Zirconium dioxide, also known as zirconia and zirconium oxide, is a crystalline metal oxide that has found its way into the ceramics industry. It is characterised by its high thermal resistivity, mechanical resistance, and abrasive properties.
One of its most popular forms is cubic zirconia, a cubic crystalline compound that is colourless and mechanically tough. Because of its optically flawless property, it serves as a low-cost alternative to diamonds in the jewellery industry.
The substance should not be confused with zircon (or zirconium silicate), a mineral that is also used in the ceramics industry and refractories.
Zirconium dioxide properties
High mechanical resistance
Zirconium dioxide is highly resistant to cracking (including further development of cracks) and mechanical stress.
- Elastic modulus = 210 GPa
- Flexural strength = 800 MPa (min, 4-point bending, 40/20mm)
- Fracture toughness = 10 MPa·√m
- Hardness, Vickers = 1220 [-]
High temperature resistance and expansion
At a melting point of 2715°C and a thermal expansion coefficient of 1.08E-5 /K, zirconium dioxide is widely known for its high resistance to heat. This is the reason why the compound has found a wide variety of uses in refractories and high-temperature industries.
Low thermal conductivity
Zirconium dioxide has a thermal conductivity of 2 W/(m·K), which makes it perfect for situations where heat needs to be contained.
The substance is chemically inert and unreactive, which works in industries that make use of several chemicals during processing. However, the compound dissolves in concentrated acids such as sulfuric or hydrofluoric acid.
Production and processing
Production of zirconium dioxide may result in three possible phases depending on the temperature:
- Below 1170°C: Monoclinic
- 1170°C to 2370°C: Tetragonal
- Above 2370°C: Cubic
This unique property of zirconium dioxide provides flexibility of use in a wide variety of purposes and industries.
Zirconium dioxide is produced through thermal treatment, although doing it in its pure form may cause abrupt phase changes that may crack or fracture the material. To combat this, doping with magnesium dioxide, yttrium oxide, or calcium oxide are added in order to keep the structure intact.
High-purity zirconium dioxide is the precursor for producing zirconium powders, through the reduction of ZrO2 with calcium hydrate. This calciothermic process is prepared under an argon atmosphere at continuous heat at about 1000°C.
Applications of zirconium dioxide
The mechanical strength and resistance of zirconium dioxide makes it a suitable component for ceramic manufacturing. This includes ceramic knives, which are noticeably tougher than steel-edged cutlery due to the high hardness factor of zirconium dioxide.
Due to its high thermal resistance, zirconium dioxide is used as a component in crucibles, furnaces, and other high-heat environments. In addition, zirconium dioxide boosts the fireproof properties of ceramics.
Thermal barrier coating (TBC)
Zirconium dioxide is applied as a coating for jet engine components which are exposed to high temperatures. This is made possible through the compound’s low thermal conductivity and high heat resistance. Studies have confirmed the effectiveness of zirconium dioxide for TBC applications, as long as the material is applied properly and uniformly.
One of the most popular uses of zirconium dioxide is in dentistry, mainly in dental restoration for bridges, crowns, and feldspar porcelain veneers and dental prostheses. Yttria-stabilized zirconium dioxide is also instrumental in producing near-permanent ceramic crowns.
Abrasive property and scratch resistance
With its elevated mechanical stability and abrasion resistance, it is being used as abrasive material. It is also useful as a protective layer for mechanical parts, due to the compound’s resistance to scratches and mechanical stress.
While other materials may experience oxidation and compromise its integrity, zirconium dioxide is stable in the presence of oxygen. In fact, it is being used in fuel cell membranes and oxygen sensing mechanisms even at elevated temperatures.
Cubic zirconia, in particular, has evolved as a viable alternative to diamond (which is extremely expensive). Aside from its durability and strong aesthetic similarity to diamond, cubic zirconia produces cuts unlike diamonds and has an optical flawlessness that appears completely colourless to the naked eye.