MATERIALS, PROCESSES AND DEFECTS

MATERIALS, PROCESSES AND DEFECTSK. SURESH SENANAYAKEBASIC MATERIALS SCIENCEAtomic structureBonding MechanismsStructure of Metals, Polymers and CeramicsStructure Property RelationshipDefects in MetalsAtomic Structure and Bonding Mechanisms in MaterialsThe atom consists of a nucleus and surrounding electrons. Each electron occupies a quantum state with a unique set of quantum numbers.With increasing atomic number, the elements in the periodic table have increasing number of protons and electrons. The order of occupation quantum state by electrons is determined by the Pauli exclusion Principle, the Hunds rule and the minimum energy criterion.

Atomic structure and bonding mechanism of materialsAtoms are connected with bondsThere mainly two types of bondsPrimary BondsSecondary BondsPrimary bonds are categorized into three typesIonic BondingCovalent BondingMetallic BondingNaClClNaIonic BondsElectrons can be transferred from an electropositive atom to an electronegative atom, producing ions of opposite sign and giving rise to the non-directional ionic bond. Occurred in inorganic materials like NaCl, MgO.OHH-++Covalent BondsSharing of electrons between neighbouring atoms results a covalent bond, which is directional. Occurred in H2O, Al2O3 (Ceramics), Plastics (Polymers).++++++++++++++++++++++++Metallic BondsA metal is an array of positive ions, which are held together in a common cloud of free electrons. The metallic bond is non directional and generally weaker than ionic and covalent bond. Occurred in all metals.Secondary BondsThese are Van Der Waals bonds (between polymer chains) and Hydrogen bonds (between water molecules).OxygenHHOxygenHHOxygenHHHydrogen bondsBond strength and properties of materialsMelting and boiling points of materials increase with increasing bond strength.Strong and directional bonds result in high strength, hard and brittle solids.Free electrons are responsible for the high thermal and electrical conductivities of metals.Structure of MaterialsCrystalline form of solidsHas long range periodic arrangement of atoms.Sharp melting pointsHigher densitiesNon-crystalline form of solids (glassy or amorphous)No long range periodic arrangement of atoms.Soften over a range of temperatureDo not give sharp diffraction pattern like crystalline solidsGrain structureIn a crystalline material a large amount of unit cells will be combined to make one GRAIN.They have the long range periodicity in all three dimensions and single orientation.In the solidification process many grains are formed with different orientations.The combination of different grains make the MICROSTRUCTURE of the material.The grains are adhere (stick firmly to each other) together with GRAIN BOUNDARIES.