2778 G N. Morscher, J. D. Cawley/Journal of the European Ceramic Society 22(2002)2777-2787 matrix crack when the boria reacts with the SiC fibers 2. The process and factors affecting intermediate tem- and matrix 0-lI coupled with B removal from the oxi- perature stress-rupture of woven BN interphase com- dation product as volatile B-containing hydrated species posites in air form in the presence of water vapor. However, the intermediate temperature stress-rupture properties of It was established in Ref. 14 from chemical analysis of Sic/SiC composites with bn interphases have been fiber fracture surfaces(degree of fiber fracture surface shown to be superior to SiC/SiC composites with C oxidation) that whole areas of fibers in a matrix crack interphases when tested in air. failed at the same time during the course of the stress Since BN interphase composites are more durable rupture experiment. It was also discerned from fracture than C interphase composites in oxidizing environments mirror analysis of failed fibers that the amount of at intermediate temperatures, bN has been selected as degradation to the fiber strength was commensurate the interphase material for the earlier mentioned com- with the expected amount from single fiber stress-rup bustor liner application. Therefore, the factors and ture data. 16 That is, no additional strength degradation mechanisms that control intermediate temperature occurred to the population of strongly bonded fibers. stress-rupture for BN interphase composites will be the Therefore, failure under stress-rupture conditions at focus of this study. A model that accounts for some of intermediate temperatures occurs by local overloading these factors will be presented. It will be shown to pre- due to the stress concentration associated with the dict the intermediate temperature rupture data. Finally, strong bonding of fibers to the matrix. Not because the recent enhancements to the microstructure of SiC/BN/ fibers are weakened. The depth of this embrittled region Sic composites resulting in improved intermediate tem- grows as the oxidation front moves deeper into the perature composite performance will be presented. All matrix crack(Fig. 1). Eventually, one of the strongly of the stress-rupture data presented in this work that bonded fibers breaks and causes all the other strongly has not been published earlier was performed in the bonded fibers to fail due to the inability to globally same manner as described in Refs. 14, 15. Woven com- share the increased stress applied to the nearest neigh- posites, 150 mm in length, were tested in tension where bor fibers and unbridged crack growth. This view is the ends of the specimens were"cold-gripped"and a strongly supported by the pattern of fracture mirrors slotted furnace with a 15 mm hot zone was inserted in the from the strongly bonded regions of near fiber contact center region between the grips. The furnace was brought in the micrograph of Fig. l, which are indicative of to temperature prior to the application of the load correlated fiber failure. If the stress transferred to the fiber x(t) BN nnnnnnnnn Embrittled rea (Loc 5)x×a.点k1: O2 and H,0 O and Ho bonded Pristine fiber Area(Global Load Sharing) matrix Fiber break Fig. 1. Idealized schematic representation of oxygen ingress in a matrix crack and an individual fiber failure that leads to failure of all strongly bonded fibers. An example of which is given in the upper left hand corner for a HN/ BN/MI SiC compositematrix crack when the boria reacts with the SiC fibers and matrix 1011 coupled with B removal from the oxi￾dation product as volatile B-containing hydrated species form in the presence of water vapor.11 However, the intermediate temperature stress-rupture properties of SiC/SiC composites with BN interphases have been shown to be superior to SiC/SiC composites with C interphases when tested in air.9,12,13 Since BN interphase composites are more durable than C interphase composites in oxidizing environments at intermediate temperatures, BN has been selected as the interphase material for the earlier mentioned com￾bustor liner application.1 Therefore, the factors and mechanisms that control intermediate temperature stress-rupture for BN interphase composites will be the focus of this study. A model that accounts for some of these factors will be presented. It will be shown to pre￾dict the intermediate temperature rupture data. Finally, recent enhancements to the microstructure of SiC/BN/ SiC composites resulting in improved intermediate tem￾perature composite performance will be presented. All of the stress-rupture data presented in this work that has not been published earlier was performed in the same manner as described in Refs. 14,15. Woven com￾posites, 150 mm in length, were tested in tension where the ends of the specimens were ‘‘cold-gripped’’ and a slotted furnace with a 15 mm hot zone was inserted in the center region between the grips. The furnace was brought to temperature prior to the application of the load. 2. The process and factors affecting intermediate tem￾perature stress-rupture of woven BN interphase com￾posites in air It was established in Ref. 14 from chemical analysis of fiber fracture surfaces (degree of fiber fracture surface oxidation) that whole areas of fibers in a matrix crack failed at the same time during the course of the stress￾rupture experiment. It was also discerned from fracture mirror analysis of failed fibers that the amount of degradation to the fiber strength was commensurate with the expected amount from single fiber stress-rup￾ture data.16 That is, no additional strength degradation occurred to the population of strongly bonded fibers. Therefore, failure under stress-rupture conditions at intermediate temperatures occurs by local overloading due to the stress concentration associated with the strong bonding of fibers to the matrix. Not because the fibers are weakened. The depth of this embrittled region grows as the oxidation front moves deeper into the matrix crack (Fig. 1). Eventually, one of the strongly bonded fibers breaks and causes all the other strongly bonded fibers to fail due to the inability to globally share the increased stress applied to the nearest neigh￾bor fibers and unbridged crack growth. This view is strongly supported by the pattern of fracture mirrors from the strongly bonded regions of near fiber contact in the micrograph of Fig. 1, which are indicative of correlated fiber failure. If the stress transferred to the Fig. 1. Idealized schematic representation of oxygen ingress in a matrix crack and an individual fiber failure that leads to failure of all strongly bonded fibers. An example of which is given in the upper left hand corner for a HN/BN/MI SiC composite. 2778 G.N. Morscher, J.D. Cawley / Journal of the European Ceramic Society 22 (2002) 2777–2787