Topics Covered
Background
Zirconia Phase Transformations
Producing Stabilized and Partially Stabilized Zirconia
Transformation Toughening in Partially Stabilized Zirconias
Typical Properties of Partially Stabilized Zirconias
Applications of Partially Stabilized Zirconias
Background
Zirconia (ZrO2) may look like a an excellent engineering ceramic on paper due to its desirable physical properties such as extremely high melting temperature, high strength and fracture toughness. However the phase changes that it undergoes during sintering are deleterious to these properties and hence pure zirconia is not a useful engineering material.
Zirconia Phase Transformations
At room temperature, zirconia exists on the monoclinic phase. When heated to about 1170°C, it undergoes a phase transformation from monoclinic to tetragonal and a volume shrinkage of more than 3-5%. Further heating produces another change to cubic at 2370°C. The cubic phase is maintained until the melting point or zirconia is reached 2680°C.
On cooling from sintering temperatures and/or high temperature exposure, zirconia undergoes the tetragonal to monoclinic transformation at 950°C and an expansion similar in magnitude of the shrinkage during heat up.
The large volumetric change associated with this phase transformation is a large enough to affect the structural integrity of the material. Repeated heating and cooling cycles would result in further erosion of mechanical integrity and properties.
Producing Stabilized and Partially Stabilized Zirconia
The additions of cubic oxides such as MgO, CaO, Y2O3, CeO2 and other rare earth oxides stabilize the high temperature cubic phase all the way back to room temperature. They also tend to decrease the transformation temperature.
In partially stabilized zirconias, similar additions are made, except, not enough to stabilize all of the material, hence the name “partially stabilized zirconia” or “PSZ”. These materials typically consist of two or more of the phases cubic, tetragonal and monoclinic.
Transformation Toughening in Partially Stabilized Zirconias
If produced properly, the resultant microstructure consists of lens or elliptical-shaped precipitates of tetragonal zirconia within the cubic grains. Normally the tetragonal phase would transform into the monoclinic phase at low enough temperatures, but the high strength of the cubic phase prevents the required expansion from happening, freezing in the tetragonal precipitates. Monoclinic zirconia may also be present in the cubic grains and at the grain boundaries.
The toughening mechanism comes into play when a crack is encountered. The cubic grains are constraining the tetragonal precipitates that want to expand and release associated energy. When these grains are faced with a propagating crack tip, the tetragonal phase is released and allowed to change back to the more stable monoclinic phase. This results in the associated volumetric expansion, effectively closing the advancing crack.
This is called transformation toughening and is a stress induced martensitic transformation to the monoclinic phase.
Typical Properties of Partially Stabilized Zirconias
Typical properties of PSZ materials include:
- Excellent fracture toughness
- Excellent wear resistance
- Excellent impact resistance
- Good resistance to thermal shock
- Good chemical resistance
- Good corrosion resistance
Applications of Partially Stabilized Zirconias
Typical application of PSZ materials include:
- Dies and tooling
- Knives, scissors and blades
- Wear resistant components including bearings and linings
- Pump parts
- Grinding media