COMPOSITES SCIENCE AND TECHNOLOGY ELSEVIER Composites Science and Technology 61(2001)1079-1082 Oxide/oxide composites with fibres produced by internal rystallis A.A. Kolchin, V.M. Kiiko, N.S. Sarkissyan, S.T. Mileik Solid State Physics Institute of the Russian Academy of Sciences, Chernogolovka, Moscow district, 142432 Russia Received 8 June 2000: accepted 16 November 2000 Sapphire fibres produced by the internal crystallization method were introduced into an alumina matrix. The fibres were coated with pyrolitic carbon to provide weak bonding to the matrix which is expected to enhance the fracture toughness of the material The composite fabrication process exploited a powder-cloth technique Evaluation of the strength and fracture toughness values of the specimens revealed an increase in the fracture toughness of the composites compared to that of the matrix, although the strength is not affected by the reinforcement. The main conclusion is that the internal crystallization-process yields single-crystal fibres suitable for the reinforcement of oxide-matrix composites. C 2001 Elsevier Science Ltd. All rights reserved Keywords: A. Oxides; A. Ceramic-matrix composites; A. Fibres; B Strength; B. Fracture toughness 1. Introduction families of oxides that provide easy cleavage either on some crystal planes or at an oxide/ oxide interface, and The high strength and high creep resistance of mono has led to the idea of using porous oxides as a crack crystalline and eutectic-oxide fibres, and the good oxi- deflector. One such family includes oxides with B-alumina dation resistance of polycrystalline oxide matrices make and magnetoplumbite structures with highly anisotropic oxide-fibre/oxide-matrix composites (OoCs) primary fracture energy that provides mica-like cleavage char candidates for structural application to serve at tem- acteristics [1]. Another family of oxides is that of the ABO4 peratures above 1200 C. As for any brittle-matrix com- composition [2], which includes a number of tungstates posite, the fracture toughness of an OoC is mainly molybdates, tantalates, niobates, and phosphates. The dependent on energy dissipation at weak interfaces in coating of fibres with such oxides is usually being carried the composite microstructure. out by using liquid- phase processes suitable for con- To trigger wider usage of OOCs, three main develop- tinuous filaments. The idea of a porous oxide matrix [3] ments of different degrees of complexity need to seems to work satisfactorily and, what is most impor accomplished tant, it can be used with any form of the fibre material On the other hand, the advance in developing oxide 1uma水 interface fibres for really high-temperature applications by using traditional methods yielding either mono-crystalline or high corrosion resistance, polycrystalline fibres is rather limited. Mono-crystalline 2. a means of generating an appropriate matrix for fibres remain too expensive to be used as reinforcements the ooc is required, and for structural composites and the use of polycrystalline 3. an appropriate fibre is needed. fibres is limited by relatively low service temperatures [3, 4. Hence, the internal crystallisation method (ICM) During the last decade, success in the first two direc- invented in the authors'laboratory [5] and then used to tions has been visible and impressive h fo make oxide fibres [6-8], which requires less energy input interface materials among oxides has revealed two in a real fibre growth process, can be considered as a basis for the production of various single-crystal and Eomt eutectic fibres. Such fibres have been shown to be effec- ail address. mileiko(@ issp ac ru(ST. Miliko) tive reinforcements for metal-matrix composites, yield- 0266-3538/01/ S.see front matter C 2001 Elsevier Science Ltd. All rights reserved. PII:S0266-3538(00)00233-5
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A.A. Kolchin et al. /Composites Science and Technology 61(2001)1079-1082 ng an increase in service temperature over Ni-based materials up to about 1150C [9, 10 In the present paper, the prospect of using ICM fibres in oxide-fibre/oxide-matrix composites is shown. The ICM fibres are obtained in the form of either bound or loose bundles of a length up to 200 mm when using existing equipment. Hence, coating processes used for continuous filaments to provide the necessary interface in a brittle-matrix composite are not suitable for ICM fibres. Also liquid-based processes need to be seriously modified to be used with such fibres. and it is clear that coatings produced from a gaseous medium should b tried with ICM fibres In the present work, a somewhat model interface, that is pyrolitic carbon, was chosen ly, mber of ways of generating an oxide matrix can now be used; however, the simplest of these based on a common powder-metallurgy route, is chosen MATRIX since the main purpose of the work is to show the pos FIBRE res duced by ICM in oxide-matrix composites to enhance the fracture toughness of the matrix material 2. Experimental CARBON LAYER 2.1. Preparation of composites 10μM Sapphire fibres produced by internal crystallisation vith the structure and mechanical properties described in detail in Ref [7] were used to make composite speci ptical micrograph of the cross-section of a sapp n(b)a fragment of the failure surface of the com mens. Typically, the fibres are characterised by an aver- posite The scale bar= 10 um. age tensile strength of about 500-800 MPa in tests on samples of I mm gauge length, with a Weibull exponent binder while heating the specimen in vacuum, Fig. la of between 3 and 5. The average characteristic cross-sec On the other hand, during the vacuum treatment the tional size of a fibre is about 0.1 mm. Pyrolitic carboncarbon interface survives perfectly as can be seen in deposited on the fibres from diethylketone by Fig. Ib. Two types of specimens were prepared, both for CVD process. The coating thickness was about 2-3 um. bending tests, Fig. 2, to evaluate the flexural strength Composite specimens were prepared by using a so- and the critical stress-intensity factor called powder-cloth technique, which is well-known as a method of fabrication of intermetallic-based composites 2. 