2D-SiC/SiC composites in which the fibers were The boron nitride interphase was infiltrated hemically treated prior to any deposition stage the within the porous fiber preforms by the icvi pro surface of these treated Nicalon fibers exhibited a cess. BCl3-NH3-H2 gas mixtures were used at a different composition over significant thickness. 3 moderate temperature of 700 C and under reduced However it has been shown that the interfacial slid pressure according to a procedure which has been g resistance increased with the interphase thick- widely described elsewhere. 3, 16 Various interphase ness at room temperature, even if a bn thickness thicknesses, in the range 0.2-0.7 um, have been variation in the 0-2-0-7 um range did not induce deposited. The BN-coated Nicalon@ preforms any differences in the ultimate strength and strain. were further densified with Sic by a classical ICVI Thus crack saturation occurred for the lowest process from methyltrichlorosilane-H2 mixtures interphase thickness, and the microcracks spacing Moreover, after the specimen manufacturing, a sic in the tensile stress direction(within the 0 fibers- coating was deposited to prevent any direct initial tows)decreased as the bn thickness increased. 12 oxygen access to the interphase and interfaces The first objective of this study was to study the Some mechanical characteristics of SiC/Sic hemical and structural evolution of the interphase composites that include a pyrocarbon(PyC) inter and interfaces of 2D-SiC/BN SiC composite when phase deposited on treated Nicalon fibers are exposed to mechanical stress in an oxidizing envi- provided for comparison. In all cases, the materials ronment. It is indeed well established that the fiber/ were overcoated by SiC after machining matrix interface mainly governs the thermo- mechanical behavior of such composites. 7, 14, 5 2.2 Mechanical tests Therefore, various mechanical tests have been per- The mechanical behavior of the 2D composites was formed such as static bending and tensile tests, as evaluated at 600C in air by static bending and well as dynamic fatigue in tension-tension. The tensile tests as well as dynamic tension-tension nfluence of the interphase thickness on the fatigue tests. For these tests, bars were machined mechanical behavior was also examined. These with their main direction parallel to one of the fiber mechanical characterizations were carried out in orientation. The geometry and dimensions of the air at a temperature of 600C which corresponds to tensile and bending specimens are presented in a moderate temperature for the occurrence of hex- Fig. 1. The static fatigue tests were performed on agonal-BN oxidation. After failure of the compo- 4-points bending machine prototype having major site, the interphase and interfaces were examined at and minor spans of 5-08 and 2.54 cm. The tensile different scales using scanning electron microScopy fatigue behavior was studied with a hydraulic but also transmission electre ppy for the machine(Instron Dynamic 8501) equipped with microstructure and energy loss spectroscopy for hydraulic grips cooled with water. These charac- the chemistry of each interfacial zone. In addition, terizations were performed within a furnace (aET)in the mechanical results obtained for the SiC/BN/ ambient air. The tensile strain was measured with an compared to the results obtained on similar mat- length. For the dynamic tests in tension-tension, the erials with a pyrocarbon interphase frequency was adjusted at the desired value, 2 or 20 Hz in the present case, after few cycles at 0.02 Hz. 2 Experimental procedures 2.3 Microstructure and chemistry of the interfacial zones 2.1 Materials The interfacial microstructures were studied by The 2D-SiC/BN/SiC composites were prepared as transmission electron microscopy (TEm)whereas rectangular plates(160×80×3mm3)from2Dpre forms consisting of stacks of fabrics made with pressed together with graphite tooling to obtain a constant nominal fiber content of about 37 vol% The Nicalon fibers(NLM 202)were previousl chemically treated (proprietary treatment) by the treated fiber presented a different composition over 9+ a significant thickness and the surface was free carbon rich. 3 At room temperature it has been checked that this treatment did not decrease the Fig. 1. Geometry and dimensions, in mm, of the(a)tensile mechanical properties of the fibers (b )bending sp716 M. Leparoux et al. 2D-Sic/Sic composites in which the fibers were The boron nitride interphase was infiltrated chemically treated prior to