Development of MoSi₂ matrix composites for high temperature structural applications

212 Pages·1992·7.7 MB·English

by  Xiao, Lingang

DEVELOPMENTOFMoSi2MATRIXCOMPOSITESFORHIGH
TEMPERATURESTRUCTURALAPPLICATIONS
BY
LINGANG XIAO
ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOL
OF UNIVERSITYOFFLORIDAINPARTIALFULFILLMENT
OFTHEREQUIREMENTSFORTHEDEGREEOF
DOCTOROFPHILOSOPHY
UNIVERSITYOFFLORIDA
1992
UWtfSITYOFFLORIDALIBRARIES
ACKNOWLEDGMENTS
I would like to express my sincerest gratitude to Dr. R. Abbaschian, my
supervisory committee chairman, for his invaluable guidance, support and
encouragementthroughouttheresearchportionofthisproject, andforhisnumerous
suggestions and corrections during the preparation of this dissertation. I am also
thankfultotherestofmycommitteemembers,Dr.R.T.DeHoff,Dr.M.J.Kaufman,
Dr. J. Mecholsky and Dr. B. V. Sankar, for their encouragement and advice
throughouttheresearchofthisproject. Theauthorwouldalsoliketoacknowledgethe
supportofthisprojectfromtheDARPAandONRthroughgrantNOOO14-9l-J-4075.
Iwouldalsoliketoextendmythankstomycolleagues,Dr.A.B.Gokhale,Dr.
Y. S. Kim, Mr. I. H. Hwang and Mr. L. Lu, with whom I had many fruitful
discussionsonvariousaspectsofthisresearch. Specialthanksmustalsobegivento
Mr.WayneAcreeforhishelpfuldiscussionsonelectronmicroscopyanalyses.Thanks
arealsoduetoDr.D.E.ClarkandMs.D.C.Folzforhelpingmetostartthesol-gel
coatingtechnique.
Finally, Iamdeeplyindebtedtomyparentsfortheirloveandencouragement
throughout my entire life, and to my loving wife and daughter for their patience,
understanding,inspirationandsupportduringthecourseofthiswork.
u
TABLEOFCONTENTS
Pages
ACKNOWLEDGMENT
ii
ABSTRACT v
CHAPTER
1.INTRODUCTION 1
2.LITERATURESURVEY 6
2.1TheoreticalBackground 6
2.2TougheningMechanismsinBrittleMatrixComposites 9
2.3DuctilePhaseToughening 22
2.4PhysicalandChemicalPropertiesofMoSi2matrix 28
3.EXPERIMENTALPROCEDURE 31
3.1MicrostructuralCharacterizationandRawMaterials 31
3.2FiberandFoilCoatingTechniques 33
3.3FabricationofMoSi2MatrixComposites 35
3.4MeasurementofIntrinsicPropertiesoftheDuctilePhase 36
3.5MeasurementofFractureEnergyofMatrix/Reinforcement
Interface 38
3.6MeasurementofToughnessoftheComposites 38
4.RESULTSANDDISCUSSION 41
4.1MicrostructuresoftheComposites 41
4.1.1MonolithicMoSi2 41
4.1.2MoSi2/uncoatedNbInterface 44
4.1.3ChemicalCompatibilityofCoatingMaterialswith
NbandMoSi2 53
4.1.4EffectivenessoftheInertCoatingsasaDiffusionBarrier 55
4.2OptimizationofCoatingProcesses 61
4.2.1CoatingviaSol-GelTechnique 62
4.2.2CoatingviaPhysicalVaporDeposition 66
4.2.3CoatingviaHotDippingandAnodizingTechnique 69
4.3FractureEnergyoftheMatrix/ReinforcementInterfaces 74
4.3.1MoSi2/UncoatedNbInterface 74
iii
4.3.2MoSi2/Al203coatedNbinterface 75
4.3.3MoSi2/Zr02coatedNbinterface 85
4.4InteractionsbetweenCracksandDuctileReinforcements 86
4.4.1RelationbetweentheExternalLoadandCrack
PropagationinMoSi2/Nbcomposites 88
4.4.2BehaviorsofCracksattheInterface 94
4.4.3FractureSurfaceObservationonMoSi2Matrix 105
4.5FlowBehaviorofConstrainedNiobium 107
4.5.1IntrinsicPropertiesoftheProcessedNb 110
4.5.2FailureMechanismandRelatedObservationon
ConstrainedNiobium 114
