Colored zirconia: It is exceedingly impressive!

In the era of 5G, the smartphone industry is undergoing a transformation, with zirconia ceramics emerging as a favored material for smartphone backs in this round of technological reshuffling. In 2018, the release of Xiaomi's phone in a color known as 'Jade Green' marked the beginning of the era of colored ceramic smartphones, gradually familiarizing the public with Colored Zirconia.

With the development of electronic communication and the improvement of people's living standards, colored zirconia ceramics, due to their excellent biocompatibility, superior metallic luster, and good mechanical properties, have found increasing applications in our daily lives, including in the fields of medical dental restorative materials, the decorative industry, and mobile smart terminal devices. However, adding 'color' to zirconia is not an easy task.

"The two coloring agents are mutually exclusive"

Currently, the colored zirconia available in the market primarily exhibits a rich variety of colors due to the addition of different rare earth elements, metallic elements, and oxides. The difficulty in preparing colored zirconia lies in the fact that the sintering temperature of zirconia ceramics typically ranges from 1550 to 1650°C. At high temperatures, many pigments or coloring agents can decompose or volatilize, losing their effectiveness; therefore, it is challenging to prepare vibrantly colored zirconia ceramics simply by adding pigments or coloring agents. Moreover, the decomposition of pigments can hinder the density of ceramic products, significantly reducing the inherent toughness of the ceramic materials, which adversely affects the quality of the finished products. Consequently, the preparation of colored ceramics and the development of new color varieties are critical issues of focus within the industry, as brightly colored, non-toxic ceramics with good mechanical properties have a very broad market application potential.

"The critical bottleneck in colored ceramics: Powder control and sintering technology"

1.Controlling Powder Quality is the key

The preparation of colored zirconia is primarily aimed at achieving a uniform distribution of the coloring agent within the zirconia matrix. This is particularly crucial for composite ceramics, especially nano-composites, as the small particle size and large specific surface area of both pigment and zirconia matrix particles lead to significant electrostatic attraction and van der Waals forces between them. Such conditions promote agglomeration of the pigment and zirconia matrix particles, a phenomenon that not only results in uneven coloration of the nano-composite ceramics but also impacts their mechanical performance.Thus, how to ensure a uniform dispersion of pigment particles within the zirconia matrix, thereby producing colored zirconia ceramics with excellent mechanical performance and color quality, hinges on overcoming the agglomeration of powder particles. To produce high-performance, diverse-colored zirconia ceramics, it is essential to identify appropriate dispersion methods. Commonly used preparation methods include the following:

• Solid Phase Mixing Method: This is the most widely used industrial method for preparing colored zirconia ceramics. It involves mixing and ball milling the coloring agents, mineralizers, and other oxide particles in a specific chemical ratio with stabilized zirconia nano-powder. Throughout this process, solid particles experience refinement, which leads to the formation of microcracks, lattice distortions, and an increased surface energy that facilitate low-temperature chemical reactions. This method boasts advantages such as simplicity, low cost, ease of operation, and potential for industrialization, yet it cannot resolve the issue of agglomeration of nano-particles.
• Chemical Co-precipitation Method: This method utilizes a mixture of zirconium salts, stabilizing agent salts, and coloring ion salts in solution, which reacts with bases or carbonates to form hydroxide or carbonate precipitates, subsequently heated to obtain zirconia composite powder. This process is relatively complex but yields high purity powders with excellent performance. It is important to note that the risk of hard agglomeration must be managed when using chemical precipitation methods.
•  Liquid Phase Impregnation Method: This is a novel approach for preparing colored zirconia ceramics. The advantage of this method lies in the uniform dispersion of coloring ions in the zirconia matrix, as well as the ability to fabricate both composite materials and gradient materials. Furthermore, different shapes of zirconia green bodies can be obtained via injection molding, which can subsequently be turned into various shapes of colored zirconia ceramics using liquid phase impregnation.

