The direct measurement of interfacial bond strength and frictional resistance to sliding in a fully-bonded SiC/SiC composite is measured. It is shown that a fiber push-out technique can be utilized for small diameter fibers and very thin composite sections. Results are presented for a 22 micron thick section for which 37 out of 44 Nicalon fibers tested were pushed-out within the maximum nanoindentor load of 120 mN. Fiber interfacial yielding, push-out and sliding resistance were measured for each fiber. The distribution of interfacial strengths is treated as being Weibull in form. 14 refs., 5 figs

Snead, L.L.

Steiner, D. (Rensselaer Polytechnic Inst., Troy, NY (USA))

Zinkle, S.J. (Oak Ridge National Lab., TN (USA))

UNT Digital Library

INTERFACIAL PUSH-OUT MEASUREMENTS OF FULLY-BONDED SIC/SlC COMPOSITES*CONF-90III 22--2Lance LSnead, Rensselaer Polytechnic Institute DP,,9I 004527Steven J Zinkle, Oak Ridge National LaboratoryDon Steiner, Rensselaer Polytechnic InstituteABSTRACTThe direct measurement of interfacial bond strength and frictionalresistance to sliding in a fully-bonded SiC/SiC composite is measured, lt isshown that a fiber push-out technique can be utilized for sma!l diameter fibersand very thin composite sections. Results are presented for a 22 micron thicksection for which 37 out of 44 Nicalon fibers tested were pushed-out within themaximum nanoindentor load of 120 mN. Fiber interfacial yielding, push-outand sliding resistance were measured for e_ch fiber. The distribution ofinterfacial strengths is treated as being Weibulllform.INTRODUCTION ,VkCeramic composite strength and fracture toughness are very sensitive tothe type and magnitude of bonding at the fiber/matrix interface1, 2. Thisinterface has the dual roles of being strong enough to transfer load betweenmatrix and fiber to take advantage of the high fiber strength and being compliantenough to inhibit crack propagation, thus adding toughness to an otherwisebrittle composite.Due to the fundamental importance of the composite interface and thenewness of direct interfacial measurement techniques, there has been growinginterest in this area. There have been several recent papers in whichinvestigators have employed microindentation testing techniques to measureinterfacial strength3,4,5, 5 . The bulk of the work for ceramic fiber compositeshas been on SiC/glass systems which, due to the mismatch in thermal expansionsand resulting debonding during process cooling, has a pre-cracked interfaceyielding only frictional resistance to fiber sliding. Marshall?,8 was the first toemploy a microindentation technique to measure the interfacial frictionalstrength by using a Vickers pyramid indentor to apply a load and push-downfibers oriented normal to a polished SiC(Nicalon)/lithium.alumino-silicate(LAS) glass surface. The interfacial friction results were shown to comparereasonably well with data taken by matrix crack-spacing measurements.lt is clear that for a fully.bonded interface there will be at least twocompeting resistances to fiber sliding. First, there will be a minimum forcerequired to initiate a crack in the interfacial chemical bond which may or maynot then have the required energy to propagate along the length of the fiber.Second, there will be a frictional component to the resistance which is caused bythe crack surfaces passing over each other as the fiber slides.,k-Research sponsored by the Office of Fusion Energy, U.S..Department of Energy, under contract DE-ACO5-840R21400 withMartin Marietta Energy Systems, Inc.l_ulhorlK_lby ! COnlrsctot ol II'_iU SGox_Kr_nt ur_ ca_t,lct No DE-I1"I _ |_'m OI Thm_ COd_lr_.JtlOr_Or I ._low otl_ws lo _o so foe U S Gov_wnrr_l /,/,NSIfiJBUIIONOF TFU_DI]CUI'rIF..NTI,_IJ .LII_I.T._,nMarshall and Oliver 9 were the first to use a microindentalion lechniqueto attempt to separate the effecls of bonding and frictional sliding in weaklybonded composites. In their experiments, they heat treated as-processed,unbonded SiO(Nicalon)/lithium-alumino-silicate (LAS) composites in air andargon between 900 C and 1250 C, forming a carbon chemical bond al