DESCRIPTION: Silicon carbide fiber-reinforced silicon carbide ceramic-matrix composites (SiC/SiC CMCs), by virtue of low density and high-temperature capability, are prime candidates for turbine engine hot-section components. However, they are thermodynamically unstable in combustion environments. Not only are they susceptible to oxidation, but the silica oxidation product volatilizes via Si(OH)x in the presence of water vapor at high temperatures and pressures.[1]

To minimize SiC and SiO2 recession due to volatilization, EBCs have been developed to protect the substrate from the products of combustion and minimize volatilization of silicon hydroxides.[2-4] While minimizing the effects of environmental degradation, it is unknown whether current EBCs are capable of protecting CMCs over their intended design life of approximately 2000 h in the combustion environment of advanced turbine engines, where component surface temperatures are predicted to reach 3000 degrees F.

Furthermore, EBCs are expected to function as a thermal barrier coating to maintain a 2700 degrees F interface and substrate temperatures for cooled CMC components. One critical aspect of the coating system is the need for a bond coat to ensure strong adhesion of the oxide EBC to the SiC/SiC CMC. State-of-the-art EBC systems rely mainly on a silicon-based alloy as the bond coat; however, the temperature of the EBC-CMC interface is expected to exceed the melting point of these alloys. Innovative materials and process solutions are sought for the development of a bond coat that will ensure survivability of advanced EBC systems on SiC/SiC CMCs for use well above the melting point of silicon.

The proposer should conduct a detailed literature search to identify key issues associated with the development of advanced EBCs for SiC/SiC CMCs. Selection and processing of a successful bond coat will be strongly influenced by the fiber and matrix constituents of the CMC substrate and the EBC system; therefore, teaming with a CMC manufacturer and turbine engine manufacturer is highly recommended. Commercialization plans and qualification requirements should be established to offer these new techniques to the aerospace industry for evaluation and qualification in Phase III. Government-furnished property will not be provided for this topic.
 

PHASE I: Identification and proof of concept of a bond coat for an advanced EBC on a SiC/SiC CMC must be demonstrated in a high moisture containing, oxidizing atmosphere (50 percent water, 50 percent air) at 2700 degrees F, which will serve as a representative combustion environment. Demonstration of proof of concept must include thermal cycling to 2700 degrees F.
 

PHASE II: Demonstration and optimization of a bond coat for an advanced EBC on a SiC/SiC CMC. Demonstration of temperature capability of 2700 degrees F under thermal cycling in simulated combustion environment such as a burner rig or actual turbine engine. Successful bond coat performance will correspond to life times on the order of 200 h at 2700 degrees F.
 

PHASE III DUAL USE APPLICATIONS: EBC bond coat technology should be made available to the turbine engine companies and CMC industry at large. CMCs are applicable to military engine hot-section components. They are also in development for commercial applications such as for power turbines and commercial aircraft engine components.
 

REFERENCES:

1. E. J. Opila, “Oxidation and Volatilization of Silica Formers in Water Vapor,” J. Am. Ceram. Soc., 86 [8] 1238-1248 (2003).
 

2. E. Eaton and G.D. Linsey, “Accelerated Oxidation of SiC CMCs by Water Vapor and Protection via Environmental Barrier Coating Approach,” J. Eur. Ceram. Soc., 22 2741-2747 (2002).
 

3. I. Spitsberg and J. Steibel, “Thermal and Environmental Barrier Coatings for SiC/SiC CMCs in Aircraft Engine Applications,” Int. J. Appl. Ceram. Technol., 1 [4] 291-301 (2004).
 

4. K.N. Lee, D.S. Fox, and N.P. Bansal, “Rare-Earth Silicate Environmental Barrier Coatings for SiC/SiC Composites and Si3N4 Ceramics,” J. Eur. Ceram. Soc., 25 1705–1715 (2005).