ICMCTF2007 Session A3-3: Thermal Barrier Coatings

Thursday, April 26, 2007 8:00 AM in Room Sunrise

Thursday Morning

Time Period ThM Sessions | Abstract Timeline | Topic A Sessions | Time Periods | Topics | ICMCTF2007 Schedule

Start Invited? Item
8:00 AM A3-3-1 Luminescence Sensing of Temperatures in Thermal Barrier Coatings
D.R. Clarke (University of California, Santa Barbara)
As the principal degradation phenomena leading to failure of thermal barrier coatings, such as bond-coat oxidation, creep, sintering and inter-diffusion, are all thermally activated (and hence exponentially dependent on temperature), precise knowledge of temperature is especially important. One approach to measuring temperature is to incorporate, within the crystal structure of the coating material itself, chromophore ions with temperature-dependent luminescence properties. The attraction of this "crystal chemistry" approach is that the other properties of the coating material can, with appropriate choice of chromophore, be are unaffected by solid-solution doping. The method of luminescence sensing of temperature in both EB-PVD and plasma-sprayed coatings will be described together with recent progress in identifying optimum chromophore doping concentrations. In addition, practical aspects in making precise temperature measurements will be presented.
8:40 AM A3-3-3 Studies of TBC Ageing by Photothermal Techniques
F. Cernuschi (Cesi Ricerca, Italy); S. Marinetti, P.G. Bison, E. Grinzato (CNR-ITC, Italy); L. Lorenzoni, A. Figari (Cesi Ricerca, Italy)

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.

9:20 AM A3-3-5 Durable APS Sensor Coatings for Remote High Temperature Detection
D.E. Mack (Forschungszentrum Jülich GmbH, Germay); R. Vassen (Forschungszentrum Jülich GmbH, Germany); J.P. Feist, S. Seefeldt (Southside Thermal Sciences (STS) Ltd., United Kingdom); S. Omar (Southside Thermal Sciences (STS) Ltd., London, U.K.)

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.

10:00 AM A3-3-7 Sensor TBCs: Remote Condition Monitoring of EB-PVD Coatings Utilising Laser Induced Phosphorescence (LIP)
R.J.L. Steenbakker (Cranfield University, United Kingdom); J.P. Feist (Southside Thermal Sciences Ltd); R.G. Wellman, J.R. Nicholls (Cranfield University, United Kingdom)

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.

10:20 AM A3-3-8 Effect of Excitation Energy on Luminescence Decays in Rare-Earth Doped High Temperature Oxides
M.D. Chambers, D.R. Clarke (University of California, Santa Barbara)
Lifetime-versus-temperature maps have been previously constructed for luminescence sensing based on rare-earth doping of high temperature oxide materials excited by 532 nm lasers. This contribution presents lifetime v. temperature maps of rare-earth doped oxides and coatings when excited using shorter wavelengths, which generally yield longer lifetimes. The possibility of extending the useful thermometric range of luminescence sensing of coatings by changing the excitation wavelength is assessed, and a mechanism for the decay from the excited state is proposed based on the assessment of the crystallographic populations of the rare-earth ions by EPR and luminescence spectroscopy.
10:40 AM A3-3-9 Hyperspectral Imaging of Residual Stress Distributions In Thermally Grown Oxides
B. Heeg, J.R. Abbiss, M. Dang (MetroLaser, Inc.); D.R. Clarke (University of California, Santa Barbara)
A recently developed method for rapid imaging of the R1-R2 luminescence spectra of Cr3+:Al2O3 is applied to measure the residual stress in various thermally grown oxides and thermal barrier coating materials. The measurement principle is based on measuring the piezospectroscopic shift at each pixel of the camera, using a tilt-tunable narrow band pass filter and digital spectrum reconstruction. The sensitivity of the method is on the order of 0.01 nm, which corresponds to a measurement error of ~ 25 MPa. One advantage of the new method is that it is several orders of magnitudes faster than scanning approaches to residual stress imaging. The principles underlying the method as well as examples of the images obtained will be presented.
11:00 AM A3-3-11 High-Temperature Vibration Damping Characteristics of Thermal Barrier Coating Systems
A.M. Limarga, T.L. Duong, G. Gregori, D.R. Clarke (University of California, Santa Barbara)
Vibration in gas turbine engines can promote mechanical fatigue of blades. Thus, it is reasonable to aim to design a multi-functional coating that not only serves as a thermal protection layer but also provides high-temperature damping. In this contribution, we present the experimental measurement of vibration damping characteristics of several high-temperature materials such as nickel-based superalloys, metallic bondcoat and thermal barrier coatings from room temperature up to 1100° also demonstrated that vibration damping measurement can be utilized to determine the elastic modulus as a function of temperature of the corresponding materials up to high temperature. Suggested referees: Drs. Carlos Levi, Ken Murphy and Remy Mevrel.
11:20 AM A3-3-10 Relationships Between Microstructural Features and Optical Properties of Yttria-Stabilized Zirconia Plasma-Sprayed Coatings
E.D. Meillot (Cea Le Ripault, France)
Heterogeneous materials exhibit different optical properties from bulk materials. Porosity, grain boundaries and others heterogeneities act as scattering centers and involve changes of optical properties. Therefore, optical properties are closely linked both to intrinsic properties of materials as well as microstructural characteristics and fabrication process. In this paper, plasma-sprayed process has been used for engineering yttria-stabilized zirconia (YSZ) coatings with significant difference in microstructure. Microstructural investigations, hemispherical reflectance and transmittance measurements over the 0.25-20µm wavelength range has been performed on plasma-sprayed coatings. The extinction coefficient which describes the decrease of light intensity due to absorption and diffusion in the matter is determined from the variation of transmittance versus coating thickness. The Kubelka-Muk four-flux model is implemented to evaluate the ratio K/S from reflectance measurement where K is the absorption coefficient and S the absorption one. Applying the Mie theory, scattering and absorption effective cross sections are obtained and correlated with microstructural features.
Time Period ThM Sessions | Abstract Timeline | Topic A Sessions | Time Periods | Topics | ICMCTF2007 Schedule