Development of novel environmental barrier coatings for next generation gas turbine engines

Ceramic Matrix Composites based on SiC fibers embedded in a SiC matrix (SiC/SiC CMCs) are regarded as the new materials of choice for the hot-section components of gas turbine aircraft engines, since they have one third density of metallic superalloys, higher temperature capability (about +200°C), good mechanical strength and excellent thermal shock resistance. These properties can be used to design aircraft engines able to meet more severe environmental, technical and economic requirements: emissions reduction, engine thrust increase and thermodynamic efficiency improvement. However, SiC/SiC CMCs are sensitive to high temperature water vapor-rich combustion gasses that induce severe surface recession phenomena and ultimately lead to the component failure. For this reason, it is necessary to design a protective coating able to efficiently protect the surface of SiC/SiC gas turbine components in combustion environments. Such a system, known as Environmental Barrier Coating (EBC), is conceived as a multi-functional system having architecture and composition arrangements designed to join thermal protection to environmental and chemical stability. Rare Earth monosilicates (RE2SiO5) and Rare Earth disilicates (RE2Si2O7) were selected for the EBC system: monosilicates are more stable when exposed to water vapor at high temperatures while disilicates exhibit a higher compliance with the substrate in terms of coefficient of thermal expansion (CTE) but a lower resistance to water vapor attack. The architecture of the coatings was designed in order to optimize the properties of the multilayer system. EBCs were processed using the Atmospheric Plasma Spray (APS) technique and the deposition parameters were optimized by analyzing the coatings microstructure in terms of porosity, presence of cracks and post deposition phases retention. Then, as-sprayed coatings were thermally aged at 1300°C and 1400°C for 40h to investigate the phase stability and the effect of heat treatment on coating microstructure. Finally, coatings were characterized by Raman Spectroscopy to assess their composition, homogeneity and crystalline quality. Deposition parameters significantly affect the microstructure, composition and crystalline phase of coatings. Although coatings manufactured show some defects like pores, microcracks and through-the-thickness cracks that vary together with process parameters, results obtained are promising. After thermal treatments, coatings show an interesting crack healing phenomenon that leads to a significant improvement of their microstructure.