chapter 5 Flashcards
define crystalline
regular layers of atoms or molecules repeating 3-dimensional pattern
define amorphous
no long range order- does not exhibit crystalline structure
define polycrystalline
where there are many regions (grains) of crystalline structure. The atoms in each grain line up in a different direction, they are non-directional
define polymers
a molecular chain, made up of a single repeating unit called a monomer
what is ionic bonding?
eg salts like NaCl: rigid, directional, stiff. the bonds are strong. the ions cannot slip so the material is hard and brittle. Little deformation before breaking. Usually brittle
what is covalent bonding?
eg, ceramics and glass: rigid, directional and stiff. the bonds are strong. the ions cannot slip so the material is hard and brittle. Little deformation before breaking. Usually brittle
what type of bonding do polymer chains have?
the monomers in a polymer chain are covalently bonded together so hard to separate making them strong. Polymer chains are often scrunched up or folded- they can be unfolded by rotating about their bonds when they are pulled making them flexible.
what affect does cross-linking bonds have on the material?
the stronger the bonds and the more of them means the more rigid the material.
what type of polymer does cross-linking happen in?
thermosetting polymers
what is the difference between thermosoftening polymers and thermosetting polymers?
thermosoftening polymers can be reshaped when hot but thermosetting polymers can only be moulded once and is heated to form cross links between chains, the resulting 3d solid structure cannot then be changed.
what type of structures can polymers have? (3)
LINEAR: allows the molecules to slide past each other, making them more flexible and dense- the longer the chain, the stronger the polymer.
BRANCHED: spreads the molecules out as they can’t pack together tightly making them less dense, less strong and have a lower melting point
CROSS-LINKED: doesn’t allow the molecules to slide past each other and prevents bond rotation from extending the chains, making them stiffer. it resists scratching so makes the polymer harder
why are polymers tough?
as the cross-links and links in the polymers take a lot of energy to break- flexible polymers are tougher as they can undergo a lot of plastic deformation before breaking such as plastic bags.
what types of polymer have higher flexibility?
longer chains have more bonds therefore through bond rotation are able to extend to a longer length under stress making them more flexible
why are polymers strong?
because the chains slide past each other when a stress is applied, eventually becoming more crystalline. Polymers have strong non-directional forces of attraction between long chains and so a lot of force is required to cause fracture
why do polymers have a low young modulus?
because when unstretched, a polymers chains are randomly arranged. the covalent bonds in chains can rotate. When a stress is applied, the polymer chains straighten out easily, resulting in a large value of strain. value of strain is therefore high relative to stress
in a ceramic, what is the arrangement of atoms and how do the grains line up?
can be crystalline or polycrystalline, where there are many regions of crystalline structure. The atoms in each grain line up in a different direction so are non-directional
when is a molten ceramic more likely to be amorphous?
the quicker it is cooled
how are atoms in a ceramic bonded?
ionically or covalently in a giant rigid structure
what makes ceramics stiff?
the strong bonds between the atoms
what makes ceramics brittle?
the rigid structure
what makes glass brittle?
glass doesn’t contain slip planes so there is no way to relive stress. Therefore, excess stress forms a crack that starts at a point where there is a surface flow. particals on the surface of the crack become separated. The stress that formed the crack is caused by particles that have fewer neighbours over which the stress can be distributed.As the crack grows, the intensity of the stress at its tip increases. This allows more bonds to break and the crack widens until the glass breaks
what are grains also known as?
crystallites- small crystals within a polycrystalline structure
what is a grain boundary?
where two grains meet. The grains on either side of the boundary are identical but align differently- where grains with different orientations meet
what’s the structure of metals?
crystalline structure- positive metal ions surrounded by a sea of delocalised electrons in a lattice. Held together by metallic bonds by electrostatic attraction
why are metallic bonds non-directional?
not bonded to another atom but held in place by the surrounding electrons- the attraction between the ions and electrons acts in every direction
metallic bonds are stronger than what forces?
intermolecular
metallic bonds are weaker than what bonds?
covalent and ionic
how are metallic solids formed?
when a metal is cooled slowly, crystals will start to form. the crystals grow until the liquid is completely solidified. this creates a metal with polycrystalline structure because grain boundaries are created as the metal cools and occurs in multiple places in teh metal
why do metals plastically deform?
because the bonds are non-directional so the atoms can make new metallic bonds with atoms once the force is applied and removed.
why do metals elastically deform?
because the atomic lattice changes size and shape when forces are applied. When forces are removed the lattice goes back to its original form.
why are metals strong?
because the electrostatic attraction is so strong
why are metals tough?
because the bonds are non-directional so the ions can move in relation to each other without crack propagation.. positive ions can slide past each other while still remaining together and so change shape and don’t break apart
what are dislocations?