2. Sintering Methods

In addition to the differences in preparation methods affecting the properties of Zirconia Powder, the sintering method also influences the performance and color of colored zirconia ceramics. With the intersection of disciplines and advancements in technology, many new sintering methods have emerged alongside traditional sintering techniques:

• Electric Discharge Sintering：Researchers have conducted tests using electric discharge sintering, where the sintering temperature is the most significant factor affecting the toughness of zirconia ceramics, followed by the sintering time. The optimal sintering temperature determined through testing is 1400°C, with the best sintering duration being 5 minutes. Zirconia ceramics produced by this method exhibit high hardness and fracture toughness.
• Microwave Sintering：Compared to traditional sintering methods, microwave sintering presents irreplaceable advantages. It is a method of uniform heating where the material converts absorbed microwave energy into molecular kinetic and thermal energy, achieving overall heating of the material. The temperature gradient within the material is minimal, significantly reducing the risk of cracking due to uneven heating. The physical properties of zirconia produced by this sintering method are notably superior.

Colored zirconia color classification

In order to meet the demand for color-variable, performance-stable colored zirconia ceramics that are environmentally friendly and pollution-free in terms of materials and preparation processes, more than ten types of colored zirconia have been developed both domestically and internationally.

1.Red Ceramic System

The research status of red ceramic has not been very satisfactory, primarily due to the instability of the dye components, which cannot withstand high temperatures, or because the dyes appear vibrant but contain harmful substances for the human body. Furthermore, there are dyes that are relatively stable and free of harmful substances, but do not exhibit vivid colors. Research has found that using iron oxide (Fe₂O₃) as a coloring agent, combined with 3YSZ as the matrix, results in orange-red colored zirconia ceramics, with the highest redness value reaching 20 and accompanied by a high yellowness value; however, its color fails to meet the requirements for red. Additionally, the incorporation of iron oxide significantly reduces the mechanical properties of the 3YSZ system, yielding a fracture toughness of only 5.0 to 6.0 MPa m1/2, greatly limiting its industrial application. Consequently, red ceramics have become one of the rarest types of ceramics that cannot be mass produced. The Huawei Mate 60 RS Extraordinary Master, released by Huawei, features a design utilizing red ceramic. It is reported that Huawei introduced CeO2 rare earth elements into the red ceramic sintering process for the first time, with 60 days of meticulous craftsmanship, followed by high-temperature calcination at 1400℃ and over 50 production processes, which fixed this shade of 'red' under specific parameters, creating a uniform red appearance inside and out. This marks a milestone as the first mass production and commercialization of red ceramic in the mobile phone industry.

2.Black ceramic system

With the development of ceramics, various improvements have been made on the basis of black glaze materials, resulting in the black ceramic pigments that are now commonly seen in daily life. In recent years, due to the scarcity and high cost of cobalt oxide, alternatives have been sought to reduce costs. As a result, black zirconia ceramic coloring agents without cobalt have been prepared using MnO2, Fe2O3, and Cr2O3 as raw materials, yielding three different colored spinels: deep brown chromite spinel, dark red ferric manganese spinel, and dark green chromic manganese spinel. By adjusting the mixing ratios to control the content of each spinel, a stable black coloring agent can be produced as the three colors influence one another, which significantly reduces costs and enhances economic benefits.

However, during the preparation process of black zirconia ceramic, the multi-phase spinel structure is prone to react with the zirconia matrix, leading to the destruction of the spinel structure, a reduction in phase stability, increased susceptibility to cracking, and difficulties in shaping. Additionally, the color of the pigments produced from reduced zirconia powders is dark gray, which poses an even greater challenge for the preparation of a bright black.

3.Blue ceramic system

Cobalt is the earliest element developed for blue glaze, and to this day, the industry still primarily relies on cobalt oxide (Co₂O₃) to synthesize blue spinel compound pigments for further production of blue ceramics. Currently, the ceramic pigments in the blue color spectrum mainly include vanadium-zirconium blue pigments as colorants, cobalt-aluminum spinels, nickel-aluminum spinels, as well as spinel-type colorants that utilize other ions to replace cobalt ions, hexaaluminates, and lanthanide-based colorants. While ensuring color performance and mechanical properties, the ongoing research focuses on exploring environmentally friendly and economical blue colorant alternatives.