Mechanical Properties and FractureBehaviour of SiTiCO Fibre/SiAlON

Composite

Takahiro Inoue&KazuoUeno

Osaka National Research Institute AIST, Midorigaoka 1-8-31, Ikeda 563, Japan

(Received 23 March 1996; accepted 29 May 1996)

Abstract: Unidirectional SiTiCO ®bre (Tyranno ®bre1) reinforced SiAlON(Si2Al4O4N4) was fabricated by the ®lament winding method. The ®bre/matrixgreen sheets were stacked and hot-pressed at 1773K for 1.8 ks under a nitrogen

atmosphere. The composite with a ®bre content of 42 vol% showed signi®cantlyhigher ¯exural strength (800MPa) than that of monolithic SiAlON ceramics(570MPa). A strength decrease due to the matrix degradation was observed at

high temperature (1273K and 1473K). The fracture toughness of the compositemeasured by SENB (single edge notched beam) method was 8.0MPa�m0.5, beingtwo times higher than that of monolithic ceramics (4.0MPa�m0.5). The work-of-

fracture of the composite measured by static and dynamic methods was 9.62 kJ/m2

and 8.29 kJ/m2, respectively. # 1998 Elsevier Science Limited and Techna S.r.l.

1 INTRODUCTION

The major intention of ®bre reinforcement forceramics is to enhance the reliability of the materialby toughening through changing its fracturebehaviour from brittle to non-brittle. The fractureresistance is expected to be extensively increased bythe ®bre reinforcement. Ceramic matrix composites(CMCs) with various inorganic ®bres, such asprecursor-derived SiC ®bre,1±12 CVD-SiC ®bre13±21

or carbon ®bre,22±24 have been reported. One of thekey points for a high-performance CMC is to pro-duce it without ®bre degradation. The polymer-derived SiC ®bre, being widely used reinforcementfor CMC, decomposes with the crystallization ofSiC grains when heated above 1500K, and itsstrength degrades signi®cantly.25,26 The tempera-ture required for sintering engineering ceramicssuch as silicon nitride or silicon carbide is too highfor the ®bre. Recently, highly heat-resistive SiTiCO®bre has become commercially available (Tyranno®bre1, Lox-M type supplied from Ube Industries,Ltd). The ®bre contains 12wt% oxygen, andretains its tensile strength of over 1GPa even afterexposure in Ar at 1773K for 3.6 ks.27 If the pro-cessing temperature is lower than 1773K, one can

fabricate the CMC with this high strength ®bre.For low-temperature processing of CMCs, the CVImethod and polymer precursor method are well-known. These methods, however, often su�er fromtime-consuming processes and/or poor mechanicalperformance of the matrix on account of residualpores. On the contrary, normal sintering based onpowder metallurgy, if it is carried out at a tem-perature su�ciently low for the ®bre, has theadvantage over these noble methods from theviewpoint of cost-performance and superior matrixproperties.In this study, unidirectionally oriented Lox-M

type Tyranno ®bre1 is incorporated into �-SiA-lON ceramics. �-SiAlON, one of the non-oxideengineering ceramics, has a thermal expansioncoe�cient comparable to that of the ®bre and itssinterability is relatively high. The ceramic isknown to have moderate strength with excellentresistance against high-temperature oxidation andmolten metal. The compound is a solid solutionin which Si and N are substituted by Al and Oin the �-Si3N4 lattice. The general formula isSi6ÿzAlzOzN8ÿz (z�4.2). Its properties stronglydepend on the composition, that is, the substitu-tion ratio z. The strength decreases and oxidation

Ceramics International 24 (1998) 565±569# 1998 Elsevier Science Limited and Techna S.r.l.

Printed in Great Britain. All rights reservedP I I : S 0 2 7 2 - 8 8 4 2 ( 9 6 ) 0 0 0 6 4 - 8 0272-8842/98 $19.00+.00

565

resistance and sinterability increase with increasingz value.28±30 �-SiAlON with z=4 (Si2Al4O4N4) isselected in this paper as the sinterability is rela-tively high at this composition. A sintering tem-perature of 1973±2073K is required for thematerial, but it is too high for the ®bre. Therefore,a sintering additive, 3wt% Y2O3, is doped to lowerthe processing temperature. Thus the SiTiCO ®bre/SiAlON composite is fabricated by hot-pressing at1773K for 1.8 ks. Its mechanical properties andfracture resistance in static and dynamic conditionsare investigated.

