Yttrium Bearing Silicon Carbide Matrices for Robust Ceramic Composites

Abstract

Application of SiC‐based ceramic matrix composites (CMCs) in combustion environments demands the use of an environmental barrier coating (EBC) to prevent volatilization of the protective SiO2 scale in flowing water vapor. The EBC only provides protection while present on the surface; cracking and spallation of the coating leaves the underlying SiC vulnerable to the oxidation–volatilization processes. A robust matrix material chemically tailored to regrow a yttrium silicate scale in the event of EBC loss has been developed by incorporating yttrium bearing species including YB2, Y2O3, and Y5Si3 into the SiC. During oxidation a borosilicate glass helps seal cracks while Y2O3 and SiO2 react to form Y2Si2O7 for environmental protection. Candidate compositions were oxidized for 10 min to 100 h at 1400°C and for 24 h at 1500°C to understand the scale growth. The prospects for effectively applying this approach in CMCs are discussed.

Vapor‐mediated melt infiltration for synthesizing SiC composite matrices

“…This could then be followed with infiltration of a Si alloy melt with the desired attributes, e.g., a eutectic that has a lower melting temperature and that can capture residual Si in the form of a high melting point silicide 11,23 . The latter may also be used to improve the oxidation resistance of the matrix, as suggested in earlier studies 24 …”

“…11,23 The latter may also be used to improve the oxidation resistance of the matrix, as suggested in earlier studies. 24 Key findings of this work are predicated on the use of amorphous C as a precursor to the SiC reaction, which leads to the formation of pores upon reaction with the Si vapor. Although the extent to which this behavior would be replicated with graphitic precursors is not known at this point, the computed reduction in volume associated with that reaction suggests that the same mechanism should be operative.…”

Reactive alloy melt infiltration for SiC composite matrices: Mechanistic insights

“…The process can create a fully dense matrix, but residual free Si limits the operating temperature of the CMC to~1300°C, below the nominal temperature capability of the most advanced SiC fibers. 4 Mo-Si alloys have been favored in the past for alloy RMI because the residual phase (MoSi 2 ) has a much higher melting temperature (2020°C) than Si. 3 Ideally, a candidate alloy for reactive melt infiltration should display: (i) a eutectic temperature significantly lower than the melting point of Si, (ii) a high melting point residual silicide, (iii) a eutectic composition suitably located between the silicide and the silicon to optimize the utilization of Si in the reaction while avoiding residual silicon in the composite, (iv) a coefficient of thermal expansion (CTE) as close as possible to that of SiC (~4 ppm/K), and (v) the ability to form an oxide scale stable in moisture-laden combustion environments.…”

“…3 Ideally, a candidate alloy for reactive melt infiltration should display: (i) a eutectic temperature significantly lower than the melting point of Si, (ii) a high melting point residual silicide, (iii) a eutectic composition suitably located between the silicide and the silicon to optimize the utilization of Si in the reaction while avoiding residual silicon in the composite, (iv) a coefficient of thermal expansion (CTE) as close as possible to that of SiC (~4 ppm/K), and (v) the ability to form an oxide scale stable in moisture-laden combustion environments. 4 Mo-Si alloys have been favored in the past for alloy RMI because the residual phase (MoSi 2 ) has a much higher melting temperature (2020°C) than Si. However, this alloy otherwise is arguably less than optimal for RMI.…”

Oxidation Behavior of SiCf/SiC Composites Modified by Layered-Y2Si2O7 in Wet Oxygen Environment

“…Because of the coefficient of thermal expansion (CTE) mismatch [10] , the peeling-off of EBCs may occur during the long-term service in aircraft engine and lead to the failure of the composites. Therefore, improving the oxidation resistance of SiC matrix to water vapor and oxygen is gradually becoming a research hotspot [11][12][13][14] . Y 2 Si 2 O 7 has similar CTE with SiC (SiC: ~4.5×10 6 K 1 , Y 2 Si 2 O 7 : ~4×10 6 K 1 ) and excellent chemical compatibility with SiC [15][16] .…”