| '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.
The usual nomenclature used to describe zirconia ceramic
alloys is as detailed below: |
|
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 microstructure
is possible by control of composition, fabrication route,
thermal treatment and final machining.
Technox® Zirconia Properties
The Technox 2000 series of zirconias have a very fine
uniform grain in comparison to Mg-PSZ's
 |
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| Typical Mg-PSZ microstructure |
Technox 2000 Microstructure |
Mechanical and Physical Properties
The fundamental properties of
zirconia ceramics 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. |
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 ceramics, 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-TZP"s
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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