Effect of heat treatment on elastic properties of separated thermal barrier coatings
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Effect of heat treatment on elastic properties of separated thermal barrier coatings D. Basu, C. Funke, and R.W. Steinbrech Institut fu¨r Werkstoffe und Verfahren der Energietechnik, Forschungszentrum Ju¨lich GmbH, 52425 Ju¨lich, Germany (Received 28 September 1998; accepted 6 August 1999)
Elastic response behavior of four different plasma-sprayed deposits has been investigated using depth-sensing micro-indentation technique. Due to the high degree of porosity and inhomogeneity of the coatings, the characteristic elastic moduli were found to be in the range of 20–75% of that of the dense bulk material (200 GPa). Considering the wide variation of properties, 150 data points were generated with five different indentation loads for each coating, and statistical tools were employed to represent the scatter of the data. The characteristic elastic moduli of all the coatings were observed to be almost doubled when the magnitude of indentation load was reduced from the highest (1000 mN) to the lowest (30 mN). The coatings were subsequently heat treated at 1100 °C, the operational temperature of a gas turbine, for 2, 25, and 100 h, and in all the coating grades the corresponding elastic moduli increased significantly. However, the stiffening effect was not uniform in two grades and was more pronounced for the smaller indentation loads. The increase in elastic modulus is attributed to elimination of fine porosity and sintering neck formation, an assumption also supported by the results of mercury porosimetry.
I. INTRODUCTION
Yttria-doped partially stabilized zirconia is increasingly plasma sprayed as thermal-barrier coating on different metallic components of high-temperature energy-conversion systems; e.g., gas turbines. Primarily the coatings are deposited to extend the operational temperature,1,2 which in turn increases the efficiency of the whole device. However, the poor reliability resulting from the inherent brittleness of ceramic coatings along with weak adhesion at the ceramic–metal interfaces still restrict a more widespread technical application. The mismatch in the stressstrain behavior and corresponding differences in thermal expansion coefficients between the ceramic coatings and metallic substrates3–6 often lead to failure of the system. In addition, delamination7,8 and cracking9,10 in the coatings are documented as the main fracture mechanisms. Evans et al.11,12 considered that buckling followed by spallation is another important failure mechanism and tried to describe the effect based on interfacial cracking. Because the mechanisms of brittle failure are related to the in-plane elastic behavior of the coatings, the elastic modulus emanates as an important parameter for coating performance during actual application. The elastic properties, however, may undergo changes during long-range exposure at high temperature due to associated sintering processes. It has been claimed by Lin et al.13,14 that the J. Mater. Res., Vol. 14, No. 12, Dec 1999
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