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Fracture toughness -Polymers generally have lower fracture toughness than metals -Thermosets generally brittle -Temp below TG(<0.75TG) - polymers hard and brittle with low fracture toughness and fail by fast fracture -If polymers used at temp well below Tg- processing controlled to reduce cracks and defects and parts designed to avoid points of stress concentration Mechanical properties -Properties of polymers depend on: -Degree of cross-linking -Type fo side-chain/branches -Molecular weight -Crystallinity -Temperature Colour in polymers -Colour and degree of transparency controlled by: -Changing side groups -Adding dyes and pigments Polymers and colour -Change colour in response to: -Applied voltage (electrochromic) -Changed in temperature (thermochromic) -Light (photochromic) of5159
Polymer Opals -Ordered structures can be made that give colour due to diffraction and interference of light Ceramics and Glasses What are ceramics -Ceramics - Inorganic, non-metallic materials. They are usually compounds of oxygen, carbon or nitrogen and a metal. -Examples: -TiO2, Al2O3, TiN, SiC, NaCl -Rocks and minerals - iron oxide, sapphire, clay -Glasses are amorphous ceramics Properties of ceramics -In general: -High hardness -High stiffness -High chemical stability (corrosion resistance) -High temperature resistance -Examples: bricks, tiles, concretes, plates, mugs, glasses -Many occur naturally - rocks, stone, minerals Atomic bonding in ceramics -Crystal structure -Ionic or covalent or both -Bond depends on electronegativity of elements in material Ionic ceramics -Ionic crystal structures - high density and most efficient packing -Always finding largest number of neighbours but constraint - must be oppositely charged ions for neighbours -Therefore, structure determined by ratio of size of cation to size of anion Covalent ceramics -Predominantly covalent - adopt the zinc blend structure -Pure covalent materials such as diamond and silicone adopt same structure -Example: Silica -Silicon atoms can form four tons to oxygen atoms in tetrahedral arrangement Natural ceramics - Silicates and carbonates -Carbonates (CO32—) - limestone, marble and seashells (calcium carbonate), dolomite rock (calcium magnesium carbonate) Polycrystallinity of5259
-Most ceramics are polycrystalline -Grain boundaries are complicated - oppositely charged ions must be next to each other. Valencies satisfied as much as possible Porosity -Many ceramics not fully dense. Defects: -Pores -Microcracks -Defects due to differences in thermal expansion during processing -Theoretically - strong due to strong interatomic bonding -Realistically - strength limited by presence of defects -High porosity reduces corrosion resistance Glasses -Silica (SiO2) based -Made of same SiO4tetrahedra as pure silica but arranged in amorphous way -Most commercial glasses contain other elements to control viscosity -Example of common commercial glass: soda-lime glass -Sodium, calcium, aluminium, other elements -Used for windows, bottles, jars -Cheap, stable, hard, easy to process, recyclable Elastic modulus -Ceramics have high