theinterface. A Nanoindentor TM was used to apply loads to fibers of thick sections ofthe as-processed and heat-treated composite. Using the Nanoindentor, anaccurate force-rs-displacement curve was generated and from the loading curveand the unload-reload hysteresis curves an estimate of the debonding andfrictional resistance could be obtained.Later researchers 10 employed a similar methodology with compositesections thin enough such that a small percentage of the fibers were "pushed-out" of the matrix, thereby simplifying analysis of the interfacial strengths.Bright et. al. 5 applied the push-out technique to fully bondedSiC(AVCO)/reaction-bonded-silicon-nitride composites and measured bothbonding and frictional interfacial strength for sections as thin as 0.5 mm,. or afiber diameter-to-thickness ratio of approximately 1/35. For fiber push-outexperiments it is generally best to have as large a fiber diameter to thicknessratio as possible while not altering the fiber/matrix interface in the thinningprocess. By doing this the calculation of the interfacial strength is simplified.The purpose of this paper is to present interfacial bond and frictionalstrengths obtained from push-out measurements for a very thin section ofSiC(Nicalon)/SiC(CVD) composite material. Ali results reported are for asingle composite section of 22 microns, which corresponds to a Nicalon fiberdiameter to specimen thickness ratio approaching unity. Whereas resultsrepresent a single composite section, they are typical of other sections tested.EXPERIMENTALThe composite specimen is a layered 30/60/90 weave of Nicalon clothinfiltrated with SiC using the ORNL Forced Chemical Vapor Infiltrationprocess. _ ! The fiber/matrix interface contains a 1 micron layer of graphiticcarbon which was deposited onto the fibers from methane gas prior to SiCinfiltration.The composite was sliced into 250 micron sections with a low speeddiamond saw and mechanically thinned and polished with 6, 1 and 1/2 microndiamond paste to a final thickness of 22 microns• The specifnen was thenmounted onto a polished aluminum holder such that the center of the specimenwas positioned over a groove of 200 microns in width. The composite sectionwas then held rigidly to the mount and fixed to the surface with Crystal BondTMm_unting wax. Care was taken during the thinning and mounting procedure toprevent specimen bending in order to avoid pre-cracking the interface.A load-controlled Nanoindentor microindentation hardness tester wasused to apply a force to fibers oriented normal to the composite surface. A gooddiscussion of the Nanoindentor can be found elsewhere. ]2 The Nanoindentor hasan applied force and depth measurement sensitivity of 0.3 IaN and 0.16 nm,respectively. The positioning table has sub-micron accuracy to insure loadingnear the center of fibers which have diameters ranging from 12 to 25 microns.DISCLAIMERThis report was prepared as an account of work _;ponsored by an agency of the United StatesGovernment. Neither the United States Government nor any agency thereof, nor any of theiremployees, makes _ny warranty, express or implied, or assumes any legal liability or responsi-bility for the accuracy, completeness, or usefulness of any information, apparatus, product, orprocess disclosed, or represents that its use would not infringe privately owned rights. Refer-ence herein to any specific commercial product, process, or service by trade ro,me, trademark,manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom-mendation, or favoring by the United States Government or any agency thereof. The viewsand opinions of authors expressed herein do not necessarily state or reflect those of theUait4'_ Sr_'t,"_ C,,':,,r,-........ ,................ ", .....A constant loading rate of 900 l.tN/sec was used with a maximum loadingcapability of 0.12 N which corresponds to approximately 700 MPa for theaverage fiber. A Berkovitch pyramidal diamond indentor tip with an aspect ralioof 1:7 was used.Once the fibers were loaded to their debond load and pushed through lhematrix (or to the maximum load of the machine while remaining intact), thespecimen was then demounted and turned over in the holder such that thedebonded fibers again lie over the 200 micron groove. The position of the fibers,which were protruding from the matrix, were found and the load reapplied withthe Nanoindentor so that the fibers passed through their original position and outthe opposite side of the composite section. The individual fiber diameters weremeasured by utilizing the Nanoindentor optics and positioning.RESULTSA total of the 44 fibers were tested of which 3/ fibers debonded withinthe machine's maximum load oi' 0.12 Newtons. Ali of the debonded fibers thatwere tested in reverse loading showed frictional sliding. Figure 1 shows the loaddisplacement curve for a fiber that did not fail, which provides a measure of thespecimen compliance. This can be crucial since if there is appreciable specimenbending present, there may be a fiber pinching effect which would affect theresults, it can be seen from Figure 1 that the maximum diamond indentor depthcorresponding to maximum machine load is 1.5 microns (marked A) of which0.4 microns (marked B) is plastic deformation of the fiber.Previous researchers have noted the problems of testing very thincomposite sections. 5,8,13 The interfacial bonding and sliding frictionstrengths can be affected by precracking of the interface during thinning and byaltering the interfacial stress state as the result of specimen compliance duringtesting. However, by careful mechanical thinning and polishing it is possible toovercome these problems with SiC composites, lt is easily shown that the 1.1micron displacement (in Figure 1) of a large section suspended over a 200micron groove will have a negligible effect on the results. Also, due to the factthat the matrix SiC becomes transparent for thicknesses less than(approximately) 50 microns, the absence of large matrix cracks were verifiedusing a transmission optical microscope.lt is also seen from Figure 1 that the load-displacement (and hyslerisis)curve has the parabolic shape characteristic of indentation hardnessmeasurements of monolithic materials, which indicates an absence of fiberinterfacial failure. When fiber interfacial cracking occurs, followed by a fiberdebonding, there will be a noticeable departure from the parabolic load curve.The bond failure can be either a partial or total debonding depending on whetherthe crack front has the energy to propagate the length of the fii:;er. The partialdebonding is measured here as 2 % departure from the parabolic load curvewhereas the total fiber debond is measured as the stress at which the fiber ispushed-out of the matrix. However, it was observed that a fiber could exibitone, both or neither of the failure mechanisms. Some fibers showed no parabolicdeviation prior to the point of complete push-out, which would correspond to avery rapidly propagating crack front, whereas other fibers showed a deviation ata relatively low load and no push-out within the r:_achine's maximum load, whichis indicative of an arrested crack front.I " I .... '' '' I ' ' '/0 ' C .. ! .... J __ _-L..-J__J_ :.'Y.I I ! .....js _j_.*0 ; ._:,_ 2; :):) 0 _;':)'3 ;.':):'.:) 3 ;, 3"_ '__;,_Figure 1. Loading curve for fiber Figure 2. Loading cLJrve for fiberremaining intact, debonding and push-out.Figure 2 shows an example of a bonding failure exhibiting distinctinterfacial yielding followed by total debcnding. As the fiber was loaded therewas a partial yield of the interface at approximately 65 mN (A) followed by atotal debonding and fiber push-out at 80 mN (B). The fiber was pushed-out ofthe composite section approximately 2.2 microns at which point the indentorcontacted the matrix surrounding the fiber. The matrix contact depth is seen inFigure 2 at an indentor depth of 3000 nm. Both partial debonding (A) and thepoint of fiber push out (B) are noted. The absence of data points between B abd Cis evidence of the rapid fiber failure indicative of the total interfacial debondingand push-out. The stress at failure is taken to be the load applied at the point ofyielding divided by the