Journal of the European Ceramic Society 18(1 C 1998 Else Printed in Great Britain, All rig PII:s0955-2219(97)00I85-4 955221998s = Oxidizing environment Influence on the Mechanical Properties and microstructure of 2D-SIC/BN/ SiC Composites Processed by ICVI M. Leparoux, a L. Vandenbulcke, a V Serin, J. Sevely, S. Goujard and C. Robin-Brossec aCNRS-LCSR, Ic avenue de la Recherche Scientifique, F.-45071 Orleans, France CEMES-LOE. CNRS. 31055 Toulouse. france Societe Europeenne de Propulsion, Les Cinq Chemins-Le Haillan, F-33165 St-Medard-en-Jalles, france (Received 11 June 1997: accepted 22 October 1997) Abstract stability in severe environments. However it is now well established that the perfo 2D-SiC/Sic composites have been elaborated with vapor processed composites is strongly dependent BN interphases of diferent thicknesses which were on each component of the material and on the deposited on treated Nicalon( fibers by isothermal- processing parameters. 1, In a recent study on the isobaric chemical vapor infiltration from BCls isothermal/isobaric chemical vapor infiltration NHgH2 mixtures. Their mechanical behavior was (ICVD)of a boron nitride interphase, it was shown investigated at 600C in air and compared to the that the temperature, the gas flow and the inlet results obtained on similar SiC/C/SiC composites. composition conditions influenced both the BN Although these materials exhibit similar stress and organization and the nature of the interface with strain values at rupture when loaded at room tempera- the substratc. 3 Thcrcforc the knowledge and the ture, whatever the interphase, their thermomechanical control of each step of the composite manufactur- resistance depends on stress type, i.e. static or dynamic. ing are of prime importance Under static fatigue the BN interphases are more effi In order to obtain high mechanical properties cient than the pyrocarbon (PyC) ones. Thin BN inter- the composites generally require a compliant inter phases tend to maintain the interfacial properties. This phase between the fiber and the matrix. 4. 5 At first it result could be explained by the larger microcrack dis- was generally composed of a thin carbon film ances in the tows supporting the main part of the load, deposited on the fiber prior to the matrix infiltra according to a lower interfacial sliding resistance In tion0-8 This interphase exhibits a structure that contrast the materials with a PyC interphase, which allows sliding between the fiber and the matrix, and lave a much higher interfacial shear resistance at room then acts as a mechanical fuse. Nevertheless, the temperature, exhibit better thermomechanical beha carbon interphase is consumed dining envi under dynamic fatigue at 600C. The mechanical ronments, unless it is protected with a self-protec characteristics are related to the evolution of the fiber- tive matrix and/ or an external sealing coating. 9, 10 matrix interfacial zone which has been studied by When the whole composite is not protected,or SEM, TEM and EELS. @)1998 Elsevier Science when it is submitted to dynamic stresses that Limited. All rights reserved maintain the microcracks opening, the interpha is at least partially replaced by a glassy phase which strongly bonds the fiber to the matrix. 4, II 1 Introduction The result is a brittle behavior of the material In a previous study, the carbon interphase was High temperature structural applications, such as successfully rcplaccd by a boron nitride coating gas turbine engines or space plane thermal protection which was expected to be more oxidation resistant systems have generated a great interest in ceramic The BN interphase was elaborated by ICVI from matrix composites(CMC). These composites exhi- BCl3-NH3-H2 mixtures at a moderate temperature bit a combination of attractive physico-chemical of 700C. Thus, high strength and high strain properties such as high strength, toughness and have been obtained at room temperature with 715Journal of the European Ceramic Society 18 (1998) 115-723 6 1998 Elsevier Science Limited PII: SO955-2219(97)00185-4 Printed in Great Britain. All rights reserved 0955-2219/98/$19.00 + 0.00 Oxidizing Environment Influence on the Mechanical Properties and Microstructure of 2D-SiC/BN/SiC Composites Processed by ICVI M. Leparoux,a L. Vandenbulcke,” V. Seriqh J. Sevely,h S. Goujard” and C. Robin-Brosse” “CNRS-LCSR, 1C avenue de la Recherche Scientifique, F-45071 Orlkans, France ‘CEMES-LOE, CNRS, 3 1055 Toulouse, France “SociCtC Europkenne de Propulsion, Les Cinq Chemins-Le Haillan, F-33 165 St-MCdard-en-Jalles, France (Received 11 June 1997; accepted 22 October 1997) Abstract ZD-Sic/Sic composites have been elaborated with BN interphases of d@erent thicknesses which were deposited on treated ‘Nicalon@‘bers by isothermal￾isobaric chemical vapor infiltration from BClr NH,H2 mixtures. Their mechanical behavior was investigated at 600°C in air and compared to the results obtained on similar SiCjCjSiC composites. Although these materials exhibit similar stress and strain values at rupture when loaded at room tempera￾ture, whatever the interphase, their thermomechanical resistance depends on stress type, i.e. static or dynamic. Under static fatigue the BN interphases are more efi￾cient than the pyrocarbon (PyC) ones. Thin BN inter￾phases tend to maintain the interfacial properties. This result could be explained by the larger microcrack dis￾tances in the tows supporting the main part of the load, according to a lower interfacial sliding resistance. In contrast the materials with a PyC interphase, which have a much higher interfacial shear resistance at room temperature, exhibit better thermomechanical behavior under dynamic fatigue at 600°C. The mechanical characteristics are related to the evolution of the$ber￾matrix interfacial zone which has been studied by SEM, TEM and EELS. 0 1998 Elsevier Science Limited. All rights reserved 1 Introduction High temperature structural applications, such as gas turbine engines or space plane thermal protection systems have generated a great interest in ceramic matrix composites (CMC). These composites exhi￾bit a combination of attractive physico-chemical properties such as high strength, toughness and 715 stability in severe environments. However it is now well established that the performance of chemical vapor processed composites is strongly dependent on each component of the material and on the processing parameters. l,* In a recent study on the isothermal/isobaric chemical vapor infiltration (ICVI) of a boron nitride interphase, it was shown that the temperature, the gas flow and the inlet composition conditions influenced both the BN organization and the nature of the interface with the substrate.3 Therefore the knowledge and the control of each step of the composite manufactur￾ing are of prime importance. In order to obtain high mechanical properties, the composites generally require a compliant inter￾phase between the fiber and the matrix.4,5 At first it was generally composed of a thin carbon film deposited on the fiber prior to the matrix infiltra￾tion.“* This interphase exhibits a structure that allows sliding between the fiber and the matrix, and then acts as a mechanical fuse. Nevertheless, the carbon interphase is consumed in oxidizing envi￾ronments, unless it is protected with a self-protec￾tive matrix and/or an external sealing coating.9,‘0 When the whole composite is not protected, or when it is submitted to dynamic stresses that maintain the microcracks opening, the interphase is at least partially replaced by a glassy phase which strongly bonds the fiber to the matrix.4,” The result is a brittle behavior of the material. In a previous study, ‘* the carbon interphase was successfully replaced by a boron nitride coating which was expected to be more oxidation resistant. The BN interphase was elaborated by ICVI from BCls-NH3-H2 mixtures at a moderate temperature of 700°C. Thus, high strength and high strain have been obtained at room temperature with