SiC composites

sbpta h&&ngicr~ et Idatedia,

Pergamon

Vol. 33, No. 6, pp. 983-988.1995 Elsmi~ScimccLtd

czopy@t01995Acta~~cdlnc. Rhk?diUthOUSA‘4llli$ltSd 09~716x/95 $9.50 + .oo

0956-716x(9spo309-6

THE 1400”C=0X1DAT10N EFFECT ON MICROSTRUCTURE, STRENGTH AND CYCLIC LIFE OF SiC/SiC COMPOSITES 0. haI*, A.J. Eckel’ and F.C. Laabs’ *AmesLaboratory, Metall~gy and Ceramics Div., Ames, IA 50010 ’ NASA Lewis Research Center, Materials Div., Cleveland, OH 44135 (Received March 20,1995) (RevisedMay 11,1995) IntrDduetion

Relatively high stmngth and excellent oxidation resistance make silicon carbide (Sic) an attractive candidate oxidiziq en+mments. However, its application in monolithic form is very much foruseinhigh~ limited due to its brittle nature. Themfom, it is otten produced and used in reinforced form. A typical example which has drawn much attention in recent years is continuous Sic fiber reinforced Sic matrix composites produced by chemical vapor in.fQtration(WI). The advantages of CVI process with respect to conventional methods are: i) it allows densification of complex shapes, ii) it has low possibility of mechanical damage to the reinforcing fibers since there is no mechanical mixing, and iii) it utilizes relatively low processing temperatures, thus thermal degradation during den&cation is minimal and high purity materials can be synthesized (1). Furthermore, the CVI processed SIC/SIC composites have shown very high t?acture toughness values which are not typical for ceramic materials (2). For instance, our prelimimuy tests have indicated that this particular material has a fracture toughness over 20 MPa.m ” . Since toughness is directly related to the level ofbonding at the fiber/matrix interface, carbon or boron nitride was found to be effective in this regard and is widely used in Sic/Sic composites. A major concern for this class of material, however, is oxidation Although Sic forms a protective silica (SiO,), carbon and boron nitride oxidize at temperatures well below theprojecteduse~. Previous work on the oxidation of Sic/Sic and C/Sic composites has shown that the formation of SiO, may close off pathways to, and limit, the oxidation of the interface (3,4). Since the primary use of Sic/Sic composites has been in structural applications at elevated temperature, it is of interest to study the oxidation effect on mechanical properties. In this paper, we will present results regarding the changes taking place in microstructure, strength and cyclic fatigue properties as a result of the oxidizing heat-treatment at 1400 “C. Jberimental

Procedure

The SiCYSiCcomposites used in this study,which were in the form of plates with about 3.2mm thickness, were obtained t?om Du Pont Lanxide Composites Inc.. Composites were 2D laminates of plain weave cloth fabricated t?om Nicalonm fiber-r?. The cloths, stacked up to provide a 0”/90” composite, were first coated witb l Du Pant Lanxide, Inc., Newa& DE, USA # Nippon Carbon, Tokyo, Japan.

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OXlDATIONEFFECTONBC/SECOMPOSITES

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-0.5pm pyrolitic carbon and then infiltrated with Sic by CVI. Resulting composites had 40% fiber content andabout1O%porosity. In~toevaluatemechanicalproperties,barswithdimensionsof3.2x3x25mm were machined from the as-received plates. Oxidation heat-treatment of the composite bars was carried out in flowing dried oxygen at 1400°C far 50 hours. Since the cut edges of the bars were not sealed after machining, they were directly exposed to the oxidizing environment. This represents an application case where the matrix is significantly cracked or machined surfaces are exposed. Mechanical testing was conducted at room temperature using a four-point bend fixture with 10 mm and 20 mm top and bottom span distances, respectively, The load in all tests was applied perpendicular to fiber layers. Prior to mechanical testing, bend tixture aud machine compliance were determined using a dummy specimen. Test results were corm&xl aocordinghl. Strength tests were carried out under displacement-control with a cross-head speed of 5 elm/s. Cyclic fatigue tests were conducted under load-control at a frequency of 2 Hz and a load ratio R (Pfi,_,,,,J = 0.1. If the specimen failure did not occur within 1O6cycles (-5.8 days), the fatigue tests were discontinued. &twlts

