Effect of Oxidation on Crack Deflection in SiC/Al2O3 LaminatedCeramic Composites

R. Krishnamurthy,*,w,z J. Rankin,* and B.W. Sheldon*

Division of Engineering, Brown University, Providence, Rhode Island 02912

Laminated composites consisting of SiC and a thin porous alu-mina interphase were exposed to air at 5001C to produce a per-sistent, nearly uniform oxidation product layer. Crack deflectionat the interface was then studied using a four-point bend testingprocedure and interfacial fracture resistances were found to de-crease with increasing oxidation times. Electron microscopy ob-servations of the fractured interface show a complex multi-phasemicrostructure. These results show that oxidation can produce asufficiently weak interface in a SiC-porous alumina interphasecomposite, in contrast to most other SiC composites where in-terface oxidation produces a strongly bonded interface whichinhibits crack deflection.

I. Introduction

IT is well established that the nature of the fiber/matrix inter-face is an important criterion in the design of ceramic matrix

composites for high temperature structural applications. Oneimportant role of the interface is to provide low resistance crackpaths that facilitate crack deflection at the interface. This can beachieved by interposing ‘‘interphases’’ with low fracture resist-ances between the fiber and the matrix in a fiber composite andbetween different laminae in a laminate.1 Traditionally C2 andBN3 have been used as interphase coatings because they createweak interfaces. However, both C and BN interphases oxidize inair to give volatile oxidation products which renders them inef-fective for high temperature applications.4–6 Furthermore, gapsintroduced in the structure because of volatilization and thestrong interfacial bonding resulting from silica glass formationupon oxidation of the SiC fiber/matrix also result in poor me-chanical properties.7 Numerous coating strategies, including theuse of fugitive layers with oxide composites,8 porous oxides,9

and C/SiC/C multilayer interphases for SiC composites10–13

have been advocated and tested in the literature to resolve thisissue.

The effectiveness of the different coating materials discussedabove in providing oxidation protection is a matter of muchdebate with some favoring porous oxides,7 and others non-ox-ides,14,15 as the interphase coating materials of choice. The ox-idation resistance of this specific system, i.e., SiC/Al2O3

laminates, was addressed by us in another study, where process-ing strategies designed to achieve oxidation protection were alsodiscussed.16 The relatively high friction coefficient of porous ox-ides as compared with C or BN can be a problem for fibercomposites, where fiber pull-out contributes significantly to the

composite toughness, but is less important for the laminate sys-tem considered here. Also, fiber coatings with high friction co-efficients can still provide considerable toughness in certaincases.10

The primary objective of this work is to consider the effects ofoxidation product layers on crack deflection at laminate inter-faces. In SiC-based composites, oxidation is typically expectedto adversely affect crack deflection, as silica reaction layers arenormally associated with strong bonding/strong interfaces.There are some experimental results in the literature that sup-port this view.17 However, no detailed study, using a well-con-trolled test geometry, has been conducted to directly investigatethe effect reaction layers have on crack deflection. In this study,we have attempted to address this issue.

Porous alumina is an excellent interphase material, having thenecessary low fracture resistance and good high temperatureproperties.18 SiC, with its excellent creep resistance, is a goodchoice as matrix material for high temperature applications.However, Al2O3 and SiC react in high temperature oxidizingenvironments with accompanying changes in the structure of theinterface and hence, in the fracture behavior. Thus, this is a goodmodel system to study the effects of interfacial reaction layers onthe fracture behavior of composites. In this paper, we reportmeasurements of the interfacial fracture resistance of these lam-inates after oxidation for different time periods at 5001C. Thischoice of the oxidation temperature was based on the findingthat a thin, nearly uniform, oxide product layer is produced afteroxidation at this temperature, as reported in another study.16

The details of the experimental methods used to process thelaminates and test their fracture behavior are given in Section II,and results from these experiments are presented in Section III.Discussion of the results of this work and conclusions are pre-sented in Section IV.