2 EXPERIMENTAL PROCEDURES

Lox-M grade Tyranno ®bre1 is supplied from UbeIndustries Ltd. The average diameter, measureddirectly in the composite cross-sectional photo, is8.0�m, and the density is 2.37Mg/m3.27 Its tensilestrength and elastic modulus are 3.5GPa and188GPa, respectively. The ®lament number peryarn is 1600.The starting powder for the matrix �-SiAlON

with z=4 is composed of Si3N4 (LC-12, HermannC. Stark Co. Ltd), Al2O3 (TM-D, Taimei ChemicalInd. Co.), and AlN (Type G, Tokuyama Soda Co.Ltd). 3wt% Y2O3 (Fine-grain grade, NipponYttrium Co. Ltd) is added to the mixture as asintering additive.The slurry for ®lament winding is prepared by

ball-milling of the powder mixture mentionedabove with polyvinylbutyral and polyethylene-glycol as binder, dibutylphthalate as plasticizer,linseed oil as de¯occulation aid and ethanol as dis-persion medium. The ®bre is continuously dippedinto this prepared slurry and wound on a windingdrum. The impregnated ®bre/matrix-powder sheetis cut into a rectangular shape with a size of40�40mm after drying. The obtained green sheets(3±5 sheets) are stacked and formed as a greencompact with the unidirectionally oriented ®bre bydie-pressing at 523K under a pressure of 30MPa.The organic additives are burned in a vacuumfurnace by heating up to 973K. Hot-pressing iscarried out at 1773K for 1.8 ks under a pressure of20MPa in a nitrogen atmosphere. Unidirectionalpressing is applied to the green compact along thedirection perpendicular to the ®bre orientationaxis. The SiAlON matrix is synthesized during thesintering. Monolithic SiAlON ceramics with thesame z value are also fabricated under the sameconditions.The sintered composite body is ground on its

surface by a diamond wheel. The bulk density ismeasured by the water immersion method. Three-

point bending strength at room temperature,1273K and 1473K was measured in air for threetest specimens of 1mm (for the composite) or 3mmthick (for monolithic SiAlON), 4mm wide and40mm long, with a span of 30mm and a crossheadspeed of 0.0083mm/s.Fracture toughness was measured by SENB

(single edge notched beam) method. A straight-through notch with a depth of about 1.5mm wasmachined by a 0.1mm thick diamond blade per-pendicular to the ®bre orientation direction on thecentre part of the test bar (4mm thick, 3mm wideand 20mm long). Four specimens were tested witha span of 16mm and a crosshead speed of0.0083mm/s. Work-of-fracture, a measure of thefracture resistance of the material, is estimated basedon the area surrounded by the load±de¯ectioncurve and the ligament area. Impact fracture resist-ance is evaluated by a Charpy impact test. The testpiece is a rectangular bar with the same size andarti®cial notch as the SENB test piece except thelength (40mm). The stress applied on the hammer-head of the Charpy impact instrument is mon-itored by a stress-sensor during the fracture. TheWOF is calculated based on the load vs displace-ment curve. Four specimens are tested.The composite microstructure was observed by

an optical microscope. Fibre volume percentagewas determined based on the number of ®lamentsembedded in a certain cross-sectional area in thephoto and the average ®bre diameter. The ®brevolume percentage is 42%.

