Thermal Oxidation-Induced Self-Healing on Yttrium Silicate Composites Dispersed with Silicon Carbides

Abstract

SiC/SiC composites are expected as new generation of gas turbine materials. However, SiC is remarkably oxidized in the combustion gas atmosphere due to existence of water vapor to lead to evaporation of SiO2 scale. In order to protect SiC/SiC composites from water vapor in the combustion gas, environmental barrier coatings (EBCs) system was proposed. Protective coating from water vapor in the EBCs system consists of oxide materials such as mullite, LaBaSiO2, Y2SiO5/Y2Si2O7, and Yb2SiO5/Yb2Si2O7. However, EBCs system loses productiveness for water vapor when EBCs were cracked penetrating into the substrate SiC/SiC composites. Nowadays, many self-healing ceramics have been developed such as SiC/Al2O3, SiC/mullite and Ni/Al2O3. In these ceramics, non-oxide dispersoids are oxidized on the crack surface to fill the crack with the oxidation product. In any cases, the oxidation product is formed to fill the crack via diffusion of cations through the matrix oxide. In this report, self-healing EBCs system is proposed with Y2SiO5 and Y2Si2O7 composites dispersed with SiC particles. Self-healing effectiveness of the composites was evaluated with crack-disappearance by high-temperature oxidation in air. In order to fit the CTE of EBCs system to that of SiC/SiC composites, mixture of Y2SiO5 and Y2Si2O7 was used. SiC/(Y2SiO5+Y2Si2O7) composite is also investigated on its self-healing effectiveness on the surface cracks. Y2SiO5 and Y2Si2O7 were prepared by reaction of powder mixture of Y2O3 and SiO2 at 1400°C in air. Powder mixture of yttrium silicates with 5 vol% of SiC powder was consolidated by using the pulsed electric current sintering at 1500°C for 5 min under 70 MPa in vacuum. In the mixture matrix composites, mixture of 30 vol% Y2SiO5 and 70 vol% Y2Si2O7 was prepared. Surface cracks (200 mm in surface length) was introduced into polished surface of the composite samples with the Vickers indentation. Figure 1 shows the surface cracks of (a) SiC/Y2SiO5, (b) SiC/Y2Si2O7 and (c) SiC/(Y2SiO5+Y2Si2O7). Thermal oxidation experiments were conducted in air at temperature of 1300°C for 1 and 6 h. Crack-disappearance after thermal oxidation was evaluated by crack length before/after thermal oxidation. Figure 2 shows surface appearance of (a) SiC/Y2SiO5 and (b) SiC/Y2Si2O7 for 1 h. Surface cracks are filled by the oxidation product Surface cracks was completely disappeared by thermal oxidation for both of SiC/Y2SiO5 and SiC/Y2Si2O7 at 1300°C for 1 h. Figure 3 shows surface appearance of SiC/(Y2SiO5+Y2Si2O7) at 1300°C for 1 h. Surface cracks of SiC/(Y2SiO5+Y2Si2O7) was completely disappeared. Self-healing effectiveness of SiC/(Y2SiO5+Y2Si2O7) is similar with the monolithic matrix composites. The authors wish to express their gratitude to the Japan Science and Technology Agency for supporting partially this study through the Advanced Low Carbon Technology Research and Development Program. Figure 1