Breviary Technical Ceramics


      Werkstoffe der technischen Keramik

 Zirconium Oxide

Zirconium oxide (ZrO2) has gained importance in the last few years due to its

  • high fracture toughness,
  • thermal expansion similar to cast iron,
  • extremely high bending strength and tensile strength,
  • high resistance to wear and to corrosion,
  • low thermal conductivity,
  • oxygen ion conductivity and
  • very good tribological properties (it is very well suited for slide rings).

Zirconium oxide occurs as monoclinic, tetragonal and cubic crystal forms. Densely sintered parts can be manufactured as cubic and/or tetragonal crystal forms. In order to stabilise these crystal structures, stabilisers such as magnesium oxide (MgO), calcium oxide (CaO) or yttrium oxide (Y2O3) need to be added to the ZrO2. Other stabilisers sometimes used are cerium oxide (CeO2), scandium oxide (Sc2O3) or ytterbium oxide (Yb2O3).

Figure 12: Zirconium oxide: cubic, tetragonal and monoclinic crystal lattices
light spheres = Zr              dark spheres = O

In fully stabilised zirconium oxide (FSZ – fully stabilised zirconia) the high-temperature cubic structure is preserved even after cooling due to the addition of the other oxides into the crystal structure. The increase in volume, undesirable for technical applications, does not take place in FSZ.

Partially stabilised zirconium oxide (PSZ – partly stabilised zirco-nia) is of great technical significance. At room temperature, the substance includes a coarse cubic phase with tetragonal regions. This state can be retained in a metastable form through appropriate process control or annealing techniques. This prevents transformation of the tetragonal phase to the monoclinic phase, and the microstructure is "pre-stressed"; this is associated with an increase in strength and toughness.

Figure 13: Microstructure of a partially stabilised zirconium oxide (PSZ)

In polycrystalline tetragonal zirconium oxide (TZP – tetragonal zirconia polycrystal) the use of extremely fine initial powders, and the application of low sintering temperatures, achieves an extremely fine-grained microstructure. Due to its extremely fine microstructure (grain size < 100 µm) and the metastable tetragonal structure, this material is characterised by extraordinary high mechanical strength, possibly even exceeding 1,500 MPa.

The very finely developed tetragonal crystal phase in PSZ and in TZP displays a phenomenon unique to high-performance ceramics: transformation of the tetragonal phase into the monoclinic phase can be prevented by pressure. When the pressure is released, e.g. through crack tips or internal tensile stress, the transformation then occurs. The pressure-controlled increase in volume involved in the metamorphosis of the crystal phases closes cracks, slowing or deflecting their growth. This behaviour is exploited technically, and is known as transformation reinforcement. In PSZ ceramics, and in particular in TZP ceramics, it leads to extremely high component strength. Depending on the stabilisation method, it can be exploited at application temperatures between 600°C and 1,100°C. Zirconium oxide ceramics are therefore favoured for use in components that are subject to high mechanical stress.

Figure 14: Microstructure of a polycrystalline tetragonal zirconium oxide (TZP)

Figure 15: Nanostructure of a polycrystalline tetragonal zirconium oxide (TZP)

Another property specific to this material is its oxygen ion conductivity. This phenomenon is used to measure the partial pressure of oxygen. Zirconium oxide is therefore the basis, for example, of the "lambda sensors" used to regulate petrol engine exhausts.



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