Refractory materials, Mechanical Engineering

Refractory Materials

These materials are made up of engineering ceramics that are quite expensive in their pure forms. Such materials are also complicated to form. Most industrial refractory materials are made of ceramic of mixture compounds. These compounds are heat resistant caused by their high melting point. Table no.1 illustrates melting point temperatures of various ceramic compounds.

                                                          Table no.1: Melting Points of Some Ceramic Compounds

Compound

Melting Point

(oC)

Titanium Carbide, TiC

3120

Tungsten Carbide, WC

2850

Magnesium Oxide, MgO

2798

Zirconium Dioxide, ZrO2

2700

Silicon Carbide, SiC

2500

Boron Carbide, B4C

2450

Aluminium Oxide, Al2O3

2050

Silicon Nitride, Si3N4

1900

Silicon Dioxide, SiO2

1715

Titanium Dioxide, TiO2

1605

The compounds are most widely utilized in industrial refractory materials are Al2O3, Si3N4, SiC and ZrO2. Various types of refractory oxides are mixed along with these compounds. Various of the thermal properties of these ceramics were demonstrated earlier. This has already been stated as: these are brittle materials and illustrates low tensile strength. They are more determined by their flexural strength and compressive strength. Their fracture toughness is also low. Table no.2 describes mechanical properties of widely utilized ceramic materials. This is seen from these data such compressive strength is more than 10 times higher than tensile strength. Though, one should remain in mind that brittle material, like ceramics are, can't yield consistent values of strength mostly tensile strength due to presence of various micro defects. Statistical approach should be adopted if load carrying member in ceramic has to be implicated. At the similar time this can also be understood that larger components along with larger surface area have higher possibility of carrying crack like defects and thus large sizes must be refused. The experimental process for tensile strength determination do not prove capable and for this reason 3 point bend tests give better strength indicator that is flexural strength. The modulus of elasticity of such materials is quite high-often as elevated as 1000 to 2000 times tensile strength. The elasticity modulus is largely dependent on the level of porosity and proportion of impurities. A significant high temperature behaviour related with ceramics is that whereas this may not sustain specific load at low temperature, the similar load may be sustained beyond specific high temperature. This phenomenon happens because of crack's tip rounding like flaw at high temperature. This flaw rounding brings down the stress concentration factor to a finite rate from infinity. The temperatures beyond that ceramics can be assists to carry stresses are calculated in respect of several ceramics. For illustration, for alumina such temperature is 1400oC while for silicon nitride it is 1600oC.

                                                 Table no.2: Mechanical Properties of some Ceramics Materials

Material

Density kg/m3

Compressive Strength (MPa)

Tensile Strength (MPa)

Flexural Strength (MPa)

Fracture

Toughness

MPa     m

Al2O3 (99%)

3850

2585

207

345

4.0

Si3N4 (Hot Pressed)

3190

3450

-

690

6.6

Si3N4 (Reaction

Bonded)

2800

770

-

255

3.6

SiC (Sintered)

3100

3860

170

550

4.0

ZrO3, 9% MgO (Partially Stablised)

5500

1860

-

690

8.0

In the given sections we illustrate a few refractory materials that are widely utilized along with signification of manufacturing utilized to bring them in usable forms.

Posted Date: 2/27/2013 1:47:38 AM | Location : United States







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