ICMCTF2004 Session A3: Thermal Barrier Coatings

Friday, April 23, 2004 8:30 AM in Room Sunrise

Friday Morning

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8:30 AM A3-1 Bond Coat Evolution and Oxidation
D.R. Clark (University of California, Santa Barbara); V.K. Tolpygo (University of California)
It is becoming increasingly clear that the life of a thermal barrier coating depends critically on the evolution of the underlying bond-coat alloy, its oxidation behavior and diffusional interactions with the underlying superalloy. Each of these, in turn, depends on temperature and, in some highly coupled manner, on the temperature-dependent plastic response of both the thermally grown oxide and the bond-coat alloy to the strains generated by oxidation and thermal cycling. In this contribution, we summarize results of experiments on the morphological and compositional evolution of bond-coats during oxidation and thermal cycling as well as the role of bond-coat surface preparation on the life of thermal barrier coatings.
9:10 AM A3-3 Measurement of TGO Stress and Surface Geometry as a Basis for Thermal Barrier Coating Life Prediction
E.H. Jordan, S. Sridharan, M. Gell (University of Connecticut)
A common failure mechanism for thermal barrier coatings is delamination driven by the stored energy in the thermally grown oxide (TGO). As a result measurement of this energy level is of fundamental interest. The bond coat surface geometry plays an important role in initiation of this type of failure, via the out of plane tensile stresses at the TGO to bond coat interface. In the present study, TGO stress was measured using photoluminescence piezo-spectroscopy (PLPS). Surface geometry was quantified using intferometric surface profilometry and TGO thickness was measured by an advanced AC impedance method, which was verified using metallography. One of the challenges is deciding how to reduce the 100,000+ number description of the surface geometry from the surface profilometer to a useful measure of the expected impact of surface geometry on life. Motivated by the relationship between mean radius of curvature and interface normal stress that comes from membrane theory a method of constructing surface radius of curvature maps was developed. Single number parameters are extracted from these maps. The results presented are consistent with the idea that these maps and the parameters extracted from them are a better surface geometry metric to use when characterizing the surface in relation to spallation failure than more standard surface metrics such as RMS roughness. Results will be reported for an experimental program involving electron beam physical vapor deposited coatings (EB-PVD) on NiCoCrAlY bond coated superalloy substrates. These samples were prepared with 3 different surface finishes. The samples were tested to failure in cyclic furnace tests at 1121°C. Comparison of achieved and predicted lives is promising. The proposed life prediction method is mechanisms-based and is applicable to specific failure modes. Correlations between failure life and PLPS measured stress values alone will also be presented.
9:30 AM A3-4 Evaluation of Interface Degradation during Cyclic Oxidation of EB-PVD Thermal Barrier Coatings
V.K. Tolpygo, D.R. Clarke (University of California, Santa Barbara); K.S. Murphy (Howmet Castings)
The typical degradation mechanism of EB-PVD thermal barrier coatings that occurs during cyclic furnace testing of a commercial 7YSZ coating on a single-crystal nickel base superalloy is analyzed. The mechanism includes local separation of the ceramic top-coat from the thermally grown oxide (TGO), growth of the separated regions along the coating-substrate interface and, upon reaching a critical size, large-scale TBC spallation. The extent of interface separation was evaluated by analyzing cross-section microstructures of the tested samples at different stages of TBC cyclic life. The morphological changes of the TGO as a result of roughening of the (Ni,Pt)Al bond coat surface were also quantified. Concurrently, the evolution of the residual room-temperature stress in the TGO during cyclic oxidation was monitored on the same set of samples using luminescence spectroscopy. In addition, the development of TBC undulations, associated with large TBC-TGO interface separations, was observed. As a result, two major characteristics of interface damage, the extent of separation and interface roughness, can be correlated with the TGO stress and TBC surface topography. In turn, both the stress and surface topography can be evaluated non-destructively, which provides the basis for the development of a TBC inspection method.
9:50 AM A3-5 The Role of Chemical Composition on the Oxidation Performance of Aluminide Coatings
B.A. Pint (Oak Ridge National Laboratory)
The durability of aluminide bond coatings is critical to the life of thermal barrier coatings. The starting chemistry of the coating is a function of processing conditions and the underlying substrate. With service, the coating composition continues to change due to oxidation and continued interdiffusion. In order to better understand the effect of coating composition on its oxidation resistance, model alloys are being studied with various levels of Al, Pt and other elements commonly found in coatings. In hypostoiciometric β-NiAl, the formation of faster-growing Ni-rich oxide is observed and this problem intensifies at lower Al contents. Platinum additions are found to decrease this problem. With two-phase Ni-Al compositions, a phase transformation upon heating to 1100°C causes macroscopic specimen deformation when the specimen is repeatedly cycled. This deformation cannot be attributed solely to stresses from the growth of the scale. The degradation of oxidation resistance with lower Al contents indicates that the loss of Al due to back-diffusion into the superalloy substrate is the critical performance-limiting problem for aluminide coatings. One strategy for improved coating performance suggested by Gleason uses higher Pt and Hf contents and lower Al contents.
10:30 AM A3-7 Investigation of Phase Stability of Y and Rare Earth Doped t' Zirconia for Thermal Barrier Applications
N.R. Rebollo, A.S. Gandhi, C.G. Levi (University of California, Santa Barbara)