interfacial area. This, however, is an oversimplificationas the load is actually concentrated at the crack front itself and not spread overthe entire interface.The failure in the reverse loadingsituation is shown in Figure 3. The...... frictional failure is characterized by_:):-' I ' I i i ] l'T-i i t i i i i i I i i, , , agraceful departure from the normalloading curve. The frictional stress is{ obtained by dividing the sliding load by_ the fiber interfacial area. In this case,"_ sliding occurrecl at 32.5 mN (A) andi was pushed a lo_al length of 4500 nm.... (B). With total fiber debonding, theapplied load required to keep the fiber.o __ ,_.,,,_,_,_, ,, _.... moving at a constant rate should remaino ,_= ,_, ,_ ,:._ ,_ ,_:_constant or changeslightly with the_'-"'_(','o_,:,',) change in interfacial area as the fiber ispushed-out.Figure 3. Loading curve forfrictional sl;dingIn Figure 3, afler the point (A) when the fiber started to slide, the forcecontinues to increase until tl_e diamond com_s in contact (B)with the channelwalls lefl behind by the sliding liber. The load increase occurs because theNanoindentor was pro,;rammed lo produce a constant loading rate during theindentation. Afler the onset of fiber sliding the load continued to increase,accelerating fiber sliding which is demonstrated by the increased spacingbetween the data points between A and B.Figure 4 shows a scanning electron micrograph of a pushed-out fiber.The fiber pictured in the bottom portion of the micrograph is the fiber of Figure1, which did not yield, and has a large pyramid-shaped imprint resulting fromplastic deformation of the Nicalon at the Nanoindentor's maximum load. Picturedabove this is a debonded fiber which was pushed-out and then reloaded and pushedin the opposile direction a total of 5 microns. Though the micrograph maysuggest that a nearest neighbor effect may have been responsible for theadherance of the lower fiber, in the inspection of hundreds of failed fibers such acorrelation was not found.For both fully bonded and unbonded interfaces, researchers have found agreat var,ability in measured interspecimen interfacial strengths. 1° lt has beenshown elsewhere, however, that a Weibuli distribution is more appropriate to_ .. -,_,_... _i ,i ...., ' =/"i I I I' " ;:.,".4,.',P-,._3 :,,f.I. _I {,,/. '' -, 20_.. ............. OI00 20 40 60 80 100 120STRESS(MPa)Figure 4. SEM micrograph of bonded Figure 5. Cumulative Weibull Dist-and pushed-out fiber, ribution of data.represent the interfacial strengths than the Iognormal, exponential or gammafunctions. ]4Here, the two parameter form of the Weibull probability distributionfunction and corresponding cumulative failure function is adopted and given by :P (13c_13-1/Oi3) e(°'/e)J3 F=I - e(C_/e)_where 13is the Weibull slope (shape parameter), e is the size parameter and c_isthe applied variable stress.Figure 5 shows the cumulative distribution for both the 2 % debondstrength and the frictional resistance to sliding. The Weibull moduli (_,) andshape parameters (e) are also given in the figure, lt is seen that the 2 % yieldstrength is not only greater that the frictional resistance but that the Weibullmodulus is less for the 2 % yield, leading to a broader distribution in strength.The Weibull mean values are calculated to be 60,8 MPa for the 2 % yield and16.5 MPa for the sliding resistance with corresponding moduli of 1 and 2.4,respectively.. Due to the relatively low values of the Weibull moduli, theprobability distribution function for the debonding yield strength and thefrictional sliding resistance are rather broad. Because of this there will benoticeable overlapping of the probability functions which would become morepronounced as the average strengths of the two distributions approach eachother. When the fibers are initially loaded, therefore, there will be somefraction of fibers (mathematically) which possess a higher frictional resistanceto sliding than debond strength. Physically this means that the stress required toinitiate and propagate a crack is less than the stress required to slide thefracture surfaces across each other. On the load curve of such fibers, thedebonding is not noticeable and gradual frictional-type failure would dominate.C4:X,,IC,L_1) lt has been shown that specimens of SiC(Nicalon)/SiC composites canbe thinned to a thickness approaching a fiber diameter ('.. 