Environmental Barrier Coatings Made by Different Thermal Spray Technologies

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.

YAlO3—A Novel Environmental Barrier Coating for Al2O3/Al2O3–Ceramic Matrix Composites

“…Because of the CTE difference between CMC and YSZ, the coating adhesion was quite weak. Furthermore, it has been reported by Vassen et al [17] that mechanical treatment of CMCs can cause severe damage to the matrix or even its failure. Corrosion rates and coefficient of thermal expansion of selected materials (blue boxes) and ceramic matrix composites (CMCs) (red circles) (data from references [5,7,18]).…”

Improved Adhesion of Different Environmental Barrier Coatings on Al2O3/Al2O3‐Ceramic Matrix Composites

“…It was suggested to use surface structuring by laser ablation instead. Another suggested option is the usage of an APS‐Y 2 O 3 layer as bond coat . Summarizing , we can say that the formation of reaction layers at the interface, as the examples of Y 2 O 3 and YAlO 3 show, may have a positive effect on the adhesion of the coating.…”

“…Another suggested option is the usage of an APS-Y 2 O 3 layer as bond coat. [32] Summarizing , we can say that the formation of reaction layers at the interface, as the examples of Y 2 O 3 and YAlO 3 show, may have a positive effect on the adhesion of the coating. Experiments with coatings where no reaction is possible, such as YSZ, show that good surface pretreatment is necessary to increase adhesion.…”

“…Contrary to the experiments with YAlO 3 , no reaction between coating and substrate was observed directly after spraying. [32] Because no high-temperature tests were conducted, whether a potential reaction at the interface favors adhesion remains unanswered.…”

“…However, Vaßen et al [32] showed recently that mechanical treatment of the brittle ceramic matrix in CMCs may lead to severe damage and premature coating failure. It was suggested to use surface structuring by laser ablation instead.…”