Silicon carbide-based ceramic composites are among the best candidates for high temperature structural components in aeronautic applications, mainly, in aircraft turbines. One major initial drawback for SiC in these applications is the volatilization of the protective silica scale during exposition to moisture at high temperature and the resulting ceramic recession [1]. Therefore, a safe use of these ceramic components depends on the development of an external protection against water vapor attack, named Environmental Barrier Coating (EBC) [2]. Many promising materials were progressively considered during the past 20 years. Today, the selected ones seem to be rare earth silicates, such as Y2SiO5 and Y2Si2O7 [3]. They are thermo-chemically and mechanically compatible with the whole architecture (SiC/SiC composites coated with a bond coat, often made in silicon). In service conditions, many other degradation phenomena of EBC were progressively highlighted and further developments remains always required. The purpose of this presentation is to review the different degradation phenomena of EBC (TBC) materials, and to define their specifications. Thus, an overview is given on the current working guidelines about the evolution of the nature of EBC materials to improve their corrosion resistance and to increase the protection efficiency over time. The durability of these coatings becomes strongly related to : (i) their corrosion resistance to molten sands or ashes (calcium magnesium alumina silicates : CMAS), ingested by the aircraft engine during flight or on the tarmac and (ii) to the growth of an oxide scale (TGO) by oxidation of the bond coat, leading to a highly stressed zone due to local variations in volumes (with the formation of SiO2 by oxidation of the Si bond coat). The sources of degradations, progressively highlighted, are introduced in modeling to assist the developments of EBC. The guideline in the modeling of chemical phenomena is focused on thermodynamic calculations of equilibria between phases in various states (solid/gaseous environment/liquid oxides). Respectively, finite element methods is often developed to evaluate the increasing local level of stresses during application time, by integrating different origins of volume variations (oxidation of the bond coat and thermal expansion coefficient mismatch), up to prevent spallation. Overall, predicting the durability and reliability of such protections becomes a complex multi-physico-chemical problem. To comply with the multi-physico-chemical specifications, complex EBC architectures are developed to obtain multi-functional properties: multi-layer EBC coating and introduction of a self-healing capability by specific adducts. The same problems are encountered for thermal barrier coatings (TBC) on metal parts.
[1] E. J. Opila, Oxidation and Volatilization of Silica Formers in Water Vapor, J. Am. Ceram. Soc., 2003, vol. 86 [8] , p.1238-48.
[2] K.N. Lee, Current status of environmental barrier coatings for Si-Based ceramics. Surface and Coatings Technology, 2000, vol.133-134, p.1-7.
[3] K.N. Lee, D. S. Fox, N. P. Bansal, Rare earth silicate environmental barrier coatings for SiC/SiC composites and Si3N4 ceramics. Journal of the European Ceramic Society, 2005, vol. 25, p.1705-1715.