Colored Zirconia-Based Ceramics without Compromise
The ability to color ceramics used in jewelry and watchmaking without compromising on the mechanical properties is vital.
Yttria-stabilized zirconia (YSZ) ceramics find applications in a wide range of industries, from super-tough, hardwearing structural ceramics in extreme environments to the intricate components and features used in the creation of jewelry and watches. In the case of jewelry and watchmaking, optical and mechanical properties are both important to ensure that a desirable and functional product is produced. In particular, the ability to color ceramics without compromising on the mechanical properties is vital.
Stabilized zirconia offers unique physicochemical, electrical, and mechanical properties that make it an extraordinary ceramic material of great interest for a wide range of applications and industries. Pure zirconia is found as a natural mineral called baddeleyite and can exist in one of three states depending on the temperature.1
At room temperature, zirconia exists in its monoclinic phase, but it transforms to a tetragonal phase at temperatures higher than 1,175°C. This transformation corresponds to altered properties that provide exceptional wear resistance, high component and flexural strength, and excellent durability. Because of these highly desirable properties, the tetragonal phase has many applications and is often used for structural ceramics in physically demanding applications. If the temperature increases beyond 2,370°C, zirconia transforms into its cubic state.
It is possible to maintain the desirable tetragonal state of zirconia through a process termed doping, which involves the addition of oxides to the zirconia crystalline structure. Different oxides can be used to stabilize the higher temperature phases, including calcia (CaO), magnesia (MgO), and ceria (CeO2). Yttria (Y2O3) is the most commonly used as a result of its high solubility in the zirconia lattice.2
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).3,4 YSZ exhibits all of the desirable properties of the zirconia tetragonal phase at room temperature, making it suitable for use at normal operating conditions.
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. For example, 3 mol% YSZ (3YSZ) is widely used in structural ceramic applications and has good fracture toughness, as well as high strength and wear resistance. Using less yttria (2 mol% YSZ (2YSZ)) results in an increase in fracture toughness.
If manufactured using a proprietary synthesis technology,* it is possible to also maintain the other desirable properties of 3YSZ. This particular 2YSZ can therefore provide an alternative to conventional 3YSZ, one that combines excellent fracture toughness with good stability and ageing resistance while maintaining high flexural strength.
*Innovnano-developed emulsion detonation synthesis (EDS).
In addition to mechanical properties such as fracture toughness, the color of a ceramic component may also be important. In jewelry and watchmaking, for example, color is a significant concern.
It is possible to modify color by exposing zirconia materials to reducing environments.5 Alternatively, zirconia color can also be tuned with small additions of various oxides to the starting ceramic powder. A number of metal oxides have been tested as dopants, with Fe2O3 and CeO2 being considered the best options. This is due to the fact that they induce the least adverse effects on the mechanical properties of zirconia ceramics.6,7
A recent study from Holz et al. evaluated the process of Fe2O3 doping on YSZ ceramics.8 The effects on color and mechanical properties were investigated to present a method for the development of a new grade of YSZ beige ceramics that doesn’t compromise on mechanical properties.
YSZ powders produced by EDS were mixed with different compositions of Fe2O3 powder. Four different samples were produced: Y-TZP0, containing 0% Fe2O3; Y-TZP01, containing 0.1% Fe2O3; Y-TZP02, containing 0.2% Fe2O3; and Y-TZP04, containing 0.4% Fe2O3. The suspensions were milled and dried before being uniaxially pressed and sintered. The sintered ceramic samples were then characterized in terms of their microstructural, structural, optical (color) and mechanical properties (see Table 1).
SEM micrographs demonstrated uniform microstructure and a relative density of > 96% that was unaffected by the addition of Fe2O3 for color modifications (see Figure 1). A complementary study by energy-dispersive X-ray spectroscopy (EDXA) confirmed a good homogenization of the elements without segregation of any secondary phase. In addition, the grain size was also shown to be unaffected by the Fe2O3 addition.
Figure 2 shows the different colors produced with different concentrations of Fe2O3 dopant. As the dopant concentration increases, the color of the sample becomes darker. Thermal treatments were carried out to ensure that no changes in color occur when samples are subjected to different temperatures. Results showed that Fe2O3 doping is a controllable and irreversible method for coloring zirconia that is suitable across a range of temperatures and conditions, including high-temperature applications.
Maintaining Mechanical Properties
The ability to maintain mechanical properties while coloring zirconia is key. The effect of Fe2O3 doping on hardness and biaxial flexural strength was investigated and showed that good mechanical properties are preserved throughout the coloring process.
Although Fe2O3 doping slightly decreased the fracture toughness of zirconia ceramics, hardness experiments (HV10) showed outstanding values and no dependence on Fe2O3 content. This suggests that flexural strength and hardness are not affected by this coloring method.
The important mechanical properties of the sintered ceramics remain largely unaffected by the addition of Fe2O3 as a result of the fracture toughness of 2YSZ produced by EDS synthesis (see Figure 3). 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. In addition, the mechanical properties of both undoped and Fe2O3-doped colored 2YSZ can be further improved with additional pressing stages, such as cold isostatic pressing (CIP) or hot isostatic pressing (HIP).
It is possible to produce colored zirconia ceramics using EDS-synthesized YSZ doped with Fe2O3 without compromising on important mechanical properties. This ability to produce components with high fracture toughness, hardness and flexural strength—in a range of colored shades—is useful in watchmaking and jewelry manufacturing. The irreversible nature of the coloring method means that the ceramic end products are suitable for use across a range of conditions, even at high temperatures.
- 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.
- J.J. Swable, “Role of Oxide Additives in Stabilizing Zirconia for Coating Applications,” US, 2001.
- B. Basu, “Toughening of Yttria-Stabilised Tetragonal Zirconia Ceramics,” Int. Mater. Rev., vol. 50, no. 4, pp. 239-256, 2005.
- R.M. Nunes Soares, “Ph.D. Thesis: Development of Zirconia Based Phospors for Application in Lighting and as Luminescent Bioprobes,” University of Aveiro, 2013.
- 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.
- 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.
- N. Wen, et al., “The Color of Fe2O3 and Bi2O3 Pigmented Dental Zirconia Ceramic,” Key Eng. Mater., vol. 435, pp. 582-585, 2010.
- L. Holz et al., “Effect of Fe2O3 Doping on Colour and Mechanical Properties of Y-TZP Ceramics,” Ceramic International, July 6, 2018, www.journals.elsevier.com/ceramics-international.