4.5.3Stress-DisplacementCurvesofConstrainedNiobium 120
4.5.4ModelingofFlowBehavioroftheConstrainedNiobium.... 123
4.5.5ComparisonbetweentheModelandExperiments 128
4.6EffectsofInterface,SizeandOrientationofDuctilePhaseson
Toughness 137
4.6.1Dual-EffectsoftheMatrix/ReinforcementInterfaceon
theToughnessoftheComposites 137
4.6.2EffectsofNiobiumSizeontheToughness 141
4.6.3EffectsofOrientationofNiobiumLaminaontheToughness 146
4.6.4StressAnalysisofChevronNotchedSpecimensofthe
MoS^/NbLaminatedComposites 151
4.6.5ComparisonbetweentheExper—imentsandAnalyticalResults 156
4.7ModelingofDuctilePhaseToughening LargeScaleBridging 159
4.7.1BasicModel 160
4.7.2ComputationoftheCrackGrowthResistance 163
4.7.3TypicalR-CurveBehaviorofDuctile-Phase-
ToughenedComposites 164
4.7.4PredictionoftheEffectsofMatrix/Reinforcement
InterfaceonToughness 170
4.7.5PredictionoftheEffectsofSizeofDuctilePhases
onToughness 170
4.7.6PredictionoftheEffectsofIntrinsicPropertiesof
DuctilePhasesonToughness 174
4.7.7PredictionoftheEffectsofVolumeFractionofDuctile
PhasesonToughness 174
4.7.8PredictionoftheEffectsofPropertiesofMatrixon ^
Toughness
5.CONCLUSIONS 1Q_
APPENDIX
REFERENCES
BIOGRAPHICALSKETCH
IV
AbstractofDissertationPresentedtotheGraduateSchooloftheUniversityofFlorida
inPartialFulfillmentoftheRequirementsfortheDegreeofDoctorofPhilosophy
DEVELOPMENTOFMoSi2MATRIXCOMPOSITESFORHIGHTEMPERATURE
STRUCTURALAPPLICATIONS
By
LingangXiao
December1992
Chairman:Dr.RezaAbbaschian
MajorDepartment:MaterialsScienceandEngineering
Ductilephase toughening ofMoSi2 matrix reinforced with Nb filaments or
laminaeforhightemperaturestructuralapplicationshasbeeninvestigated.Toimprove
thehightemperaturestabilityofthecomposites,AI2O3coatingbetweenthematrixand
reinforcementswasidentifiedasaneffectivediffusionbarrierandthermomechanically
compatiblewithboththematrixand thereinforcements. Thetechniquesinvestigated
forproducingtheAI2O3coatingonthesurfacesofniobiumreinforcementsincluded
sol-gelprocessing, physicalvapordeposition, andhotdippinginamoltenaluminum
followedbyanodizingtoformalumina. Theoptimizationofthesecoatingtechniques
wasconductedandtheresultsindicatedthatthebestcoatingwasprovidedwithphysical
vapordeposition,followedbythesol-geltechnique.
In order to optimize the ductile phase toughening, the interactions between
cracks and ductile reinforcements were examined. The effects of various material
parameters, such as size of the ductile phase, bonding strength of the
matrix/reinforcementinterface,andorientationoftheductilelaminaeonthetoughening
werealsoexplored.Theexperimentalresultsindicatedthattheductilephasetoughening
inthepresentsystemwaslarge-scalebridginginnature; thecrackgrowthresistance
increasedwithsizeofductilephasesforlargeinitialcracks;ductile-laminae-reinforced
composites provided a two-dimensional toughening; and the matrix/reinforcement
interfaceexhibitedadual-effectonthetoughness,thatis,astronginterfacewouldgive
risetoahighdamagetoleranceandlowworkoffracture,whileaweakinterfacewould
provideahighworkoffractureandlowdamagetolerance.
Based on the experimental observation, a rigorous analytical model was
proposed todescribethelarge-scale-bridging (LSB) oftheductilephasetoughening.
