Development of ODS Coating for High Temperature Turbine Components Using DED Additive Manufacturing

Abstract

Current and future designs for advanced turbine systems, such as Integrated Gasification Combined Cycle (IGCC), advanced Natural Gas Combined Cycle (NGCC), and the emerging supercritical CO2 (SCO2) systems require increasing turbine inlet temperature (TIT), which is well beyond the substrate melting temperature. The well-known approach is coating the turbine blade with thermal barrier coatings (TBC) combined with internal cooling channel in the substrate. However, due to thermally grown oxide (TGO) and thermal expansion mismatch stresses, TBC spallation failure is a major concern. Furthermore, neither the ceramic coating layer nor the metallic bond coat in current TBC system can provide structural support to house the internal cooling channels. In this research, a method to fabricate high temperature protective structural coating on top of critical gas turbine components by additive manufacturing (AM) technique using oxide dispersion strengthening (ODS) metal powder is presented. A novel combined mechanochemical bonding (MCB) plus ball milling process is utilized to produce near spherical and uniformly alloyed ODS powders. AM-processed ODS coating by direct energy deposition (DED) method on MAR-247 substrate, with laser powers from 100W to 200W were carried out. The ODS coated samples were then subjected to thermal cyclic loadings for over 2200 cycles. For comparison, in our earlier studies, under the same cyclic testing condition, typical tested TBC coupons showed spallation failure after ∼400 cycles. Correlation of the measured ODS coating Young’s modulus using a unique non-destructive micro-indentation testing method with evolution of the ODS microstructures are studied to identify optimum AM processing parameters for best performance of the ODS samples. In particular, stability of secondary γ′ phase in the ODS coating after thermal cycles is analyzed. Test results revealed a thin steady durable alpha alumina oxide layer on the best performance ODS samples. After 2,200 thermal cycles, strong bonding at ODS/substrate interface is also maintained for most of the ODS coated samples. Test results also showed stable substrate microstructure due to the protective ODS coating even after 2,200 thermal cycles. These preliminary test results showed strong potential for applications of AM-assisted ODS coating on advanced gas turbine components.