复合材料6-(composite-materials6)

1.4REINFORCEMENTMATERIALSPARTICLESIdealRequirements•Goodstiffness(orotherdesirableproperty)•Goodbondingwithmatrix–Roughsurfacebetter–Smallparticlesbetter–Coatings(stearicacid,silanes)IdealRequirements•Chemicalandthermalstability•Lowcost•LowhealthandenvironmentalconcernParticlefillers•CalciumcarbonateCaCO3,includingchalk–Generalpurpose/lowcostfiller–Givesgoodstiffness,whitenessandUVprotection•SilicaSiO2–Goodstiffness,goodbondingsogoodstrength–CangivewhitenessParticlefillers•Calciumhydroxide(talc)–Plate-like,givingreasonablestiffness.Itallowseasiercolouringthansilicaandcalciumcarbonate.•Mica–alsoplate-likeinnature,andeasilyfracturesalongtheplates–cancausestrengthreductionsandsoislesscommon.Particlefillers•Kaolins–tendtogivebettercrackresistancethantheotherfillers,buttheydoimpartadarkercolourtothematerial.•Wollastonite–hasaneedle-likemorphology,andcanalmostbeconsideredtobeafiberreinforcement.–Assuch,itdoestendtogivegoodstrengthproperties.Particlefillers•Glassspheres–canbeusedtogivereflectivityandsmoothsurfaces,althoughthebondingtothepolymertendstobeworsethanfortherougherparticleshapes.–Hollowspheresproducedbyincludingagas-formingagentdissolvedinaglassparticlewhichexpandsonheating.Particlefillers•Titaniumdioxide–alsoquitewidelyusedasafiller,althoughitisprimarilypresentforitscolouringbehaviour.GLASSFIBRESGlassFibreStructure•Basedonsilicatetrahedra–Siatomsbondedto4Oatoms–GivesoverallcompositionofSiO2–Wouldbeveryhardtoprocess–Structuremodifiedwithotheratoms–Na,Mg,Ca,K,Al,BM+M+M+M+M+M+M+M+M+=SiO4TypesofGlassFibres•E-glass(electrical)–Bestprocessibilityandgoodgeneralpurposefibres•C-glass–Betterchemicalresistance•S-glass–Higherstrength•ECR&ARglass–GoodacidandalkaliresistanceE-GlassC-GlassS-GlassECR-GlassAR-GlassSiO25565655863ZrO2----14TiO2---2-Al2O315425110.7B2O37.55-0.1-Fe2O30.30.5-0.3-CaO18.514-226MgO3.23102.40.3ZnO---3-Na2O0.38.5-0.810K2O0.20-0.46GlassFibreProperties•Stiffness:70–85GPa•Strength:Upto2GPa(E-glass)Upto3GPa(S-glass)•Density:2,500kg/m3•Toughness:0.75MPa.m1/2•Strengthlimitedbybrittleness•Howthindoesafibreneedtobetonevercrack?Toughness•K1c:fracturetoughness=0.75MPa.m1/2•sc:stressrequiredtogetcracktopropagate•a:cracklength•(Yieldstress=2GPa)aKcCs1.11ProcessingofGlassFibres•Simplymeltedandpassedthroughcirculardies(204normally!)•Pulledintofibres(typically10mmdiameter)•Coatedwitha“size”–Toprotectthesurfacefromdamage–TogivebetterbondingtomatrixCARBONFIBRESGraphite-likeStructureGraphiteGraphite-likeStructureCarbonFibreProductionofCarbonFibres•Cannotprocessgraphite–Toobrittle–Meltingpoint:3675°C•Startwithfibreofcarbon-basedmaterial–PANmostcommon–Pitchalsoused•Convertthistographite-likestructurebyheattreatment•Polyacrylonitrile(PAN)•Stretchedandheated(inairto280°C)toproduceorientedladder-likemolecules•Oxidationto“OxyPAN”•Heatinginargonupto2800°Ccauses“graphitisation”TypesofCarbonFibre•Moreheattreatmentleadstobetteralignmentofgraphitestructure–Greaterstiffness–Moredefectsandsolowerstrength–“HMfibres”•Lessheattreatmentgivesconverse–“HSfibres”CarbonFibreProperties•Stiffness:230(HS)–380(HM)GPa•Strength:4.5GPa(HS)2.7GPa(HM)•Density:1,800kg/m3•Toughness:1–1.5MPa.m1/2POLYMERFIBRESPolyaramids•Kevlar(DuPont)ismostfamous•Rigidpolymerchains•Liquidcrystallinepolymers(orderinliquidphase)CNONCOHHnPolyaramids•Processedbyextrudingfibresfromsolution•Thenextendedandheated•GivesveryhighlycrystallinefibresPolyaramids•Toughfibres•Donotmelt•Limitedtotemperaturesbelow300°C•Lowflammabilityforapolymer•Dodegradeinsunlight(UV)AramidFibreProperties•Stiffness:130GPa•Strength:2.5GPa•Density:1,450kg/m3•Toughness:5MPa.m1/2Polyethylene•Theoretically,polyethylenecouldhavestiffnessofupto300GPa•Forthis,needstobe100%crystallineand100%oriented•CangetclosetothiswithPEfibres•Gel-spun•Tradenames:“Spectra”,“Dyneema”Polyethylene•Hightoughness,highstrength,highstiffnessandlowdensity!•Limitedtotemperaturesbelow100°C(meltsat130°C)•Poorbondingtomatrixmaterials•ExpensivePolyethyleneFibreProperties•Stiffness:170GPa•Strength:2.8GPa•Density:970kg/m3•Toughness:5MPa.m1/2CERAMICFIBRESSiliconCarbide•Covalentbondinggives–highstiffness,–highstrength,–hightemperatureresistance,–lowtoughnessSiliconCarbide•Chemicalvapourdeposition(CVD)•Usesatungsten(orcarbon)core•ManysmallcrystalsofSiCgrownoncoreCH3SiCl3(gas)SiC(solid)+3HCl(gas)SiliconCarbide•CVDmethodproduceslargediameter(150mm)“monofilaments”SiliconCarbide•Canuseapolymerfibrepre-cursor(aswithCfibres)•Polycarbosilane(PCS)mostcommon•ConvertedtoSiC(andsomeSiO2)•Comprisesmanysmallcrystals•“Nicalon”mostwidelyusedcommercialversionSiliconCarbideWhiskers•Theonlytypethataresinglecrystals(andsogetbeststrengthandstiffness)•Producedfromchemicalreactionsingasphase•Orextractedfromricehusks!•Small(1mmdiameter,50mmlength)SiliconCarbideProperties•Stiffness:250GPa(Nicalon)–700GPa(Whisker)•Strength:2.2GPa(Nicalon)–8GPa(Whisker)•Density:2,600kg/m3(Nicalon)–3,200(Whisker)BoronFibres•Producedbychemicalvapourdepositionontoatungstencore•Produceslargediametermonofilaments,similartoSiC•2BCl3(gas)+H2(gas)2B(solid)+6HCl(gas)BoronFibrePrope