and Discuesio~

Since it is well established that oxidation of Sic can take place readily under high temperature and high P,, the possibility that the phases present in the Sic/Sic composite may have changed during the oxidation was first checked by x-ray di&action (XRD). XRD traces were recorded from both the as-received and oxidized samples in the same angular range using CuK=radiation. Peak locations in Figure 1a reveal that both the Sic matrix and the Sic Nicalon fibers in the as-received composite have the p-form with a cubic crystal structure (ma&d by “ g”). As can be seen in Figure 1b, with oxidation at 1400°C cristobalite (SiOa with a telragonal crystal structure farms on the specimen surface (mark4 by “C”). Note also in Figure. lb that the primary pSic phase in the as-received sample is retained after oxidation. Figure 2 shows the optical microscopy images of the as-received and oxidized SiCYSiCcomposites. It is obvious that, as a result ofoxidation, the surface morphology of the as-received composite changed considerably The light contrast regions in the oxidized composite correspond to the cristobalite phase as indicated by the XRD trace in Figure. 1b. Notice also in Figure. 2 that large surface porosity is present on the as-received composite. This porosity is not sealed by the newly formed cristobalite phase on the oxidized composite. Weight gain studies in this particular material, which will be published elsewhere (5), indicated that the oxidized composite lost about 2% of its weight. Although cristobalite formation is due to “passive” oxida-

” ,.. 20

.I....,....,....(....,....( 30 40 50

60

70

60

28 (degree) Figurelax-raydifbctiontrace~hthe

as-received SiClSiCcomposh. Marker“p” stmdsfor B-Sic.

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OXIDATIONEFfZCT ON Sic/Sic COMFO!GTES

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4000

3000 0 E !!? +

2000

1000

0

20

30

40

50 60 28 (degree)

lb. X-my-ditlMimlmce taken from the oxidized SiC/SiC cmpsite. Sic doea not trmsfm during oxidation.

Fii

70

80

Marker “C” stands for cdobalii

(SiQ).

Notice that p-

tion (6) and therefore, increases the weight of composite, the experimentally measured 2% weight loss indi-

cates that sign&ant oxidation must have also been operative during the heat-katment since unsealed pores on the surface of the oxidized composite in Figure. 2b act as paths for oxygen di&sion it is not surprisingl that oxidation, and thus weight loss, took place in these composites. Although we do not know all of the details about the reactions leading to the weight loss, as will be shown later, fractography studies suggest that oxidation of pyrolytic carbon present on the Sic fiber surface is the most likely reaction. Figure 3 shows the comparison of typical load versus displacement records of both as-received and oxidized composites. Both curves were corrected for compliance, but to show curves clearly, the one corresponding to the as-received composite was displaced by -20 pm. It is clear that the as-received material has a much higher load carrying capability than its oxidized counterpart_ While the peak load in Figure 3 for the

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OXIDATION EFFECT ON Sic/Sic

COMPOSITES

I -e--

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As-received

.

800

600 5 B cj 400

0 0

50

100

150

200

Displacement Figure 3. Comparison

250

300

350

400

(micron)

of load versus displacement curves for as-remived and oxidized

corn-

as-received sample is -840 N, that for the oxidized sample is -395 N, corresponding to more than a 50% reduction_ Moreover, the load drop at?erpeak-load in the as-received sample is in general much more gradual as opposed to that in the oxidized sample. As a result, the as-received composite maintains its load carrying capability for much higher deflection values; peak-load in this case, for instance, drops to its 85% value at about 350 t.tmwhile the same percentage of peak-load drop in the oxidized sample is achieved at a deflection of only 100 F. Finally, since the area under the load/deflection curve is proportional to the ability of a material to absorb energy during testing, Figure 3 clearly shows that the toughness of the as-received composite is gmatly reduced as a result of oxidation. Precise fracture energy measurements on these composites were also made using &actummechanics test specimens with known crack size and then tested in three-point bending. In order to calculate fracture energy: i) the area under the experimental load/deflection curve was determined by numerical integration, and ii) the area, which corresponds to total energy expended during ti-actumprocess, was divided by twice the area ofcracked ligament. The effect of oxidation on elastic moduhrs was determined indirectly by measurement of stitlhess. The results of strength, t?acture energy and the stitlhess measurements are summarized in Table 1. This summary indicates that fracture stress is reduced by -50% as a result of oxidation; fracture energy, on the other hand, is reduced by -75%. However, the changes in strength and fracture energy values have shown a similar tendency with the oxidation heat-treatments at 950,1100,1250 and 1400°C. These results will be published in an extended fotm at a later date (7). The cyclic life (S-N curve) of both as-received and oxidized Sic/Sic composites under constant load amplitudewithR=0.1isshowninFigure4. They-exisinthefigureshowsthemaximumstress,a_,applied

TABLE 1 Summtuy of Monotonic Mechanical Test Results Oxidation Temperature (“C) AS-received

1400

FmctureStress Wa) 370 f 30 189* 16

Fracture Energy (J/m’)

s*ess (106N/m)

