NASA-TM-106992 19960000619 NASA Technical Memorandum 106992 CVD Silicon Carbide Monofilament Reinforced SrO-A1203-2SiO2 (SAS) Glass-Ceramic Composites Narottam P. Bansal Lewis Research Center Cleveland, Ohio I i i : I i August 1995 ! q': ! _: ..... _-p . 1.-3 I I r,t_l F',' n°- .... --- , a National Aeronautics and Space Administration https://ntrs.nasa.gov/search.jsp?R=19960000619 2018-03-26T09:26:17+00:00Z
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only limitedimprovementin strengthover SASmonolithic.The SCS-0/SAScompositehaving
a fibervolumefractionof 0.24andhotpressedat 1400°Cexhibiteda first matrixcrackingstress
of -231 _+_20 MPa and ultimatestrengthof 265 _ 17 MPa. From fiberpush-outtests, the
fiber/matrixinterracialdebondingstrength(z_d) and frictionalsliding stress (Tfrietio_) in the
SCS-6/SASsystemwere evaluatedto be -6.7 + 2.3 MPa and 4.3 ___0.6 MPa, respectively,
indicatinga weak interface. However,for the SCS-0/SAScomposite,much higher values of
- 17.5 4- 2.7 MPa for l"ao_dand 11.3 -I-1.6 MPa for zf,_cao,respectively,were observed;some• of the fibers were so strongly bonded to the matrix that they could not be pushed out.
Examinationof fracture surfacesrevealedlimited short pull-outlength of SCS-0fibers. The
for the matrixcrackingstressin a compositeconsistingof a lowfailurestrainmatrixreinforced
with high failurestrain continuousfibers:
ay = [(12rf_¢ao,I'mV2 EfE¢2)/{r_l-Vf)Em2}]_n (7)
where I'mis the matrix fracture surface energy, E_ -- VfE¢+ VmF__,and other terms have the
same meaning as above. It is apparent from this equation that the first matrix cracking stress can
be enhanced by increasing fiber-matrix interfacial sliding stress, by using fibers of smaller
radius, and by increasing the volume fraction of fibers. It might also be increased, less easily,
by increasing the ratio Ef/F__.The matrix microcracking may also be suppressed by placing the
matrix in compression through choosing af > am,although for isotropic fibers this will result
in contraction of the fibers away from the matrix and a potential decrease in fiber-matrix shear
strength. It is important to optimize the fiber-matrix bond strength carefully as too strong a bond
will result in a brittle composite with low toughness. Typical values of F, for ceramics range
from 20 to 40 J/m2as quoted by Briggs and Davidge.34Values of I'mfor calcium aluminosilicate
and lithium aluminosilicate glass-ceramic matrices have been reported to be 25 and 20-30 J/m2,
respectively. Taking 1", -- 25 Jim2 for the SAS glass ceramic matrix, and values of other
15
parameters as shown above, the ay for the SCS-6/SASand SCS-0/SAScomposites were
calculatedfrom eq. (7) to be 179MPaand 185MPa, respectively,withoutany correctionsfor
the expectedresidual stressesin the matrix. In spite of higher fiber volume fraction in SCS-
6/SAScomposite,the calculated%is higher for the SCS-0/SAScomposite.This is becauseof
much higher value of sliding frictional stress at the interface, _'f,_c_o,,for the SCS-0/SAS
compositewhich, accordingto eq. (7), shouldresult in higher or. These calculatedvalues of
Cryare close to thoseobtainedfrom eq. (4) but much lower than the values of 290 4- 40 MPaand231 4-20 MPameasuredin 3-pointflexurefor thetwocomposites,respectively.However,
it may be pointed out that generally the tensile strengthsare lower than those measuredin
bending and the tensile test results, rather than the flexural data, are more meaningfulfor
comparisonwith the predictionsof the micromechanicalmodels. Also, the effects of internal
residualstressesarisingfrom thethermalexpansionmismatchbetweenthe fiberand the matrix,
whichhavebeen neglectedin the calculationsof the abovemodels,must be takeninto account.
