ICMCTF1998 Session A3-2: Thermal Barrier Coatings (2)
Time Period TuA Sessions | Abstract Timeline | Topic A Sessions | Time Periods | Topics | ICMCTF1998 Schedule
Start | Invited? | Item |
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1:30 PM |
A3-2-1 Mechanical Properties of Zirconia Thermal Barrier Coatings Measured using Instrumented Indentation
D.T. Smith, J.S. Wallace (National Institute of Standards and Technology) Instrumented, or depth-sensing, indentation was used to probe the mechanical properties of several air plasma sprayed yttria-stabilized zirconia thermal barrier coatings. Indentation loads ranging from 10 mN to more than 10 N using both sharp and spherical indenters permitted the measurement of elastic and plastic response over a broad range of length scales. High-load indentation on surfaces both perpendicular and parallel to the spray direction was used to study bulk elastic anisotropy of the coatings. In addition, low-load indentation was used to probe spacial variations in mechanical properties that are shown to correlate with local yttria concentration. The effects of thermal aging and sintering are also reported. |
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1:50 PM |
A3-2-2 Strain Gradients in Plasma Sprayed Zirconia TBCs
P. Scardi, M. Leoni (Universita' di Trento, Italy); L. Bertini (Universita' di Pisa, Italy); F. Cernuschi (ENEL - Centro Ricerche Ambiente e Materiali, Italy) Neutron diffraction was used to measure the residual strain field in plasma sprayed zirconia Thermal Barrier Coatings (TBCs). Data were collected at the British spallation source of ISIS (Didcot), on ENGIN, a recently installed TOF (Time-of-Flight) station designed to measure strains in materials, and to perform residual strain depth profiling in particular. The used geometry permitted one to directly measure the interplanar distances of crystallographic planes laying parallel to the component surface 1; zero strain data were also collected on ENGIN, by using annealed samples of the three present phases: zirconia top coat, Ni-base bondcoat, and metal substrate (Cu or Al). In this way ε33, the strain component perpendicular to the sample surface, was determined at several position inside the component, for all the present phases. The results of this analysis, consisting of a strain profile throughout the entire cross-section of the coated component, were integrated by those obtained by a destructive testing, performed after TOF data collection, consisting in the measurement of elongation and curvature change of the ceramic after substrate removal by chemical attack. All the experimental results were evaluated in the light of a suitable mechanical model of the coated component, and the results were discussed considering the thermal history of the studied samples and previous results obtained by X-ray diffraction techniques 2,3. Neutron diffraction proved a unique tool to study residual stress in plasma sprayed TBCs, provided that the ceramic coating is not too thin (≥1 mm), and suitable precision is achieved in measurements of both coated components and zero-strain samples of the present phases. 1. P. Scardi, M. Leoni and I.B. Harris, "Residual Stress Gradient in Ceramic TBCs", in ISIS 97, Annual report 1996-97, RAL-Didcot (UK). In press. 2. P. Scardi, M. Leoni, L. Bertini, L. Bertamini, " Residual stress in partially-stabilised-zirconia TBCs: experimental measurement and modelling" Surf. and Coat. Technol. (1998). In press. 3. P. Scardi, M. Leoni, S. Veneri, "Residual stress analysis of ceramic coatings by means of synchrotron radiation XRD", Advances in X-Ray Analysis, 40 (1997). In press. |
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2:10 PM | Invited |
A3-2-3 Mechanical and Thermophysical Properties of Thick PYSZ Thermal Barrier Coatings - Correlation with Microstructure and Spraying Parameters
D. Schwingel, R. Taylor (UMIST, United Kingdom); T. Haubold (BMW Rolls-Royce, Germany); J. Wigren (Volvo Aero Corporation, Sweden); C. Gualco (Ansaldo Ricerche, Italy) Current state of the art TBC systems are two layer systems consisting of a 0.1 to 0.15 mm thick bond coat and a thermal barrier coating (TBC) about 0.25 mm thick of ZrO2 partially stabilised with 6 - 8 wt.% Y2O3. For future generations of combustion engines, higher requirements with respect to efficiency and environmental compatibility will have to be taken into account. An improved insulation may enable higher combustion temperatures, or more realistically an augmented cooling effectiveness which will also improve the overall engine efficiency. Within an international project financially supported by the Commission of the European Community, TBC's up to 2 mm have been developed for combustor applications. For the successful industrial application of such coatings, a good understanding of the material properties is necessary. Besides thermophysical properties such as the thermal conductivity and thermal expansion, mechanical properties have also to be taken into account. While the thermal diffusivity and thus the thermal conductivity determine the insulation effect of the coating, the Young's modulus is considered to be a critical value determining the life time of the coatings. Failue causing stresses resulting from the thermal mismatch between coating and substrate may be controlled by an adequat manipulation of the elastic modulus. For the TBC's examined this manipulation of the thermophysical properties as well as of the mechanical ones was achieved by the modification of the coating microstructure. The latter was adjusted by variation of the plasma spraying parameters. Namely the influence of coating porosity, microcracking and coating segmentation on the mechanical and thermophysical properties was subject of the examinations performed. Within the proposed paper, a complete material characterisation with respect to the thermophysical and mechanical properties will be presented. The correlation to the production parameters and to the microstructure developed during the spraying will also be shown. This work has been funded by the CEC under the contract number BRE2-CT94-0936. |
