Papers

ICMCTF2002 Session A3-2: Thermal Barrier Coatings

Tuesday, April 23, 2002 1:30 PM in Room Esquire/Towne

Tuesday Afternoon

Start	Invited?	Item
1:30 PM		A3-2-1 Microstructure Stability Issues in Searching for New TBC Materials R.M. Leckie, C.G. Levi (University of California, Santa Barbara) Materials based on rare-earth oxide (REO) additions to zirconia are of interest in the drive to enhance the thermal resistivity and/or long term stability of TBCs. In general, these materials fall into two groups: those based on the t´ form of zirconia, and those based on pyrochlore-type zirconates. Both groups of materials are metastable in TBC applications, the former because of the eventual partitioning of the t´ phase, and the latter because of the potential evolution of interphases by interaction with the TGO. These problems can be further aggravated by corrosion. Both groups are also susceptible to morphological evolution of their pore structure when exposed to high temperature, with attendant degradation of the compliance and thermal resistivity of the coating. These issues will be discussed in the context of ongoing research at UCSB and collaborating institutions..
2:10 PM		A3-2-3 The Effects of Oxidation on the Failure of Thermal Barrier Coatings N.M. Yanar, F.S. Pettit, G.H. Meier (University of Pittsburgh) The current state of the art thermal barrier coatings consist of yttria stabilized zirconia deposited either by air plasma spray (APS) or electron beam physical vapor deposition (EBPVD) techniques. MCrAlY or platinum aluminide coatings are usually used as bond coats. In this study, the oxidation and failure behavior of the current state of the art EBPVD TBC systems as well as these TBC systems with various modifications were examined under isothermal and cyclic conditions in laboratory air. The modifications included compositional variations (e.g applying Pt overlayers before TBC deposition) and also surface modifications (e.g. polishing and/or pretreatment). Tentative failure mechanisms of these TBC systems will be given and the possible reasons for the improved performance of the TBC systems with various modifications will be explained. Furthermore, additional processing changes that can further improve the performance of these TBC systems will be suggested.
2:30 PM		A3-2-4 Directed Vapor Deposition of Platinum Modified Nickel Aluminide Bond Coat Layers K.P. Dharmasena, D.D. Hass (University of Virginia); Y. Marciano (The Nuclear Research Center-Negev, Israel); H.N.G. Wadley (University of Virginia) An oxidation resistant bond coat layer is an integral part of an effective thermal barrier coating system. Here we report on an electron beam based directed vapor deposition technique to deposit platinum modified nickel aluminide bond coats on superalloy substrates. A high scan frequency (100 kHz) electron beam is rapidly scanned between multiple elemental melt pools to create an alloy vapor stream that is entrained in an inert carrier gas flow directed towards the substrate. Factors governing the compositional homogeneity of the vapor cloud will be discussed. Bond coats with a wide range of controllable alloy compositions have been deposited by changing the residence time of the beam in each melt pool and the source material feed rates. Dense coatings of platinum modified nickel aluminide conducive to the formation of an α-alumina scale have been obtained using a hollow cathode plasma positioned just below the substrate.
2:50 PM		A3-2-5 Development of Cyclic Displacement Instabilities in Pt-modified Bond Coats A.M. Karlsson, A.G. Evans (Princeton University) A thermal barrier system, consisting of a super alloy substrate, a bond coat, a thermally grown oxide (TGO), and a thermal barrier coating (TBC), constantly evolves during its life where the TGO is a reaction product that forms by oxidation of the bond coat. This study investigates a class of thermal barrier systems based on Pt-modified NiCoCrAlY bond coats, coated with EB-PVD ZrO₂. For this class, the failure mechanisms are closely linked to morphological features of the TGO/BC interface that develops during thermal cycling. When these features assume a critical size, they act as nucleation sites for cracks and are eventually associated with large scale buckling and spallation of the TBC. The geometric instability of a morphological feature is caused by several factors. Primarily, these are (i) thermal mismatch strain between bond coat and TGO, causing a large compressive stress at ambient temperature (3-6 GPa), (ii) oxidation of the TGO leading to a growth stress in the TGO, (iii) thermal cycling, (iv) yielding in the bond coat during the cooling and reheating, and (v) initial imperfections in the bond coat/TGO/TBC interface. A numerical scheme, utilizing the finite element method, has been developed which has established and qualified these parameters. An additional factor that may accelerate the morphological instability is the local volume change in the bond coat due to phase transformation (β transforms to γ') of grains depleted of aluminum. The volumetric shrinkage has been added to the finite element model, and is seen to have a minor effect on the development of the morphological features.
