复合材料8(composite materials8).ppt

Sol-Gel Processing Sol-gel is a thick dispersion of very small (~10 nm) particles Normally produced by chemical precipitation Sol-gel can be simply poured through preform of fibres and then heated Often needs several cycles to get good density Sol-Gel Processing Or can be used with short-fibres in a similar manner to slurry methods Small particles give better bonding on heating (larger surface area) Chemical Vapour Infiltration Like CVD Fibre preform used Gas mixture (carrier gas + reacting gases) passed through preform Ceramic deposits on preform Complex temperature control can be used to deposit ceramic progressively through preform Heating Elements PerforatedLid Reactant Gases Water-cooled Graphite Liner Preform Colder (800°C) Hotter (1000°C) Exhaust Gases Chemical Vapour Infiltration Preform Composite * Powder Metallurgy Inert atmosphere needed to prevent oxidation and explosion Pressing can be done as part of rolling or forging, or simple pressing followed by secondary shaping Limited to short fibres and low volume fractions (typically 30% max) Co-extrusion and Drawing Only possible where both matrix and fibres are ductile Used for multi-filament superconducting composites Liquid State Processing Requires higher temperatures Fibres may need coatings for protection and wetting C reacts with Al to form Al4C3 (brittle soluble). Ni coating applied by CVD. Possible to use continuous fibres and higher fibre volume fractions Conventional Casting Fibres (short) stirred into molten metal and cast. Problem of different densities can lead to settling or floating. Rheocasting can be used where the metal is partly solid, partly liquid. More fibre breakage Conventional Casting SiC / Al alloy casting Squeeze Casting Preform used Liquid metal pushed through preform with a ram Can now use continuous fibres Squeeze Casting PREFORM RAM DIE Squeeze Casting MOLTEN METAL Squeeze Casting Squeeze Casting Squeeze Casting EXCESS MACHINED AWAY Squeeze Casting Size of component l