CATERTALIA Pergamon Acta mater.48(2000)46194628 www.elsevier.com/locate/actamat INTERFACIAL CHARACTERIZATION OF A SLURRY-CAST MELT-INFILTRATED SiC/SIC CERAMIC-MATRIX COMPOSITE .. J. breNnaN United Technologies Research Center, East Hartford, CT 06108, USA being developed for combustor applications under the High Speed Civil Transport(HSCT) Enabling Propul on Material(EPM) Program. A major part of this effort has dealt with the characterization and optimizatio cussed in this paper include an overview of the differences in composite microstructure between the EPM SiC/SiC material and a more conventional CVI SiC/SiC composite material, the microstructure/property relationships for the EPM SiC/SiC composite with two different types of Sic fiber(High- Nicalon and Sylramic ), and the effect of moist, high-temperature environments on the stability of the BN interface. o 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Ceramic composites; Fibers; Interface; Microstructure; Mechanical properties 1 INTRODUCTION SiC-fiber-reinforced SiC matrix offers the best comb The Enabling Propulsion Material(EPM) Program is nation of high-temperature capability and high ther aimed at developing a gas turbine engine combustor mal conductivity. Chemical vapor deposited(CVD) liner so that an environmentally acceptable and econ- SiC has been reported to possess a thermal conduc- omically viable High Speed Civil Transport(HSCT) tivity of up to 325 Wim K [I], which is higher than for some metal superalloys. The most common aircraft can be achieved. One of the prime objectives SiC/SiC CMC system consists of a chemically vapor of this program has been to develop and demonstrat infiltrated(CVI) SiC matrix around a woven SiC-fiber a material system, design concept and manufacturing preform. The resultant microstructure of this CVI process that can meet the HSCT combustor s environ- Sic/Sic composite system, however, is not dense mental, thermal, structural, economic and durability requirements. Among these requirements are an No, instead it contains rather large regions of matrix emissions index <5 g/kg, a material that can with- porosity. This porosity lowers the thermal conduc- stand temperatures to 1200 C under a tensile stress tivity of the composite to an unacceptable level fo of up to 100 MPa, and be able to meet an 18,000 h the HSCT combustor liner. Therefore, it was decided life requirement. The combustor concepts under con- to concentrate SiC/SiC composite developmental sideration do not permit the use of film cooling. efforts on the system that consists of a woven SiC- which traditionally has been used to reduce combus- fiber preform( five-harness satin) that has a BN fiber tor liner temperatures to manageable levels for met- interface coating applied to it by CVI, followed by a allic combustor liners. Therefore. high-thermal-con- relatively thin(1-4 um)CVI coating of SiC. This ductivity materials with a significantly higher relatively porous rigidized preform is then infiltrated temperature capability than those of current metallic with a slurry of SiC particles, dried, and then infil- combustor liners are required for this application. trated with molten silicon metal. The result is a full Ceramic-matrix composites(CMCs)have been ident- dense matrix(except within the fiber tows)of Si/Sic. ified as having the highest potential to satisfy the as shown for the melt infiltration(MI) SiC/SiC com- design requirements and service conditions posite in Fig. 1, which is compared with a typical Of the CMC systems available, the system of an CVI SiC/SiC composite with its inherent matrix porosity. Both composites have a fiber volume frac tion of-0.35. Besides the higher thermal conductivity nnanjj@aol.com(J.J.Brennan)oftheMISiC/SiCcomposite,thelackofmatrix 1359-6454100/520.00@ 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved PI:S1359-6454(00)00248-2Acta mater. 48 (2000) 4619–4628 www.elsevier.com/locate/actamat INTERFACIAL CHARACTERIZATION OF A SLURRY-CAST MELT-INFILTRATED SiC/SiC CERAMIC-MATRIX COMPOSITE J. J. BRENNAN* United Technologies Research Center, East Hartford, CT 06108, USA Abstract—An SiC-particulate, silicon-metal melt-infiltration-matrix composite reinforced with SiC fibers is being developed for combustor applications under the High Speed Civil Transport (HSCT) Enabling Propul￾sion Material (EPM) Program. A major part of this effort has dealt with the characterization and optimization of the boron nitride (BN) based fiber/matrix interface. BN was chosen as the primary interfacial material due to its inherently weak structure and thus good crack-deflecting ability, ease of deposition by chemical vapor infiltration (CVI) into woven fiber preforms, and relatively good environmental stability. Topics dis￾cussed in this paper include an overview of the differences in composite microstructure between the EPM SiC/SiC material and a more conventional CVI SiC/SiC composite material, the microstructure/property relationships for the EPM SiC/SiC composite with two different types of SiC fiber (High-Nicalon and Sylramic), and the effect of moist, high-temperature environments on the stability of the BN interface.  2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Ceramic composites; Fibers; Interface; Microstructure; Mechanical properties 1. INTRODUCTION The Enabling Propulsion Material (EPM) Program is aimed at developing a gas turbine engine combustor liner so that an environmentally acceptable and econ￾omically viable High Speed Civil Transport (HSCT) aircraft can be achieved. One of the prime objectives of this program has been to develop and demonstrate a material system, design concept and manufacturing process that can meet the HSCT combustor’s environ￾mental, thermal, structural, economic and durability requirements. Among these requirements are an NOx emissions index ,5 g/kg, a material that can with￾stand temperatures to 1200°C under a tensile stress of up to 100 MPa, and be able to meet an 18,000 h life requirement. The combustor concepts under con￾sideration do not permit the use of film cooling, which traditionally has been used to reduce combus￾tor liner temperatures to manageable levels for met￾allic combustor liners. Therefore, high-thermal-con￾ductivity materials with a significantly higher temperature capability than those of current metallic combustor liners are required for this application. Ceramic-matrix composites (CMCs) have been ident￾ified as having the highest potential to satisfy the design requirements and service conditions. Of the CMC systems available, the system of an * E-mail address: Brennanjj@aol.com (J.J. Brennan) 1359-6454/00/$20.00  2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S13 59-6454(00)00248-2 SiC-fiber-reinforced SiC matrix offers the best combi￾nation of high-temperature capability and high ther￾mal conductivity. Chemical vapor deposited (CVD) SiC has been reported to possess a thermal conduc￾tivity of up to 325 W/m K [1], which is higher than for some metal superalloys. The most common SiC/SiC CMC system consists of a chemically vapor infiltrated (CVI) SiC matrix around a woven SiC-fiber preform. The resultant microstructure of this CVI SiC/SiC composite system, however, is not dense; instead it contains rather large regions of matrix porosity. This porosity lowers the thermal conduc￾tivity of the composite to an unacceptable level for the HSCT combustor liner. Therefore, it was decided to concentrate SiC/SiC composite developmental efforts on the system that consists of a woven SiC- fiber preform (five-harness satin) that has a BN fiber￾interface coating applied to it by CVI, followed by a relatively thin (1–4 µm) CVI coating of SiC. This relatively porous rigidized preform is then infiltrated with a slurry of SiC particles, dried, and then infil￾trated with molten silicon metal. The result is a fully dense matrix (except within the fiber tows) of Si/SiC, as shown for the melt infiltration (MI) SiC/SiC com￾posite in Fig. 1, which is compared with a typical CVI SiC/SiC composite with its inherent matrix porosity. Both composites have a fiber volume frac￾tion of |0.35. Besides the higher thermal conductivity of the MI SiC/SiC composite, the lack of matrix