Influence of an O2 Type Oxidizing Environment on SiCf/SiC Composites: Properties/Microstructure Relationship

Abstract

Abstract. Ceramic matrix composites have been identified as a potential material of core structure for the IV th generation of fission nuclear reactors. Regarding its excellent mechanical behavior in very harsh conditions (high temperature and high irradiation flux), the CVI-SiC f /SiC composites with pyrocarbon interlayer are of prime interest for the fuel cladding in the Gas-cooled Fast Reactor (GFR). Although the working atmosphere is helium in these advanced reactors, presence of oxidizing impurities could have a significant role on the mechanical behavior of materials for a long time exposure. Within this framework, this study is intended to investigate the influence of oxidation on the SiC f /SiC composites mechanical properties. Different pre-damage states were intentionally operated by mechanical tensile tests on plate specimens before performing an oxidation treatment of 1000 hours at 1000°C under helium with 10 ppm of O 2 . The degradation of the composite was determined from the mechanical behavior of post-exposure specimens. Results were correlated both with microstructural observations of the damaged tows and with characterizations of the generated oxides at the surface of the composites. The most severe decline of mechanical properties occurs for the higher predamaged loadings. A degradation mechanism, depending on the damage state, is proposed to explain the mechanical behavior change in accordance to the different examinations.

SiC fiber strength after low pO2 oxidation

“…Severe strength degradation is common after passive oxidation with formation of a crystalline scale . Severe degradation is also observed after active oxidation, which is a concern for SiC‐based CMCs used in hypersonic or nuclear applications . Suggested degradation mechanisms are stress corrosion cracking under load, unspecified damage developed during active oxidation, or decomposition of an SiOC glass second phase .…”

“…Active oxidation produces rough SiC fiber surfaces that have been characterized by SEM, but characterization by TEM that could specifically identify fine‐scale microstructural changes is lacking. SiC fiber degradation in oxidized CMCs is more complex, and may also depend on matrix porosity, microcracking, and the nature of the carbon or BN fiber‐matrix interphase …”

“…14,23 Severe degradation is also observed after active oxidation, [18][19][20]22,[23][24][25][26] which is a concern for SiC-based CMCs used in hypersonic or nuclear applications. [27][28][29][30] Suggested degradation mechanisms are stress corrosion cracking under load, 19 unspecified damage developed during active oxidation, 20,24 or decomposition of an SiOC glass second phase. 18,23,25,26 Active oxidation produces rough SiC fiber surfaces that have been characterized by SEM, but characterization by TEM that could specifically identify fine-scale microstructural changes is lacking.…”

“…SiC fiber degradation in oxidized CMCs is more complex, and may also depend on matrix porosity, microcracking, and the nature of the carbon or BN fiber-matrix interphase. 22,[30][31][32] Hi-Nicalon TM -S SiC fiber is used in many of the most advanced SiC-SiC CMCs. [33][34][35][36][37] The fiber properties are described in many publications.…”