any deposition stage. The within the porous fiber preforms by the ICVI pro￾surface of these treated Nicalonm fibers exhibited a cess. BC13-NH3-H2 gas mixtures were used at a different composition over significant thickness.13 moderate temperature of 700°C and under reduced However it has been shown that the interfacial slid- pressure according to a procedure which has been ing resistance increased with the interphase thick- widely described elsewhere.3a16 Various interphase ness at room temperature, even if a BN thickness thicknesses, in the range 0.2-0.7 pm, have been variation in the 0.24.7pm range did not induce deposited. The BN-coated Nicalon@ preforms any differences in the ultimate strength and strain. were further densified with SIC by a classical ICVI Thus crack saturation occurred for the lowest process from methyltrichlorosilane-H2 mixtures. interphase thickness, and the microcracks spacing Moreover, after the specimen manufacturing, a SIC in the tensile stress direction (within the 0” fibers- coating was deposited to prevent any direct initial tows) decreased as the BN thickness increased.12 oxygen access to the interphase and interfaces. The first objective of this study was to study the chemical and structural evolution of the interphase and interfaces of 2D-SiC/BN/SiC composite when exposed to mechanical stress in an oxidizing envi￾ronment. It is indeed well established that the fiber/ matrix interface mainly governs the thermo￾mechanical behavior of such composites.7,14,‘5 Therefore, various mechanical tests have been per￾formed such as static bending and tensile tests, as well as dynamic fatigue in tension-tension. The influence of the interphase thickness on the mechanical behavior was also examined. These mechanical characterizations were carried out in air at a temperature of 600°C which corresponds to a moderate temperature for the occurrence of hex￾agonal-BN oxidation. After failure of the compo￾site, the interphase and interfaces were examined at different scales using scanning electron microscopy but also transmission electron microscopy for the microstructure and energy loss spectroscopy for the chemistry of each interfacial zone. In addition, the mechanical results obtained for the SiC/BN/ SIC composites with various BN thicknesses were compared to the results obtained on similar mat￾erials with a pyrocarbon interphase. Some mechanical characteristics of Sic/Sic composites that include a pyrocarbon (PyC) inter￾phase deposited on treated Nicalon@ fibers are provided for comparison. In all cases, the materials were overcoated by SIC after machining. 2.2 Mechanical tests The mechanical behavior of the 2D composites was evaluated at 600°C in air by static bending and tensile tests as well as dynamic tension-tension fatigue tests. For these tests, bars were machined with their main direction parallel to one of the fiber orientation. The geometry and dimensions of the tensile and bending specimens are presented in Fig. 1. The static fatigue tests were performed on a 4-points bending machine prototype having major and minor spans of 5.08 and 2.54cm. The tensile fatigue behavior was studied with a hydraulic machine (Instriin Dynamic 8501) equipped with hydraulic grips cooled with water. These charac￾terizations were performed within a furnace (AET) in ambient air. The tensile strain was measured with an extensometer (2620-603 Instron) with a 25 mm gauge length. For the dynamic tests in tension-tension, the frequency was adjusted at the desired value, 2 or 20 Hz in the present case, after few cycles at 0.02 Hz. 2 Experimental procedures 2.1 Materials The 2D-SiC/BN/SiC composites were prepared as rectangular plates (160x 80x3 mm3) from 2D pre￾forms consisting of stacks of fabrics made with Nicalon@ fibers. These stacks were maintained and pressed together with graphite tooling to obtain a constant nominal fiber content of about 37 ~01%. The Nicalon@ fibers (NLM 202) were previously chemically treated (proprietary treatment) by the Sociite Europeenne de Propulsion (SEP). The treated fiber presented a different composition over a significant thickness and the surface was free carbon rich.13 At room temperature, it has been checked that this treatment did not decrease the mechanical properties of the fibers. 2.3 Microstructure and chemistry of the interfacial zones The interfacial microstructures were studied by transmission electron microscopy (TEM) whereas 20 I- I- (4 3 60:s 60 P 60 (b) Fig. 1. Geometry and dimensions, in mm, of the (a) tensile and (b) bending specimens