Introductory Chapter: Introduction to Zirconia Ceramic – A Versatile and Durable Material with a Wide Range of Applications

1. Introduction

Zirconium oxide, also known as zirconia, is a ceramic material that has many important properties, including high strength, good toughness, and excellent corrosion resistance [1, 2]. It is a white crystalline material made from the mineral zircon found in various parts of the world. Zirconia exhibits numerous attributes that hold significance within industrial applications including use in the manufacture of ceramics, abrasives, and refractories, and as a structural material in the aerospace and automotive industries [3]. In addition to its practical uses, zirconia is also known for its beautiful, diamond-like appearance, which has made it a popular use in jewelry and other decorative items [4].

Zirconia possesses a multitude of distinct properties that render it a compelling material for a diverse range of applications [5]. With an exceptionally elevated melting point, zirconia demonstrates remarkable resistance to elevated temperatures and thermal shocks. Furthermore, its exceptional performance as an electrical insulator and its minimal coefficient of thermal expansion contribute to its resilience against thermal stresses. Beyond these qualities, zirconia’s notable strength and hardness make it a prime candidate for structural applications. Moreover, its pronounced corrosion resistance positions it as a superb option for deployment in demanding environmental conditions [3].

When it comes to its manufacturing process, zirconia is created by subjecting zircon sand to exceptionally elevated temperatures within a specialized furnace. This procedure yields zirconium oxide as the final product [6]. The technique employed is known as sintering, which yields a compact and durable substance amenable to various configurations. Once crafted, zirconium oxide finds diverse applications based on particular traits and needs. Notably, it finds utility in crafting cutting implements and components resistant to wear, and it serves pivotal roles within the aerospace, automotive, and medical sectors. Its biocompatibility renders it valuable for producing medical equipment, including dental implants and other healthcare devices [7].

2. History of zirconia

Zirconia possesses a captivating and extensive historical background. Its origins trace back to the eighteenth century, marked by the pioneering work of German scientist Martin Heinrich Klaproth. He succeeded in extracting zirconia from the mineral zircon, an enduringly precious gemstone with a centuries-old legacy [6, 8]. In the nineteenth century, zirconia was first used as an abrasive, and it was later found to possess several unique properties that made it useful for a wide variety of applications. One of the earliest uses of zirconia was in the manufacture of abrasives and it was used to make grinding wheels and other abrasive tools. In the twentieth century, zirconia began to be used in ceramic manufacture, and proved to be an excellent material for use in structural applications due to its high strength and toughness. Zirconia has also been shown to have excellent corrosion resistance and a low thermal expansion coefficient, making it an ideal material for use in harsh environments. Zirconia has continued to be a popular and widely used material in the twenty-first century [9]. In the medical field, zirconia has been used to manufacture dental implants, as well as other medical devices such as prosthetics and surgical tools. In the aerospace industry, zirconia has been used as a structural material due to its high strength and corrosion resistance. It has also been used in the production of cutting tools, wear-resistant parts, and other industrial components [10, 11].

3. Structure of zirconia

Zirconia displays polymorphism across varying temperatures, manifesting itself in three distinct shapes (Figure 1): monoclinic or baddeleyite (from room temperature up to 1170°C), tetragonal (1170–2370°C), and cubic (2370–2700°C, aligning with its melting point) [12]. At room temperature, zirconia exists in a monoclinic crystalline structure, but it can also exist in other crystalline structures depending on the temperature and pressure conditions. One of the unique properties of zirconia is that it can undergo a phase transition from monoclinic structure to a tetragonal structure under severe stress [13]. This alteration in phase brings about heightened toughness and ductility in the material, making it an ideal material for use in structural applications where it may be subjected to high stresses. Beyond its crystalline arrangement, zirconia’s notable attributes encompass elevated density and hardness, qualities attributed to robust chemical bonds. These characteristics collectively bestow zirconia with remarkable potency and resilience, positioning it as an optimal choice across an extensive array of applications [14].

Figure 1.

Structure of zirconia.

Zirconia boasts an intricate crystalline arrangement, composed of zirconium oxide molecules arranged in a repetitive pattern [15]. This material exhibits refractory qualities, indicating a remarkable resistance to chemical reactions and an elevated melting point. At ambient conditions, zirconia takes on a monoclinic crystalline structure, characterized by the intersection of two axes at an angle. This configuration remains stable at room temperature and is marked by low density and strength. Yet, under substantial loads, zirconia can undergo a reversible phase transition, shifting from monoclinic to tetragonal. This phase transition is reversible and occurs when the material is stressed beyond a certain threshold [16]. The tetragonal structure exhibits superior density and strength compared to the monoclinic form, and it also demonstrates greater ductility, rendering it more resilient against cracks and fractures when burdened. Beyond its crystalline arrangement, zirconia is distinguished by high density and hardness, attributes attributed to robust chemical bonds within the material. These intrinsic characteristics bestow upon zirconia exceptional strength and endurance, rendering it an optimal choice for a wide array of applications. Moreover, zirconia functions as an outstanding electrical insulator and exhibits a low coefficient of thermal expansion, which safeguards it against thermal strains and positions it as a prime candidate for high-temperature environments [17]. Collectively, zirconia’s intricate structure bequeaths it with a distinctive amalgamation of traits, establishing its significance across diverse industries.

Zirconia’s distinct attributes are also influenced by a range of structural properties [18]. These encompass its microstructure, grain size, and defects. A material’s microstructure consists of the arrangement of its atoms and their spatial relationships. In the case of zirconia, its microstructure can exhibit variability contingent upon the specific production conditions employed. For instance, when zirconia is manufactured via sintering, its microstructure often assumes a more porous nature, yielding a heightened surface area. Conversely, the application of hot isostatic pressing yields zirconia with a denser microstructure and a diminished surface area [19]. Notably, zirconia’s microstructure holds considerable sway over its attributes, including strength and toughness [20].

