This chapter describes how ceramic thermal barrier coatings (TBCs) are usually applied on metal components of aircraft engines and land-based gas turbines, with the purpose to extend their lifetime as well as to increase performance and durability, by increasing the operating temperature. The TBCs have to satisfy basic requirements in terms of low thermal conductivity, high stress compliance, high sintering resistance as well as high resistance to the environmental attack promoted by oxygen, molten salts and CMAS (calcium-magnesium-alumino-silicate) deposits. This chapter is aimed at analyzing the state-of-the-art, the recent developments and the future perspectives in the field of TBCs, focusing the attention on advanced materials and new architectures as well as explaining the mechanisms affecting the failure of TBC systems.
TopBackground
The components of aircraft engines and land-based gas turbines typically operate in harsh service environments because of the presence of extreme temperatures, as well as of oxidative, corrosive and erosive agents, that reduce their durability and lifetime, thus involving high costs for maintenance, dismantling and substitution (Boyce, 2002; Davis, 2004; Pawlowski, 2008).
This also implies any restrictions on the operating temperature, imposing any technological barriers toward the development of new systems with enhanced performance, reduced fuel consumption and lower emissions of NOx and CO.
The application of thermal barrier coatings (TBCs) is able to protect the surface of metal components from the surrounding environment (stationary guide vanes, rotating blades, blade outer air-seals and shrouds in the high-pressure section behind the combustor, afterburners in the tail section of jet engines). Their first aim was to extend the lifetime of stationary engine parts, then, at the end of 1980s, the TBCs were applied to the rotating blades.
These insulating ceramic coatings are able to reduce the temperature on the surface of the component, as well as to protect it from the attack promoted by environmental contaminants, thus increasing both the temperature capability and durability, without compromising the mechanical strength of the structural component (Zhu & Miller, 2000; Padture et al., 2002; Xu & Guo, 2011).
TBCs typically allow to reduce the substrate temperature between 100 and 300 °C for thicknesses of 120-400 µm.
The enhanced temperature capability allows to increase the inlet temperature and the efficiency of the turbines. TBCs are generally applied on internally air-cooled components, such as combustor parts, transition ducts of stationary and rotation airfoils.
Ceramic TBCs have thickness from several hundreds of micrometers to >2mm. A TBC system generally consists of a metallic substrate, a MCrAlY-type bond coat and a ceramic top coat (Miller, 1997). The application of the intermediate layer increases the adhesion of the upper ceramic layer, by reducing the thermal stresses arising from different thermal expansion of ceramic and substrate, and also provides high resistance against infiltration of oxygen and molten salts (Saremi et al., 2008; Richer et al., 2010). Ceramic materials with low thermal conductivity are potential candidates for developing next-generation TBCs, because of their high temperature capability and thermal gradient tolerance (Curry et al., 2011; Cao et al., 2004; Hetmanczyk et al., 2007; Bast & Schumann, 2002).
The industrial gas turbines have a market share of 25%, the total contribution of the aero held sums up to 35%. The global TBC market was estimated to be US$12.4 bn in 2015 and is expected to reach US$22.4 bn by the end of 2024. The rise of the global market between 2016 and 2024 has been measured at CAGR (Compound Annual Growth rate) of 6.6% (TMR, 2017).
The growth of the automotive industry is feeding the demand for TBCs. The aerospace industry, as main end-user of TBCs, is contributing significantly to the expansion of the market. The rising number of applications for gas turbines in energy, aerospace and defense is also expected to dramatically increase the demand. The rise in the price of raw materials, coupled with the high cost of using superior manufacturing technologies, is able to restrain the growth of the global market.
The competitive position of the main players in TBC market is well protected due to license restrictions by original equipment manufacturing companies for sharing TBC solutions for hot-section gas turbines for aircrafts.