ICMCTF2005 Session A3-2: Thermal Barrier Coatings

Tuesday, May 3, 2005 1:30 PM in Room Sunrise

Tuesday Afternoon

Start	Invited?	Item
1:30 PM	Invited	A3-2-1 Thermal Barrier Coating Life Improvement Options K.S. Murphy (Howmet Castings and Alcoa Business) Thermal Barrier Coating Life Improvement Options. The platinum aluminide (Pt-Al) aerospace thermal barrier coating (TBC) system is comprised of 4 components (Ni-based surperalloy, Pt-Al bond coat, Thermally Grown Oxide (TGO), and Electron-Beam Physical Vapor Deposited (EB-PVD) TBC). The typical failure mode of the system is separation of the TGO from the bond coat or TBC. Methods to resist this separation and hence extension of TBC life are discussed. Methods investigated include the modification of the Ni-based superalloy, bond coat chemistry, bond coat surface preparation, and TBC composition. Results indicate that proper selection of these variables can greatly increase TBC life in cyclic furnace testing.}
2:10 PM	Invited	A3-2-3 Enhancing the Reliability of Thermal Barrier Coatings by Advanced Process Control S. Sampath, A.N. Vaidya, T. Streibl, M. Friis (State University of New York) Thermal sprayed coatings, especially ceramic thermal barrier coatings have found wide spread applicability in both aero and power generation turbine engines. These coatings have found wide spread applicability in both aero and power generation turbine engines. In the past, coatings have been principally added as an after thought with the goal of life extension of engineering components, however, as the need for prime reliant coatings grow, so do the requirement of reliability and reproducibility. Depositing reproducible coatings is an implicit requirement for the application of prime reliant coatings, so as to reduce/eliminate infant mortality of coatings. Advances process diagnostics and property extraction for these lamellar materials has significantly enhanced our understanding of the process dynamics during thermal spray. This has enabled improving performance as well as assessing process sensitivity, establishing optimizing protocols and reducing variability. In this presentation, we will discuss a fundamental approach to understanding the process through complete plume diagnostics, singe and multiple point measurement of particle state and measuring deposit properties insitu. The outcome of this effort is displayed in what is referred to as 1st order (process-particle interactions) and 2nd order (particle-coating interactions) process maps. These maps allow reducing variability for deposition of ceramic thermal barrier coatings. Critical parameters in the processing space have been identified, and their ability to control the microstructure and microstructure variability is explored. New coupled parameters to describe the melting status of the ceramic particles in-flight has been developed and demonstrated in first order process maps. For example, these maps allow reducing variability for deposition of ceramic thermal barrier coatings. Implications of these maps towards industrial applications will be addressed. Acknowledgements: Supported by the NSF MRSEC program DMR 0080021 and through the Industrial Consortium for Thermal Spray Technology at Stony Brook University
2:50 PM		A3-2-5 Formation of Vertical Cracks In Solution Precursor Plasma Spray Thermal Barrier Coatings M. Gell, L. Xie, D. Chen, F. Wu (University of Connecticut); X.Q. Ma (Inframat Corporation); E.H. Jordan, N.P. Padture, A. Ozturk, B.M. Cetegen (University of Connecticut) Vertical, through-coating-thickness cracks or columns can confer thermal cycle strain tolerance and superior durability to thermal barrier coatings (TBCs). These strain-tolerant features are critical to the performance of two production coatings: dense vertically cracked (DVC) and electron-beam physical vapor deposited (EB-PVD)TBCs and the novel, developmental TBC: Solution Precursor Plasma Spray (SPPS). SPPS TBCs are made using commercial plasma spray equipment, with atomized precursor liquid injected into the plasma plume instead of powder. The resultant coating microstructure consists of a homogeneous distribution of nanometer and micrometer porosity, ultra-fine splats and vertical cracks. The SPPS TBCs with this microstructure have excellent durability resulting from strain tolerance provided by the vertical cracks and porosity and the resistance to spallation and ceramic cracking provided by the ultra-fine splats. In this research, a series of controlled experiments have been conducted to determine the mechanism(s) for vertical crack formation. SPPS TBCs have been deposited at various thicknesses ranging from 30 to 300 micrometers. The presence of vertical cracks has been determined before and after heat treatment. SPPS TBCs at various thicknesses have also been deposited on substrates with varying thermal expansion coefficients. The mechanism(s) of SPPS TBC vertical crack formation deduced from these experiments will be described in detail in the presentation.
3:10 PM		A3-2-6 The Effect of Oxidation Pre-Teatment on the Cyclic Life of EB-PVD Thermal Barrier Coatings with Platinum-Aluminide Bond Coats V.K. Tolpygo, D.R. Clarke (University of California Santa Barbara) This work describes a simple and effective method of improving durability of EB-PVD thermal barrier coatings (TBC) in an oxidizing environment at high temperatures. Two major types of the oxidation-induced failure upon thermal cycling, which are typical for the TBC systems with platinum aluminide bond coats, are considered. One involves a gradual separation between the TBC and the thermally grown alumina scale (TGO), whereas the other occurs as a result of an abrupt spallation of the TGO. It is demonstrated that a substantial increase of the TBC cyclic life can be achieved by preliminary oxidation of the bond coat. Although both types of failure still occur, this treatment leads to a two-fold increase of the TBC cyclic life in air at 1150°C. This dramatic improvement is associated with a reduction in the extent of the interface damage during cyclic oxidation. Microstructural characterization indicates that the alpha-alumina scale formed on the bond coat surface prior to TBC deposition is less susceptible to separation along the TBC-TGO interface and also exhibits a lower growth rate in comparison with the TGO that develops on a similar bond coat without pre-oxidation. Based on these and related observations, some key principles and requirements of the oxidation pre-treatment as well as the origin of its beneficial effect are discussed.