The axial residual stress in the matrix, Cr,,,in the compositeas a result of coolingfrom the
6) fiber-reinforcedSASglass-ceramicmatrixcompositescan be obtainedby hot pressingat
3.9
1250 - 1400 °C for 2 h at 20 - 27 MPa (3 - 4 KSI). Also, uncoatedSCS-0 fiber is not
appropriatefor the reinforcementof the SASglass-ceramicmatrix. CVDSiCSCS-6fibersand
the SASmatrixare chemicallycompatibleevenat temperaturesas high as 1400°C.The current
theoreticalmodelsdo not appear to be appropriatein predictingthe matrixmicrocrackingstress
or the ultimate strength of the large diameterCVD SiC fiber reinforced SAS glass-ceramic
matrixcomposites.
ACKNOWLEDGMENTS:Thanksare due to JohnSetlock,Dan GoricanandRichardFirst for
their assistancein compositeprocessingand testing.
20
REFERENCES
1. Bansal, N.P., Ceramic Fiber Reinforced Glass-Ceramic Matrix Composite, U. S. Patent• 5,214,004, May 25, 1993.
2. Bansal, N.P., Method of Producing a Ceramic Fiber-Reinforced Glass-Ceramic MatrixComposite, U. S. Patent, 5,281,559, January 25, 1994.
3. BansaI, N.P., SiC/Celsian Glass-Ceramic Matrix Composites, HITEMP Review 1991:Advanced High Temperature EngineMaterials Technology Program, NASA CP-10082, 1991,pp. 75-1 to 75-15.
4. Wawner, F.W., Teng, A.Y., and Nutt, S.R., Microstructural Characterization of SiC (SCS)Filaments, SAMPE Quart., 1413] 39-45 (1983).
5. Eldridge, J.I., Bhatt, R.T., and Kiser, J.D., Investigation of Interfacial Shear Strength inSiC/Si3N4Composites, NASA TM-103739, 1991.
6. Bansal, N.P., and Drummond, C.I-I., III, Kinetics of Hexacelsian-to-Celsian PhaseTransformation in SrA12Si2Os,J. Am. Ceram. Soc., 7615] 1321-1324 (1993)
7. Bansal, N.P., Fiber-Reinforced Refractory Glass-Ceramic Composites, in HITEMP Review1993: Advanced High Temperature Engine Materials Technology Program. Vol. UI TurbineMaterials-- CMCs, Fibers, NASA CP 19117, p. 63-1 to 63-13 (1993).
8. Marshall, D.B., and Evans, A.G., Failure Mechanisms in Ceramic-Fiber/Ceramic MatrixComposites, in "Ceramic Containing Systems", A.G.Evans, Ed., Noyes Publications, ParkRidge, NJ, 1986, pp. 90-123.
9. Singh, R.N., Influence of Testing Method on Mechanical Properties of Ceramic MatrixComposites, J. Mater. Sci. , 26123] 6341-6351(1991).
10. Bleay, S.M., Scott, V.D., Harris, B., Cooke, R.G. and Habib, F.A., Interface Characteriza--tion and Fracture of Calcium Aluminosilicate Glass-Ceramic Reinforced with NicalonFibers, J. Mater. Sci., 27[10] 2811-2822 (1992).
11. Prewo, K.M., Tension and Flexural Strength of Silicon Carbide Fiber Reinforced GlassCeramics, J. Mater. Sci., 21[10], 3590-3600 (1986).
12. Bhatt, R.T., and Hull, D.R., Microstructural and Strength Stability of CVD SiC Fibers inArgon Environment, NASA TM 103772, 1991.
13. Aveston, J., Cooper, G.A., and Kelly, A., Single and Multiple Fracture, in "The Propertiesof Fiber Composites", IPC Sci. and Technol. Press, Guildford, 1971, pp. 15-26.
14. Wang, S.-W., and Parvizi-Majidi, A., Mechanical Behavior of Nicalon Fiber-ReinforcedCalcium AluminosilicateMatrix Composites, Ceram. Eng. Sci. Proc., 1119-10] 1607-1616
21
(1990).
