Nov_Dec_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 2 0 4 4 iTSSe TSS iTSSe TSS SEGMENTED THERMAL BARRIER COATING SOLUTIONS FOR TURBINE APPLICATIONS Segmented thermal barrier coatings (TBCs) exhibit enhanced thermal cyclic behavior and erosion resistance, along with improved application economics, over conventional TBCs. M.R. Dorfman, G. Dwivedi, C. Dambra, D. Chen, and R. Rocchio-Heller Oerlikon Metco (US) Inc., Westbury, New York I n recent years, gas turbine engines constituted a global in- dustry valued at $95 billion, of which 70% was allocated for aviation and the remainder for Iand-based industrial gas turbine (IGT) applications [1] . The IGTmarket produces approxi- mately 26%of global electricity demand, while the aero indus- try produces almost 100% of the propulsion power for large commercial andmilitary aircraft. Improvements in engine efficiency, which increase the electricity output of land-based turbines or the thrust-to- weight ratio and durability of jet engines, have and will contin- ue to rely on the development of TBCs. In addition to the direct impact on the thermal efficiency of these engines, other key benefits include lower environmental impacts, longer mainte- nance intervals for repair, weight reduction, and reduced fuel consumption. In particular, the thermal efficiency of a gas turbine en- gine is directly related to the inlet gas temperature of its tur- bine section, as shown in Fig. 1 [2] . For this reason, making improvements to the cooling and thermal protection systems have been major contributors to the success of gas turbines. The thermal protection of critical components is achieved through the use of thermal barrier coatings that are typical- ly comprised of an oxidation resistant metallic bond coat (MCrAlY) and a top coat of an insulating ceramic that has low thermal conductivity. For many applications, this ceramic is zirconium oxide- based, which was introduced several decades ago. Even today, the majority of legacy applications utilize 7-8 wt% yttrium-stabilized zirconia (YSZ) top coats. However, as en- gine operating temperatures continue to rise, multilayers and unique coating architectures with modified zirconia chemis- tries as top coats are being developed to address the design limitations of standard 7-8 wt% YSZ coatings. These limits in- clude, but are not restricted to, high-temperature sintering of the ceramic, thermal conductivity changes during engine ser- vice, phase stability, erosion resistance, and CMAS corrosion of YSZ-based coatings. CMAS (calcia-magnesia-alumina-silica sand) can be ingested as airborne particles through the air intake of an engine. It then melts and re-solidifies onto various engine sections. This is particularly detrimental to TBCs since the surface temperature of these coatings is typically hotter (> 1200°C) than others in the engine. These multifaceted challenges encountered by today’s TBCs demand that the next generation of TBCs is designed through an integrated approach that relies on strong materials know-how and processing advancements compared to conventional legacy TBCs. SEGMENTED MICROSTRUCTURES Although conventional air plasma- sprayed TBCs have been used since the 1970s and 1980s, it was only during the late 1980s that aerospace companies start- ed shifting toward electron beam-physi- cal vapor deposition (EB-PVD) coatings for commercial aerospace components. The Fig. 1 — Dependency of the overall efficiency of combined cycle power plants on the temperature of the gas entering the turbine module of the gas turbine engine. 8 FEATURE