refractory materialsthese materials are made up

Question

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 1400^oC while for silicon nitride it is 1600^oC.

                                                 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.