Colored Ceramics Without Compromises

Yttria-Stabilized Zirconia (YSZ) ceramics are utilized in a number of industries – from intricate components and features used in jewelry and watch-making, to extremely tough, hardwearing structural ceramics used in extreme environments.

Mechanical and optical properties are both key in the case of watch-making and jewelry to guarantee a functional and desirable product is made. The capability to color ceramics without compromising on the mechanical properties is especially important.

Why Zirconia?

Stabilized zirconia possesses unique mechanical, physicochemical, and electrical characteristics that make it an exceptional ceramic material of considerable interest for a wide range of industries and applications.

Pure zirconia exists as a natural mineral called baddeleyite and can be observed in one of three states dependent on its temperature¹ (Figure 1).

1. At room temperature, zirconia exists in its monoclinic phase
2. At temperatures over 1,175 °C it transforms to a tetragonal phase. This transformation corresponds to modified properties that offer exceptional wear resistance, excellent durability, and high component and flexural strength. Due to these advantageous properties, the tetragonal phase has many applications and is often used for structural ceramics in physically demanding applications.
3. Zirconia transforms into its cubic state if the temperature rises above 2,370 °C.

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

Through a doping process, the highly sought after tetragonal state of zirconia can be kept. This requires oxides to be added to the zirconia crystalline structure. Various oxides can be employed to stabilize the higher temperature phases, including ceria (CeO₂), calcia (CaO), and magnesia (MgO), but yttria (Y₂O₃) is used most often, because it has high solubility in the zirconia lattice³.

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

Depending on the required properties of the ceramic end-product, the quantity of yttria dopant employed to stabilize the zirconia can be changed to produce different crystalline structures. So, 3 mol % YSZ (3YSZ) is used extensively in structural ceramic applications and is strong, has wear resistance, and good fracture toughness.

Using a smaller amount of yttria (2 mol % YSZ (2YSZ)) results in an increase in fracture toughness. If made using Emulsion Detonation Synthesis (EDS), the other desirable properties of 3YSZ can also be maintained. Therefore, this 2YSZ can offer an alternative to conventional 3YSZ; one that combines excellent fracture toughness with good stability and aging resistance, while maintaining high flexural strength.

Coloring Ceramics

As well as mechanical properties like fracture toughness, the color of an end-product ceramic may also be important. This is a significant concern in jewelry and watch-making for example. Color can be modified by exposing zirconia materials to reducing environments⁶. Another option is to tune the zirconia color with small additions of various oxides to the starting ceramic powder.

Many metal oxides have been evaluated as dopants, with Fe₂O₃ and CeO₂ being seen as the best options. They cause the least adverse effects on the mechanical properties of zirconia ceramics^7,8.

Holz et al⁹ carried out a recent study to evaluate the process of Fe₂O₃ doping on YSZ ceramics. This was to clarify the effects on color and mechanical properties, to create a method for the development of a new grade of YSZ beige ceramics that doesn’t affect mechanical properties.

YSZ powders produced by EDS were mixed with different compositions of Fe₂O₃ powder. Four different laboratory 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.

Next, 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 (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

Composition	% Fe2O3	Grain size (nm)	HV10 (MPa) ± STD	σflexural (MPa) ± STD
Y-TZP0	0	398	1235 ± 18	1050 ± 125
Y-TZP01	0.1	378	1225 ± 10	1070 ± 118
Y-TZP02	0.2	372	1226 ± 11	853 ± 131
Y-TZP04	0.4	368	1213 ± 18	1136 ± 97

SEM micrographs (Figure 2) illustrated uniform microstructure and a relative density of >96% that was unaltered by the addition of Fe₂O₃ for color modifications. A study by energy-dispersive X-ray spectroscopy (EDXA) confirms a good homogenization of the elements, without segregation of any secondary phase (Figure 1). Furthermore, the grain size was also observed to be unchanged 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.

The different colors created with different concentrations of Fe₂O₃ dopant are observed in Figure 3. As the concentration of dopant increases, the sample color gets darker. Thermal treatments were performed to confirm that no color changes happen when samples are subjected to different temperatures.

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

Results showed that Fe₂O₃ doping is an irreversible and controllable technique for coloring zirconia which can be used across a variation of temperatures and conditions, including high-temperature applications.

Maintaining Mechanical Properties

Conserving the mechanical properties while coloring zirconia is key. The effect of Fe₂O₃ doping on biaxial flexural strength and hardness was explored and it was found that good mechanical properties are maintained throughout the coloring process.

Whilst Fe₂O₃ doping slightly reduced the fracture toughness (Table 1) of zirconia ceramics, outstanding values and no dependence on Fe₂O₃ content were shown by hardness experiments (HV10). This proposes that flexural strength and hardness are unaffected by this method of coloring.

The key mechanical properties of the sintered ceramics are unaffected by the addition of Fe₂O₃ because of the outstanding fracture toughness of 2YSZ produced by EDS synthesis (Figure 4). A defined cycle of pressures, high temperatures, and rapid quenching is utilized by a fully automated system, based on the detonation of two water-in-oil emulsions in a single step reaction. The energetic nature of EDS assists the zirconia stabilization, a process that has been extensively tested.

The end powders have a nanostructure – with higher specific surface area due to reduced grain sizes – to which the improved structural properties of hardness, resistance to thermal shock, fracture toughness, and flexural strength are attributed.

2YSZ (and other ceramics powders) is made with increased mechanical properties that can then be colored, whilst maintaining these highly sought after properties by utilizing EDS. Furthermore, the mechanical properties of both undoped and Fe₂O₃-doped, colored 2YSZ can be enhanced further with more pressing stages such as cold isostatic pressing (CIP) or hot isostatic pressing (HIP).

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

Colored zirconia ceramics can be produced by using EDS-synthesized YSZ doped with Fe₂O₃ without affecting important mechanical properties. The capability to create components in a range of colored shades, with high fracture toughness, hardness and flexural strength is very useful in jewelry manufacturing and watchmaking. The ceramic end-products are suitable for utilization across a range of conditions, even at high-temperatures, due to the irreversible nature of the coloring method.

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., Cassir, 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.