Matrix Development for Water Vapor Resistant SiC-Based Ceramic Matrix Composites

SiC fiber-reinforced SiC matrix Ceramic Matrix Composites (SiC/SiC CMCs) have entered service in gas turbine engines to increase engine efficiency by operating at higher temperatures and reducing the weight of engine components. In this environment the SiC fibers and matrix oxidize to form a SiO2 scale that reacts with water vapor to produce volatile Si(OH)4 (g). Formation of volatile Si(OH)4 (g) leads to recession of SiC fibers and matrix which will shorten the life of SiC/SiC CMC engine components. This work explores three alternative matrix material concepts for improving the water vapor resistance of SiC-based CMCs. These candidate matrix materials include 1) yttrium disilicate (Y2Si2O7), 2) SiC particulate – Y2Si2O7 and 3) yttrium silicides. The thermochemical stability of these matrix candidates was investigated in oxidizing and high-velocity water vapor environments.

The thermochemical stability of Spark Plasma Sintered (SPS) Y2Si2O7 was assessed at temperatures of 1000 – 1200°C for times up to 250 hours at steam velocities of 130 – 180 m/s. These exposures resulted in the selective volatilization of SiO2 to form volatile Si(OH)4 (g) and porous Y2SiO5. SiO2 depletion from Y2Si2O7 followed parabolic kinetics at 1200°C. Mechanisms contributing to the overall depletion reaction were evaluated and include gas diffusion through pores, Y2SiO5 coarsening and development of tortuosity in the pore network.

The diffusivity of oxygen in a candidate matrix material must be sufficiently low to limit the oxidation of SiC fibers and matrix within the composite. Oxygen diffusion coefficients in SPS Y2Si2O7 and SPS Y2SiO5 were determined using the oxygen tracer diffusion technique. The 18O diffusion concentration profiles were measured after exposure at diffusion temperatures of 1000 – 1300°C using Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). Oxygen tracer diffusion coefficients were obtained by fitting the semi-infinite solution of the diffusion equation to the concentration profiles. Oxygen diffusion coefficients in yttrium silicates ranged from 10-12 – 10-14 cm2/s. Preferred transport of 18O along grain boundaries, pores and in some grain orientations (anisotropic diffusion) was observed.

The thermochemical stability of SPS 60 vol% SiC particulate – 40 vol% Y2Si2O7 in oxidizing and high-velocity water vapor environments was investigated at temperatures from 1000 – 1200°C for times of 60 – 250 hours. Oxidation in air resulted in the formation of a multiphase oxide scale consisting of SiO2 and Y2Si2O7. Parabolic oxidation kinetics were observed at 1200°C indicating the oxidation rate was controlled by the diffusion of oxygen through the SiO2 scale. Slower oxide growth at 1000°C prevented the rapid sealing of pores and cracks resulting in higher overall weight changes. In high-temperature high-velocity water vapor, SiC particulate – Y2Si2O7 specimens suffered from rapid material loss due to the preferential oxidation/volatilization of SiC and erosion of remaining Y2Si2O7 particles.

The ability of yttrium silicides to form yttrium silicates (phases with greater stability in high-temperature water vapor than SiO2) in high-temperature oxidizing environments was investigated. Yttrium silicides with compositions of 41, 67 and 95 at% Si-Y were fabricated using arc melting and oxidized at 1000°C and 1200°C for times up to 24 hours in air. Oxidation resulted in the rapid formation of a non-protective Y2O3 scale and rejected Si with additional minor oxide phases. Thermodynamic and kinetic considerations indicate that rare-earth silicides are unsuitable as matrix materials for CMCs.

The findings in this study demonstrate that the Y2Si2O7 matrix is the most promising matrix concept as it possesses sufficient thermochemical stability in high temperature water vapor and sufficiently low oxygen diffusivities.