Introduction of Zirconia Ceramics

AUTHOR : Foyo Valve Date : 11/10/2013 5:46:17 PM
 ‘Zirconia – Ceramic steel’. The title of the first scientific paper to highlight the possibilities offered by the “transformation toughening” mechanism which occurs in certain zirconia ceramics. Since the publication of this seminal work in 1975, considerable research, development, and marketing effort has been expended on this single material which offers the traditional ceramic benefits of hardness, wear resistance, and corrosion resistance, without the characteristic ceramic property of absolute brittleness.

Mechanical and Physical Properties

The fundamental properties of   zirconia ceramic 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

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The usual nomenclature used to describe zirconia ceramic alloys is as detailed below

• TZP tetragonal zirconia polycrystals
• PSZ partially stabilised zirconia
• FSZ fully stabilised zirconia
• TTC transformation toughened ceramics
• ZTA zirconia toughened alumina
• TTZ transformation toughened zirconia

The common notation used in zirconia literature involves placing the cation symbol of the stabilising oxide before the TZP or PSZ abbreviation. In some cases the amount expressed as mol% of the stabilising oxide will be indicated by a number before the cation symbol, e.g. zirconia containing 3 mol% yttria will be denoted as 3Y-TZP.
Symbols corresponding to non-stabilising additions are placed behind the abbreviation. These additions are given as weight percentages,

e.g. 3 mol% yttria – zirconia with 20 wt% alumina = (3Y-TZP)20A.

To use zirconia to its full potential, the properties of the oxide have been modified extensively by the addition of cubic stabilising oxides. These can be added in amounts sufficient to form a partially stabilised zirconia (PSZ) or to form a fully stabilised zirconia which has a cubic structure from its melting point to room temperature.

The addition of varying amounts of the cubic stabilising oxide, particularly MgO, CaO and Y2O3, has allowed the development of novel and innovative ceramic materials which have brought about considerable technological change.

The range of materials has been expanded by the use of specific rare earth additions, notably cerium oxide, this material shows unusual “toughness” which could have significant implications for the design of engineering ceramics.

Zirconia-based ceramics have now been developed to the stage where design of micro structure is possible by control of composition, fabrication route, thermal treatment, and final machining.

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Zirconia Ceramics Properties

The zirconia ceramics have a very fine uniform grain in comparison to Mg-PSZ’s

In common with all other engineering ceramics, the attainment of the above properties is largely dependent on both the starting powders and the fabrication techniques.

All common ceramic consolidation techniques have been applied to zirconia ceramic, including dry pressing, iso-static pressing, injection moulding, extrusion and tape casting.

Flaw elimination at all process stages is crucial for not only high strength but also reliability.

With critical flaws for a Y-TZP of the order of 40 µm, clean-room processing has been shown to significantly enhance both mean strengths and the distribution of strengths in Y-TZPs as measured by the Weibull modulus.

Values of over 1000 MPa bending strength with Weibull Moduli of > 30 have been recorded for these materials.

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