20 Bm) and push-outtests can be performed without specimen compliance limitations or matrixcracking.2) The debond yield strength and fiber push-out strength for these smallfibers can be obtained using a Nanoindentor with a high percentage of interfacialdebonding, l"he frictional resistance to sliding can likewise be determined byreapplying the load to the opposite end of the pushed out fiber.3) The debond yield, debonding push-out and fiber sliding stress can beadequately separated by inspection of the load-displacement curve. Thedebonding is characterized by an abrupt failure while the fiber sliding is a moregracefull failure.4) The result of the analysis has shown that the interfacial frictionalstress has a higher Weibull modulus than the debond stress, thus a less "brittle"failure.ACKNOWLEDGMENTSThe authors would like to thank R. A. Lowden of Oak Ridge NationalLaboratory for providing the SiC/SiC composite material used and also K. B.Alexander for the scanning erectron micrograph.REFEEENGES1 A.G. Evans and D.D. Marshall. "The Mechanical Behavior ofCeramic Matrix Composites," Acta metall. V. 37. No 10. 2567-2583 (1989).2 R. J. Kerans, R. S. Hay, N. J. Pagano, T. A Parthasarathy. "TheRole of the Fiber-Matrix Interface in Ceramic Composites," CeramicBulletin, V. 68, No. 2, 429-442 (1989).3 D. B Marshall and W. C. Oliver, "Measurement of InlerfacialMechanical Properties in Fiber-Reinforced Ceramic Composites," J. Am.Ceram. Soc., V. 70, No. 8, 542-548 (1987).4 T. P. Weihs and W. D. Nix, "Direct Measurement of the FrictionalResistance to Sliding of a Fiber in a Brittle Matrix," ScriptaMetallurgica, V. 22, 271-278 (1988).5 j. D. Bright, et al. "Interfacial Bonding and Friction in SiliconCarbide (filament)-Reinforced Ceramic and Glass-Matrix Composites," J.Anl. Ceram. Soc., V. 72, No. 10, 1891-1898. (1989).6 C. J. Yang, S. M. Jeng, J. M. Yang, "Interfacial PropertiesMeasurement for SiC Fiber Reinforced Titanium Alloy Composites,"Scripta Metallurgica, V. 24, 469-474. (1990).7 D. B. Marshall and A. G. Evans, "Failure Mechanisms inCeramic-Fiber/Ceramic-Matrix Composites," J. Am. Ceram. Soc., V.68, No5, 225-231. (1981).8 D. B. Marshall, "An Indentation Method for Measuring Matrix-Fiber Frictional St_'esses in Ceramic Composites," J. Am. Ceram. Soc., V.67, No. 12, C-259 - C 260. (1984).9 D.B. Marshall and W. C. Oliver, "Measurement of InterfacialMechanical Properties iri Fiber Reinforced Ceramic Composites," J. Am.Ceram. Soc., V. 70, No. 12, 542-548. (1987).10 T. P. Weihs and W. D. Nix, "In Situ Measurements of theMechanical Properties of Fibers, Matrices and Interfaces in MetalMatrix and Ceramic Matrix Composites," Mechanical and PhysicalBehavior of Metal and Ceramic Comp. Proc. of Ninth RISO InternationalSymposium on Mettalurgy and Material Science Sept. 5-9 1988.11 D.P. Stinton, A. J. Caputo and R. A. Lowden, "Synthesis ofFiber-Reinforced SiC Composites by Chemical Vapor Infiltration",Ceramic Bulletin, V. 65, No. 2, 347-350. (1986).. 12 W. C. Oliver, ct. al. "Measurements of Hardness at IndentationDepths as Low as 20 Nanometers", pp. 90-108 in ASTM Special TechnicalPublication No. 889, Microindentation Techniques in Material Scienceand Engineering. Edited by P. J. Blau and B. R. Lawn. American Societyfor Testing Materials. Philadelphia Pa. (1985).13 T. P. Weihs, C. M. Dick, W, D, Nix, "The Frictional Resistance toSliding of a SiC Fiber in a Brittle Matrix", pp. 247-252 in High-Temperature/High-Performance Composites. F. D. Lemkey ct. al. Eds.Mat. Res. Soc. Symp., V. 120, (1988).14 L.L. Snead, ct. al. J. of Mat. Res.. Submitted (1990)

Interfacial push-out measurements of fully-bonded SiC/SiC composites

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Interfacial push-out measurements of fully-bonded SiC/SiC composites

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