ES:M1:Materials

Classification of Materials

Metals, Polymers & Elastomers, Ceramics and Composites.

Metal

Have high electrical and thermal conductivities

Can be ductile

Have reasonable stiffness, toughness and strength

Lustrous when oxide is removed

Usually solid at room temperature

Polymers

Commonly called plastics & rubbers. They are all organic materials heavily modified for desired properties.

Ceramics

Hard and brittle

Have high compressive and low tensile strength

Low electrical and thermal conductivity

Chemically inert

Composites

Made by bonding two materials to achieve desired properties

Generally have a high specific strength (strength/mass)

Porosity

amount of pores in a material

Moisture Content

amount of matter in a given volume

Density

amount of moisture in a material

Ductility

ability of a material to be stretched or drawn into wire by a tensile force without fracturing

Malleability

ability of a material to be flattered by a compressive force without failing

Hardness

Ability to resist scratching or indentation

Elasticity

Ability to return to its original position after removal of the deforming force

Toughness

Ability to withstand an impact force and deform plastically before fracturing

Strength

Ability to withstand a load or force without failure

Protons , nuetrons Electrons

Protons + neutrons (no) Electron -

Primary bonds

Ionic, covalent and metallic bonds

Secondary bonds

Van der wall’s forces (dispersion and dipole-dipole) and hydrogen bonds. They have around 1% of the strength of a primary bond

Ionic Bonding

One atom tran sfers electrons to another atom to achieve a full outer shell configuration. Ionically bonded materials are poor conductors of electricity and typically have high melting points

Covalent bonding

One atom shares electrons with another to achieve a full outer shell configuration.

Covalently bonded materials typically have high melting points and are good insulators.

Metallic bonding

The electrons detach from the atom to form an electron ‘cloud’ or ‘sea’.

Metallically bonded materials (metals) usually are good conductors of electricity and have high strength.

Van der Waals Forces

Van der Waals forces is a general term used to describe intermolecular forces between molecules. There are two types of Van der Waals forces, dispersion forces and dipole-dipole forces.

Hydrogen bonding

Hydrogen bonding is a special type of dipole-dipole force which forms a strong bond.

Body-centred cubic (BCC

This structure has an atom in each corner of a cube with one in the centre. The total volume of atoms in this structure is 2 due to part of the corner atoms not being included in the structure

This structure has a low amount of slip planes, which results in low ductility

Face-centred cubic (FCC)

This structure has an atom in each corner of a cube and an atom at the centre of each face

The total volume of atoms in an FCC structure is 4

This structure has a lot of slip planes, therefore having higher ductility

Close-packed hexagonal (CPH)

This structure has atoms at each corner of a hexagonal prism and 3 atoms in a triangle in the middle

The total volume of atoms in this structure is 6

This structure has less slip planes than an FCC structure, which results in a lower ductility than an FCC structure

Crystalline and non-crystalline structure

Crystalline - ordered and non-crystalline - unordered

Ferrous alloys

Ferrous alloys contain iron as their main constituent

Non-ferrous alloys

Non-ferrous alloys are alloys that do not have iron as their base element

Hot rolling

Hot rolling is done above the metal’s recrystallization temperature and produces a refined, unstressed and smaller grain structure

Cold rolling

Cold rolling is done below the metal’s recrystallization temperature and gives a smoother and more accurate finish than hot rolling. Compared to hot rolling, it produces a harder, stronger and less ductile product

Extrusion

Extrusion is the process of metal being forced through a die, like squeezing toothpaste from a tube.

Lead, brass, bronze, copper, aluminium and steel are commonly extruded to increase strength, toughness and hardness.

Fabricating

Fabricating is the construction of projects form components

Thermosoftening Polymers

These are made up of long molecules that are entangled together like spaghetti

The bonds along the chain are covalent bonds, while the bonds across the chains are weaker secondary bonds, which include Van der Waals bonds, hydrogen bonds and dipole-dipole interactions

At low temperatures, all the molecules are held together

When the plastic is heated, it softens due to the secondary bonds weakening, which allows it to be reshaped. On cooling, the bonds re-establish into new positions

Thermoplastics can be softened by heat many times before it affects the strength of the polymer

Thermosetting polymers

These are similar to thermoplastics, but covalent bonds are formed across the chains instead of the weak secondary bonds

This prevents the molecules from moving independently, making the polymer unable to be softened after it is hardened

Thermosets are generally stronger than thermoplastics

Extrusion

Moist ceramic is forced through a shaped die and then cut to length. Product has a constant cross-section. Used for pipes, bricks, long, thin rods etc.

Timber

Concrete

Concrete is made up of:

Aggregate - Occupies 60-70% of the concrete and provides strength to it. Should not be larger than 20% of the thickness of the structure and should be angular

Sand - fills in the voids between the aggregate to give a high packing factor and reduces the likelihood of concrete disintegration

Cement and water - Forms the glue to bind the sand and aggregate together through complex hydration reactions

Advantages of non-ferrous alloys

• Often greater ductility;

• generally lower density;

• higher thermal and electrical conductivity;

• often lower melting points;

• good resistance to corrosion;

• generally non-magnetic (except Ni and Co alloys)

Advantages of ferrous alloys

Strength and stiffness are generally higher; better for welding