Zirconia A Wear Resistant Engineering Ceramic
Zirconia or Zirconium Oxide (ZrO2) is an important engineering ceramic used primarily for its high impact resistance or toughness and resistance to thermal shock. Coupled with relatively high wear resistance, corrosion resistance and high temperature stability it can be used in a wide range of applications and industries from laboratory, to metallurigcal, chemical and refractory.
The most common form of Zirconia is stabilised with Yttria (usually 3YZ or Y-PSZ) as this dopant allows for the enhanced properties of toughness and thermal shock resistance by preventing crack propagation (a common failure mechanism in ceramics) thus making it ideal for use in harsh conditions where other engineering ceramics may not cope.
It is also widely used as additive in other engineering ceramics in the form of particles, fibre or whiskers allowing its beneficial properties to be used alongside other engineering properties of the matrix ceramics.
Zirconia is characterised by properties such as:
- High hardness
- Good abrasion and wear resistance
- Chemical inertness – not attacked by acids or alkalis
- High temperature stability – Can safely work at temperatures >1500°C
- High thermal shock resistance
- High fracture toughness
Applications of Zirconia
- Pump parts, dies, knives, scissors
- Cutting blades, spikes
- Grinding media
- Screws and bolts
- Crucibles, batts, tubes
- Thermal insulation
- Golf club faces
- Medical devices
- Electrical devices
- Optical fibre sleeves
Zirconia or Zirconium Dioxide (ZrO2) is known to exist in three well known polymorphs; monoclinic, tetragonal and cubic, with a lesser known high pressure orthorhombic polymorph.
The monoclinic phase is stable up to about 1170°C (2140°F) at which point it transforms to the tetragonal phase up to about 2370°C (4300°F). After which the cubic phase exists up to the melting point of 2680°C (4855°F).
Crystallographic and physical properties of zirconia are provided below.
Monoclinic to Tetragonal
Tetragonal to Cubic
Cubic to Liquid
|Crystallography - Monoclinic|
|Crystallography - Tetragonal|
|Crystallography - Cubic|
|Thermal Expansion Co-Efficient|
|Heat of Formation||-1097.73||K|
During the cool down phase for pure Zirconia from the tetragonal to monoclinic states, there is a significant volume expansion (approx 3-5%). This volume expansion, even in small grain sized particles is significant such that elastic and fracture limits of the material are exceeded and the Zirconia sample or grain will crack.
As such, in order aid the manufacturing process of the Zirconia and to ensure stability and integrity of the crystal and grain structure, it is often the case that dopants or additives are used. The most common being those of calcia (CaO), magnesia (MgO), yttria (Y2O3) and ceria (CeO2) and these work to stabilise in the tetragonal and cubic phases.
The most popular of these dopants is the yttria stabilised Y-PSZ because it not only stabilises the tetragonal and cubic forms, but it also lowers the tetragonal to monoclinic transformation temperature. The practical advantage of this being the larger particles of the yttria stabilised zirconia are retained in the metastable tetragonal form.
The main properties of zirconia which are useful in industry are; high mechanical strength, good wear resistance, chemical corrosion resistance, electrical insulation, good surface finish and high fracture toughness.
Stablised Zirconia ceramics are generally resistant to attack by most molten metal (titanium and alkali metals the exception) and are therefore useful as crucibles. The other common application is in refractories where they can be fabricated as bricks foam or wool.