2:50 PM |
A3-2-5 Microstructure Formation in Plasma Sprayed Functionally Graded NiCoCrAlY/Yttria Stabilized Zirconia Coatings
Z.L. Dong, K.A. Khor, Y.W. Gu (Nanyang Technological University, Singapore) Functionally graded NiCoCrAlY/Yttria stabilized zirconia coatings were fabricated by plasma spraying using mechanically alloyed, plasma spheroidized NiCoCrAlY/Yttria stabilized zirconia (YSZ) composite powders. Scanning electron microscopy (SEM) and energy dispersive x-ray spectrometry (EDS) analysis results revealed that oxidation of aluminum in NiCoCrAlY alloy occurred in the high temperature plasma torch during powder preparation and coating deposition. The oxidized products subsequently mixed with zirconia at elevated temperatures in a wide composition range. As a result, fine lamellae structures in the coating with different gray levels in SEM images were formed. Within the above lamellae structures, the regions containing high aluminum oxide concentration appeared darker, whereas the regions with high zirconia concentration appeared brighter. It was observed that Al2O3 and ZrO2 formed preferential combination in the coating. Dendritic structures were frequently observed in the coating inter-layers and were considered to be formed during the solidification process of the molten splats. The observations on coating cross sections also showed that the resultant coatings were very dense and had no clear interface between adjacent layers. It is believed that the gradient distributions of the coating compositions help to decrease the high residual thermal stresses, and hence contribute positively to the bond strength of the coating. These FGM coatings have demonstrated superior tensile adhesive bond strength to duplex coatings. |
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3:10 PM |
A3-2-6 Break
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3:30 PM |
A3-2-7 Phase Stability in Scandia, Yttria-stabilized Zirconia TBCs
M. Leoni, P. Scardi (Universita' di Trento, Italy); R.L. Jones (U.S. Naval Research Laboratory) The evolution in phase composition of several scandia,yttria stabilized zirconia (SYSZ) samples which underwent high temperature ageing treatments was followed by X-ray Diffraction (XRD). XRD data were processed by Whole Powder Pattern Fitting (WPPF), a recently proposed method for a simultaneous refinement of phase composition and microstructure, based on the Rietveld method.1,2 Studied samples were sol-gel powder samples and free-standing plasma spray TBCs prepared from the corresponding powders. Results concerning both powder and TBC samples clearly demonstrated the better stability of SYSZ samples, as compared with state-of-the-art yttria-stabilzed-zirconia (YSZ) powder and TBCs. All the as-prepared samples were single-phase tetragonal (t) zirconia. However, no monoclinic phase was observed in SYSZ coatings (6.57mol%Sc2O3, 1.00%Y2O3) subjected to a 100 hour heat treatment in air at 1673 K, followed by a further 24 hour treatment at 1753 K; on the contrary, the phase stability of YSZ coatings after the same heat treatment was seriously compromised, with a 4% monoclinic phase, and formation of a second tetragonal phase in addition to the originally observed t-phase. The comparable results obtained for the studied powder samples and data from recent literature3 were discussed together with further information from the WPPF procedure, regarding lattice parameters, crystalline domain size and microstrain. 1. P. Scardi, M. Leoni, L. Bertamini, "Influence of phase stability on the residual stress in partially stabilized zirconia TBC produced by plasma spray.", Surf. & Coat. Techn. 76-77 (1995) 106-112. 2. P. Scardi, "A new whole powder pattern approach", in: X-ray Powder Diffraction Analysis of Real Structure of Matter, eds. H,-J Bunge, J. Fiala, R. L. Snyder (IUCr series, Oxford Univ. Press, 1997). In press. 3. R.L. Jones, D. Mess, "Improved tetragonal phase stability at 1400C with scandia, yttria-stabilzed zirconia", Surf. & Coat. Techn. 86-87 (1996) |
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3:50 PM |
A3-2-8 Sintering and Creep Behavior of Plasma-Sprayed Zirconia and Hafnia Based Thermal Barrier Coatings
D. Zhu (NASA Lewis Research Center); R.A. Miller (Consultant) The sintering and creep of plasma-sprayed ceramic thermal barrier coatings are complex phenomena under cyclic high heat flux/high thermal gradient conditions. Changes in thermomechanical and thermophysical properties and in the stress response of these coating systems as a result of the sintering and creep processes are detrimental to coating thermal fatigue resistance and performance. Therefore, determination of coating sintering/creep behavior and mechanisms is of great importance. In this paper, the sintering and creep characteristics of several zirconia and hafnia based thermal barrier coatings are investigated using dilatometry and laser high heat flux sintering(creep) techniques. The coating creep mechanisms and dopant effect on coating sintering/creep rates are also discussed based on experimental observations. |