Coloured Zirconia Ceramics

Yttria-Stabilized Zirconia (YSZ) ceramics have uses in a variety of industries – from intricate components and features used in the creation of jewelry and watches, to super tough, hardwearing structural ceramics for extreme environments. In the case of watch-making and jewelry, mechanical and optical properties are critical to ensure a functional and desirable product is created. The ability to color ceramics without compromising on the mechanical properties is particularly important.

Why Zirconia?

Stabilized zirconia has unique physicochemical, mechanical and electrical properties which make it an extraordinary ceramic material of huge interest for a wide variety of applications and industries.

Pure zirconia can exist in one of three states depending on the temperature¹(Figure 1). Zirconia exists in its monoclinic phase at room temperature, but at temperatures higher than 1,175 °C it transforms to a tetragonal phase.

The transformation corresponds to altered characteristics that provide high component and flexural strength, exceptional wear resistance, and excellent durability. These highly desirable properties mean the tetragonal phase has a number of useful applications and it is often employed for structural ceramics in physically demanding applications. If the temperature rises beyond 2,370 °C, zirconia transforms into its cubic state.

Figure 1. Zirconia phase transformations. As temperature increases, zirconia transforms from monoclinic (a), to tetragonal (b) to cubic (c)².

The highly desirable tetragonal state of zirconia can be maintained through a doping process, involving the addition of oxides to the zirconia crystalline structure. Different oxides can be utilized to stabilize the higher temperature phases, such as magnesia (MgO), calcia (CaO), and ceria (CeO₂), although yttria (Y₂O₃) is the most frequently employed because of its high solubility in the zirconia lattice².

In the course of the doping process, some of the Zr⁴⁺ ions are substituted in the crystal lattice for the slightly larger Y³⁺ ions to create yttria-stabilized zirconia (YSZ)^4,5. YSZ maintains the tetragonal phase at room temperature, meaning that it exhibits all of the desirable characteristics of the zirconia tetragonal phase at room temperature, which makes it okay suitable for use at normal operating conditions.

The amount of yttria dopant needed to stabilize the zirconia can be adjusted to create different crystalline structures, depending on the properties required of the ceramic end-product. For good fracture toughness, high strength and wear resistance for example, 3 mol % YSZ (3YSZ) is widely used in structural ceramic applications.

Reducing the yttria (2 mol % YSZ (2YSZ)) results in higher fracture toughness. It is possible to also maintain the other desirable properties of 3YSZ if manufactured using Emulsion Detonation Synthesis (EDS), which is a proprietary process from Innovnano. This 2YSZ can therefore be a great alternative to conventional 3YSZ, which combines good stability and aging resistance with excellent fracture toughness, while maintaining high flexural strength.

Coloring Ceramics

The color of an end-product ceramic may also be important, as well as mechanical properties such as fracture toughness. In watch-making and jewelry for example this is significant.

The color can be changed by exposing zirconia materials to reducing environments⁶. Alternately, zirconia color can also be tuned with small additions of various oxides to the starting ceramic powder. Various metal oxides have been badessed as dopants, with CeO₂ and Fe₂O₃ being considered the strongest options because they induce the least adverse effects on the mechanical properties of zirconia ceramics^7,8.

The process of Fe₂O₃ doping on YSZ ceramics was evaluated in a recent study from Holz et al⁹. The effects on mechanical properties and color were explored to present a method for the development of a new grade of YSZ beige ceramics without compromising on mechanical properties.

YSZ powders created by EDS were mixed with different compositions of Fe₂O₃ powder. Four different samples were produced – Y-TZP02 containing 0.2% Fe₂O₃, Y-TZP0 containing 0% Fe₂O₃, Y-TZP01 containing 0.1% Fe₂O₃, and Y-TZP04 containing 0.4% Fe₂O₃. The suspensions were then milled and dried before being uniaxially pressed and sintered. The sintered ceramic samples were then characterized by their structural, microstructural, optical (color) and mechanical properties (Table 1).

It should be noted that for the purpose of this experiment, laboratory samples were used that did not contain any binder, which usually helps during the pressing stage. The slight deviation in some of the mechanical properties could be due to a defect in the laboratory sample. Completing the full sample preparation and powder treatment procedures on an industrial scale with binder and spray drying the powder will significantly improve the mechanical properties.

Table 1. Summary of some key properties of the sintered ceramics

A complementary study by energy-dispersive X-ray spectroscopy (EDXA) confirms a good homogenization of the elements, without segregation of any secondary phase (Figure 2). SEM micrographs (Figure 2) showed uniform microstructure and a relative density of >96% that was unaffected by the addition of Fe₂O₃ for color modifications. Additionally, the grain size was also found to be unaffected by Fe₂O₃ addition.

