Zhangwei Chen

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 LSCF6428 were investigated by nanoindentation of porous bulk samples and porous films sintered at temperatures from 900-1200C. 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 33.8-174.3 GPa and hardness from 0.64-5.32 GPa as the porosity decreased from 45-5% after sintering at 900-1200C. 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 900-1200C. 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 to 0.2. The elastic moduli of the films and bulk materials were approximately equal for a given porosity. The 3D microstructures of films before and after indentation were characterized using FIB-SEM tomography. Finite element modelling of the elastic deformation of the actual microstructures showed excellent agreement with the nanoindentation results.

Crack formation in ceramic films used in solid oxide fuel cells

Abstract The manufacture of solid oxide fuel cells (SOFCs) involves fabrication of a multilayer ceramic structure, for which constrained sintering is a key processing step in many cases. Defects are often observed in the sintered structure, but their formation during sintering is not well understood. In this work, various ceramic films were fabricated by screen printing and a variety of defects observed. Some films showed “mud-cracking” defects, whereas others presented distributed large pores. “Mud cracking” defects were found to originate from a network of fine cracks present in the green film and formed during drying and binder burn-out. Control of these early stages is essential for producing crack-free films. In order to investigate how defects evolve during sintering, artificial cracks were introduced in the green films using indentation. It was observed that crack opening always increased during constrained sintering. In contrast, similar initial cracks could be closed and healed during co-sintering.

Fracture Toughness of Porous Material of LSCF in Bulk and Film Forms

Fracture toughness of La0.6Sr0.4Co0.2Fe0.8O3‐δ (LSCF) in both bulk and film forms after sintering at 900°C to 1200°C was measured using both single‐edge V‐notched beam (SEVNB) 3‐point bending and Berkovich indentation. FIB/SEM slice‐and‐view observation after indentation revealed the presence of Palmqvist radial crack systems after indentation of the bulk materials. Based on crack length measurements, the fracture toughness of bulk LSCF specimens was determined to be in the range 0.54–0.99 MPa·m1/2 (depending on sintering temperature), in good agreement with the SEVNB measurements (0.57–1.13 MPa·m1/2). The fracture toughness was approximately linearly dependent on porosity over the range studied. However, experiments on films showed that the generation of observable indentation‐induced cracks was very difficult for films sintered at temperatures below 1200°C. This was interpreted as being the result of the substrate having much higher modulus than these films. Cracks were only detectable in the films sintered at 1200°C and gave an apparent toughness of 0.17 MPa·m1/2 using the same analysis as for bulk specimens. This value is much smaller than that for bulk material with the same porosity. The residual thermal expansion mismatch stress measured using XRD was found to be responsible for such a low apparent toughness.

Surface quality improvement of porous thin films suitable for nanoindentation

Abstract The reliability of perovskite material La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3− δ (LSCF) to be used as cathode parts in solid oxide fuel cells (SOFCs) also relies on its mechanical properties. Adequate surface conditions (i.e. flat and crack-free) are desired when the as-sintered porous thin films are subjected to nanoindentation for mechanical property determination. In this study, extensive cracks and considerable surface roughness were found in the LSCF films after sintering at high temperatures. This would significantly scatter the nanoindentation data and results in unreliable measurements. Various attempts including the comparison of film deposition methods, drying and sintering processes, and reformulating the ink were made to improve the surface quality. Results revealed little dependence of cracking and surface roughness on deposition methods, drying and sintering processes. It was found that the critical factor for obtaining crack-free and smooth LSCF films was the ability of the ink to be self-leveling in the earlier wet state. Reproducible nanoindentation measurements were obtained for the films with improved surface quality.

Nanoindentation of Porous Bulk and Thin Films of LSCF

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 LSCF6428 were investigated by nanoindentation of porous bulk samples and porous films sintered at temperatures from 900-1200C. 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 33.8-174.3 GPa and hardness from 0.64-5.32 GPa as the porosity decreased from 45-5% after sintering at 900-1200C. 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 900-1200C. 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 to 0.2. The elastic moduli of the films and bulk materials were approximately equal for a given porosity. The 3D microstructures of films before and after indentation were characterized using FIB-SEM tomography. Finite element modelling of the elastic deformation of the actual microstructures showed excellent agreement with the nanoindentation results.

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

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 LSCF6428 were investigated by nanoindentation of porous bulk samples and porous films sintered at temperatures from 900-1200C. 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 33.8-174.3 GPa and hardness from 0.64-5.32 GPa as the porosity decreased from 45-5% after sintering at 900-1200C. 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 900-1200C. 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 to 0.2. The elastic moduli of the films and bulk materials were approximately equal for a given porosity. The 3D microstructures of films before and after indentation were characterized using FIB-SEM tomography. Finite element modelling of the elastic deformation of the actual microstructures showed excellent agreement with the nanoindentation results.