Interesting Figures about Ceramics.

Interesting Figures about Ceramics.

1) Hardness and wearing resistance: Ceramics such as silicon carbide and alumina are the hardest materials next to diamond. The vicker’s hardness for silicon carbide and alumina are 2500 MPa and 1600 MPa respectively, as compared with only 900 MPa for high speed steel. The wearing resistance for silicon carbide ceramic is superior to those for steels. In practical terms, the life of a silicon carbide ceramic component in wearing resistance applications is 10 to 14 times longer than that of a similar steel component

2) Mechanical Strength: Ceramics are generally considered as weak materials because of their brittleness. In technical terms, ceramics have very low toughness values (K_1C).

The toughest ceramic practically available is the zirconia type, which has a fracture toughness of around 10 MPa.m^1/2, similar to that of cast iron. This means it is still breakable when hit hard with a hammer.

Typical bending strengths (MOR) for ceramics are in the range of 100–500 MPa which are comparable to those for metals. However, in engineering practice, good strength alone is not enough and a high toughness is considered more important. This is the reason why technical ceramics are only used in heat, corrosive and wearing resistant applications.

In the last 20 years, material scientists and engineers world-wide have tried to develop a tough ceramic for hot engine without success.

3) Extremely Thermal Shock Resistance: Warm or hot ceramic articles can be shattered when cooled suddenly. However in recent years several types of ceramics that do not have this problem have emerged. An example is that when a red-hot (1200^oC) ceramic component made of fused silica is suddenly dropped into cold water, nothing will happen to the ceramic component. This is because the ceramics of this type have almost zero thermal expansion coefficients, as a result, no stress is created during the rapid cooling process

4) Extremely Corrosive Resistance: Some ceramics like boron nitride have now become available, which are not wetted by some molten metals and chemicals. This means that there are no corrosion effects on these ceramics. In practice, the containers made of, or coated with such ceramics can have extremely long life in these applications.

5) Surface Areas in Porous Ceramics: Porous ceramics processed by sol-gel technique have extremely large surface areas, ranging from 200 to 500 square meters per gram (a handful of such materials will provide a surface area as large as a football pitch!). The ceramics of this kind provide enormous surface areas for chemical attachments or chemical immobilisation in high performance catalyst applications, and very recently, in biological applications.

6) Pore Size in Porous Ceramics: Ceramics manufactured by different techniques have different pore sizes:

Conventional powder sintering: 10 - 200 microns.

Micronized powder compact and sintering: 0.1-10 microns.

Sol-gel method: 1-100 nm.

Porous silica glass prepared by phase separation and acid leach technique have relatively narrow pore size distributions with pore size generally in the region between 10 to 100 nm