ICMCTF1998 Session A3-2: Thermal Barrier Coatings (2)

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.

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.

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)