ICMCTF2003 Session A3-3: Thermal Barrier Coatings

Wednesday, April 30, 2003 8:30 AM in Room Sunset

Wednesday Morning

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8:30 AM A3-3-1 Bi-directional Thermal Conductivity Measurements: Development of a Characterization Tool
T.D. Bennett, F. Yu (University of California, Santa Barbara)
A non-destructive measurement is developed to determine bidirectional heat transfer properties of thermal barrier coatings. The measurement is made entirely from the front surface; no intrinsic sample geometry requirements are imposed and no optical coatings are used. The film is heated with a modulated laser beam and the phase of thermal emission is interrogated to determine the properties of the film. A numerical model of anisotropic diffusion is developed to calculate the temperature field and corresponding thermal emission using a complex number representation. Varying the heating beam diameter relative to the film thickness enables across-plane and in-plane components of thermal conductivity to be resolved. The degree of diffusion anisotropic may be a good measure of important aging characteristics of thermal barrier coatings, and the measurement technique developed in this work a suitable tool for monitoring these changes over the service lifetime of a film.
9:10 AM A3-3-3 Influence of Aging on the Structure and Thermal Conductivity of Ypsz and Yfsz Ebpvd Coatings
A. Azzopardi (Snecma Moteurs, France); R. Mevrel (ONERA, DMMP, France); B. Saint Ramond (Snecma Moteurs, France)

The surface temperature of the ceramic layer of thermal barrier coatings is likely to increase in future, due to the introduction of systems with lower thermal conductivity. This study aims at describing and understanding how the crystallographic structure and the microstructure of EBPVD coatings can change after excursion at high temperature, and how the thermal conductivity of the ceramic layer is affected.

The structures of two EBPVD yttria-zirconia coatings have been studied (one partially stabilized, the other fully stabilized) by XRD, EBSD and image analysis after heat treatments at temperatures ranging between 1000°C and 1500°C up to 300 hours. Their thermal diffusivity has been determined up to 1000°C by a laser flash method. The results show that:

- No or little monoclinic phase is detected in the YPSZ coatings after aging,

- No grain growth or recrystallization occurs within the columnar structure,

- The shrinkage due to the constrained sintering of the coating results in transverse cracks, an effect function of the ceramic coating composition.

The thermal conductivity results will be discussed in relation with these structural evolutions.

9:30 AM A3-3-4 Electrochemical Impedance Spectroscopy of Thermal Barrier Coatings as a Function of Isothermal and Cyclic Thermal Exposure
B. Jayaraj, S. Vishweswaraiah, V.H. Desai, Y.H. Sohn (The University of Central Florida)
Electrochemical impedance spectroscopy (EIS) was employed to non-destructively examine thermal barrier coatings (TBCs) as a function of thermal exposure during isothermal and cyclic oxidation. TBCs examined in this study include electron beam physical vapor deposited (EB-PVD) and air-plasma sprayed (APS) ZrO2-7wt.%Y2O3 on NiCoCrAlY or (Ni,Pt)Al bondcoats. Impedance response of specimen at room temperature is analyzed with an equivalent model circuit based on the multiplayer constituents of TBCs. Physical and microstructural characteristics of TBCs were examined by optical and electron microscopy. The resistance and the capacitance of YSZ and TGO increased, and decreased, respectively, as YSZ sinters and TGO thickens with thermal exposure. Significant changes in resistance and capacitance were observed near failure corresponding to the penetration of electrolyte solution down to the metallic bondcoat. The change in electrochemical impedance is correlated to the damage initiation and growth observed by microstructural characterization.
9:50 AM A3-3-5 High Temperature Mechanical Property Characterisation of Thermal Barrier Coatings
J.R. Nicholls (Cranfield University, United Kingdom); J.F. Smith (Micro Materials Ltd., United Kingdom); S.A. Impey (Cranfield University, United Kingdom)
Many investigations of small-scale mechanical properties of thermal barrier coatings have been carried out at room temperature. Although it has been clear for some time that in many cases the materials properties are strongly influenced by temperature, performing sub-µmm mechanical property measurements has hitherto been problematical because of inherent thermal drift in the apparatus. An advanced high temperature stage for use with nanoindentation equipment was developed for the present work. This permitted virtually drift-free measurements to be performed at high temperature. Using this stage, thermal barrier coatings were investigated over a temperature range from room temperature to 800°C. For the measurement of hardness and elastic modulus, continuous indentation techniques were used. For the measurement of impact resistance, a repetitive pendulum impulse method was employed in which impact energy absorption could be determined. Variations in indentation data were related both to temperature and to microstructural characteristics determined by SEM. In general, higher temperatures led to lower hardness and modulus values for the coatings. Also, higher temperatures led to a reduced propensity to fracture during loading, consistent with an increase in micro-plasticity, and an important result relative to erosive wear resistance. This was explored at different temperatures by comparing fracture during slow loading of an indenter with impact wear rates.
10:10 AM A3-3-6 Isothermal Oxidation of Aluminide Diffusion Coatings
H. Svensson (Chalmers University of Technology and Goteborg University, Sweden); J. Angenete, K. Stiller (Chalmers University of Technology and Göteborg University, Sweden)

