CERAMICS

Introduction ~ keramikos - burnt stuff in Greek. ~ Ceramics are inorganic and non-metallic materials that are commonly electrical and thermal insulators, brittle and composed of more than one element (e.g., two in Al 2 O 3 ). ~ Traditional ceramics – based on clay (china, bricks,tiles, porcelain), glasses. ~ Advanced ceramics - consist of carbides (SiC), pure oxides (Al2O3), nitrides (Si3N4), non-silicate glasses and many others. ~ “New ceramics” for electronic, computer, aerospace industries.

Structure ~ Ceramics usually have a combination of stronger bonds called ionic and covalent. ~ there are many structures in ceramics : 1) Ceramics have crystal structure which is similar to a metal structure. 2) Atoms of ceramics are ionically bonded or covalently bonded. 3) Ionically bonded ceramics are hard,rigid and posses high chemical stability. Example:- Nacl. 4) Covalently bonded ceramics are also hard,rigid and posses high electrical resistance. Example:- SiO 5) Combinedly bonded ceramics are similar in properties and show better characteristics than both. Example :- MgO.

Properties Of The Ceramics

Properties :- The following properties of ceramic materials make them useful for Engineering Applications :- Mechanical Properties Mechanical Properties Electrical Properties Electrical Properties Chemical Properties Chemical Properties Optical Properties Optical Properties Thermal Properties Thermal Properties

~ The properties of ceramic materials, like all materials, are dictated by the types of atoms present, the types of bonding between the atoms, and the way the atoms are packed together. 1) Mechanical Properties :- i) Ceramic materials have low tensile strength. ii) They generally fail due to stress concentration on cracks, pores etc. iii) Ceramics posses great hardness and resistance to wear and can be used for grinding. iv) Ceramics posses high compressive strength. v) Most ceramics posses low fracture strength and fail in a brittle manner. vi) Values of Modulus of Elasticity for ceramics ranges from 7 x 1010 N/m2 to 40 x 1010 N/m2.

2) Electrical Properties :- i) Ceramics are often used for electric insulation. ii) Some ceramics conduct electrically well and are used as Semiconductors i.e N.T.C & P.T.C resistors. iii) Many ceramics have a dielectric constant value upto 100 and very low dielectric losses. iv) Some ceramics also exhibit piezoelectric properties and can transfer mechanical deformations in to voltage changes.

3) Chemical Properties :- i) Majority of ceramics are highly resistant to all chemicals and organic solvents. ii) Ceramics are completely resistant to oxidation even at high temperature. iii) Glazed porcelain ceramics is used for chemical vessels.

4) Optical Properties :- i) Many types of glasses are used for windows and optical lenses. ii) They also find use in selective transmission or absorption of certain wavelengths. 5) Thermal Properties :- i) Ceramic materials do not have enough electrons for bringing about electronic thermal conductivity.

PropertyCeramicMetalPolymer HardnessVery HighLowVery Low Elastic modulusVery HighHighLow Thermal expansionHighLowVery Low DuctilityLowHigh Corrosion resistanceHighLow Wear resistanceHighLow Electrical conductivityDepends on materialHighLow DensityLowHighVery Low Thermal conductivityDepends on materialHighLow MagneticDepends on materialHighVery Low Note: For general comparison only; specific properties depend on the material’s specific composition and how it is made.

Applications

Automotive Computers Consumer Uses Medical (Bioceramics) Military Other Industries Communications Aerospace