Zirconium Dioxide (Zirconia): Properties, Production and Applications
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.
First used in the medical industry in 1969, zirconia has demonstrated exceptional biocompatibility, with good tribological properties, good aesthetic, and high mechanical properties. It is used quite pre-eminently in dental procedures, as in zirconia crowns and zirconia-based implant abutments [1].
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.
Zirconium dioxide should not be confused with zircon (or zirconium silicate), a mineral that is also used in the ceramics industry and refractories.
Here, you will learn about:
- What zirconia is
- Properties of zirconia
- How zirconia is produced and processed
- The different application areas where zirconia excels
Dental drilling process.
What is zirconia?
Zirconia is a crystalline solid that is white in colour, but can be produced in different colours to be used as an alternative gemstone to diamond or as ceramic dental crowns in medical applications. Naturally, it occurs as the translucent (sometimes transparent) mineral baddeleyite, a rare mineral that has a monoclinic prismatic crystal structure; i.e. a mineral having unequal vectors. Also known as “ceramic steel”, this oxide of zirconium is chemically inert and is considered as one of the highly auspicious restorative materials, due to its excellent mechanical properties.
Out of all advanced ceramic materials, zirconia has the highest toughness and strength at room temperature. At high temperatures, zirconia may go through substantial change in volume during phase transformation. As a result, it is difficult to obtain stable zirconia products during sintering, which is why stabilisation of zirconia is generally required. Partially stabilised zirconia (PSZ) adds to the exceptional mechanical properties and chemical inertness a high level of chemical stability, even in harsh environments. It is used as a substitute for alumina in biomedical applications such as dental implants, thanks to its superior mechanical properties, and is comparable with teeth in terms of mechanical strength [2]. Other relative materials to PSZ include yttria-stabilised zirconia (YSZ), calcia-stabilised zirconia (CSZ), and magnesia-stabilised zirconia (MSZ).
Properties of zirconia
Zirconia’s exceptional strength, toughness, biocompatibility, high fatigue and wear resistance render it optimal for dental applications. Zirconium (Zr), in particular, is in fact one of the two most commonly used metals in dental implants, alongside titanium, as they both show very good physical and chemical properties and they allow the growth of osteoblasts, the cells that actually form bones [3]. Here’s a list of zirconia’s most prominent physical and chemical properties. Notice how these properties are high enough to allow zirconia to be an effective material for many applications, especially for refractory and dentistry purposes.
High mechanical resistance
Zirconium dioxide is highly resistant to cracking (including further development of cracks) and mechanical stress. Other outstanding mechanical properties of zirconia are shown in the table below.
|
Mechanical properties |
Values for ZrO2 |
|
100 – 250 GPa at 20ºC |
|
|
180 – 1000 MPa at 20ºC |
|
|
330 MPa at 20ºC |
|
|
10 MPa·√m at 20 °C |
|
|
1220 |
|
|
8 – 8.5 |
High temperature resistance and expansion
With a melting point of 2700ºC and a thermal expansion coefficient of 1.08×10-5 K-1, 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. Here are the different temperature ranges of melting point for zirconia, based on its temperature-dependent forms.
|
Zirconia’s temperature-dependent form |
Melting point |
|
Monoclinic, baddeleyte |
20 – 1170ºC |
|
Tetragonal |
1170 – 2370ºC |
|
Cubic |
2370 – 2700ºC |
Upon heating, however, zirconia may undergo phase change, especially in its tetragonal form, where internal stresses arise, and cracks begin to develop. In order to resolve and correct this weakness, stabilisers such as yttria are added to make up a more stable yttria partially stabilised zirconia (or yttria tetragonal zirconia polycrystal, YTZP) [4].
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.
Chemical resistivity
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 of zirconia
Production of zirconium dioxide may result in the aforementioned three possible phases depending on the temperature: monoclinic, tetragonal, and cubic. This unique property of zirconium dioxide provides flexibility of use in a wide variety of purposes and industries.
Zirconia is produced through thermal treatment, or thermal dissociation, although doing it in its pure form may cause abrupt phase changes that may crack or fracture the material. That is when doping with stabilisers, such as magnesium oxide, yttrium oxide, and calcium oxide, is applied to keep the structure intact. This thermal process is also referred to as calcination, where heating to high temperatures is performed within an oxygen or air medium.
Zirconia can also be produced by decomposing zircon sand via fusion with compounds such as calcium carbonate, calcium oxide, sodium carbonate, magnesium oxide, and sodium hydroxide (also known as caustic soda).
Chlorination of zircon also leads to the production of zirconia, where the resulting zirconium tetrachloride is calcined at a high temperature (~900ºC), producing a commercial grade of zirconia. Another way is to dissolve the collected zirconium tetrachloride in water to form crystallised zirconyl chloride. This resultant is then thermally treated at a high temperature to produce high-purity zirconia [5].
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
Zirconia’s high mechanical properties, chemical inertness, high-temperature stability, corrosion resistance, and high quality have put this ceramic steel on the radar in many industries and application areas. Many products of today, ranging from refractory to medical products, pigments, electronics, coatings, and ceramics, have been based on zirconia due to its superior characteristics and advantages as compared to other materials. Some of the typical applications of zirconia include dies for hot metal extrusion, oxygen sensors, membranes in fuel cells, deep well valve seats, and marine pump seals. Here is a list of some of zirconia’s most common applications areas and uses.
Ceramics
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 zirconia.
Refractory purposes
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. Refractory bricks and armour plates are examples of zirconia-based refractory applications. Furthermore, when added to melted quartz, zirconia can be used to produce siloxide glass, a harder and more stress resistant glass than quartz opaque glass [6]. Zirconia can also be added to aluminium oxide to be used in components for steel casting process.
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.
Dental industry
Due to its biocompatibility, good aesthetics, and high mechanical properties, one of the most popular uses of zirconium dioxide is in dentistry, mainly in dental restorations for bridges, crowns, and feldspar porcelain veneers and dental prostheses. Yttria-stabilized zirconium dioxide is also instrumental in producing near-permanent zirconia crowns.
Scratch resistant and abrasive material
With its elevated mechanical stability and abrasion resistance, zirconia is being used as an abrasive material. It is also useful as a protective layer for mechanical parts, due to the compound’s resistance to scratches and mechanical stress.
Oxygen-rich systems
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.
Jewellery industry
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. It is commonly referred to as a diamond imitation rather than a synthetic diamond, as it resembles natural diamond visually but does not have the same chemical properties. Examples of zirconia-based jewellery include cubic zirconia rings and cubic zirconia earrings.
A ceramic knife.
Sources
[1] "Zirconia in Dentistry", [Online] Available from: https://www.ddslab.com/zirconia-in-dentistry/, 2020.
[2] K. Shanmugam & R. Sahadevan, "Bioceramics: An Introductory Overview", Fundamental Biomaterials: Ceramics, Woodhead Publishing Series in Biomaterials, pp. 1-46, 2018.
[3] A.D. Bonna et al, "Zirconia as a Dental Biomaterial", Materials (Basel), MDPI, 8(8) pp. 4978–91, 2015.
[4] "Zirconia (ZrO2 or Zirconium Oxide)", [Online] Available from: https://www.ceramcoceramics.com/materials/zirconia/zirconia.php
[5] "Making Zirconia by Zirconium Chlorination", [Online] Available from: https://www.911metallurgist.com/making-zirconia/
[6] "The Use of Zirconia as a Refractory Material", Nature, 99, pp. 375–6, 1917.