Printed in norie m irela atl ihs served MICROSTRUCTURE AND MECHANICAL PROPERTIES OF HI-NiCALON/BN/a-SILICON-NITRIDE CERAMIC-MATRIX COMPOSITES Monssef Drissi-Habti, *t Kikuo nakano The National Industrial Research Institute of Nagoya, 1-1 Hirate-cho, Kita-ku, Nagoya 462, Japan (Received 19 March 1996; revised 5 March 1997; accepted 21 March 1997) Abstract interface with carbon or boron nitride viz. mechanical Unidirectional ceramic-matrix composites consisting of tests performed at high temperatures along with post BN-coated Hi-Nicalon (Sic) fibre in a silicon nitride test observation under a microscope have proved that matrix were fabricated by a slurry-impregnation/hot- limited success has been achieved to date. Starting pressing route. The microstructures of the composites from the considerations addressed above, the scope of vere investigated by scanning and transmission electron this work is to provide an alternative to available microscopies and atomic force microscopy, while the CMCs by associating the a silicon nitride ceramic mechanical parameters of the composites were eval- matrix with Hi-Nicalon ceramic fibers. uated by flexure and micro-indentation tests. The effect Silicon nitride has excellent characteristics for use in of the processing route on the constituents is evaluated. high-temperature structural materials owing to its high Owing to the small size of the specimens required tability up to 1400C and its low density. However, it latter technique seems to be suitable for evaluation of is brittle as monolithic ceramics. The major concern in the in situ mechanical properties of the matrix and utilizing silicon nitride is to improve its R-curve fibers of a given composite. C 1997 Elsevier Science behavior. This could be achieved by using continuous Limited ramic fibers and this work presents the first results obtained by combining a silicon nitride with Hi- Keywords: A. ceramic-matrix composites; Hi-Nicalon Nicalon fibers(<0 5% oxygen content, *30 mol% fibres; B. mechanical properties; B. microstructure free carbon). This composite was manufactured by the polymer pyrolysis route, chosen on the basis of cost effectiveness considerations. 1 INTRODUCTION 2 MATERIALS AND EXPERIMENTS Continuous-fiber ceramic-matrix composites(CMCs) are considered to be promising candidates for high The material investigated is made of Hi-Nicalon fibers temperature application in structural parts, mainly in with 14 um mean diameter, 500 filaments/yarn, 2.74 g aerospace and military applications. However, despite cmdensity, Si: 63. 7%, C: 35-8%, O: 0 5% in weight their attractiveness and the great effort expended with a boron nitride coating of 0 5 um thickness. The during the last decade towards the improvement of longitudinal Youngs modulus of the virgin fibers is their processing routes and the prediction of their Er=250 GPa and the volume fraction of fibers is mechanical behavior, CMCs are still not totally V=0-48. The reinforcement is imprcgnated with convincing for large-scale manufacturing. This is slurry of a Si3n4 containing 5%Y2O3 and 5% Al2O3 mainly due to the numerous problems which remain to as sintering additives (referred to as the filler) be solved. In addition to the high cost of available uniaxial fiber prepreg was made by filament winding processing routes, the oxidation sensitivity of com- The slurry consisted of the filler, polysilazane(Chisso lonly used fibers as a consequence of their high NCP 201)and toluene. The prepreg was cut into oxygen and/or free carbon content is one of the main coupons of equal size to form a post-form such that limiting parameters. Even the relevant attempts the fibers were all arranged in the same direction. In directed towards the protection of the fiber/matrix order to make post-for To whom correspondence should addressed, at: 19 Avenue pyrolyzed(850C in nitrogen)and hot pressed(1550oC Horatio Smith. 