2. Specimen testing (see e.g. Ref. [llD. Thin slurry films containing alumina powder of average size 0.5 um and raw rubber as an The tests were performed at room temperature. The organic binder(powder cloth) were used as a precursor fo strength was evaluated by testing specimens shown in the matrix. The average film thickness was about 160 um. Fig. 2a in 3-point bending. The value of the critical The fibres were arranged on a film of the matrix pre- stress-intensity factor was calculated by testing the spe- cursor by hand. The stack of slurry films and fibres was cimens shown in Fig 2b and using the following inter placed in a closed graphite die, heated in vacuum to polation formula [12] burn out the organic binder, and then sintered under pressure. The temperature-time-pressure conditions were 1350-1400oC, 1-3 h: 70 MPa. The specimen cross- 功1(x) sections are illustrated in Fig. 1. It should be noted that the alumina matrix seems to be saturated with carbon here o' is maximum force and that has not been completely burnt out from the organic x=c/h slurry films were supplied by Drs G.A. Fomina and SD pin of Moscow State University of Aviation Technology, MATl, Ru Y(x)=3√x(1.93-3.07x+14.5x2-251x3+25.8x)
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A.A. Kolchin et al. /Composites Science and Technology 61(2001)1079-1082 l081 h= about 5 mm Q ch=0.37-042 Fig. 2. Composite specimens to measure (a) bending strength and (b)critical stress-intensity factor Sintering time 1 h 12 ●3h E350 30 13601370138013901400 Sintering temperature /C Fibre volume fraction Fig3. Bending strength of alumina matrix versus sintering tempera- Fig. 5. Critical stress intensity factor of sapphire-fibre/alumina-matrix ture. Sintering was done under a pressure of 70 MPa composite versus fibre volume fraction. Sintering regime is 1400.C 70 MPa-3 h c450 400 Sintering time o 1 h 350 v300 8250 0.000.020.040.060.080.100.12 Fibre volume fraction 100μ m Fig. 4. Bending strength of sapphire-fibre/alumina-matrix composite rsus fibre volume fraction. Sintering temperature is 1400.C. Fig. 6. The failure surface of a composite specimen To choose the fabrication parameters for making composite specimen, a number of matrix specimens 3. Discussion and conclusions were made and tested, Fig 3, and the temperature-time conditions were determined as 1400C for 3 h. Depen Two obvious conclusions can be drawn First. rein- dencies of the strength and fracture toughness on the forcing the alumina matrix with ICM sapphire fibres fibre volume fraction are presented in Figs. 4 and 5. does not essentially change the material strength that
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A.A. Kolchin et al. /Composites Science and Technology 61(2001)1079-1082 can be expected owing to the presence of a relatively low [2] Marshall DB. Davis JB, Morgan PED, Waldrop JR, Porter JR. interface strength and low fibre strength when measured Properties of La-monozite as an interface in oxide composites. Z on long fibre lengths Metallkd1999902:1048-52. Secondly, the fracture toughness of the composites is 3 Levi CG. Zok Fw, Yang J-Y, Mattoni M. Lofvander JPA Microstructural design of stable porous matrices for all-oxide clearly higher than that of the matrix, certainly as a ceramic composites. Z Metallkd 1999: 90(12): 1037-47 esult of the processes occurring at the weak fibre/ Deleglise F, Berger MH, Bunsell AR. In: Crivelli Visconti I, edi- matrix interface; in particular, fibre pull-out can be seen tor. Proc. of 8th European Conf. Compos. Mater. Napoli, June on the failure surface of the composite, Fig. 6, although 98. Cambridge: Woodhead, 1998, Vol 4, p. 175 ne length of pull-out is not large 5 Mileiko ST, Kazmin VI Structure and mechanical properties of oxide fibre reinforced metal matrix composites produced by the It is clear that achieving sufficiently high energy dis- sipation at the interface and, hence, high fracture toughness of the brittle-fibre/brittle-matrix composites [6 Mileiko ST, Kiiko VM, Sarkissyan NS, Starostin MYu, Gvo requires careful adjustment of properties of all the con- deva SI, Kolchin AA, Strukova GK. Microstructure and prop- tituents, which has not been the objective of the present Al2Or-AlsY3012 fibre produced via internal piece of work. The aim has been to demonstrate a major rystallization route. Comp Sci Tech 1999: 59(11): 1763-72. [7 Kurlov VN, Kiiko VM, Kolchin AA, Mileiko ST Sapphire fibres possibility of using ICM oxide fibres in oxide/oxide grown by a modified internal crystallization method. J Cryst composites, and this objective has been achieved Growth1999204(4):499-504 [8 Mileiko ST, Kiiko VM, Starostin M.Yu, Kolchin AA, Kozhev nikoy ls. fabrication and mullite fibers. Scripta Materialia, submitted Acknowledgements [9 Mileiko ST. Oxide fibres produced by internal crystallization method and their usage in composite technology, CD-ROM The work was supported by INTAs and RFBR ( Project #95-599). The authors are thankful to Dr M. Yu. Starostin and Mr. A.Ya. Mizkevich. for their [10] Mileiko ST, Kiko VM, Kolchin AA, Serebryakov AV, Korzhor VP. Starostin MYu, Sarkissyan NS. High temperature strength help in the experimental work and creep properties of oxide-fibre/ni-based matrix composites, [l Kumar Ks, Bao G. Intermetallic-matrix composites: an over- References iew. Comp Sci Tech 1994: 52(2): 127-50 [12] Kovchik SE, Morozov EM. Characteristics of short time fracture Cinibulk MK. Magnetoplumbite compounds as a fiber coating in toughness of materials and methods of their evaluation. Kiev oxide/oxide composites Ceram Eng Sci Proc 1994: 15:721-8. Naukova Dumka, 1988(in Russian)
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