ICMCTF2007 Session A3-3: Thermal Barrier Coatings

The always more demanding operation conditions of gas turbine for power generation surely affect the life of the overlay coatings such as TBC. The availability of non destructive techniques able to give information useful to assess the integrity of these coatings is a primary step toward an estimation of their residual life.

The Thermally Grown Oxide (TGO) at the interface between the bondcoat and the TBC, as well as the TBC sintering phenomena taking place at high temperature are the main responsible for TBC delamination and its consequent spallation from the substrate.

An overview of non destructive thermographic techniques applied to the detection of coatings delaminations, with specific attention to TBC, will be given. In particular, the performances of different data reduction algorithms aimed to improve the delamination detect ability of pulsed thermography will be discussed. In fact in-field coating inspection of serviced components by pulsed thermography can suffer of surface disturbances and thckness variations making non trial the discrimination of delaminations.

Monitoring the evolution of mechanical properties of TBC caused by sintering phenomena taking place during operation could furnish information about the real strain compliance of this top protective layer. Since it appears to be difficult to estimate non destructively mechanical parameters like microhardness and elastic modulus, the approach presented consists in measuring thermal diffusivity of TBC deposited onto components by photothermal non destructive techniques and correlated the values so obtained with those of elastic properties as results of a laboratory destructive activity. In particular, two different photothermal techniques and some preliminary results on aged samples will be illustrated.

The future gain of efficiency for gas turbines is coupled with an increase in turbine entry temperatures clear above 1400°C. This will make it necessary to improve thermal barrier coatings (TBC) for the protection of the metallic base materials as those thermal protection will become critical for safe operation of the turbines. A key issue to the durability of ceramic TBC systems is their temperature-induced degradation which gives rise to the demand for an accurate online monitoring of surface and sub-surface temperatures. The thermo-luminescent response of phosphorescent ceramics has been shown to be suitable for such application. In this study thermographic sensor coatings consisting of todays standard material YSZ with a minor incorporation of a phosphorescent phase doped with a rare-earth activator have been successfully produced using a standard air plasma spray (APS) process.

Sensor coatings as well as standard YSZ TBC, which have been coated with identical parameters, were tested by thermal cycling tests with thermal gradients in a burner rig facility and showed comparable lifetimes. Microstructural analysis of coatings by means of optical microscopy, XRD and SEM in as-sprayed and cycled conditions revealed no indication of detrimental changes of microstructure or phase stability due to interactions between YSZ and the phosphorescent phase. Calibration of the coatings using the lifetime decay response mode showed them to have a dynamic range for temperature measurement extending to just under 1300°C for the complete lifetime of the coatings. APS sensor coatings have shown lifetime decay response up to 1500°C. Results will be presented in respect to lifetime and evolution of microstructure and thermo-luminescent properties in as-coated and aged condition.

Thermal Barrier Coatings (TBC) are used to reduce the actual working temperature of the high pressure turbine blade surface. Knowing the temperature across a TBC and at the interface with the thermally grown oxide (TGO) under realistic condition is highly desirable. As the major life-controlling factors for TBC systems are linked with temperature this would provide useful data for a better understanding of these phenomena and to assess the remnant life-time of the TBC. This would also enable the design of advanced cooling strategies in the most efficient way using a minimum amount of air. Further the integration of a sensor coating into an on-line temperature detection system will enable the full potential of TBCs to be realised due to improved precision in temperature measurement and early warning of degradation. This in turn will increase fuel efficiency and reduce CO₂ emissions.

The temperature measurements were made using the temperature dependence of the lifetime decay of the laser induced phosphorescence process. Thermographic phosphors are an innovative way of measuring temperatures remotely at different depths in the coating. The sensing coating consists of a layer of standard YSZ sandwiched between two phosphorescent layers. A rare-earth doped YSZ sublayer was used to measure the temperature at the interface between the TGO and the ceramic top coat. The luminescent top layer is based on a different host material so temperature measurements in excess of 1200°C could be achieved.

The coating was tested in a burner rig under thermal gradient conditions and compared with optical pyrometry measurements. Details of the measurements, the influence of aging, the composition and the fabrication of the sensing TBC will all be discussed in this paper. The work and data presented in this paper were partly sponsored by the European Community under the project ASTERIXE and by The Carbon Trust under the project SATURN.