14000 Caen. france in nitrogen). To allow a comparison to be made t On leave from LERMAT, URA CNRS 1317. ISMRa between the composite behavior and that of the Caen. france constituents, monolithic silicon nitride matrix materialComposites Science and Technology Sl(l997) 1483-1489 0 1997 Elsevier Science Limited Printed in Northern Ireland. All rights reserved ELSEVIER PII: SO266-3538(97)00070-S 0266-3538/97/$17.00 MICROSTRUCTURE AND MECHANICAL PROPERTIES OF HI-NICALONlBNh-SILICON-NITRIDE CERAMIC-MATRIX COMPOSITES Monssef Drissi-Habti,*t & Kikuo Nakano The National Industrial Research Institute of Nagoya, I -I Hirate-cho, Kita-ku, Nagoya 462, Japan (Received 19 March 1996; revised 5 March 1997; accepted 21 March 1997) Abstract Unidirectional ceramic-matrix composites consisting of BN-coated Hi-Nicalon (Sic) fibre in a silicon nitride matrix were fabricated by a slurry-impregnationlhot￾pressing route. The microstructures of the composites were investigated by scanning and transmission electron microscopies and atomic force microscopy, while the mechanical parameters of the composites were eval￾uated by flexure and micro-indentation tests. The effect of the processing route on the constituents is evaluated. Owing to the small size of the specimens required, the latter technique seems to be suitable for evaluation of the in situ mechanical properties of the matrix and fibers of a given composite. 0 I997 Elsevier Science Limited Keywords: A. ceramic-matrix composites; Hi-Nicalon fibres; B. mechanical properties; B. microstructure 1 INTRODUCTION Continuous-fiber ceramic-matrix composites (CMCs) are considered to be promising candidates for high￾temperature application in structural parts, mainly in aerospace and military applications. However, despite their attractiveness and the great effort expended during the last decade towards the improvement of their processing routes and the prediction of their mechanical behavior, CMCs are still not totally convincing for large-scale manufacturing. This is mainly due to the numerous problems which remain to be solved. In addition to the high cost of available processing routes, the oxidation sensitivity of com￾monly used fibers as a consequence of their high oxygen and/or free carbon content is one of the main limiting parameters. Even the relevant attempts directed towards the protection of the fiber/matrix * To whom correspondence should addressed, at: 19 Avenue Horatio Smith, 14000 Caen, France. ’ On leave from LERMAT, URA CNRS 1317, ISMRa, Caen, France. interface with carbon or boron nitride, viz. mechanical tests performed at high temperatures along with post￾test observation under a microscope, have proved that limited success has been achieved to date.’ Starting from the considerations addressed above, the scope of this work is to provide an alternative to available CMCs by associating the CY silicon nitride ceramic matrix with Hi-Nicalon ceramic fibers. Silicon nitride has excellent characteristics for use in high-temperature structural materials owing to its high stability up to 1400°C and its low density. However, it is brittle as monolithic ceramics. The major concern in utilizing silicon nitride is to improve its R-curve behavior. This could be achieved by using continuous ceramic fibers and this work presents the first results obtained by combining (Y silicon nitride with Hi￾Nicalon fibers ( < 05% oxygen content, = 30 mol% free carbon). This composite was manufactured by the polymer pyrolysis route, chosen on the basis of cost￾effectiveness considerations.* 2 MATERIALS AND EXPERIMENTS The material investigated is made of Hi-Nicalon fibers with 14 pm mean diameter, 500 filaments/yarn, 2.74 g cmp3 density, Si: 63.7%, C: 358%, 0: 0.5% in weight, with a boron nitride coating of 0.5 pm thickness. The longitudinal Young’s modulus of the virgin fibers is3 Ef = 250 GPa and the volume fraction of fibers is V,= 0.48. The reinforcement is impregnated with a slurry of (Y S&N, containing 5% Y,O, and 5% Al,O, as sintering additives (referred to as the filler). A uniaxial fiber prepreg was made by filament winding. The slurry consisted of the filler, polysilazane (Chisso NCP 201) and toluene. The prepreg was cut into coupons of equal size to form a post-form such that the fibers were all arranged in the same direction. In order to make a composite, the post-form was pyrolyzed (850°C in nitrogen) and hot pressed (1550°C in nitrogen). To allow a comparison to be made between the composite behavior and that of the constituents, monolithic silicon nitride matrix material 1483