15. Evans, A.G., The MechanicalPerformanceof Fiber-ReinforcedCeramic MatrixComposites,in "Mechanicaland PhysicalBehaviorof Fiber-ReinforcedCeramicMatrixComposites', editedby S.I. Andersen,H. Lilholt, and O.B. Pedersen, Riso, Denmark,1988, pp. 13-34.
16. Dharani, L.R., Rahaman,M.N., and Wang, S.-H., InterfacialShear Stressin SiC FiberReinforcedCordierite,J. Mater. Sci., 2613]655-660(1991).
17. Weihs, T.P., and Nix, W.D., ExperimentalExaminationof the Push-DownTechniqueforMeasuringthe SlidingResistanceof SiliconCarbideFibers in a CeramicMatrix, J. Am.Ceram.Soc., 7413]524-534(1991).
24. Singh,R.N., Influenceof InterfacialShearStress on First-MatrixCrackingStress inCeramicMatrixComposites,J. Am. Ceram.Soc., 73110]2930-2937(1990).
25. Budiansky,B., Hutchinson,J.W., and Evans, A.G., MatrixFracture in Fiber-ReinforcedCeramics,J. Mech. Phys. Solids, 3412]167-189(1986).
26. Phillips,D.C., Sambell,R.A.J., and Bowen,D.H., The MechanicalPropertiesof CarbonFiber ReinforcedPyrex Glass,J. Mater. Sci., 7, 1454-1464(1972).
27. Curtin,W.A., Theory of MechanicalPropertiesof Ceramic-MatrixComposites,J. Am.Ceram.Soc., 74111]2837-45(1991).
22
28. Draper, S.L., Brindley, P.K., and Nathal, M.V., Effect of Fiber Strength on the RoomTemperature Tensile Properties of SiC/Ti-24Al-llNb, Metall. Trans. A, 23A, 2541-48(1992).
29. Bansal, N.P., Processing and Properties of CVD SiC Fiber-Reinforced BaA12Si20s Glass-Ceramic Matrix Composites, in Proc. of 17th Conference on Metal Matrix, Carbon, andCeramic Matrix Composites, Cocoa Beach, FL, Jan. 10-15, 1993; NASA CP 3235, Part 2,pp 773 - 797 (1994).
30. Murthy, V.S.R., and Lewis, M.H., Matrix Crystallization and Interface Structure in SiC-Celsian Composites, Br. Ceram. Trans. J., 8915] 173-174 (1990).
31. Martineau, P., Lahaye, M., Pailler, R., Naslain, R., Couzi, M., and Cruege, F., SiCFilament/Titanium Matrix Composites Regarded as Model Composites. Part 1. FilamentMicroanalysis and strength Characterization, J. Mater. Sci., 1918] 2731-48 (1984).
32. MacKay, R.A., Draper, S.L., Ritter, A.M., and Siemers, P.A., A Comparison of theMechanical Properties and microstructures of Intermetallic Composites Fabricated by TwoDifferent Methods, Metall. Trans. A, 25A[7] 1443-1455 (1994).
34. Briggs, A. and Davidge, R.W., Borosilicate Glass Reinforced with Continuous SiliconCarbide Fibers: A New Engineering Ceramics, Mater. Sci. Eng., A109, 363-372 (1989).
35. Bansal, N.P., Strontium Aluminosilicate Glass-Ceramic Composites Reinforced WithUncoated CVD SiC Fibers, NASA TM 106672 (1994).
36. Bansal, N. P., McCluskey, P., Linsey, G., Murphy, D., and Levan, G., Processing andProperties of Nicalon-ReinforcedBarium Aluminosilicate (BAS)Glass-Ceramic Composites,Paper Presented at 19th Annual Conference on Composites, Materials, and Structures(Restricted Sessions), Cocoa Beach, FL; January 9-10, 1995.
37. Dawson, D. M., Preston, R. F., and Purser, A., Fabrication and Materials Evaluation ofHigh Performance AlignedCeramicFiber-Reinforced Glass Matrix Composite, Ceram.Eng.Sci. Proc., 817-8] 815-21 (1987).
38. Hegeler, H. and Bruckner, R., Fiber-Reinforced Glasses: Influence of Thermal Expansionof the Glass Matrix on Strength and Fracture Toughness of the Composites, J. Mater. Sci.,25111] 4836-46 (1990).