II. Experimental Procedure

SiC bars (CVDRohm and Haas, Woburn, MA) of dimensions 2mm� 4 mm� 70 mm were lapped with 1 mm diameter grit to asurface finish of Ra 0.05 mm, and coated with a uniform layer ofhigh purity porous alumina (499.95%), 2–5 mm thick, using anaerosol spray deposition technique.9,19 Porous alumina coatedSiC bars were hot-pressed with matching, but uncoated, SiCbars to form SiC/porous alumina laminates, using a proceduredescribed by O’Brien and Sheldon9 and Krishnamurthy andSheldon.19 All samples used in this study were hot-pressed at14001C and 10.4 MPa. A schematic of one of these laminatesalong with typical dimensions is shown in Fig. 1. These lami-nates were oxidized for varying time periods at 5001C to pro-duce a thin, persistent oxidation product layer.16

A combined Vickers indentation and three-point bendingprocedure9 was used to introduce a pre-crack in the top SiClamina of the oxidized SiC/alumina laminates (see Fig. 1). Un-der three-point bend loading, the pre-crack in the top SiC lam-ina naturally self-arrests on reaching the interface, and thespecimen is immediately unloaded. These pre-cracked laminates

Journal

J. Am. Ceram. Soc., 88 [5] 1362–1365 (2005)

DOI: 10.1111/j.1551-2916.2005.00320.x

1362

D. J. Green—contributing editor

Supported by the MRSEC Program of the National Science Foundation under AwardNumber DMR-0079964.

*Member, American Ceramic Society.wAuthor to whom correspondence should be addressed. e-mail: rkrishna@princeton.eduzCurrently at Princeton Institute for the science and technology of materials, Princeton

University.

Manuscript No. 11254. Received August 6, 2004; approved December 28, 2004.

are subsequently tested using a four-point bend test procedureto obtain interfacial fracture resistance values (implementationdetails are described).9,19–21 This test was continued beyond thesteady state loading regime until the specimen completely de-bonded at the interface to allow for microscopic examination ofthe fracture surface. The tests were repeated using a secondsample processed (and oxidized) under the same conditions toconfirm the validity of the fracture resistance measurement. Af-ter the fracture tests, two pieces from the bottom, uncrackedlamina (see Fig. 1) were bonded interlayer to interlayer to pre-pare cross-sectional transmission electron microscopy (TEM)specimens with the debond interface (i.e., the uncracked SiClamina/porous interlayer interface) in the middle. A Au–Pdsputter coat placed atop one of the unbonded pieces was usedas a marker to locate the debond interface.

III. Results

In a previous study, laminate specimens hot-pressed in vacuumat 14001C and 10.4 MPa exhibited the maximum measured in-terfacial fracture resistance, Gi (units of J/m

2), where interfacialcrack deflection was possible (i.e., higher temperature and/orpressure created interfaces that were too strong to allow crackdeflection).19 In the current study, laminates processed underthese conditions were oxidized at 5001C for different times. In asample oxidized at 8001C, the deflected crack debonded the in-terface completely without arresting after the three-point bendtest used in the pre-cracking procedure. This demonstrates thatthis interface is sufficiently weak, however, as described in otherwork,16 this higher temperature produces non-uniform interfaceoxidation (the likely cause of this fracture result), and furthermechanical testing at this temperature was not pursued. Figure 2shows that Gi decreases with prolonged oxidation exposures ofthe laminates at 5001C. After a 40 h exposure, the laminate de-bonded completely during the pre-cracking stage indicating thatthe interface is very weak. These observations are in contrastwith the traditional view that silicate layers create a stronglybonded interphase. However, this conventional wisdom strictlyapplies only when the matrix and the fiber are strongly bondedtogether by an oxide plug. It may not necessarily be applicablewhen the oxide layer does not extend across the entire thicknessof the interface to form a strong fiber/matrix bond, as is likelythe case for 5001C oxidation, where only a thin oxide productlayer is expected to be formed.

