低溫常壓活化逆反應(yīng)燒結(jié)β-SiN陶瓷的性能與結(jié)構(gòu)研究

Inordertoresolvetheproblemsofhighproductioncostandlowoutputinthetraditionalmakingprocessofα－Si3N4ceramic,theβ－Si3N4powdercomposedbyflashcombustionnitridingmethodwasusedastherawmaterials,addingAl2O3、Y2O3andmetalAlassinteringadis,moldingedbyisostaticpressuremethod,thereversereactionsinteringatnormalpressurewasusedformakinglowcostandhighqualityβ－Si3N4ceramics,whichwouldexpandthefieldofapplicationofSi3N4ceramicenormously.Thereversereactionsinteringprocessisanewconceptraisedaccordingtotheprocessmechanism.TheclouistosacrificeafewofSi3N4forforminganeonatalcombinedphaseofSi-Al-O-NsystembyoxidationreactionofSi3N4.ThekeyiscontroltheoxidationofSi3N4byregulatetheoxygenpartialpressure,notonlyavoidingtheexcessiveoxidized,butalsoachieveactivesintering.Ithasbeenstudiedasfollow:discussingthefeasibilityofusingthereversereactionsinteringtoprepareβ－Si3N4ceramicbythermodynamicsanalyse;ascertainingtheoptimaloxygenpartialpressureandsinteringsystembycomparingthesinteringproducepropertiesofdifferentsystemwhichsinteredinvaryoxygenpartialpressureat1550℃,andtheinfluenceofcontentofAl2O3orY2O3onsinteringproperties;discussingtheinfluenceofnano-Al2O3anditscontentonsinteringpropertiessinteredat1600℃,andtheinfluenceofcontentofAlpowdersinteredat1650℃;finally,discussingtheinfluenceoftemperatureonsinteringpropertiesbythesinteringexperimentsat1550、1600、Theresultsindicatedthat:Itwasfeasibleusingthereversereactionsinteringtoprepareβ－Si3N4ceramic.Theoptimaloxygenpartialpressurewas0.002MPa,theoptimalsinteringsystemwasSi3N4-Al2O3-Y2O3-Al,theoptimalcontentofAl2O3wasw%=10.ThemoreY2O3wasadded,themoreexcellentwerethepropertieswhensinteredat1550℃.Thecontributionofnano-Al2O3tocoldcrushingstrengthwasenormous,andthecontributionwasenhancedwiththeincreasingofthecontentwhensinteredat1600℃.ThesinteringpropertieswasbestwhentheadditionofAlpowderw%=2andsinteredat1650℃.Thepropertiesoftheproducewereimprovedwiththerisingofthesinteringtemperaturefrom1550to1650℃.Thevolumedensitymaximum3.14g/cm3,thecoldcrushingstrengthachievedmaximum470MPa,sothesampleachievedcompactsinteringwhensinteredat1650℃.MgO是人們最早使用的燒結(jié)助劑，引入MgO作添加劑，可以保障液相形成和制得高密度Si3N4材料。液相的形成是由于MgO與SiO2(始終以Si3N4粒子表面膜形式參與)相互作用的結(jié)果，起初人們認(rèn)為，所形成液相的組成大致與MgSiO3－SiO2共晶體相符，但后來確定，它是具有Mg－Si－O－N四種成分的較復(fù)雜組成。冷卻時(shí)，這種液相在Si3N4晶界上形成玻璃相，高溫下晶界軟化和變形使Si3N4強(qiáng)度和韌性都大幅度下降。引入稀土元素氧化物添加劑時(shí)，由于復(fù)雜氧化氮化物的形成，在陶瓷結(jié)構(gòu)中產(chǎn)生熱強(qiáng)晶間相。這種Si3N4材料在較高溫度下具有極高強(qiáng)度，稀土是合金化和摻雜組元的理想元素，作為添加劑一般多存在于Si3N4的晶界處，經(jīng)過熱處理易于析出二次小晶粒。文獻(xiàn)REF_Ref118471607\r\h[34]在使用MgO－CeO2做燒結(jié)助劑的過程中發(fā)現(xiàn)，燒結(jié)初期(1450～1550℃)MgO－CeO2會(huì)與Si3N4粉末表面的SiO2反應(yīng)形成硅酸鹽液相，促進(jìn)燒結(jié)致密化，使材料具有優(yōu)異的常溫性能；在燒結(jié)后期高于1550℃時(shí)，會(huì)出現(xiàn)MgO的自動(dòng)析晶現(xiàn)象，減少了對(duì)高溫性能有害的玻璃相，大大提高了氮化硅陶瓷的高溫性能。一般來說，燒結(jié)助劑的作用機(jī)理是：在燒結(jié)過程中燒結(jié)助劑與Si3N4顆粒表面的SiO2反應(yīng)形成液相，促進(jìn)燒結(jié)致密化。但是，冷卻后這些液相往往形成玻璃相分布在晶界，大大降低了Si3N4陶瓷所固有的高溫性能。因此，對(duì)氮化硅陶瓷的研究有很大一部分集中在晶界玻璃相的晶化上，被形象地稱為晶界工程。目前，主要通過以下途徑來改善材料的高溫性能：形成