3346rt318 774 f 105

6.8 f 0.98 4.9 f 0.61

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OXIDATION EFFECT ON Sic/Sic COMPOSITES

to the specimens, which was calculated based on elastic beam theory, while the x-axis shows the number of cycles to spe&nen failure (IV). Each marker in the plot cormqonds to an individual test. The arrows at 1O6 cycles (-5.8 days) indicate run-out. The solid lines under the data points of the as-received and oxidized specimens were drawn to indicate the stress levels below which the given material did not fail for the experimental conditions used. Although the composites contained about 10% porosity and the volume of the test bar is small, the relatively small scatter seen in Figure 4 is remarkable. As a result, the change of cyclic life with stress for both composites can clearly be seen. which the as-received composite can withstand without failure hrFigure4atN=10,themaximumo, is about 330 MPa; at this point, however, the maximum a_ for the oxidized composite is about 180 MPa, a drop of-G%. At N = lo”, the maximum o, for the as-received and the oxidized composites are 283 MPa and 130 Mpa, respectively. The trend in cyclic life versus applied stress for both materials is similar, increasing with decreasing applied stress. The endurance limits for both materials appears to be around 75% oftheira_atN= 10. TheendurancelimitfarasimilarmaterialtestedintensionandR=-l wasreported to be as 63% of its ultimate stress (8). These results clearly demonstrate that oxidation heat-treatment of Sic/ Sic composites reduces the cyclic life by about 50%, similar to the range of degradation observed in strength. Fractography by SEM also showed some distinct discrepancies between as-received and the oxidized samples. Although it was not distinguishable macroscopically, the fracture surfaces clearly showed microscopic di@erences. The obvious one was that the fracture surface of as-received composites were rougher, Figure Sa Most imporh&y the Sic fiber surface in the as-received composites invariably had broken matrix fragments on it This indicates that the fiber/matrix interface was strong. No such features could be seen on the fibers in the oxidized composites; instead, the fiber surface in these samples was smooth, Figure 5b. The formation of a smooth fiber surface is attributed to the loss of bonding at the fiber/matrix interface resulting from oxidation of the pyrolytic carbon present at the interface. This change of the interface in turn negatively affected the load canying capability of oxidized Sic/Sic composite as illmated by the mechanical test data. Deterioration of the mechanical properties due to the oxidation of various fiber coatings was reported in similar materials by others (9). The fracture surface of the oxidized composites also exhibited distinct dense layered areas displaying brittle hacture features surrouudingthe fiber bundles as can be seen in Figure 5b. These layered areas are rather common, particularly in regim close to the specimen surface. Chemical analysis on dense areas showed that they are rich in oxygen thus indicating that it is the cristobalite phase. The cristobalite fation in these

100

1000

lo4

lo5

IO6

lo7

Cycle Figure 4. comparison

ofstressvenus

cyclic life (S-N) cu~ea for as-&ved

aad oxidked cxuqmh.

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OXIDATION EFFECTON SiC/SiCCOMPOSlTES

Figure 5. SEM miuqraphs of hcture surfacesin: a) as-kved

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and b) oxidized cumposim. See.text for discussion

composites is believed to have had two opposing effects: i) promoted brittle tm&re by providing strong bondingwithinthe matrix, ii) protected composite properties by limiting oxygen diffusion to interior, as also pointed out by others (10). ConclyqippB

1.

2. 3.

Oxidation heat-treatment of Sic/sic composites at 1400°C for 50 hours leads to the formation of the “passive” oxidation product of cristobalite (SiO& However, cristobalite does not seal surface completely, including pores, and therefore, it is not protective. Mechanical test results clearly demonstrate that oxidation reduces both the fracture stress and the cyclic life, at a given stress level, by about 50%. The degradation of the mechanical properties appears to be related to the preferential oxidation of pyrolytic carbon present at the fiber/matrix interface.

The work of two of us (0. U. and F. C. L.) was funded by Ames Laboratory which is opemted for the U.S. Department of Energy by Iowa State University under contract No. W-740%ENG-82. Useful information provided by Joe Halada of Du Pont Lanxide Composites Inc. is greadully acknowledged. Furthermore, we thank Dr. 0. Buck for his comments.

1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

w. s. St&k, ham. Fag Sci. Pmt. 14,1045 (1993). P.J.Lamic+G.ABemba&M.M.Daud1k,andJ.G.Macc,Am.Corun.Soc.Bull.65,336(1986). LFiiG.Camw,audRNaslain,J.hham.Soc.77,459(1994). F.Lmomux,G.~aadJ.~J.Am.Cacun.Soc.77,2049(1994). AEddmd0.Unal,iaptqmti~ S. C. Siqbd, J. Mater.Sci. 11,1246 (1976).

O.Ud,AEdcel,andF.Ldm,inpmpmtio~~ W. R Moecbslle.Ccmm. Eq. Sci Proc. 15.13 (1991). P.F.Tatadi,S.Nijlmwan,LRicata,mdRA~Cuam.lhg&i.Fmc.14,358(1993). RA~rmdRD.krm,RooArm.~POlLilEaergyM;lda.S~(rdasdbyN.C.Cole~RRJuQinsX1,33,NTIS, Spk@dd, VA(lPPl>