Intermediate Temperature Internal Oxidation of a SiC/SiCN Composite with a Polymer‐Derived Matrix

Abstract

SiC-based composites exhibit oxidative embrittlement at intermediate temperatures. Although the mechanisms of internal oxidation in composites with initially hermetic matrices have been studied extensively, comparable studies on composites with semipermeable matrices, such as those produced by polymer infiltration and pyrolysis, have not been reported. The present article focuses on the latter class of composites, specifically a SiC f /SiCN m with a dual BN/Si 3 N 4 fiber coating. It describes detailed SEM and TEM analyses of the microstructure before and after oxidation in dry air or water vapor at 800°C. The results show that internal oxidation is more aggressive in water vapor and occurs appreciably even in the absence of an applied stress. The sequence of oxidation of the constituent phases appears to be consistent with the underlying thermodynamic hierarchy for the respective oxidation reactions. Notably, contrary to existing models based on preferential oxidation of BN coatings, oxidation occurs first on the SiC fiber surfaces and the Si 3 N 4 overcoat; crystalline BN remains even after significant fiber and matrix oxidation has occurred. The results are discussed in terms of rate-controlling kinetic processes, the effect of oxidant type, and applied stress.

Degradation of a SiC‐SiC composite in water vapor environments

“…This observation is consistent with that previously reported for a SiNC/Hi‐Nicalon TM composite . It is also supported by thermodynamic calculations of the relative propensity for oxidation of SiC and BN in oxygen or water vapor at the temperatures of interest . Thus, in the early stages of the process, very thin silica scales form at the interfaces between the matrix and the BN and between the BN and the fibers; at this point, the oxide scales on the fibers are expected to be passivating.…”

“…Strategies to mitigate this problem remain elusive Oxidants in the gas stream can permeate through cracks in both the EBC and the matrix and subsequently react with both BN coatings present on the fibers and the fibers themselves . Consumption of BN occurs through formation of low melting‐point borosilicate glasses and subsequent formation and volatilization of borohydroxide gases.…”

“…Oxidants in the gas stream can permeate through cracks in both the EBC and the matrix and subsequently react with both BN coatings present on the fibers and the fibers themselves. [26][27][28][29][30][31] Consumption of BN occurs through formation of low melting-point borosilicate glasses and subsequent formation and volatilization of borohydroxide gases. Once the BN is consumed locally (eg in the vicinity of a matrix crack), silica remains and continues to form through further reaction, eventually bonding the fibers to the matrix and preventing sliding during subsequent cyclic loading.…”

Boria Effects on the High Temperature Oxidation of Silicon Carbide

“…A crack in the CMC is a condition that limits the availability of O2 12 . The SiC will getter the O2 in low O2pressure conditions and oxidize to form SiO2 while the BN interphase remains intact unless the O2 pressure in the crack is enough to trigger BN oxidation 40,101 . Although higher O2 pressures are needed to oxidize BN, the BN oxidation kinetics are faster than SiC oxidation kinetics because O2 diffusion is very rapid in B2O3 95 .…”

M2YSi (M=Rh, Ir): Theoretically predicted damage‐tolerant MAX phase‐like layered silicides

“…One urgent need for the Y and Si containing MAX phase-like layered materials is to use them as interphase materials of SiC f /SiC composites since the internal oxidation and embrittlement of BN based interphase materials in stem-laden environments has become one of the most critical issues influencing the durability of SiC f /SiC composites. 35,36 The main role of interphase materials in SiC f /SiC composites include: (i) to ensure the integrity of SiC f /SiC composites, (ii) to protect SiC fibers by deflecting cracks from matrix (mechanical fuse), (iii) to maximize the load transfer after deflection, and (iv) to protect the SiC fibers from oxygen and water vapor attack. 37 Thus, the Y and Si containing layered materials must have low shear deformation resistance, volume expansion upon oxidation, and the oxidation products must be stable in water vapor environment.…”