• 39. Shetty,D. K., Pascucci,M. R., Mutsuddy,B. C., and Wills,R. R., SiC Monofilament-ReinforcedSi3NaMatrixComposites,Ceram.Eng. Sci. Proc., 617-8]632-45(1985).
40. Xu, H. H. K., Ostertag, C. P., Braun, L. M., and Lloyd, I. K., Effect of Fiber VolumeFraction on Mechanical Properties of SiC-Fiber/Si3N4-MatrixComposites, J. Am. Ceram.
SCS-0/SASComp. [Vf = 0.24; SAS 6-9-93]1.44 17.5 (2.7) b 11.3 (1.6)
aMean value for 8-10 fibers. Values in parenthesis are standarddeviation.bValuesin table are for fibers that debonded. However, one fiber showed 7"d_bond
of 56.1 MPa which is not included in the average; some fibers did not debondup to a load of 40 N, the upper limit of the apparatus, resulting in _'O_bo,d> 62 MPa.
TableIV. Effect of Hot Pressing Time at 1450 °C, 24 MPa on Fiber/MatrixInterface Shear Strength, Sliding Frictional Stress and Flexure Strength
of CVD SiCf (SCS-6)/SAS Composites
Hot press time, min (Yy,MPa a CYu,MPaa 1;debond,MPab "Cfriction,MPab
Figure5.mPowder x-ray diffractionpatternof SCS-0/SAScompositehotpressedfor2 hr at 1400°C at 27.6 MPa.The peak at 2e = 35.6 is due to _-SiC. The remainingpeaks correspond to monoclinic celsian.
1000 --_u = 824 MPa--_
'_ r-_y = 248 MPa\
\\800 \
300 _ .-'-'- _u = 285 MPa
Q.600 -- Test -"
stopped -_'" 200P
_- 400m_ ._y = 289 MPa _
100
200SAS monolithic1200°C hotpressed
0 09
Displacement Displacement(a) (b)
Figure 6.--_tress-displacement curves. (a)SASmonolithic hot pressed at 1200°C for 2 hr under 24 MPa and aunidirectional SCS-6/SAScomposite (Vf= 0.24)hot pressed at 1400°Cfor 2 hr under 24 MPa. (b)A unidirectionalSCS-0/SAScomposite (Vf = 0.24) hot pressed at 1400 °Cfor 2 hr under 27.6 MPa.The monolithic ceramicwastested infour-point bending and the composites in three-point bending, respectively.
31
700
600 -- []
strengthn
cc_500--
f-
400-0
t-.-
300 -- / Firstmatrixcrackingstress
2000 10 20 30 40
Test spanto thicknessratio,L/d
Figure 7.--Effect of test span length to sample thickness ratio (l/d) on first matrix cracking stress and ultimatestrength, measured in three-point flexure, for a unidirectionalSCS-6/SAScompositehotpressedat 1350 °Cfor2 hrat 24 MPa; VI = 0.25. One tesVdata point except for I!d = 32 where five specimens were tested.
1000 ---- 140
800 -- Ultimatestrength -- 120
-- 100
600 --P -- 80
Q-- 60
__ 400-o FirstmatrixcrackingstressQ.
- 40€-v- 200 --
SASmonolithic -- 20
0 I I I I I I1150 1200 1250 1300 1350 1400 1450 1500
Hot pressingtemperature,°C
Figure8.--Effect of hotpressingtemperatureonfirstmatrixcrackingstressand ultimatestrength,measuredat roomtemperatureinthree-pointflexure,fora unidirectionalCVDSiCf(SCS-6)/SAScompositehotpressedfor2 hr at 24 MPa;Vf = 0.25 ± 0.01.Alsoshownisthe four-pointbendstrengthof a SAS monolithicsamplehotpressedat 1200°C for2 hr.
32
1000 ---- 140
• -- 120800 --n
=- Ultimate_-strength __ 100
600 --80
==X
I 60_ 400 --0
_ _-- 40v- 200 --
-- 20
o I I I I0 30 60 90 120
Hotpressingtime,min
Figure9.--Effect of hotpressingtime onroomtemperaturethree-pointflexuralstrengthofunidirectionalSCS-6/SAScompositeshotpressedat 1450°C at24 MPa; Vf = 0.24 _.%0.01.Thelinesdrawnare onlyguideto the eyes.
