listed in Table 1. However, when a defect is introduced in the ceramic component, using the same applied load, the parts prepared with the 2YSZ product present higher flexural strength values when compared to those of benchmark 3YSZ products.
Due to the higher fracture toughness of 2YSZ synthesized with the proprietary process (14 MPa.m0.5), the creation of the defect also results in smaller crack length for the same applied loads when compared to 3YSZ (see Figure 2C). This is consistent with theoretical predictions that materials with high fracture toughness have an enhanced energy absorption ability and therefore a lower crack propagation rate.
The biaxial flexural strengths of proprietarily synthesized 2YSZ and benchmark 3YSZ products can therefore be compared as a function of defect crack length (see Figure 2B) to further show the superior damage tolerance capabilities of this 2YSZ. For the same crack lengths, 2YSZ products synthesized with the proprietary process have a biaxial flexural strength that is 1.5-1.8 times higher than for 3YSZ.
Using proprietary synthesis manufacturing technology, it is possible to produce a product with outstanding potential for structural ceramics applications. 2YSZ synthesized in this way combines the benefits of high fracture toughness with desirable aging and damage tolerance capabilities.
For more information, visit www.innovnano-materials.com.
- Swab, J.J., “Role of Oxide Additives in Stabilizing Zirconia for Coating Applications,” Army Research Laboratory, Aberdeen Proving Ground, U.S., September 2001.
- Basu, B., “Toughening of Yttria-Stabilised Tetragonal Zirconia Ceramics,” International Materials Reviews, vol. 50, no. 4, Kanpur, India, pp. 239-256, 2005.
- Nunes Soares, R.M., “Ph.D. Thesis - Development of Zirconia Based Phospors for Application in Lighting and as Luminescent Bioprobes,” University of Aveiro, 2013.
- Chaves Souza, Renato, et al., “Performance of 3Y-TZP Bioceramics under Cyclic Fatigue Loading,” Materials Research, Vol. 11, No. 1, 89-92, 2008.
- Carter, C.B., and M.G. Norton, M.G., Ceramic Materials–Science and Engineering, Springer, 2007.