Zirconia ceramics

Dr. Dmitri Kopeliovich

Zirconia Ceramic is a ceramic material consisting of at least 90% of Zirconium Dioxide (ZrO₂).

Zirconium Oxide is produced from natural minerals such as Baddeleyite (zirconium oxide) or zirconium silicate sand.

Pure zirconia changes its crystal structure depending on the temperature:

At temperatures below 2138 ºF (1170 ºC) zirconia exists in monoclinic form.

At the temperature 2138 ºF (1170 ºC) monoclinic structure transforms to tetragonal form which is stable up to 4300 ºF (2370 ºC).

Tetragonal crystal structure transforms to cubic structure at 4300 ºF (2370 ºC).

Structure transformations are accompanied by volume changes which may cause cracking if cooling/heating is rapid and non-uniform.

Additions of some oxides (MgO, CaO,Y₂O₃) to pure zirconia depress allotropic transformations (crystal structure changes) and allow to stabilize either cubic or tetragonal structure of the material at any temperature.

The most popular stabilizing addition to zirconia is yttria (Y₂O₃), which is added and uniformly distributed in proportion of 5.15%.

Depending on sintering temperature and other processing parameters, the following forms of stabilized zirconia may be prepared:

1. Fully stabilized zirconia (FSZ) with cubic crystal structure;
2. Partially stabilized zirconia (PSZ) with mixed structure (cubic+tetragonal);
3. Polycrystalline tetragonal zirconia (TZP) with metastable tetragonal structure of very fine zirconia grains sintered at low temperature.

The following characteristics are typical for Zirconia Ceramics:

• High density – up to 380 lb/ft³ (6.1*10³ kg/m³);

• Low thermal conductivity – 10% of that of alumina ceramics;

• High fracture toughness;

• Very high flexural strength and hardness;

• High maximum service temperature – up to 4350 ºF (2400 ºC).

• coefficient of thermal expansion similar to that of cast iron;

• Modulus of elasticity similar to steel;

• High chemical resistance;

• High resistance to molten metals;

• Good wear resistance;

• Low coefficient of friction;

• Oxygen ion conductivity (used for oxygen sensors and high temperature fuel cells).

Zirconia ceramics are used for manufacturing Extrusion dies, powder compacting dies, cutting tools, balls and seats for ball valves, thread and wire guides, pump seals, impellers and shaft guides, engine parts, oxygen sensors, fuel cells membranes,high temperature heaters for electric furnaces, bearings (e.g., bearings for submersible pumps).

(Materials Data)

• Zirconia Ceramic YTZP (yttria stabilized polycrystalline tetragonal zirconia)
• Zirconia Ceramic MgPSZ (magnesia partially stabilized zirconia)

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