4. Types of zirconia

There are different types of zirconia used in different applications. The most common types of zirconia include: (1) Monoclinic zirconia: This is the most stable form of zirconia at room temperature and is widely used in grinding and cutting tools, as well as in ceramic and refractory applications, (2) Tetrahedral zirconium: This form of zirconium undergoes a phase transition from monoclinic to tetragonal at a certain temperature known as the “transition temperature” [21]. Quadrilateral zirconia has higher strength and toughness than monoclinic zirconia, making it suitable for applications where these properties are important, (3) Cubic zirconia: This type of zirconia has a high refractive index and is often used as a substitute for diamond in jewelry. It is also used in high temperature thermal protection tubes and other grinding and high temperature applications, and (4) Stable zirconia: This type of zirconia is made by adding small amounts of other elements such as yttrium or magnesia to improve the stability and strength of the material. Stabilized zirconia is widely used in abrasive and high temperature applications, as well as in fuel cell electrodes and other advanced technologies. The type of zirconia used in a particular application depends on the specific properties and performance requirements for that application [22].

5. Production of zirconia

Zirconia, also referred to as zirconium dioxide, is synthesized through a procedure known as calcination. This process entails subjecting natural zircon, a mineral containing zirconium, to elevated temperatures in the presence of oxygen [23]. This chemical reaction transforms zircon into zirconia (Figure 2). The initial stage in zirconia production involves the extraction of zircon from natural deposits, usually undertaken within mining operations that involve excavating the earth to access zircon reservoirs [25]. Once extracted, zircon is purified to eliminate impurities. The refined zircon is then finely ground and combined with other substances like alumina or magnesia to generate a uniform mixture. This blend is introduced into a calcination furnace, where it is subjected to high temperatures (typically around 2000°C) in an oxygen-rich environment. During this calcination process, zircon reacts chemically with oxygen, resulting in its transformation into zirconia. Subsequently, the zirconia powder formed is cooled and collected for subsequent procedures [24]. Following calcination, additional processing steps, such as pressing and sintering, may be applied to the zirconia powder. These steps help shape the material into its desired form while achieving specific properties. Eventual zirconia products find application across a broad spectrum of uses, encompassing abrasives, ceramics, refractories, and cutting-edge technologies.

Figure 2.

The production processes of zirconia [24].

Multiple techniques exist for producing zirconia, with their suitability contingent upon the distinctive attributes and performance prerequisites of the product. Further insights into zirconia production encompass:

Following calcination, the zirconia powder can undergo additional refinement via “hot pressing” or “hot isostatic pressing.” This technique entails pressing the zirconia powder under elevated pressure and temperature, resulting in a compact, robust structure with heightened strength and toughness. An alternate method, termed “sintering,” involves subjecting the zirconia powder to elevated temperatures (typically around 1600 to 1800°C) in a dedicated sintering furnace. As the powder heats, its particles coalesce, yielding a consolidated piece characterized by improved density and strength. Beyond hot pressing and sintering, another avenue for zirconia processing is “reaction bonding.” In this approach, zirconia powder is blended with a metal oxide like alumina or magnesia, and the mixture is then subjected to high temperatures. The ensuing chemical reaction establishes a bond between the zirconia and the metal oxide, yielding a sturdy, fortified piece with heightened strength and toughness [26]. The selection of a specific zirconia production method hinges upon the distinct attributes and performance requisites of the final product [27]. Considerations encompass parameters like density, strength, toughness, and various other properties that influence the optimal manufacturing route.

6. Future developments in zirconia technology

There are several areas of research and development that are focused on advancing the technology of zirconia. Some potential future developments in zirconia technology include [28]:

1. Enhanced Strength and Toughness: Scientists are exploring novel processing techniques and additives to heighten the strength and toughness of zirconia. This endeavor aims to render it more apt for applications necessitating structural integrity and load-bearing capacities.

2. Augmented Corrosion Resistance: While zirconia boasts exceptional corrosion resistance, researchers are investigating avenues to further enhance this trait. This could potentially involve the development of fresh coatings or innovative additives.

3. Elevated Electrical and Thermal Conductivity: Zirconia’s prowess as an electrical insulator is recognized, yet researchers are delving into additives or doping methods to elevate its electrical conductivity for specific applications. Parallelly, efforts are underway to bolster zirconia’s thermal conductivity, perhaps through the incorporation of conductive additives or novel processing techniques.

4. Biomedical Innovations: Zirconia holds promise in the realm of biomedicine, potentially finding utility in medical implants and prosthetics. Researchers are directing their efforts toward optimizing zirconia’s surface attributes and biocompatibility for these medical applications.

7. Conclusion

Zirconia ceramic emerges as a resilient and versatile substance, renowned for its multifaceted properties such as elevated melting point, robust strength, notable toughness, minimal thermal expansion, chemical resilience, and remarkable wear resistance. This amalgamation of attributes equips it for a diverse array of applications, spanning industrial and biomedical domains. Its utility encompasses cutting tools, wear-resistant coatings, dental prosthetics like crowns and bridges, refractory components, catalysts, and abrasives. Furthermore, the burgeoning interest in leveraging zirconia within fuel cell technology is fuelled by its aptitude for oxygen ion conduction at elevated temperatures. Overall, zirconia stands as a highly advantageous and invaluable material, exerting its influence across an extensive spectrum of fields.