Figure 2. SEM micrographs of sintered ceramics Y-TZP0 (A), Y-TZP01 (B), Y-TZP02 (C) and Y-TZP04 (D) show a uniform microstructure. EDXA correlated individual maps of iron (D1) and zirconium elements (D2) show a good homogenization of the elements.

Figure 3 exhibits the different colors generated with different concentrations of Fe₂O₃ dopant. The color of the sample becomes darker as the dopant concentration increases. Thermal treatments were undertaken to make sure that no changes in color happen when samples are subjected to different temperatures.

Results established that Fe₂O₃ doping is a permanent and controllable way to color zirconia that is suitable for a variety of conditions and temperatures, including high-temperature applications.

Figure 3. Digital photographs of the Fe₂O₃ doped YSZ samples.

Maintaining Mechanical Properties

The effect of Fe₂O₃ doping on biaxial flexural strength and hardness was studied, and showed that good mechanical properties are maintained throughout the coloring process. Conserving mechanical properties while coloring zirconia is crucial. Fe₂O₃ doping decreased the fracture toughness slightly (Table 1) of zirconia ceramics, while hardness experiments (HV10) showed no dependence on Fe₂O₃ content and outstanding values, suggesting that hardness and flexural strength are not affected by this coloring method.

As a result of the outstanding fracture toughness of 2YSZ produced by EDS synthesis¹, the critical mechanical properties of the sintered ceramics remain largely unaffected by the addition of Fe₂O₃ (Figure 4). Using this unique synthesis approach, a defined cycle of rapid quenching, high temperatures, and pressures is carried out by a fully automated system, which is based on the detonation of two water-in-oil emulsions in a single step reaction.

The energetic nature of EDS helps towards the stabilization of the zirconia, a process that has been tested extensively. The resultant powders have a nanostructure – with increased specific surface area because of smaller grain sizes – to which the improved structural properties of fracture toughness, hardness, flexural strength and resistance to thermal shock are attributed.

Using EDS, 2YSZ (and other ceramics powders) is created with enhanced mechanical properties which can be colored while keeping its highly desirable characteristics. Additionally, the mechanical properties of both Fe₂O₃-doped and undoped, colored 2YSZ can be improved further with additional pressing stages such as hot isostatic pressing (HIP) or cold isostatic pressing (CIP).

Figure 4. Schematic representation of Emulsion Detonation Synthesis – EDS. This proprietary process to Innovnano uses high temperatures and high pressures for the production of nanostructured ceramic powders.

Summary

It is possible to produce colored zirconia ceramics without compromising on important mechanical properties by using EDS-synthesized YSZ doped with Fe₂O₃. This ability to produce components with flexural strength, high fracture toughness, and hardness in a range of colors, is very useful in watch-making and jewelry manufacturing.

The permanent nature of the coloring method means that the ceramic end-products are available for utilization across a range of conditions, even at high temperatures.

References

1. S. Shukla and S. Seal, “Mechanisms of room temperature metastable tetragonal phase stabilisation in zirconia,” Int. Mater. Rev., vol. 50, no. 1, pp. 45–64, 2005.
2. Ricca, C., Ringued, A., Cbadir, M., Adamo, C. & Labat, F. A comprehensive DFT investigation of bulk and low-index surfaces of ZrO2 polymorphs. J. Comput. Chem. 36, 9–21, 2015.
3. J. J. Swable, “Role of Oxide Additives in Stabilizing Zirconia for Coating Applications,” US, 2001.
4. B. Basu, “Toughening of yttria-stabilised tetragonal zirconia ceramics,” International Materials Reviews, vol. 50, no. 4, Kanpur, India, pp. 239–256, 2005.
5. R. M. Nunes Soares, “Phd Thesis – Development of Zirconia based phospors for application in lighting and as luminescent bioprobes,” University of Aveiro, 2013.
6. H. Zhang, B. Kim, and K. Morita, “Effect of sintering temperature on optical properties and microstructure of translucent zirconia prepared by high-pressure spark plasma sintering,” Sci. Technol. Adv Mater., vol. 55003, 2011.
7. I. Denry and J. R. Kelly, “State of the art of zirconia for dental applications,” Dent. Mater., vol. 24, no. 3, pp. 299–307, 2008.
8. N. Wen et al., “The Color of Fe₂O₃ and Bi₂O₃ Pigmented Dental Zirconia Ceramic,” Key Eng. Mater., vol. 435, pp. 582–585, 2010.
9. L. Holz et al., “ Effect of Fe₂O₃ doping on colour and mechanical properties of Y-TZP ceramics,” Ceramic International IN PRESS

This information has been sourced, reviewed and adapted from materials provided by Innovnano.

For more information on this source, please visit Innovnano.