3 RESULTS AND DISCUSSION

3.1 Density andmicrostructure

The bulk density and open porosity of monolithicSiAlON and SiTiCO ®bre/SiAlON compositemeasured by the water immersion method areshown in Table 1. The monolithic ceramic has avery small amount of open porosity, suggestingthat it has been sintered almost fully. The openporosity of the composite is 9.34%, being sig-ni®cantly higher as compared to that of the mono-lith. The ®bre incorporation apparently hinders thematrix sintering. The relative density of the com-posite, de®ned as the ratio of its bulk density to thepore-free density, is 91.0%. The pore-free, idealbulk density of the composite is calculated byassuming that the matrix has the same density asthat of monolithic SiAlON. The calculated relativedensity is consistent with themeasured open porosity.The microstructure of the composite cross-section

shows that pores exist in the places where the ®bres

566 T. Inoue, K. Ueno

gather closely, as indicated by the arrows[Fig. 1(a)], but the matrix seems to be well densi-®ed. Most of the ®lament is su�ciently surroundedby the matrix. In the microstructure observed bylower magni®cation [Fig. 1(b)], we ®nd that thereexist ®bre-less layer structures as indicated by thearrows in the photo. This non-uniform ®bre incor-poration is derived by excess slurry on the impreg-nated green sheet, exuded under the tensile stresson the winding drum during the ®lament winding.X-ray di�ractometry reveals that the composite

matrix consists of �-SiAlON, con®rming that thesynthesis of the compound was completed duringthe sintering process.

3.2 Flexural strength at ambient and hightemperatures

Figure 2 shows the temperature dependence of thebending strength of monolithic SiAlON and thecomposite. For the test specimen with a thicknessof 3mm, shear fracture due to interlamellardelamination takes place. The shear strength is alsoshown in Fig. 2.The room temperature strength of the composite

is 794MPa, which is signi®cantly higher than thatof the monolithic ceramics (568MPa). Its fracturebehaviour is a typical non-brittle type. Even afterfracture takes place from the tensile surface at themaximum load, the composite does not showunstable rapid fracture but can bear the load up togreater deformation of the specimen.When the test is carried out at high temperature,

the strength of the monolithic ceramics and ofthe composite both degrade. The strength of thecomposite at 1273K and 1473K is 499MPa and403MPa, respectively, both being higher thanthose of monolithic ceramics by 150±200MPa. Onthe contrary, the shear strength of the compositeshows no such thermal degradation, as shown inFig. 2. The shear stress is applied so as to delami-nate along the ®bre/matrix interface, so that theshear fracture takes place from the inner part ofthe test specimen, while the bending strength initi-ates from the tensile surface of the specimen. This

Table 1. Bulk density, open porosity and relative density of monolithic SiAlON and SiTiCO fibre/SiAlONcomposite

Bulk density(Mg/m3)

Open porosity(%)

Relative density*

(%)

Monolithic SiAlON 3.28 0.15 öSiTiCO fibre/SiAlON 2.64 9.34 91.0

*Relative density of the composite is the ratio of its bulk density to theoretical density, which is calculated by assuming that thematrix full density is 3.28Mg/m3, the bulk density ofmonolithic SiAlON.

Fig. 1. Microstructure of SiTiCO(f)/SiAlON composite. Thecomposite cross-section perpendicular to the ®bre orientationdirection is observed by an optical micrometer with (a) higher

and (b) lower magni®cations.

Fig. 2. Temperature-dependence of the bending of monolithicSiAlON (&) and SiTiCO(f)/SiAlON composite (*), and the

interlamellar shear strength of the composite (*).

SiTiCO ®bre reinforced SiAlON 567

fact suggests that the bending strength degradationis attributable to some surface damage.The tested specimens of the monolithic ceramics

and the composite are then examined. Both have awhite layer on their surfaces. Figure 3 shows across-sectional view of the composite after thebending test at 1273K. The arrows in the photoindicate the reaction front of the surface layer.Many ®bres exist in the surface layer having athickness of about 100�m. Hasegawa et al. repor-ted that the oxide layer was built on the surface ofSiAlON ceramics when heated in an oxygen streamat high temperatures.29 When the compound withz=4 is oxidized, they reported that only mullitewas formed. The surface of monolithic SiAlONbefore and after the bending test at 1473K wasexamined by an X-ray di�ractometer. No new lineappears except a hump at 2�=20±30�, which isthought to re¯ect amorphous silica probably con-taining yttrium. This oxide layer may cause thethermal degradation of the strength.