A dislocation is adefectin a metallatticewhen a few ions in a layer are missing. This causes the neighbouring layers to be displaced slightly
what are alloys?
a metal made by combining 2 or more metallic elements to give greater strength or resistance to corrosion. The different sized crystals and atoms mean that layers slide over each other less easily
examples of alloys
brass- copper and zinc
steel- copper and iron
why are metals weaker than we predict?
because dislocations mean that less bonds need to be broken than in theory. very small samples will have fewer dislocations and so will give a different strength to larger samples
what are lattice defects?
imperfections in the crystalline lattice
when are metals weaker than theory?
where there are few lattice defects in a structure
what type of defect are grain boundaries?
large defects as they block dislocations from moving through the structure
what are edge dislocations?
dislocations in response to an external stress. As soon as critical sheer stress is reached, the dislocation starts moving and deformation is no longer elastic but plastic because the dislocation will not move back after the stress is removed
what happens when alloys are made?
they add different atoms to the lattice so create more lattice defects- which block the movement of dislocations through the lattice
what do the different sized atoms in the lattice cause the properties to be?
harder
less ductile
less malleable
stronger- withstand a far greater stress before yeilding
explain dislocations in elastic deformation
when stress is applied to a metal, the dislocations in the lattice are able to move through the structure. Elastic deformation is the movement of these layers of atoms in the lattice such that they return to their original position when stress is removed
explain dislocations in plastic deformation
plastic deformation occurs when the atoms move enough to break the metallic bonds between layers. this can either induce dislocations or cause the atoms to rearrange and form new bonds. this leads to a permanent change of shape when stress is removed
when is a metal less tough?
when it has small grains because the dislocations can’t travel far through the metal without being stopped at a grain boundary therefore less plastic deformation occurs before failure
when is a metal tougher?
when it has large grains because the dislocations can move through the metal therefore it undergoes more plastic deformation before failure
what are screw dislocations?
when enough force is applied making the metal ‘rip’ apart
what are substitutional alloys?
alloy containing metal ions of similar size that form by atom exchange in the crystalline structure- makes them malleable and ductile
why do alloys generally have a high young modulus?
alloys experience less elastic deformation but can withstand more stress before reaching their elastic limit. This generally gives them a higher modulus than the metals they are made up of
how are insulators held together?
by strong, directional bonds (ionic and covalent)
how are metals held together?
by strong, non-directional bonds
what happens when a metal or ceramic stretched elastically?
the bonds between neighbouring atoms are extended very slightly
what happens when a polymer is stretched elastically?
the atoms rotate about their bonds
why are metals ductile?
because of the presence of dislocations which can move through the material
why are ceramics brittle?
because cracks and other flaw limit their strength
why are metals opaque and shiny?
because their conduction electrons scatter light back- they absorb and re-emit light
why are insulators like glass and Perspex transparent?
because they have no free electrons to scatter light back again
how does conductivity vary with temperature in metals and semi-conductors?
decreases gradually with metals with increasing temp while semiconductors increase rapidly
how can the conductivity of a semiconductor be increased?
by doping with atoms of other elements nearby in the Periodic Table
what is a composite material?
materials made from 2 or more constituent materials with different physical properties that when combined produce a material with different physical characteristics from the original materials and individual components remain separate.
what is a matrix?
holds the components of a composite together and gives it its bulk form. It transfers the load onto the reinforcement so that it doesn’t do as much of the work
what is a reinforcement?
the material in the composite which provides most of the desirable properties of the composite e.g tensile strength or toughness. usually in the form of fibres or particles
what is cermet?
ceramic metal composite- particle reinforced composite used to create greater strength within the material. the particles provide a physical barrier to stop cracks propagating and prevent the particles of the matrix from dislocating and slipping- making the composite harder
what is in a polymer matrix composite?
resin as matrix and the reinforcement could be: glass, carbon or natural fibres
what is in a metal matrix composite?
metal as matrix eg aluminium
reinforcement= fibres of silicon carbide
what is in a ceramic matrix composite?
ceramic- matrix
reinforcement- short fibres of silicon carbide
explain the difference in the bonds of 2 materials in a composite and the bonds within each material
the adhesive bonds between 2 materials are not as strong as the bonds within each material but most composites protect each others material so that overall the material is stronger
define anisotropic
fibres go in one direction meaning that the strength depends on the direction of the fibres
define isotropic
fibres go in different directions, meaning the direction doesn’t affect the properties
why are alloys generally less ductile than pure metals?
because in a pure metal dislocations are free to move and so slip occurs easily whereas in an alloy, the alloy atom pins dislocations and slip is more difficult.
