TECHNOLOGY AREAS: Air Platform, Materials/Processes

ACQUISITION PROGRAM: Joint Strike Fighter

OBJECTIVE: Develop and demonstrate thin, low cost coatings for ceramic matrix composites for use at temperatures up to 2000 degrees Fahrenheit to protect against oxidation and moisture degradation.

DESCRIPTION: Ceramic matrix composites (CMCs) are being explored for turbine engine applications in a number of military platforms because of potential weight and performance benefits. Significant development of complex environmental barrier coatings (EBCs) has been undertaken for extremely high temperature CMC applications (>2000 degrees F) [ref 1,2]. These are typically multilayer coatings and are hundreds of microns thick. CMCs used for lower temperature engine applications (<2000 degrees F maximum) may also benefit from protective coatings, but there is a need for thinner (micron level), simpler, less expensive coatings. Often these CMCs are somewhat porous, so some level of infiltration may be necessary or desirable. Degradation is generally associated with oxidation of the interface coating, as opposed to the moisture induced matrix erosion which is the primary problem in the higher temperature applications referenced above. Often the most severe oxidation occurs in what is called the intermediate temperature range [ref 3], roughly 1200 degrees F-1500 degrees F, where the self sealing properties of SiC based matrices are not as effective as at higher temperatures. Moisture exposure (liquid or vapor) exacerbates the problem, so testing should include cyclic humidity and thermal exposures in order to somewhat simulate the engine environment. The possibility of a coating which can be periodically reapplied so as to extend the protective benefit is also of interest.

PHASE I: Demonstrate the feasibility of applying a thin, inexpensive CMC coating to protect against oxidation, particularly at intermediate temperatures. Include mechanical property measurements after 100 hours of exposure to intermediate temperature and after cyclic humidity and high temperature exposures.

PHASE II: Optimize the selected coating composition and process. Validate the coating through extensive environmental exposure. Testing should include cyclic exposure to moisture and periodic mechanical conditioning to induce microcracking typical of that seen in service. Evaluate and demonstrate the technology and the long term benefit. Estimate the cost of adding the coating.

PHASE III: Transition the developed technology to endorsing platforms.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: The technology developed here will be applicable to these CMCs with applications ranging from commercial aircraft engine components to industrial heat treatment, wear, and corrosion control.

REFERENCES:
1. Lee, K. N., Fox, D. S., Robinson, R. C., and Bansal, N. P, "Environmental Barrier Coatings for Silicon-Based Ceramics. High Temperature Ceramic Matrix Composites," High Temperature Ceramic Matrix Composites, Edited by W. Krenkel, R. Naslain, H. Schneider, Wiley-Vch, Weinheim, Germany, 224-229 (2001).

2. Lee, K.N.; Fox, D.S.; Bansal, N.P, "Rare Earth Environmental Barrier Coatings for SiC/SiC Composites and Si3N4 Ceramics," J. Eur. Ceram. Soc., 25 [10] 1705-1715 (2005).

3. Mechanical, Thermal and Environmental Testing and Performance o f Ceramic Composites and Components, Ed: Jenkins, Lara-Curzio, & Gonczy, ASTM STP 1392, ASTM Int., pp. 185-320 (2001).

KEYWORDS: high temperature; ceramic matrix composites; environmental barrier coatings; environmental protection; moisture resistance; oxidation protection;

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