Ceramics as a materials classification are inherently very strong in compression.
This is due to the type of bonding which holds the atoms together being either covalent or ionic or a combination therein.
The direct consequence of this structure gives ‘ceramic’ materials very high melting points and relatively low electrical conductivity together with other key benefits.
Whereas in metal systems there are slip planes where crystal lattices can be moved which give malleability and ductility, ceramics reach a limit and fail catastrophically. The limit at which they fail is generally noted as the modulus of rupture (MOR) which is a statistical statement based on many results. Zirconia unlike many ceramics has a very high fracture toughness value which enables it to be functionally used without the fear of failure. It also has a thermal expansion co-efficient similar to that of steel which enables composite systems of metal and ceramic to be utilised to enable massive performance gains.
Zirconia is proving an exciting novel material which is gaining in popularity due to its specific key advantages over other more traditional ceramics. It does not have the malleability of metals but does have significantly high fracture toughness – K1c value. This is due to additions being made to zirconia which ‘toughen’ the material.
The primary function of the additive is to stabilise the tetragonal phase. If done correctly this high temperature phase becomes metastable at room temperature.
Metastable is where a given phase within the material is present and stable under certain conditions. When the conditions change it becomes energetically favourable for the crystal structure to change.
Indeed it is the very nature of this phase being metastable that causes the increase in fracture toughness.
When the tetragonal phase reverts to monoclinic it does so under a martensitic type transformation. This gives an associated volume change of between positive 3% and 5%. This volume expansion tends to close the crack, thus relieving stresses at the tip. This in turn is what causes the increase in K1c value.
Indeed it is this transformation behaviour which gives this material the interesting advantages for the modern engineering world.
There are four zirconia systems that as engineers we are interested in …
- SZ full stabliised cubic system 8% mol
- TZP Tetragonal Zirconia Polycrystal (2-4mol% Yytria)
- PSV partially stabilised zirconia. Plasma coatings 5-6mol%
- Approx 15 wt% far more than CSV used for Ion carriers in fuel cell systems
Oil and Gas Industry
As the material, zirconia is wear resistant and inert and can therefore be utilised in areas where traditional metallic systems would not be able to function.
In sub-sea systems where metallic systems would corrode, zirconia excels.
Zirconia is used as an oxygen sensor due to the free charge carriers within the structure. Specific grades of zirconia can become electrically conducting at high temperatures due to the energy imputed into the material which liberates the charge carriers to carry the current.
Zirconia Pumps, Pistons & Liners
Zirconia has great wear resistance, which combined with the high fracture toughness properties makes it an ideal material for pumps.
General Zirconia Components
As zirconia is so hard, it is the perfect material from which to manufacture knives and blades – cutting edges remain sharp for much longer.