Nanoindentation of porous bulk and thin films of La0.6Sr0.4Co0.2Fe0.8O3-δ

In this paper we show how reliable measurements on porous ceramic films can be made by appropriate nanoindentation experiments and analysis. Room-temperature mechanical properties of the mixed-conducting perovskite material La0.6Sr0.4Co0.2Fe0.8O3-delta (LSCF6428) were investigated by nanoindentation of porous bulk samples and porous films sintered at temperatures from 900 to 1200 degrees C. A spherical indenter was used so that the contact area was much greater than the scale of the porous microstructure. The elastic modulus of the bulk samples was found to increase from 33.8 to 174.3 GPa and hardness from 0.64 to 5.32 GPa as the porosity decreased from 45% to 5% after sintering at 900-1200 degrees C. Densification under the indenter was found to have little influence on the measured elastic modulus. The residual porosity in the "dense" sample was found to account for the discrepancy between the elastic moduli measured by indentation and by impulse excitation. Crack-free LSCF6428 films of acceptable surface roughness for indentation were also prepared by sintering at 900-1200 C. Reliable measurements of the true properties of the films were obtained by data extrapolation provided that the ratio of indentation depth to film thickness was in the range 0.1-0.2. The elastic moduli of the films and bulk materials were approximately equal for a given porosity. The 3-D microstructures of films before and after indentation were characterized using focused ion beam/scanning electron microscopy tomography. Finite-element modelling of the elastic deformation of the actual microstructures showed excellent agreement with the nanoindentation results. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.