3:30 PM		A3-2-7 Measuring the Mechanical Properties of Platinum Aluminide Bond Coats for Thermal Barrier Coatings Deng Pan (The Johns Hopkins University); J.A. Pfaendtner, P.K. Wright (GE-Aircraft Engines, Materials and Process Engineering Department); K.J. Hemker (The Johns Hopkins University) The thermal cyclic durability of a TBC is strongly dependent on the physical and mechanical properties of the bond coat layer. Attempts to measure these properties for as-deposited and thermally cycled diffusion aluminide bond coats have been greatly inhibited by their limited thickness (~60 ïm). In the present study, a novel high temperature microsample tensile testing technique is employed to measure the mechanical properties of a diffusion aluminide bond coat before and after thermal cycling in the temperature range of 25°C to 1150°C. Experimental measurements of the coefficient of thermal expansion (CTE), elastic modulus (E), yield strength and stress relaxation behavior will be presented. Thermal cycling has been found to have a significant effect on the physical and mechanical properties of these bond coats, and this change in properties can be related to microstructure evolution that results from thermal cycling. The mechanical properties of bond coats deposited on different substrates will also be compared and correlated to their underlying microstructures. The support of General Electric Aircraft Engines and the National Science Foundation (Grant No. DMR9986752) are gratefully acknowledged.
3:50 PM		A3-2-8 Influence of Bondcoat Pre-treatment and Surface Topology on the Lifetime of EB-PVD TBCs on IN100+PtAl BC H. Lau, C. Leyens (German Aerospace Center (DLR), Germany); U. Kaden (Siemens Power Generation, Germany); U. Schulz (German Aerospace Center (DLR), Germany); J. Muenzer (Siemens Power Generation, Germany); C. Friedrich (MTU Aero Engines, Germany) It is generally accepted that the thermally grown oxide (TGO) that forms at the interface between the bondcoat (BC) and the yttria partially stabilized zirconia (Y-PSZ) thermal barrier coating plays a major role in system lifetime. The present study aimed at comparing TBC lifetime of differently pre-treated bondcoats. Two types of bondcoats were used, a NiCoCrAlY overlay coating produced by electron beam physical vapor deposition (EB-PVD) and a platinum aluminide diffusion coating. The coatings were deposited onto IN100 substrate alloy and pre-treated in vacuum and ArH atmosphere as well as by other pre-treatment procedures prior to the deposition of an EB-PVD Y-PSZ top coat. The specimens were then thermal cyclically tested at 1100°C in air (50 min at 1100°C / 10 min forced air cooling). Scanning electron microscopy (SEM) helped to examine the microstructure of the TGO layers prior and after testing. Heat treatment in ArH significantly improved lifetime of the TBC system with (Ni,Pt)-Al BCs, while earlier results demonstrated an opposite behaviour on NiCoCrAlY BCs. Furthermore, the results demonstrated that early TBC failure was associated with short undulation wavelength and high aspect ratios, while longest lifetimes were observed with smoother interfaces, suggesting that surface roughness has significant effect on TBC durability.
4:10 PM		A3-2-9 Composite and Nanolaminate Ceramic Coatings Prepared by PVD Techniques for High Temperature Applications V. Teixeira, A. Monteiro, A. Portinha (Universidade do Minho, Portugal); R. Vassen, D Stoever (Forschungszentrum Juelich, Germany) In this contribution we studied the structural properties of ZrO₂/Al₂O₃ nanolayered coatings prepared by magnetron sputtering. These coatings were deposited by DC reactive magnetron sputtering with O₂ an Ar gas mixture, for a constant temperature, bias and working pressure. Besides the nanolaminates structures we also produced and studied unsabilised zirconia, zirconia-alumina and yttria stabilised zirconia composites coatings were produced by reactive magnetron sputtering. X-ray diffraction measurements were used to characterize the film structure. The surface microtopography was analyzed by atomic force microscopy (AFM). EDX was used to get thin film composition. SEM was used to measure the film thickness and to observe microstructure of the film cross-section. The ZrO₂/Al₂O₃ films are composed by nanolayers with 3/3.5, 6/7 and 12/14 manometers each, and the total thickness is 2.2 microns.The nanolayered films present a ZrO₂ polycrystalline phase (monoclinic and tetragonal phases depending in the ratio of thicknesses of the nanolaminated structure) and an Al₂O₃ amorphous phase. The ZrO₂ high temperature tetragonal phase content increases as the nanolayers in the structure get thinner. The Al₂O₃ nanolayers are used to stabilize the ZrO₂ tetragonal phase at room temperature. After high temperature annealing the alumina continues in amorphous state and the quasi-amorphous tetragonal zirconia nanosized grains crystallizes to tetragonal phase without any monoclinic transformation.
4:30 PM		A3-2-10 Measuring Fatigue Crack Propagation in Plasma Sprayed Y-PSZ Ceramic Coatings J. Lira-Olivares, M. Brito (Centro de Ingenieria de Superficies, Venezuela); Y. Mutoh (Nagaoka University of Technology, Japan); M. Takahashi (Toshiba Corporation, Japan) A series of Mode I and Mode II fatigue tests were designed for Thermal Barrier Coatings (TBC) of Partially Stabilized Zirconia (PSZ), applied by plasma spray. The tests were carried out at room temperature and 900°C, using original and heat treated specimens. The results showed that under Mode I tests, specimens presented a crack propagation in the external TBC, following a function that depended on the maximum stress intensity factor. The crack propagation was influenced by previous defects localized near the interface coating-substrate. In contrast, the Mode II tests results showed an extensive microcrack phenomena instead of the single large cracks observed in Mode I. In both cases, high temperature enlarged the same crack mechanisms, but enhanced the Young’s Modulus.