3:30 PM		A3-2-7 Directed Vapor Deposition of Al-Ni-Cr Bond Coats Z. Yu, H. Wadley (University of Virginia) Next generation Thermal barrier coating (TBC) systems with improved high temperature properties must be developed to meet the demands of advanced engines. As the critical part of the TBC system, the bond coats drive more interests in new fabrication technology and potential materials. In this paper, a novel directed vapor deposition (DVD) processing approach has been used to deposit compositional and morphologically controlled bond coat layers. With this technique, fully dense and chemically homogeneous Al-Ni-Cr bond coats have been deposited. Combinatorial method is deployed to study the role of coating layer composition on the microstructure and properties of the coatings.
3:50 PM		A3-2-8 Thermal Cycling Life of Thermal Barrier Coatings using Cryomilled NiCrAlY Powders as Bond Coat Material L. Ajdelsztajn, F. Tang (University of California, Davis); G. Kim (Perpetual Technologies, Canada); V. Provenzano (National Institute of Standards and Technology); J.M. Schoenung (University of California) This work describes recent progress in improving the thermal cycling life of thermal barrier coatings (TBC) using cryomilled powders as a feedstock material for NiCrAlY bond coats. NiCrAlY powders were cryomilled and low pressure plasma sprayed (LPPS) onto a Ni-based alloy. SEM, XRD, TGA and TEM were used to characterize the material after milling, thermal spraying and thermal cycling. Thermal cycling tests were conducted at 1121C with a 50 min hold at temperature followed by a 10 min air cooling period. A significant improvement (300%) in thermal cycling life was observed on the cryomilled samples when compared to conventional ones sprayed and tested under the same conditions. The nanostructured characteristic of the dispersoids formed during the cryomilling process and its effect on the thermal cycling behavior of the TBC system is discussed in this work.
4:10 PM		A3-2-9 Using Thermal Phase Shift Method for Investigating the Influence of Deposition Process on Thermal Diffusivity of EB-PVD Thermal Barrier Coatings F. Yu, T.D. Bennett (University of California, Santa Barbara); K. Lawson, J.R. Nicholls (Cranfield University, United Kingdom) A thermal phase shift method is developed for determining the thermal properties of thermal barrier coatings (TBC) nondestructively. In this method, the TBC sample is heated periodically with a CO₂ laser. The resulting temperature field is interrogated through its thermal emission. The phase of thermal emission is related to thermal and optical properties of the films. Both the thermal conductivity and volumetric heat capacity of the film can be determined from the measurements. A set of specimens deposited with varying process parameters are measured by the thermal phase shift method. The influences of film thickness, rotation rate, deposition rate and deposition tilt on the thermal properties of films are studied. The effect of the process parameters on microstructure and thermal properties are compared and discussed.
4:30 PM		A3-2-10 Plasma Sprayed Thick Thermal Barrier Coatings with Improved Thermal Shock Resistance H. Guo (National Institute for Materials Science, Japan); H. Murakami (The University of Tokyo, Japan); S. Kuroda (National Institute for Materials Science, Japan); R. Vassen, D. Stoever (Juelich Research Center, Germany) Thick TBCs with different segmentation crack density were produced using plasma spraying. High substrate temperature gave rise to increased segmentation crack density and excellent interslpat bonding. The segmentation crack density for the coatings sprayed at 850 °C was measured to be 3.6 mm^-1. Segmentation cracks hardly initiated during the sintering. The coatings sprayed at high substrate temperature exhibited a relatively high thermal diffusivity compared to the coatings sprayed at low substrate temperature. High temperature ageing caused a significant increase in thermal diffusivity. The Young's modulus of the coatings with higher segmentation crack density ranged from 5 GPa to 15 GPa, whereas that of the coatings with lower segmentation crack density was as high as 40 GPa. Also, the Young's modulus increased significantly with increasing the sintering times. Thermal shock tests were conducted in a gas burner rig setup. The TBC with a segmentation crack density of 2.7 mm^-1 had a lifetime of more than 1770 cycles at 1238 °C (coating surface temperature).
4:50 PM		A3-2-11 Effect of Morphology of EB-PVD PYSZ TBCs on Thermal Conductivity A. Flores Renteria, H.-J. Raetzer-Scheibe, U. Schulz, B. Saruhan (German Aerospace Center (DLR)-Cologne, Germany) The PYSZ top coat of thermal barrier coatings (TBC) manufactured by EB-PVD is a crucial part of a system which protects the turbine blades situated at the high pressure sector of aero engines and stationary turbines under severe service conditions. These materials have a high strain tolerance due to their particular coating morphology presenting individual weakly bonded columns. Growth of these columns occurs in a preferred crystallographic orientation producing inter-columnar spacing in between them. Furthermore, the presence of a feather-like sub-columnar structure all around the primary column surfaces, inclined toward their growing axe, increases the opened porosity. Finally, rotation of the specimens during the vapor deposition process produces an additional intra-columnar closed porosity inside of the primary columns. Variation of the process parameters alters the morphology of the coatings. These morphology changes affect primarily the thermal conductivity which is one of the physical key properties of this material. The analysis of the contribution to thermal insulating capabilities of each pore type (i.e. feather-arm, intra- and inter-columnar pores) in TBCs would provide a guideline for engine manufacturers for designing more efficient coatings with optimized thermal properties only by controlling the manufacturing process parameters. This study investigates five different morphologies of PYSZ EB-PVD TBCs in terms of their morphology, pore types and thermal conductivity in as-received coatings and their sintering behaviour after annealing at various temperatures and holding times. The changes in total porosity and its influence on thermal property are evaluated.