The LSB model allowed for the prediction ofthe typical R-curve behavior ofthe
composites, encompassing the effects of the bonding strength of the matrix/
reinforcement interface, the size of ductile phases, and the intrinsic mechanical
propertiesofthereinforcementandmatrix. TheLSB model, whichwasfoundtobe
consistentwiththeexperimentalresults,providesaguidelineforthedesignofductile-
phase-toughenedcomposites.
VI
CHAPTER
1
INTRODUCTION
Theneedtoincreasetheoperatingtemperatureofheatenginestoimprovetheir
efficiencyhasprovidedalargeimpetusin recentyears forthedevelopmentofhigh
temperature structural materials such as carbides, nitrides, aluminides and silicides.
Theseceramicsandintermetallics havevery high melting temperatures, high elastic
moduli, low densities, and can withstand hostile environments. Their structural
application,however,iscurrentlylimitedbytheirtendencytofailcatastrophicallyfrom
smallsurfaceand/orinternalflaws,andbythermalshock.
Accordingly, much effort has been made to improve the toughness ofthese
materials. Two general approaches have received a great deal of attention: (1)
increasing toughness through composite approaches (e.g., introduction ofa second
phasein theform ofparticles, flakes, whiskers, fibers, oreven voids toproducea
ceramic matrix compositeorintermetallic matrix composite); (2) reducing flaw size
and population through improved processing (e.g., elimination of agglomerates,
pressure sintering, etc.). Although significant improvements in minimizing strength-
degradingdefectshavebeenachievedthroughimprovedprocessinginrecentyears,this
approachisreachingitslimitation sincethecritical-sized flawscannotbeeliminated
entirely. Incontrast, compositeapproacheshaveproved tobeeffectiveinimproving
thetoughnessandhaveincreasedtheapparentfracturetoughnessofbrittlematerialsto
usefullevels(about20MPa.m1/2). However, furtheradvancesareneededtodevelop
ceramics and intermetallics with the desired level of reliability for structural
applications.
1
2
It has been established that the mechanisms responsible for composite
tougheningconsistofcrackdeflection, crackbowing, crackbridging, crackblunting
and crack shielding (i.e., phase transformation toughening and microcracking
toughening).Amongthesemechanisms,crackbridginghasbeenshowntobethemost
effectivetoughening mechanism in the sensethat itcould increasethe toughness of
compositestoausefullevelandprovidesuchtougheningatambienttemperaturesas
well as at elevated temperatures. Crack bridging is achieved generally by the
incorporation of ceramic whiskers or fibers, or ductile fibers or particles. Much
exploration has been made in the area of ceramic fiber- or whisker-reinforced
composites. For example, SiC whiskers have been used to reinforce alumina [1],
mullite[2], silicon nitride [3-6], glass [7,8], magnesia-alumina spinel [9], zirconia
[10,11], and mullite-zirconia [12,13]. ContinuousSiC fibers havealsobeen used to
reinforcesiliconcarbide[14-17], siliconnitride[18]andglass[19], Oneofthemost
importantfindingsinthebrittle-fiber-orwhisker-reinforcedbrittlematrixcompositesis
thatabrittlematrixbondedtoabrittlereinforcementbyabrittleinterfaceresultsina
toughmaterial,aresultdefyingintuition.Anotherfindingisthatthetoughnessoffiber-
orwhisker-reinforcedcompositesisflaw-sizeindependent.Incontrast,investigationon
ductilephase toughening has received relatively less attention than ceramic fiberor
whiskertoughening.Toobtainanoptimalductilephasetoughening,itisnecessaryto
considerthefollowingeffectsonthetoughness:(1)matrix/reinforcementinterface,(2)
morphologyofductilephases, (3)sizeofductilephases,and(4)intrinsicmechanical
propertiesofductilephases. Besides, mostofthetheoriesofductilephasetoughening
[20-26]arebasedonfracturemechanicsofsmall-scalebridgingwhichisusuallynot
representative ofductile phase toughening. Therefore, more accurate models (i.e.,
models of large-scale bridging) are needed to predict the overall mechanical
performance of the composite from the properties of the constituents (matrix,
reinforcement, interface). Finally, there are often deleterious interactions between
3
ductile reinforcements and intermetallic matrices during the processing and/or at
elevatedservicetemperature. Thus, itisnecessarytoestablishacoatingtechniqueto
preventtheinteractionsandmaintaintheintegrityofthereinforcement.