1000 -- -- 140
-- 120800 --0.
-_ -- 100
600-80
,=
= -- 60400-
Firstmatrixcrackingstress
_-- 40200 ---- 20
o l I l l I1.5 2.0 2.5 3.0 3.5 4.0 4.5
, Pressureof hotpressing,Psi
Figure10.--Effect of hotpressingpressureonroomtemperaturethree-pointflexuralstrengthof unidirectionalSCS-6/SAScompositeshotpressedat 1450°Cfor 2 hr;Vf = 0.26 _+0.02.
33
1000 _ _ 140
vf_-026_o " _ 120n "
800 _ ",
-- 100 .e-
P 600-- 80
.=X "
= -- 60400 --
o
_ _ 40_- 200_- Firstmatrixcrackingstress _ 20
0 I I I I 00 1 2 3 4 5
Pressureof hotpressing,ksi
Figure11.--Effect of hot pressingpressureon roomtemperaturethree-pointflexuralstrengthof unidirectionalSCS-6/SAScompositeshotpressedat 1250°C for2 hr;,Vf = 0.22 _+0.01.
3,1 m
O3E 3.0 w
2.9 m
.Jffl
._ 2.8 m
0 1 2 3 4Pressureof hot pressing,ksi
Figure12.DEflect of hotpressingpressureondensitiesof unidirectionalSCS-6/SAS composites hot pressed at 1250 °C for 2 hr, VI = 0.22 + 0.01.
Figure20.---SEMmicrographsshowingin-placeandpushedout fibers inCVDSiCf/SAScomposites.(a)SCS-6/SAScom-posite hotpressedat 1500°C for 2 hrat 24 MPa.(b)SCS-0/SAScompositehotpressedat 1400°C for 2 hrunder27.6 MPa.
38
Figure21.--SEM micrographand x-raymapsof variouselementsat thefiber-matrixinterfaceof the polishedcross-sectionof a unidirectionalCVDSiCf(SCS-0)/SAScompositehotpressedat 1400°Cfor 2 hrunder 27.6 MPa.
39
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Unidirectional CVD SiC fiber-reinforced SrO.AI203.2SiO2 (SAS) glass-ceramic matrix composites have been fabricatedby hot pressing at various combinations of temperature,pressure and time. Both carbon-rich surface coated SCS--6anduncoated SCS--0fibers were used as reinforcements. Almost fully dense compositeshave been obtained. Monocliniccelsian, SrA12Si208, was the only crystalline phaseobserved in the matrix from x-ray diffraction. During three pointflexure testing of composites, a test span to thicknessratio of-25 or greaterwas necessary to avoid sample delamination.Strong and tough SCS--6/SAScomposites having a first matrix crack stress of-300 MPa and an ultimate bend strength of-825 MPa were fabricated. No chemical reactionbetween the SCS-6 fibers and the SAS matrix was observed after hightemperatureprocessing. The uncoated SCS-0 fiber-reinforcedSAS composites showed only limited improvement instrengthover SAS monolithic. The SCS-0/SAS composite having a fiber volume fraction of 0.24 and hot pressed at1400°Cexhibited a first matrix cracking stress of-231+20 MPa and ultimate strength of 265+17 MPa. From fiberpush-out tests, the fiber/matrix interfacial debonding strength (Xdebond)and frictional sliding stress (xfriction)in the SCS-6/SASsystem were evaluated to be -6.7+_2.3MPa and 4.3+0.6 MPa, respectively,indicating a weak interface. However, for theSCS--0/SAScomposite, much higher values of-17.5+2.7 MPa for'Cdebondand 11.3_+1.6MPa for'tfrictionrespectively,were observed; some of the fibers were so stronglybonded to the matrix that they could not be pushed out Examinationof fracture surfaces revealed limited short pull-out length of SCS-0 fibers.The applicability of various micromechanicalmodels for predicting the values of first matrixcracking stress and ultimate strength of these composites were examined.