Emerging TBC's with thermal conductivity lower than the standard 6-8 wt% yttria stabilized zirconia (YSZ) are often based on the strategy of co-doping of zirconia with yttria and one or more rare earth oxides (REO). Thermal stability of the metastable t' solid solution in such compositions is a necessary condition, although a host of other factors would also influence the overall coating durability. The influence of REO addition on t' stability has been studied by phase evolution experiments on precursor derived powders heat treated isothermally as well as at successively higher temperatures. XRD, electron microscopy, Raman spectroscopy and thermal analysis have been used for phase identification and microstructural characterization. The stabilizing efficiency of individual dopants has been compared by heat treatments on ZrO2-7.6mol% REO1.5 powders where RE = Sc, Yb, Y, Gd, Sm, Nd and La in the order of increasing ionic radius. The relevant phase transformations for t' destabilization are its partitioning into equilibrium tetragonal and either cubic or pyrochlore phases at high temperature, and the tetragonal to monoclinic transformation upon cooling. The influence of dopant ionic radius on t' stability has been correlated to thermodynamic driving forces for these transformations and kinetic factors such as cation diffusivities. The t' compositions based on co-doping with 7.6% of YO1.5 and 7.6%REO1.5 appear to be more stable than single-doped compositions, with a weaker correlation between stability and dopant ionic radius. The results indicate that phase stability is not significantly compromised by modest additions of rare earth cations.
10:50 AM A3-8 Nano and Micro Hardness Testing of Aged EB-PVD TBCs
R.G Wellman, H. Tourmente, S.A. Impey, J.R. Nicholls (Cranfield University, United Kingdom)
Previous studies on the erosion of EB PVD TBCs has shown that aging the coatings at between 1100°C and 1500°C before erosion testing results in a significant increase in the erosion rates of the coating. These changes in the erosion rate were attributed to a number of different factors including changes to the nano porosity within the coatings as well as the sintering together of the columns of the coatings and to phase changes within the coatings. Since such changes in the morphology of the coatings should be measurable as changes to the hardness of the coatings it was then decided to ascertain the effect that the aging had on the hardness of the coatings. Since, during erosion, the size of the interaction zone between the impacting particle and the coating is in the same range as the size of the individual columns of the coating it was decided to measure the change in the hardness of the columns as well as the coating as a whole. It was found that the aging increased the hardness of both the coating as a whole and the individual columns of the coatings. The micro hardness of the coating was found to increase from 2.5-3.5GPa in the as received condition to 4.5-6GPa after 100hrs at 1100°C and to 7.5-8GPa after 24hrs at 1500°C. The nano hardness of the individual columns on the other hand was found to increase from 18GPa in the as received condition to 35GPa after aging. Due to the large scatter of the results obtained form the nano indentation tests it was necessary to use statistical methods in order to de-convolute the results. The paper discusses these increases in hardness due to aging in terms of the sintering and morphological changes that occur in the coating due to the aging. While the difference in the nano and micro hardness results are discussed in terms of the relative size of the indents and column size and the associated interactions that occur with the different indenter heads.
11:10 AM A3-9 Graded Thermal-barrier Coatings, Deposited by EB-PVD
B.A. Movchan, K.Yu. Yakovchuk (International Center for EB-PVD Technologies of Paton Electric Welding Institute, Ukraine)
Variants of graded TBC are given, which consist of bond coats of NiAl or MCrAlY+NiAl and YSZ-based outer ceramic layer, produced in one-step cycle by evaporation of the composite ingot. A design of the composite ceramic ingot is considered, as well as the ability to regulate in a broad range the composition and structure of the bond coats, outer ceramic layer, as well as the transition barrier zones of the substrate/bond coat and bond coat/outer ceramic layer. Distributions of chemical elements in the coating/substrate system and microstructure after deposition and after heat treatment are shown. Results are given of furnace thermal cycling tests of various types of graded TBC at 1150°C. Design of a new generation of electron beam units for deposition of the above coatings and cost-effectiveness of the one-step deposition process are described.
11:30 AM A3-10 Luminescence Sensing of Thermal Barrier Materials and Coatings
M.M. Gentleman, D.R. Clarke (University of California, Santa Barbara)
Luminescence from appropriately doped thermal barrier coatings (TBC) can be used to sense information about the temperature, temperature gradients, and erosion of coatings. We have been carrying out basic studies of the effects of temperature and concentration [0.005-5.0 atomic percent] on the luminescence lifetime and intensity of rare earth doped yttria stabilized zirconia (YSZ) and gadolinium zirconate (GD2Zr2O7) to identify appropriate dopants and concentrations. We discuss the results of monitoring various emission lines for several rare earth ions in both host materials from room temperature to the temperature where luminescence is extinguished. We also demonstrate two sensor concepts: a "red-line" coating, which allows the TBC-bond coat interface to be probed, and a rainbow sensor, which can be used to measure erosion.
11:50 AM A3-11 The Status and Potential Of Highly Durable Thermal Barrier Coatings Made By The Solution Precursor Plasma Spray Process
M. Gell, E.H. Jordan, N. Padture, L. Xie (University of Connecticut); X. Ma, P. Bryant (Inframat Corporation)
Thermal barrier coatings(TBCs)made by the Solution Precursor Plasma Spray (SPPS)Process exhibit a unique microstructure consisting of (a)uniformly distributed micrometer and nanometer size pores, vertical microcracks, and ultra-fine "splats." The SPPS porosity and vertical cracks provide excellent TBC strain tolerance and the ultra-fine splats provide a ceramic with improved toughness and bond strength. As a result, SPPS TBCs exhibit cyclic spallation lives that are 2.5 times that of air plasma spray TBCs on the same substrate and bond coat and 1.5 times that of very good electron beam physical vapor deposited TBCs. This presentation will descibe the mechanisms of formation of these desirable microstructural features and will introduce a new feature called an "inter-pass" boundary that shows potential to provide further reductions in thermal conductivity.
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