TEM cross-section specimens with the freshly exposed sur-face (i.e., exposed after the fracture test) of the uncracked, bot-tom SiC lamina and any residual porous interlayer sticking on it(i.e., the debond interface) in the middle of the cross-section

were prepared by the procedure outlined in Section II. TEMexamination of the cross-section samples shows that oxidationof the laminates produces an interphase that includes a glassyphase and alumina grains (see Figs. 3(a) and (b)). X-ray diffrac-tion and scanning electron microscopy (SEM) also confirmedthat the oxidation product was amorphous.16 Considering thelow oxidation temperature, the amorphous phase is likely to besilica with some Al incorporated into it. The incorporated Alcan lead to a faster rate of SiC oxidation.6 SEM examination ofthe oxidized samples revealed that the interlayer also containedsome retained porosity (see Krishnamurthy and Sheldon16 fordetails). The TEM micrographs in Figs. 3(a) and (b) correspondto a sample oxidized for 6 h; samples oxidized for other timeperiods also show similar features. Basic fracture mechanics ar-guments that do not consider the effect of the porous interlayermicrostructure would require this debond interface to be a cleanSiC surface. The presence of alumina grains and a glassy phasein the cross-section of the TEM samples is a clear indication thatthe oxidized porous interlayer has an effect on the location ofthe actual debond interface in this system. In contrast, a com-paratively clean SiC surface with some pulled-out aluminagrains constituted the debond surface for the non-oxidized sam-ple (the pertinent micrograph can be found in an earlier publi-cation, Krishnamurthy and Sheldon19). While this does notnecessarily eliminate the possibility that a glassy phase wasformed after hot-pressing, any such layer is likely to be thincompared with glassy layers formed after oxidation. The pres-ence of the alumina grains in the examined region of the TEMsamples can be explained if oxidation produces a complex mul-tiphase interphase rather than just a layer of glassy phase nextto the SiC lamina. However, possible microstructural inhomo-geneities produced during processing of the laminates could alsolead to pulled-out alumina grains.

IV. Discussion and Conclusions

An important theoretical consideration in the study of interfa-cial fracture is the He-Hutchinson criterion.22 This states thatcrack deflection will occur in preference to crack penetration ifthe ratio of the interface and substrate fracture resistances,Gi=Gf , is less than a threshold value (this value is B1/4 whenthere is no elastic mismatch between the two layers). To assessthe results in Fig. 2, changes in the interface because of oxida-tion must be considered. For very thin reaction layers, crackdeflection may still occur at the SiC/Al2O3 interface and conse-quently, the measured fracture resistance is close to the initialSiC/Al2O3 interfacial fracture resistance (i.e., the He-Hutchin-son criterion must be applied to this interface to determine

4 mm

2 mm

2 mm

porous alumina interphaseµ2−3 m

70 mm

Fig. 1. A schematic depiction of the SiC/porous Al2O3 laminate used inthis study.

oxidation time (hours)

Γ i (J

/m2 )

Γ i /

Γ f

0 5 10 15 200

2

4

6

8

10

12

14

16

0

0.1

0.2

0.3

0.4

0.5

0.6

Fig. 2. Gi versus time of air exposure.

May 2005 Communications of the American Ceramic Society 1363

whether crack deflection or penetration should occur). Forthicker reaction layers, crack deflection is more likely to occuraway from the SiC interface. The limited microscopy resultspresented in Fig. 3 clearly indicate that both alumina grains andamorphous glassy phase are stuck to the lower, uncracked SiClamina, on fracture. This can be explained if debonding occurredat an interface within the porous interlayer that is terminated byboth the porous alumina and the glassy phase. It is also possiblethat the pulled-out porous alumina grains result from micro-structural inhomogeneities produced during the processing ofthe laminates, and that low temperature oxidation only allowsfor a finer control of the interface fracture resistance. Neverthe-less, either mechanism is consistent with debonding occurring atan interface other than the uncracked SiC lamina/porous inter-layer interface. For thicker reaction layers, this is even more

likely to happen and a simple application of the He-Hutchinsoncriterion may no longer be valid. The experiments suggest thatcrack deflection in the reaction layer/Al2O3 region leads to areduction in the measured value for Gi.

