transformation toughening of partially stabilized zirconia

V Krishnan Anirudh

2008A4PS284H

ADVANTAGES OF CERAMICS

V KRISHNAN ANIRUDH 2

DISADVANTAGES OF CERAMICS


Ceramics are inherently resilient to oxidation and deterioration at elevated temperatures.

But they are very prone to brittle fractures, else, some of these materials would be ideal for use in high temperature and severe stress applications.

Such applications also include automobile and aircraft gas turbine engines


Research has been conducted to improve toughness.

Hot pressing with additives and reaction bonding improve toughness


Zirconium dioxide is one of the most studied ceramic materials.

Pure ZrO2 has a monoclinic crystal structure at room temperature and transitions to tetragonal and cubic at increasing temperatures.

The volume expansion caused by the cubic to tetragonal to monoclinic transformation induces very large stresses and will cause pure ZrO2 to crack upon cooling from high temperatures.

Several different oxides are added to zirconia to stabilize the tetragonal and/or cubic phases: MgO, Y2O3, CaO and Ce2O3, amongst others

ZIRCONIUM DIOXIDE (Zirconia)


CRYSTAL STRUCTURES

ZrO2 exists in 3 structures. Monoclinic :

Up to 11700C Tetragonal:

1170 – 23700C Cubic:

above 23700C


Zirconia is very useful in its 'stabilized' state.

If sufficient quantities of the metastable tetragonal phase is present, then an applied stress, magnified by the stress concentration at a crack tip, can cause the tetragonal phase to convert to monoclinic, with the associated volume expansion.

This phase transformation can then put the crack into compression, retarding its growth, and enhancing the fracture toughness.

This mechanism is known as transformation toughening, and significantly extends the reliability and lifetime of products made with stabilized zirconia.


The process of "transformation toughening" is based on the assumption that zirconia undergoes several martensitic phase transformations (cubic → tetragonal → monoclinic) between room temperature and practical sintering temperatures.

Thus, due to the volume restrictions induced by the solid matrix, metastable crystalline structures can become frozen in which impart an internal strain field surrounding each zirconia inclusion upon cooling.

This enables a zirconia particle to absorb the energy of an approaching crack tip front in its nearby vicinity


Stabilizing of zirconia can be achieved by adding 3 to 7 wt.% of CaO.

At this composition, at about 10000C, both cubic and tetragonal phases will exist.

Upon cooling to room temperature under normal conditions, the monoclinic and CaZr4O9 phases don’t form (as predicted from the phase diagram)

Consequently, the cubic and tetragonal phases are retained and crack formation is circumvented

STABILIZATION OF ZIRCONIA


Stabilizing of zirconia can also be achieved by adding 3.6 wt.% of Y2O3.

At this composition, at about 11000C, tetragonal phase is pretty stable.

Upon cooling to room temperature under relatively rapid cooling, the tetragonal phase is retained.

From the phase diagram it is evident that a monoclinic ZrO2 is the stable phase, which coexists with this now unstable tetragonal phase in metastable equilibrium.



Stabilizing of zirconia can also be achieved by adding 9 mol% of MgO.

At this composition, sintering at about 18000C, and then rapidly cooling it to room temperature, will result in the all-metastable cubic structure.

If this material is reheated to 14000C, and held for sufficiently long time, a fine metastable submicroscopic precipitate with the tetragonal structure is precipitated.



Single crystals of the cubic phase of zirconia are commonly used as diamond simulant in jewellery.

Visually discerning a good quality cubic zirconia gem from a diamond is difficult, and most jewellers will have a thermal conductivity tester to identify cubic zirconia by its low thermal conductivity (diamond is a very good thermal conductor).

This material is also used in the manufacture of sub-frames for the construction of dental restorations such as crowns and bridges, which are then veneered with a conventional feldspathic porcelain

USES OF CUBIC ZIRCONIA


The cubic phase of zirconia has a very low thermal conductivity, which has led to its use as a thermal barrier coating or TBC in jet and diesel engines to allow operation at higher temperatures.

Thermodynamically the higher the operation temperature of an engine, the greater the possible efficiency (see Carnot heat engine). As of 2004, a great deal of research is going on to improve the quality and durability of these coatings.

Also, it is used as a refractory material, in insulation, abrasives, enamels and ceramic glazes.

Stabilized zirconia is used in oxygen sensors and fuel cell membranes because it has the ability to allow oxygen ions to move freely through the crystal structure at high temperatures. This high ionic conductivity (and a low electronic conductivity) makes it one of the most useful electro-ceramics.

USES OF CUBIC ZIRCONIA


transformation toughening of partially stabilized zirconia

Documents