Ni-base alloys exposed to high temperature have to be covered with coatings, to protect them against corrosion and oxidation in order to prolong their lifetime. Aluminide diffusion coatings belong to the most frequently used category of such coatings. Addition of Pt to the coatings is known to improve their corrosion resistance, but the exact role of Pt for this improvement is not yet fully established.

Three commercial Pt modified coatings (RT22, SS82A and MDC150L) and one conventional aluminide diffusion coating, without Pt (PWA73) applied on the Ni-based single crystal superalloy (CMSX4), have earlier been investigated after isothermall oxidation in laboratory air at 1050°C from 20h to 20 000 hours [1-5]. RT22, SS82A and PWA73 are inward grown coatings (high activity), while MDC150L is an outward grown coating (low activity). In that investigation all samples were oxidized in an "as-received condition", i.e. the coating surface was not polished. It is known however, that the initial surface roughness highly affects the oxidation growth rate and the formation of the stable α-Al2O3, which is one of the most protective oxides at higher temperatures (above ~900°C). Since the surfaces of the four coatings were very different it was difficult to compare the obtained results and draw conclusions about the influence of Pt on the oxide growth in these materials. Moreover, the investigation revealed the necessity of investigation of the initial stages of the oxidation, because they affect the further development of the oxide scale. Therefore, study of the mentioned coatings that have been polished to the same surface fineness and exposed in laboratory air at 1050°C for only 1 hour has been undertaken. All the coatings were then investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

The first comparison of the different oxide phases present in the materials reveals that the presence of Pt seems to suppress the formation of spinel phases that allow outward diffusion of cations and contribute to faster oxidation rate. This result is also consistent with previous investigations [4, 5]. On the other hand, Pt seems to have no such effect on the formation of NiO phase. Moreover, no effect of Pt on formation of voids at the oxide/metal interface was observed. Further discussion of the results from microstructural studies of oxide scales will be presented.

1J. Angenete and K. Stiller, Mater. Sci. Eng. A, A316, (2001) 182.

2J. Angenete and K. Stiller, Surf. Coat. Technol., 150, (2002) 107.

3J. Angenete, K. Stiller and E. Bakchinova, Accepted to Surf. Coat. Techn., (2002)

4J. Angenete, K. Stiller and V. Langer, Submitted to Oxid. Met., (2002)

5J. Angenete and K. Stiller, Submitted to Oxid. Met., (2002) Keywords: commercial coating, Al2O3, microstructure, high temperature oxidation, XRD

10:30 AM A3-3-7 Directed Vapor Deposition: An Emerging Tool for Thermal Coating System Deposition
H.N.G. Wadley (University of Virginia)

A new technology for depositing both metallic bond layers and ceramic top coats for current and second generation thermal barrier coating systems is emerging. The approach utilizes electron beam evaporation from single or multiple, closely spaced sources. In marked contrast to conventional EB evaporation approaches, the evaporant(s) is (are) immediately entrained in a rarefied, supersonic helium gas jet and transported to the substrate for deposition. Oxygen (or other reactive gases) can be added to create metal oxides from their metal vapors. DVD top coats have a columnar microstructure, but the gas jet dynamics control the morphology of the coating. The column width, inter-column spacing and relative density of the column are are all controllable via the jets density and velocity. The density and velocity of the gas jet also determines the extent of vapor mixing. Rarefied conditions enable the deposition of homogeneous multi-component coatings. Higher density jets enable phase spread samples to be synthesized. These are of significant utility for combinatorial experimental studies that seek to discover second generation coatings. Multi-layered coatings are easily achieved by temporal variation of the electron beam power deposited in each melt pool. The use of a gas jet based approach has created other characteristics of the process that cannot be achieved by a high vacuum process. For example, it possible to ionize the jet and its evaporant and use ion assisted deposition to control coating morphology. This enables deposition of pore free bond coats at low substrate temperatures. It is also possible to create conditions that promote non line of site deposition.