3.3 Fracture resistance

Fracture toughness and WOF of monolithicSiAlON and the composite are shown in Table 2.The toughness of the composite (8.0MPa�m0.5) isjust two times greater than that of monolithicceramics (4.0MPa�m0.5). Typical load±de¯ectioncurves are shown in Fig. 4. Before the load reachesthe maximum load point, the curve for the com-posite indicates a deviation from the linear rela-tionship. This departure from the elastic response

means that the crack has already propagated fromthe notch tip. Direct observation of the test speci-men reveals that the delamination fracture startsfrom the notch tip, running parallel to the ®breorientation direction. This delamination at thenotch tip greatly reduces stress intensi®cation. Atthe maximum load point, the tensile fracture takesplace but it is arrested immediately by the next®bre layer. In the later stage after this point, quitea lot of delamination and partial tensile fractureoccur simultaneously up to the ®nal fracture. TheWOF of the composite, therefore, includes all ofthese microfractures during the test, so that thevalue is far greater than that of monolithic cera-mics that fail in a brittle manner (as shown inFig. 4).Impact fracture resistance is elucidated by a

Charpy impact test. Dynamic WOF is based on thehammer-head load vs de¯ection curve. The resultsare shown in Table 2. Di�ering from the staticfracture test, the composite fails in a tensile mannerwithout delamination. Figure 5 shows the test spe-cimen after SENB and Charpy impact test. The

Fig. 3. Cross-sectional view of SiTiCO(f)/SiAlON compositeafter the bending test at 1273K.

Fig. 5. Test specimens after the SENB test (upper) andCharpy impact test (lower).

Fig. 4. Load±de¯ection curves of SiTiCO(f)/SiAlON andmonolithic SiAlON recorded during the SENB test.

Table 2. Fracture toughness and WOF (work-of-fracture) of monolithic SiAlON and SiTiCO fibre/SiAlONcomposite, measured by SENB (single edge notched beam) method

Fracture toughness(MPa�m0.5)

Work-of-fracture (J/m2)

Static Dynamic

Monolithic SiAlON 4.0 (3.7, 4.2) 148 (129, 164) 430 (421, 439)SiTiCO(f)/SiAlON 8.0 (7.5, 8.6) 9617 (8831, 10300) 8289 (7822, 8640)

*The three figures in each columnare an average (theminimum, themaximum) data.

568 T. Inoue, K. Ueno

SENB test specimen deforms by extensive delami-nation. The static WOF, therefore, contains quitelarge amounts of the energy associated with sheardeformation. While a Charpy test specimendemonstrates that it fractures with a large amountof ®bre pull-out. The dynamic WOF re¯ects moresimple tensile fracture as compared to static data.These results indicate that the composite has veryhigh fracture resistance against both static defor-mation and mechanical impact.

4 CONCLUSION

Unidirectionally oriented SiTiCO ®bre/SiAlONcomposite is fabricated by the ®lament windingmethod, followed by hot-pressing. Remarkablestrength improvement from ambient temperatureup to 1473K is achieved by the ®bre incorporation.The fracture behaviour of the composite is non-brittle with quite a large amount of energy con-sumption. Fracture toughness and fracture resistanceare both improved by the ®bre reinforcement. Thecomposite is a tough and very damage tolerablematerial. It also shows toughness against impactfracture. All these enhanced mechanical perfor-mances are derived from the ®bre, that is intactand strong enough to bear high stress. Therefore,the concept to fabricate CMCs at a temperature aslow as possible, but high enough to bring about asatisfactory level of matrix properties, is promising.Although SiTiCO ®bre/SiAlON composite is foundto be a very tough material at ambient temperature,one problem is that the e�ort for low processingtemperature often con¯icts with the high temperatureperformance of the composite. Further improvementin the processing technology for CMCs is desirable.

REFERENCES

1. CHRISTIN, F., NASLAIN, R. & BERNARD, C., InProc. 7th Int. Conf. on CVD, ed. T. O. Sedwick & H.Lynin. Electrochem. Soc., Princeton, 1979, p. 499.