It is the goal ofthis research to explore ductilephase toughening in brittle
matrices for structural applications above 1200^C. Owing to the projected use
temperatures, the composites must exhibit adequate mechanical properties—high
specificstrength, toughness, good thermal shockresistance, creepresistance, fatigue
resistance-aswellasstabilityinoxidizingatmospheres. Thus, molybdenumdisilicide
(MoSi2)hasbeenchosen asthematrix material in thepresent study, becauseofits
excellentoxidationresistance, highmeltingpoint(2020^0)andrelativelylowdensity
(6.31g/cm3).Similarly,thesuitableductilereinforcementsarelimitedtotherefractory
metals, notablyW, Ta, MoandNb. Nbisofparticularinterestbecauseitsthermal
expansion coefficientis closest to that ofMoSi2, reducing the potential for matrix
crackinguponthermalcyclingduringprocessingandinservice.Theworkpresentedin
thisdissertationconsistsoffourmajorelements:
(1)establishmentofaprocessingtechniqueandtheprocesscontrolparameters
forproducingadiffusionbarrierbetweentheMoSi2matrixandNbreinforcement;
(2) study ofthe flow behavioroftheconstrained Nb and theeffects on the
toughening;
(3)investigationoftheeffectsofvariousmaterialparameters,e.g., sizeofthe
ductilephase, bondingstrengthofthematrix/reinforcementinterface, andorientation
oftheductilelaminaeonthetoughening;
(4)developmentofmodelsofthebridging micromechanicsandguidelinesfor
thedesignofductile-phase-reinforcedcomposites.
Forcontrollingthematrix/reinforcementinteraction,chemicalcompatibilitiesof
variouspotentialcoating materials (mullite, AI2O3, and Z1O2) with MoSi2 and Nb
were conducted via microstructural and compositional analyses. Based on these
4
analyses, AI2O3 was chosen as the final diffusion barrier coating because it was
chemicallycompatiblewithbothMoSi2andNbplusitalsohadathermalexpansion
coefficientclosetothoseofboththematrixandreinforcement.Thetechniquesusedfor
producingtheAI2O3coatingontheNb surfacesincluded (1) sol-gelprocessing; (2)
physical vapor deposition; (3) hot dipping the reinforcement in molten aluminum,
followed by anodizing to form AI2O3. The process control parameters for the
techniquesandtheeffectivenessofthecoatingsasadiffusionbarrierwereevaluated.
StudyoftheflowbehavioroftheconstrainedNbwasconductedusingatensile
test on a single Nb lamina imbedded in an MoSi2 matrix. The effects of the
matrix/reinforcement interface, the mechanical properties (yield strength and work
hardening rate) and sizeofthe ductile phase on the flow behavior were evaluated.
Basedontheobservationsfromthetensiletests, anapproximatemodelwasproposed
which gave insight into the influence of yield strength, work hardening,
matrix/reinforcementinterfacialbondingstrengthandsizeoftheductilereinforcement
ontheflowbehavior.Thestress-displacementfunctiondevelopedwasusedtocalculate
thebridgingstressacrossthecracksurfaceinthelarge-scale-bridgingmodelproposed
inthisstudy.
The role of the matrix/reinforcement interface in the fracture toughness of
MoSi2/Nb composites was assessed via four-point bend tests on chevron notched
specimens. Variationoftheinterfacialbonding was obtained by depositing different
oxidecoatings(AI2O3andZrC>2)orbythedevelopmentofareactionproductlayer
between the matrix and reinforcement. Measurement of fracture energy (bond
toughness)oftheinterfaceswascarriedoutonachevronnotchedshortbarspecimen.It
was established that whether or not a strong interfacial bonding was conducive to
toughnessdependedonthecriterionusedtodescribethetoughnessofthecomposites.
Four-pointbendtestswerealsousedtoevaluatetheeffectsofthesizeandorientation
ofductilelaminaeonthetoughnessofthecomposites. Theresultsindicated thatthe