Zirconia

Chemical Formula

ZrO₂

Topics Covered

Background

Key Properties

Applications

Knives and Scissors

Seals, Valves and Pump Impellers

Orthopaedic Implants

Refractory Applications

Electronic Applications

Synthetic Gemstones

Other Applications

Background

Zirconia as a pure oxide does not occur in nature but it is found in baddeleyite and zircon (ZrSiO₄) which form the main sources for the material.  Of the two of these, zircon is by far the most widespread but it is less pure and requires a significant amount of processing to yield zirconia.

The processing of zirconia involves the separation and removal of undesirable materials and impurities - in the case of zircon - silica, and for baddeleyite, iron and titanium oxides.  There are several routes to the extraction of zirconia from zircon including:

·         Chlorination

·         Alkali oxide decomposition

·         Lime fusion

·         Plasma dissociation

Pure zirconia exists in the monoclinic form at room temperature.  Cubic and tetragonal phases are also stable at higher temperatures.  The transformation of monoclinic to cubic zirconia occurs at 800-1000°C and is accompanied by a large change in lattice size.  A consequence of this phase change is a large volume expansion on cooling which make the fabrication of pure zirconia ceramics impossible.

In order to manufacture zirconia components, it is necessary to 'lock' the material wholly or partially into the cubic form by the use of additives or stabilising agents.

The addition of varying amounts of cubic stabilisers such as CaO, MgO, and Y₂O₃ allows the formation of partially stabilised zirconias which combined with processing variations can result in ceramics which demonstrate exceptional properties and have found a number of application areas.

Key Properties

Typical properties exhibited by zirconia that are commonly utilised include:

·         High strength

·         High fracture toughness

·         Excellent wear resistance

·         High hardness

·         Excellent chemical resistance

·         High toughness

·         Very refractory

·         Good oxygen ion conductor

The properties exhibited by zirconia ceramics depend upon the degree and type of stabilisation and on the processing used. Typical properties for various zirconias are given in table 1.

Table 1. Mechanical properties for zirconia.

Applications

Knives and Scissors

A major problem with metal knives and scissors when faced with tough materials such as Kevlar or when cutting large quantities of paper (which often contain dispersed minerals) is the abrasion or blunting of the cutting edge. Zirconia materials in this application retain their edge and stay sharp longer.

The key properties which make zirconia a suitable material for this application are:

·         High strength and fracture toughness coupled with high hardness (harder than the materials being cut)

·         Fine grain size so that a microscopically sharp edge may be achieved

·         The effect of transformation toughening at the machined edge of the blade which enhances toughening.

Other similar applications include blades for cutting of plastic film, magnetic tape and other tough or abrasive materials. Zirconia is also used in composite cutting tools and abrasive wheels.

Seals, Valves and Pump Impellers

The handling and transport of slurries and aggressive chemicals present a difficult materials problem.  High temperatures and high pressure flow lead to highly reactive and abrasive conditions. The key properties which make zirconia a suitable material for this application are:

·         Chemical resistance

·         High hardness for wear resistance

·         Good surface finish to resist fouling and to minimise friction on sliding surfaces

·         High toughness to prevent damage during assembly or by impact in operation.

Orthopaedic implants

Zirconia is used as a femoral head component in hip implants. High strength and high toughness allow the hip joint to be made smaller which allows a greater degree of articulation. The ability to be polished to a high surface finish also allows a low friction joint to be manufactured for articulating joints such as the hip.

The chemical inertness of the material to the physiological environment reduces the risk of infection. For this reason, only zirconias manufactured from low radioactivity materials can be used in this application.

Refractory Applications

Zirconia (monoclinic and partially stabilised) powder is used in refractory compositions to enhance thermal shock resistance and abrasion resistance. These materials are used in severe applications such as sliding gate plates for pouring steel, and in steel immersion applications such as stopper rods and as components in submerged entry nozzles.

Other refractory applications for zirconia include insulating fibre and thermal barrier coatings.

Electronic Applications

The electronic and electrical applications of zirconia rely on the relatively high electronic conductivity and the high oxygen ion conductivity of the fully stabilised material.

This allows the material to be used as:

·         Resistive heating elements - which can be operated in air beyond the temperature range of convention elements

·         Oxygen sensors which operate on the Nernst cell principle

·         Electrolyte in solid oxide fuel cells

Synthetic Gemstones

By melting zirconia with dopants and recrystallising on cooling, gemstones can be produced which simulate diamond, ruby, topaz and emerald. These materials are commonly known as cubic zirconias.

Other Applications

Due to its excellent wear resistance, zirconia is used as in thread guide, cams and wire drawing dies. It is also used as a cathode for plasma torches and a nucleating agent for glass ceramics.

Primary author: Ceram Research

For more information on this source please visit Ceram Research Ltd.