2. PREWO, K. & BRENNAN, J., J. Mater. Sci., 15 (1980)463±468.

3. RICE, R. W., BECKER, P. F., FREIMAN, S. W. &MCDONOUGH, W. J., Ceram. Eng. Sci. Proc., 1 (1980)424±443.

4. PREWO, K. & BRENNAN, J., J. Mater. Sci., 17 (1982)1201±1206.

5. BRENNAN, J. & PREWO, K., J. Mater. Sci., 17 (1982)2371±2383.

6. MAH, T., MENDIRATT, M. G., KATZ, A. P., RUH,R. & MAZDIYASNI, K. S., J. Am. Ceram. Soc., 68(1985) C27±30.

7. FITZER, E. & GADOW, R., Am. Ceram. Soc. Bull., 65(1986) 326±335.

8. STINTON, D. P., CAPUTO, A. J. & LOWDEN, R.,Am. Ceram. Soc. Bull., 65 (1986) 347±350.

9. LOWDEN, R. A. & STINTON, D. P., Ceram. Eng. Sci.Proc., 9 (1988) 705±722.

10. YAMAMURA, T., ISHIKAWA, T., SATO, M., SHI-BUYA, M., OHTSUBO, H., NAGASAWA, T. &OKAMURA, K., Ceram. Eng. Sci. Proc., 11 (1990)1648±1660.

11. YANG, J., LIN, W., SHIH, J., KAI, W., JENG, S. M. &BURKLAND, C. V., J. Mater. Sci., 26 (1991) 2954±2960.

12. KIM, Y., SONG, J., PARK, S. & LEE, J., J. Mater. Sci.,28 (1993) 3866±3868.

13. LINDLY, M. W. & GODFREY, D. J., Nature, 229(1971) 192±193.

14. SHETTY, D. K., PASCUCCI, M. R., MUTSUDDY, B.C. & WILLS, R. R., Ceram. Eng. Sci. Proc., 6 (1985)632±645.

15. BHATT, R.T., Tailoring Multiphase and CompositeCeramics, Material Science Research, Vol. 20. PlenumPress, New York, 1985, pp. 675±686.

16. SINGH, R. N. & GADDIPATI, A. R., J. Am. Ceram.Soc., 71 (1988) C100±103.

17. KODAMA, H., SAKAMOTO, H. & MIYOSHI, T., J.Am. Ceram. Soc., 72 (1989) 551±558.

18. BHATT, R. T. & KISER, J. D., Report no. NASA-TM-103098, 1990.

19. BHATT, R. T., J. Am. Ceram. Soc., 75 (1992) 406±412.

20. HUANG, C. M., XU, Y., ZOU, D. & KRIVEN, W. M.,Ceram. Eng. Sci. Proc., 15 (1994) 1154±1163.

21. SINGH, D., SHINGH, P., MAJUMDAR, S., KUPPER-MAN, D. S., COWDIN, E. & BHATT, R. T., J. Am.Ceram. Soc., 77 (1994) 2561±2568.

22. GUO, J., MAO, Z., BAO, C., WANG, R. & YANG, D.,J. Mater. Sci., 17 (1982) 3611±3616.

23. FITZER, E., FRITZ, W. & GADOW, R., In Proc. Int.Symp. on Ceramic Component for Engines, Japan, 1983,pp. 505±518.

24. LUNDBERG, R., POMPE, R. & CARLSSON, R.,Ceram. Eng. Sci. Proc., 9 (1988) 901±905.

25. MAH, T., HETCH, N. L., MCCULLUM, D. E., KIM,H. M., KATZ, A. P. & LIPSITT, H. A., J. Mater. Sci.,19 (1984) 1191±1201.

26. FAREED, A. S., FANG, P., KOCZAK, M. J. & KO, F.M., Am. Ceram. Soc. Bull., 66 (1987) 353±358.

27. Ube Industries, Ltd, Technical Report no. 20002003.28. GAUCKLER, L. J., PRIETZEL, S., BODEMER, G. &

PETZOW, G., Nitrogen Ceramics, ed. F. L. Riley.Noordho�, 1977, pp. 529±538.

29. HASEGAWA, Y., HIROTA, Y., YAMANE, N.,MITOMO, M. & SUZUKI, H., Yogyo Kyokaishi, 89(1981) 50±57.

30. UMEBAYASHI, M., KISHI, K., KOBAYASHI, K.,MIYAZAKI, K. & OOYAMA, T., Yogyo Kyokaishi, 90(1982) 615±621.

SiTiCO ®bre reinforced SiAlON 569