Smart overlay coatings are a functionally gradient coating system designed to provide high temperature corrosion protection over a wide range of operating conditions.

The "SMARTCOAT" design consists of an MCrAlY base, enriched first in chromium, then aluminium to provide a chemically graded structure. At elevated temperatures, above 900°C (1650°F), the coating oxidises to form a protective alumina scale. However, at lower temperatures this alumina scale does not reform rapidly enough to confer protection under type II hot corrosion conditions. The coating is therefore designed with an intermediate chromium rich interlayer, which permits the rapid formation of chromia healing for areas of type II corrosion damage.

Laboratory and burner rig tests have been carried out on a series of developmental smart overlay coatings. These have shown that the development of an intermediate chromium rich phase provides protection under low temperature hot corrosion conditions, while the aluminium rich surface layer provides resistance to high temperature oxidation and type I hot corrosion. Thus the single application of "SMARTCOAT" permit operation over a broad range of industrial and marine turbine conditions.

Gas turbine designers are increasingly using Electron-Beam Physical Vapor Deposited (EB-PVD) Thermal Barrier Coatings (TBC) to meet the challenge of higher efficiency gas turbine engine requirements. A key feature for expanding the use of TBC's is increased spallation life and reduced spallation life variability. Such a coating system comprises a substrate (Ni-based single crystal alloy), a bond coat (Diffusion aluminides or MCrAlY), a ceramic (7 wt.% yttria stabilized zirconia), and a thin Thermally Grown Oxide (TGO) between the bond coat and the ceramic. The TGO is intended to be α-alumina but evidence suggests that in some cases the as-deposited TGO may not be entirely α-alumina.

The thin nature of the as-deposited TGO (<0.5 microns) makes analysis of the phases present and morphology difficult. Advancements in TEM sample preparation and Photo-stimulated Luminescence Spectroscopy (PSLS) have allowed higher quality and easier interrogation of the TGO.

EB-PVD TBC coatings were applied to platinum-aluminide bond coats on a Ni-based superalloy. One PVD process variable was selected and coatings were made at three levels of this variable. STEM and PSLS results are reported for each level of the process variable. An explanation for the creation of the mixed oxide zone found in some TGO morphologies is presented.

A complex approach to the simulation of the entire thermal coating process and modellinfg results are presented. The simulation tool consists of physical models and their computational realizations for all major elements of the process such as a plasma torch, sub-sonic and supersonic plasma flows, particle motion and heating, plasma-substrate and particle-substrate interaction, formation of the coating layer. Simulation results are compared with experimental data for VPS and APS.

Using a simplified thermo-mechanical splat model and statistical model of particle deposition some aspects of interface and microstructure formation are studied. Influence of particle size, temperature and speeds as well as spray angle and standoff distance on the coating structure and surface texture is investigated in detail.

Influence of the plasma torch design, gas composition, motion control and process parameters on the spray patterns, coating porosity and microstructure, substrate and coating temperature is discussed. Several examples of sensitivity analysis and coating optimization for those parameters are presented.

Advantages and drawbacks of the complex simulation of the spray process are discussed.

The adhesion of an EB-PVD thermal barrier coating is mainly caused by the thermally grown oxide TGO in between the bond coating and the ceramic top coat. Coating failures occur nearly always in this TGO.

For creating optimal properties the TGO has to be a dense thin hexagonal α-alumina layer. The formation of this interlayer is critical and proved to be dependent on the material combination, base metal and bond coat and all preparation and coating steps for the EB YPSZ ceramic top coating.

Especially on MCrAlY type of bond coatings the initial oxide is a chromium oxide, and the final oxide is always α-alumina. However if the initial chromium content in the TGO is high, this will influence the life time of the coating very negative. The change between these two oxides is created by a combination of chemical thermodynamics of the TGO - metal interface and reaction kinetics and diffusion processes in the metal and the TGO. This complex oxide formation process is very sensitive for all kind of process variations.

By careful control of all process parameters it is not only possible to improve the TGO formation, but also to increase the life time of a thermal barrier coating system, and to reduce the variation in the results dramatically.

New advances in the use of conformable Meandering Winding Magnetometer (MWM^TM) eddy current sensors provide nondestructive inspection capabilities for condition monitoring of protective coatings on turbine blades. Metallic overlay coatings and thermal barrier coatings (TBCs), including a metallic bond coat and the ceramic topcoat, allow engine operation at higher temperatures and provide protection from severe oxidation and high-temperature corrosion for turbine blades and vanes. Effective condition monitoring and remaining life assessment is essential to address both safety and cost concerns; however, this monitoring requires accurate and practical nondestructive methods that provide relevant information about thickness and degradation of the coatings as well as degradation of the substrate.

A nondestructive method for inspecting these coatings using spatially periodic conformable eddy-current sensors, such as the MWM, and quantitative model-based inversion algorithms is described in this paper. For TBCs, multiple frequency measurements and data inversion methods provide simultaneous and independent determination of multiple unknowns, such as metallic bond coat thickness, metallic bond coat porosity, and ceramic topcoat thickness. This measurement method is suitable for manufacturing quality control and in-service inspection of non-magnetizable materials. Similar methods have been applied to thermally aged samples for estimation of depletion zone thickness and other representative features of coating degradation. This paper provides a brief review of the MWM technology, a description of improved multiple frequency quantitative inversion methods, and representative measurements on both as-manufactured and aged coating systems.