Elastic mismatch between the oxidation-affected interlayerand the SiC lamina and residual stresses at the interface alsoaffect crack deflection. As oxidation results in a part of the porespace being filled with a glassy phase, the elastic mismatch be-tween the SiC and the interlayer is likely to be reduced, resultingin a reduced threshold for crack deflection.23 Also, the debondinterface will experience a greater tensile stress for the same rea-son (as the effective thermal expansion coefficient of the inter-layer is increased upon oxidation), and this too produces a lowerthreshold for crack deflection.24 Both factors favor crack de-flection occurring at ‘‘weak’’ interfaces located away from thebottom uncracked SiC surface. The He-Hutchinson criterion isstrictly applicable only for bi-material interfaces and for a smalldeflected/penetrated crack. Our previous study of interfacialfracture in Al2O3/porous Al2O3 laminates provides a verygood fit to the He-Hutchinson criterion.9 However, experimen-tal results from another study with a non-oxidized SiC/Al2O3

system show crack deflection for interfaces stronger than thethreshold limit set by the He-Hutchinson criterion,19 with thedeviation from this threshold being larger than can be explainedby finite interphase thickness effects. The effect of multi-layeredinterphases on crack deflection has been analyzed using isotrop-ic linear elasticity,25 and cracks are found to be attracted to theweakest interface within the multi-layered interphase.

Our results with oxidized SiC laminates seem to be in contrastto the general opinion held in the case of fiber-reinforced SiCcomposites, where crack deflection is observed to be adverselyaffected by the formation of SiO2 at the fiber–matrix interface.4

While most of this work considers C and BN interface layersthat volatilize during oxidation, our observations also appear todiffer from fiber-reinforced SiC composites with alumina inter-phases, where fiber pull-out on fracture surfaces is not observedin composites that have been exposed to an oxidizing environ-ment at elevated temperature.17 This result was attributed to thestrong interfacial bonds that are formed because of reaction be-tween the Nicalont fibers and the SiC matrix. Note, however,that the strong fiber/matrix interface bond requires an oxideplug to be formed by reaction, and the traditional view does notapply to situations where the oxide layer does not extend com-pletely across the thickness of the interphase. In considering ourcurrent results, it is interesting to note that reaction layers atinterfaces that are undesirable because of their deleterious effecton the environmental resistance of the fiber may actually be de-sirable with regard to fracture resistance.

The oxidation-induced decrease in fracture resistance inFig. 2 is interesting and unexpected; more detailed informationon these types of effects is desirable. In particular, it is importantto consider the effect of higher oxidation temperatures and/or longer oxidation exposures on both oxidation resistanceand mechanical properties. For good oxidation resistance, rap-id pore/crack sealing at the ends of the interphase is desirable.Our results show that the presence of a thin reaction layer isresponsible for the deflection behavior seen in these systems. Inview of these two arguments, an effective strategy for producinga weak and oxidation resistant interphase in this system mightinclude limited low temperature oxidation to produce a thinoxide layer and sufficiently rapid pore/crack sealing at the sur-face to provide oxidation protection. Such a strategy has beenproposed in the literature,7 and has been explored by us throughtheoretical calculations presented in another study.16 Note, how-ever, that SiC composites are often considered for applicationswhere fatigue resistance and reliability under thermal cycling arenecessary requirements. More work is required to evaluate thesuitability of this strategy when such effects are significant.

SiC composites are generally considered for use at tempera-tures higher than the processing temperatures used here. Athigher temperatures, the pore space in the interlayer may berapidly filled up and/or reactions between Al2O3 and SiC can

Fig. 3. Transmission electron microscopy micrographs of the debondinterface for a laminate oxidized at 5001C for 3 h, showing (a) An amor-phous reaction layer at the SiC/Al2O3 interface and (b) An aluminagrain and the amorphous layer stuck to the debond interface. The un-labeled areas in (a) and (b) represent void space.

1364 Communications of the American Ceramic Society Vol. 88, No. 5

occur, thus altering the interfacial fracture resistance dramati-cally. Clearly, other systems and/or mechanisms will then beneeded to obtain optimal fracture and oxidation properties.26–28

In conclusion, SiC/porous alumina interphase laminates wereoxidized in air at 5001C and the measured interfacial fractureresistance of these laminates decreased with prolonged oxidationexposures. This suggests some interesting processing strategiesfor fabricating fracture and oxidation resistant composites.

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