Technical ceramics mechanical engineering

Ceramic materials are inorganic and non-metallic.Usually they are shaped from the green body at room temperature and acquire their typical properties during a sintering process at high temperatures.Technical ceramics means ceramic products made for technical application.

In accordance with their chemical composition the technical ceramic materials can be divided into main groups:

Silicate ceramics

These materials from natural raw materials combine the basic electrical, mechanical and thermal properties of technical ceramics.Fundamental ingredients of this multi-phase material are clay, kaolin, as well as feldspar and soapstone as silicate carrier.Further ingredients such as alumina and zicronia(ZrSiO4) are added to achieve specific properties.

Oxide ceramics

The material of this group consist 90% of single-phase and single component metal oxides. These materials are glass-phase low or glass-phase free.Synthetic raw materials with a high level of purity lead to an even structure with very good properties at a very high sintering temperature.

High purity starting materials(powders) are prepared using mineral processing techniques to produce a concentrate followed by further processing(typically wet chemistry) to remove unwanted impurities and to add other compounds to create the desired starting composition. This is the most important stage in the preparation of high performance oxide ceramics.

As these are generally high purity systems minor impurities can have a dynamic effect, for example small amounts of MgO  can have a marked effect upon the sintering behaviour of alumina.

Various heat treatment procedures are utilised to create carefully controlled crystal structures.These powders are generally ground to an extremely fine or ultimate crystal size to assist ceramic activity.Plasticisers and binders are blended with these powders to suit the preffered method of forming(pressing,extrusion,slip casting, etc.) to produce the raw material.

Both high and low pressure forming techniques are used. The raw material is formed into the required “green” shape or precursor and fired to high temperatures in air or a slightly reducing atmosphere to produce a dense product.

Non-oxide ceramics

Ceramic materials e.g. compounds of silicon and aluminium with nitrogen or carbon belong to this group.In general non-oxide ceramics demonstrate a high share covalency bonding which provides them with very good mechanical properties, even when being used at high temperatures.Amongst non-oxide ceramics are carbide and nitride.

The production of non-oxide ceramics is usually a three stage process:prepaation of the precursors or starting powders, the mixing of these precursors and the forming and sintering of the final component.

Similar to oxide ceramics, carefully controlled purities and crystalline characteristics are needed to achieve the desired final ceramic properties.

Precise tolerances

 

ALUMINA Al2O3 ZIRCONIA ZrO2 NITRIDE Si3N4 CARBIDE SiC
MATERIAL PROPERTIES
Density gr/cm3 3.8 6 3.3 3.2
Porosity Vol % 0 0 0 0
MECHANICAL PROPERTIES
Hardness N/mm2 20000 12000 16000 25000
Compressive strength MPa 1700-2500 2000 2800 2500
E-modulus GPa 300-350 200 275 410-450
 
Fracture toughness MPa*m 4 9-15 6-7 3-4
 
Bending strength N/mm2 300-340 800-1400 750-850 300-550
THERMAL PROPERTIES
Max. use temperature °C In air
In inert gas		 1650-1900
1650-1900		 900-1200
900-1200		 1000-1400
1300-1800		 1400-1600
2000
 
Thermal expansion 10-6/K 7.0-9.0 8.0-11.0 3.0-4.0 4.0
 
Thermal conductivity W/mK at 20°C 20-30 2-3 35 110
 
ELECTRICAL PROPERTIES
Electrical resistivity Ohm.cm at 400°C 1012 1010 1011 10-50

 

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