Fracture toughness polymers generally have lower

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Fracture toughness - Polymers generally have lower fracture toughness than metals - Thermosets generally brittle - Temp below T G (<0.75T G ) - polymers hard and brittle with low fracture toughness and fail by fast fracture - If polymers used at temp well below T g - 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) of 51 59
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: - TiO 2 , Al 2 O 3 , 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 (CO 3 2— ) - limestone, marble and seashells (calcium carbonate), dolomite rock (calcium magnesium carbonate) Polycrystallinity of 52 59
- 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 (SiO 2 ) based - Made of same SiO 4 tetrahedra 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

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