Description:
OBJECTIVE: Develop strategies and viable, affordable production methods for CMAS-resistant thermal barrier coatings (TBCs) for turbine engine components prone to such attack. This should include the creation and development of a computational code that describes processing to create relevant structures or chemical or thermo-mechanical-based structure/property relationships relevant to CMAS resistance that could be incorporated into a larger, multi-code integrated computational materials engineering (ICME) processing protocol. In Phase I, several strategies will be considered, matured and downselected. Maturation will include cost and demonstration of small-scale production and scalability of competing technologies with a downselect of the optimal technology. DESCRIPTION: The remarkable increase in efficiency of gas turbine engines over the last 60 years has been achieved in significant measure by elevation of the engine gas operating temperature. This was enabled by (i) the development of superalloys that were increasingly resistant to creep, hot corrosion and oxidation, (ii) the invention of novel blade cooling techniques integrated into single crystal airfoil fabrication processes and (iii) the emergence of oxidation and hot corrosion resistant metallic bond coats and TBCs that reduced their temperature. Improved TBC systems for hot section air foils are of critical importance as the turbine inlet gas temperatures continue to rise [1]. For example, increases in the combustion chamber and turbine inlet temperatures of military gas turbine engines have resulted in the melting of dust particles upon contact with hot components with potential catastrophic consequences. These calcium aluminum magnesium silicate (CMAS) dust and volcanic ash particles melt on the surface of the thermal barrier coatings applied to these components. The coatings are then susceptible to dissolution and reaction with these liquid deposits. The resulting liquid glasses are able to wet the surface and internal interconnected pores within the coatings enabling their rapid penetration and dissolution of the protective coating. Upon cooling, the glass and reaction product phases solidify and the void structure that is utilized to reduce thermal conductivity and provide the strain compliance are lost leading to delamination of the affected region of the coating. Rare earth zirconates have been found to provide resistance to CMAS attack, but are unstable with thermally grown alumina TGO layer which requires yttria-stabilized zirconia to be applied adding to the complexity and cost. The zirconates are also less resistance erosion and foreign object damage. This topic seeks proposals that investigate novel concepts for mitigating the effects of CMAS upon current and future thermal barrier coating systems. Proposals are sought that (i) identify promising mitigation concepts, (ii) develop the ability to synthesize coatings and (iii) demonstrate that CMAS mitigation can be achieved without degradation to other essential coating properties (especially high through thickness thermal resistance, thermo-mechanical life, erosion and impact damage resistance). PHASE I: A number of promising strategies for the mitigation of CMAS effects will be investigated including the use of coating compositions that are not as readily wetted by CMAS, the use of compositions that promote precipitation of high melting temperature reaction phases that impede the penetration of the liquid glass, compositions that increase the viscosity or melting temperature of the glass upon reaction within the coatings, and the use of multi-layered coating architectures including those that contain layers with reduced interconnected pore fractions (dense layers) or utilize non reactive metallic layers to impede CMAS penetration. Maturation will include showing the feasibility to demonstrate small-scale laboratory production (actual demonstration to be done in Phase II) and the production scalability of competing technologies and demonstrate a CMAS-resistant TBC with a downselect of the optimal, affordable technology. A framework for creating an ICME code for specific processing-structure or structure-property relationship needs to be demonstrated. Interactions with turbine engine OEMs (original equipment manufacturers) is strongly encouraged. PHASE II: Develop a detailed test plan to scale-up processes for TBC production from a laboratory size to an industrial size. Demonstrate a small-scale laboratory production of the CMAS-resistant TBCs and assess the consistency of materials properties. Conduct material property tests at the level required to provide data for the design of a suitable component for demonstration in an OEM or DoD core engine test. The ICME method chosen in Phase I needs to be matured and validated. Provide material for manufacture and test of the suitable component. PHASE III: Transition the material production methodology to a suitable industrial material producer. The ICME code needs to transitioned to the commercial entity for potential incorporation of a more comprehensive ICME code. Commercialize the material for use in DoD and commercial markets. PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: Commercial Aviation industry would benefit from this technology when flying the sand-ingested areas such as the Middle East. and would provide some added protection for aircraft against the effects of volcanic ash as there are similarities chemically with CMAS and volcanic ash.
