Abstract
Environmental barrier coatings (EBCs) are essential to protect ceramic matrix composites against water vapor recession in typical gas turbine environments. Both oxide and non-oxide-based ceramic matrix composites (CMCs) need such coatings as they show only a limited stability. As the thermal expansion coefficients are quite different between the two CMCs, the suitable EBC materials for both applications are different. In the paper examples of EBCs for both types of CMCs are presented. In case of EBCs for oxide-based CMCs, the limited strength of the CMC leads to damage of the surface if standard grit-blasting techniques are used. Only in the case of oxide-based CMCs different processes as laser ablation have been used to optimize the surface topography. Another result for many EBCs for oxide-based CMC is the possibility to deposit them by standard atmospheric plasma spraying (APS) as crystalline coatings. Hence, in case of these coatings only the APS process will be described. For the EBCs for non-oxide CMCs the state-of-the-art materials are rare earth or yttrium silicates. Here the major challenge is to obtain dense and crystalline coatings. While for the Y2SiO5 a promising microstructure could be obtained by a heat-treatment of an APS coating, this was not the case for Yb2Si2O7. Here also other thermal spray processes as high velocity oxygen fuel (HVOF), suspension plasma spraying (SPS), and very low-pressure plasma spraying (VLPPS) are used and the results described mainly with respect to crystallinity and porosity.
Highlights
Ceramic materials often show unique high temperature capability
Monolithic ceramics suffer from an intrinsic low fracture toughness and, for demanding high temperature applications as blades, vanes, shrouds or transition ducts of turbine engines reinforced ceramics have to be used [1]
Even sensitive parts can be made from such ceramic matrix composites (CMCs) and this has been demonstrated in the last years by different industries especially by GE [2]
Summary
Ceramic materials often show unique high temperature capability. monolithic ceramics suffer from an intrinsic low fracture toughness and, for demanding high temperature applications as blades, vanes, shrouds or transition ducts of turbine engines reinforced ceramics have to be used [1]. In the thermal spray process, the molten particles will impinge on the substrate, the deformed droplets (“splats”) will cool down quickly and generate large tensile stresses which typically introduce cracks [12] These cracks form an open network which allow the diffusion of gas species through it and such coatings will delay the water vapor corrosion but can hardly avoid it. In order to form gas-tight coatings rather hot spraying conditions with high substrate temperatures (above 500◦ C) and complete melting of the feedstock are beneficial. Such conditions can lead to a re-melting of already deposited splats and by this improve the bonding in the coating.
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