Acknowledgements: The material described above is the result of collaborations with Derek Hass, James Groves, Kumar Dharmasena and David Wortman. The research has been supported by the ONR (Steve Fishman, Program Manager).

11:10 AM A3-3-9 Effects of Ion Beam Assisted Deposition, Beam Sharing and RF Biasing in EB-PVD Processing of Graded Thermal Barrier Coatings
D.L. Youchison, M.A. Gallis, R.E. Nygren, J.M. McDonald, T.J. Lutz (Sandia National Laboratories)
The development of advanced thermal barrier coatings of yttria stabilized zirconia (YSZ) that exhibit lower thermal conductivity through better control of electron beam - physical vapor deposition (EB-PVD) processing is of prime interest to both the aerospace and power industries. Recently, we have developed the processing technology to create graded TBCs by coupling ion beam assisted deposition with RF biasing in the alumina-YSZ system. The Electron Beam - 1200 kW PVD system1 was used to deposit a variety of TBC coatings with layered microstructures and reduced thermal conductivity. In addition to the process technology, the results of Direct Simulation Monte Carlo plume modeling, spectroscopic characterization of the PVD plumes, and physical properties of the coatings will be discussed.

1D.L. Youchison and R.E. Nygren, "Adaptation of Sandia's EB-1200 Electron Beams for EB-PVD Research," Electron Beam Melting and Refining, State of the Art 1997, Bakish Materials, 1997, pp.99-113.

11:30 AM A3-3-10 Morphological Evolution in Thermal Barrier Systems
A.S. Gandhi, C.G. Levi (University of California)
The performance and durability of yttria stabilized zirconia (YSZ) thermal barrier coatings (TBC) made by electron beam physical vapor deposition (EBPVD) can be significantly influenced by the morphological evolution of the columnar microstructure, compounding the deleterious effects of the TGO thickening. The microstructural features responsible for the low conductivity of EBPVD TBC’s, the feathery surface and intra-columnar porosity, evolve with time at high temperatures leading to a decrease in overall porosity and a consequent increase in thermal conductivity. The high compliance characteristic of the EBPVD microstructure may also be compromised by sintering together of the columns in certain parts of a turbine blade, as well as stiffening of the individual columns. The present investigation involves model high temperature experiments aiming to relate the evolution of TBC microstructures, in terms of open or closed porosity and surface area, to factors such as surface diffusion and thermal stresses.
11:50 AM A3-3-11 Measurement of the Elastic Modulus of a Plasma-sprayed Thermal Barrier Coating using Spherical Indentation
M. Eskner (Royal Institute of Technology, Sweden)
Failure in plasma-sprayed thermal barrier coatings systems mostly takes place in the ceramic topcoat or at the interface between the topcoat and the bondcoat. The failure normally occurs by spallation of the topcoat, either in the form of buckling or pre-buckling local deflection, at shutdown operations from high temperatures where compressive thermal mismatch stresses are induced in the topcoat. In order to determine the residual stress situation in the coating system, knowledge of the elastic modulus of the different components in the coating system is required. In this work, a spherical indentation method has been used for room-temperature measurement of the elastic modulus of both the topcoat (300 μm thick air plasma-sprayed ZrO2-8 wt% Y2O3) and the bondcoat (150 μm thick air plasma-sprayed Ni-23Co-17Cr-12Al-0.5Y). This method gives the compressive modulus, which for the topcoat may be more adequate as it has different modulus in tensile and compressive stress states due to its microstructure. Measurements were made on specimens in as-sprayed condition and after heat treatment (1500 hrs at 1000°C). A significant increase in elastic modulus of the bondcoat was observed as a result from the heat treatment, which could be explained by the internal oxidation at the initial weak inter-splat regions. No sintering effect and change in elastic modulus was observed for the topcoat.
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