Thermal barrier coatings (TBCs) on components in the high-pressure turbine allow essential increase in gas inlet temperature. They are applied on first and second stage blades and vanes in the new generation of high by-pass ratio aero engines as well as in modern gas turbines for power generation. Extended use of TBCs is envisaged in future engines to increase their efficiency. This demands a fundamental understanding of all relationships that rule the lifetime of coating systems, e.g. coating process, microstructure, and failure mechanism. The thermal barrier coating system typically consists of a MCrAlY (M= Ni, Co, Ni-Co) bond coat and an insulating Y2O3 partially stabilized ZrO2 as a ceramic top coat. The ceramic deposition is accomplished either by plasma spraying (PS) or electron beam physical vapor deposition (EB-PVD).
Thermal barrier coatings (TBC) are widely used to provide thermal and oxidation protection to nickel base superalloy components used in gas turbine engines. In these systems, a metallic bond coat is used to provide a strong, oxidation protection layer for the superalloy substrate. The bond coat's oxidation resistance is achieved by the use of sufficient aluminum to result in the formation of α-alumina upon high temperature oxygen exposure. For internally cooled components, this temperature can be reduced by applying a low thermal conductivity ceramic layer to the bond coat. The durability of the resulting TBC system depends on the ability of a candidate bond coat alloy to form an α-alumina layer with minimal intermediate phase formation during oxidation, the adherence of the resulting alumina layer to the bond coat and the high temperature strength / creep resistance of the bond coat. Attempts to make such NiAl thin films using magnetron sputter deposition and ion-assist sputter deposition have been reported. In both approaches, NiAl compound targets or co-axial disk targets were used to obtain the required deposition composition coating. Both of these sputtering processes suffer from a low deposition rate (which prolongs the deposition time) and result in strictly line-of-sight coating, which can result in some parts of a component having too thin coating.
Gas turbine blades work in high temperature and hot corrosion atmospheres. MCrAlY coatings (M=Ni, Co or its combination) are deposited on the blades surface in order to protect them from high-temperature oxidation and corrosion to prolong their lifespan. The protective oxidation scales such as Al2O3 and Cr2O3 form on the MCrAlY coatings by interacting with the high temperature environment. The oxidation rate of blades decrease greatly due to their isolation from the corrosive environment by the dense oxide layer.
Development of MCrAlY coatings with improved oxidation resistance at elevated temperatures is a major challenge. One approach to overcome this challenge is to develop an additional coating or coating system, either as an external layer over the MCrAlY coating or as a diffusion barrier between superalloy substrate and the MCrAlY. In recent years, coating compositions, viz. TiN and TiC or the Al–O–N system, have been considered for this purpose.
The thermal barrier coatings (TBCs) used here can be considered as four-layer materials systems, consisting of: (1) a superalloy substrate; (2) an oxidation-resistant metallic bond coat, usually MCrAlY or a platinum aluminide coating; (3) a thermally grown oxide (TGO), typically α-alumina, formed during heat treatment and/or in service; and (4) the ceramic top coating, usually 6–8 wt.
% yttria-stabilized zirconia deposited either by plasma spraying or electron beam physical vapor deposition. The zirconia top coat has excellent thermal shock resistance, low thermal conductivity and a relatively high coefficient of thermal expansion (CTE). The MCrAlY bond coats provide a rough surface for mechanical bonding of the ceramic top coat, protect the underlying alloy substrate against high temperature oxidtion corrosion, and minimizes the effect of CTE mismatch between the substrate and ceramic top coat materials. TBCs, however, have a tendency to spall, or debond, under cyclic high temperature conditions. It is believed that spallation of the ceramic component in TBCs is a result of the stresses generated in service and further that the performance of TBCs is affected by thermal expansion mismatch between the ceramic and the metal, thermal stresses generated by the temperature gradients in the TBC, ceramic sintering, phase transformations, corrosive and erosive attack and residual stresses arising from the deposition process.
Due to their ability to form dense, adherent Al2O3 scales, MCrAlY coatings are widely applied to blades and other hot gas turbine components, both as overlays and as a bond coating for TBCs. Presently there is a desire to increase the operating temperature of gas turbine engines to improve their efficiency. Recently a sputtered NiCrAlY coating was aluminized using pack cementation process to further improve its oxidation and corrosion resistance. The high-temperature oxidation behavior and hot-corrosion behavior of the sputtered coating with and without aluminizing have been investigated. A coating of Ni–30Cr–8Al–0.5Y (wt.%) approximately 40 μm in thickness was then deposited onto the substrate by magnetron sputtering. The sputtered coating displayed a columnar structure the grain size of which was less than 100 nm. The sputtered coating was aluminized using the pack cementation method. There was not much difference between the mass gains of both sputtered coatings with and without aluminizing at 1000 °C. However, at 1100 °C the aluminized coating showed lower oxidation rate than the as-sputtered coating. Corrosion kinetics of the alloy and coatings in molten salt at 900 °C showed that the sputtered coating could not improve hot-corrosion resistance significantly.