Oct 9 2001
Zirconias, when partially stabilized with either Yttria or Magnesia, exhibit excellent mechanical properties at high temperatures and are relatively inert to adverse environments.
They are used in refractory applications, thanks to their good thermal shock and impact resistance. Since zirconias are resistant to attack by most molten metals, they are often used as crucible materials.
Their excellent hardness and wear resistance make them suitable for high temperature nozzles and extrusion dies. At higher temperatures, zirconias become conducting and are therefore used as heating elements. They also find application as thermal barrier coatings for gas turbine blades.
Background
The fundamental properties of zirconia ceramics which are of interest to the engineer or designer are:
• High strength,
• High fracture toughness,
• High hardness,
• Wear resistance,
• Good frictional behaviour,
• Non-magnetic,
• Electrical insulation,
• Low thermal conductivity,
• Corrosion resistance in acids and alkalis,
• Modulus of elasticity similar to steel,
• Coefficient of thermal expansion similar to iron.
Types of Zirconias
There are many different types of zirconias. These have evolved as researchers and manufacturers sought to exploit the different properties of the various phases. Some of the phases are stable at high temperatures and need to be “frozen” in such that they can be used at room temperatures, while others exploit toughening mechanisms that are only found in these and few other materials. Some of these materials are listed below along with their typical abbreviations.
Table 1. The different types of zirconias.
| Material |
Abbrev. |
| Tetragonal Zirconia Polycrystals |
TZP |
| Partially Stabilised Zirconia |
PSZ |
| Fully Stabilised Zirconia |
FSZ |
| Transformation Toughened Ceramics |
TTC |
| Zirconia Toughened Alumina |
ZTA |
| Transformation Toughened Zirconia |
TTZ |
Materials (oxides) added to stabilise or toughen the zirconia will also be noted as a prefix to the abbreviations listed in table 1. They will sometimes be used in conjunction with numbers which indicate the amount of the stabilising agent added. Typical examples include Y, Ce, Mg and Al which correspond to yttria (Y2O3), ceria (CeO2), magnesia (MgO) and alumina (Al2O3) respectively. So a material denoted as 3Y-TZP would tetragonal zirconia polycrystal with an addition of 3mol% Y2O3 as a stabiliser.
Property Comparison
Table 2. lists properties for various grades of zirconia and has been compiled from a variety of sources. However, as with most ceramic materials properties are dependent on many factors such as starting powders and fabrication techniques. Most ceramic fabrication techniques have been applied to zirconias such as dry pressing, isostatic pressing, injection moulding, extrusion and tape casting. Addition of impurities during processing may also introduce flaws and degrade properties.
Table 2. Typical properties of various types of zirconia.
| Property |
Y-TZP |
Ce-TZP |
ZTA |
Mg-PSZ |
3Y20A |
| Density (g.cm-3) |
6.05 |
6.15 |
4.15 |
5.75 |
5.51 |
| Hardenss (HV30) |
1350 |
900 |
1600 |
1020 |
1470 |
| Bend Str. (MPa) |
1000 |
350 |
500 |
800 |
2400 |
| Compressive Str. (MPa) |
2000 |
- |
- |
2000 |
- |
| Young’s Modulus (GPa) |
205 |
215 |
380 |
205 |
260 |
| Poisson’s Ratio |
0.3 |
- |
- |
0.23 |
- |
| Fracture Toughness (MPa.m-1/2) |
9.5 |
15 - 20 |
4 - 5 |
8 - 15 |
6 |
| Thermal Exp. Co-Eff (x10-6 °C-1) |
10 |
8 |
8 |
10 |
9.4 |
| Thermal Conductivity (W.m-1.K-1) |
2 |
2 |
23 |
1.8 |
3 |
Primary author: AZoM.com