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Elastic modulus and internal friction of Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) and La0.58Sr0.4Co0.2Fe0.8O3-δ (LSCF) were determined from resonance frequencies and damping behavior, respectively, using an impulse excitation method. An elastic anomaly for BSCF around 476 K, with a corresponding peak in the internal friction, is attributed to the experimentally confirmed spin transition of Co3+. LSCF is in a ferromagnetic state below ∼220 K and in a paramagnetic state above ∼250 K. The elastic modulus of LSCF exhibits an anomaly between 473 and 1113 K, which is attributed to a transition from rhombohedral to cubic symmetry.
Based on a comparison of relationships between energy dissipated during indentation and the ratio of hardness to reduced elastic modulus, Malzbender has recently proposed a procedure to determine both hardness and elastic modulus from a single loading–unloading indentation curve. However, a more accurate analysis of this relationship shows that, in fact, it suffers from the same limitations as the well-known Oliver and Pharr's method; i.e., in general, the true projected imprint area has to be measured in addition to the load–penetration curve or at least two such curves obtained with different indenter geometries are to be used to unequivocally determine the hardness and the elastic modulus of a material.
Based on a comparison of relationships between the energy dissipated during indentation and the ratio of hardness to elastic modulus, a procedure is outlined to determine hardness and elastic modulus from the ratio of the elastic to total energy dissipated during an indentation cycle for non-ideal indenters.
A relationship is derived for spherical indentation relating the dissipated energy to the ratio of hardness to elastic modulus and the ratio of indentation depth to radius. The result agrees with recent findings obtained on the basis of scaling relationships in combination with finite element simulations. Furthermore, relationships are given for hardness, elastic modulus and contact area, which permit a determination of these properties independent of the strain hardening characteristics and independent of pileup and sink-in.
A notched bimaterial bar bend test was applied to identify weak interfaces that influence the thermomechanical performance of solid-oxide fuel cell (SOFC) stacks with planar design. The experiments were focused on the weakest interface of the multilayered cells and on the rigid glass ceramic sealants between metallic interconnects of SOFC stacks. The fracture energies of these interfaces were determined. To test interfaces within the cells, they were glued to steel strips, and the notched cell was used as a stiffener in the test. The weakest part of the cells with composite cathodes was the interface between the functional part of the cathode and the remaining current collector. Values for the interfacial fracture energies of composite cathodes both freshly prepared and after aging were determined. Taking advantage of the crack extension within the anode from the notch-tip to the interface, the fracture energy of the oxidized and reduced anodes was calculated. Sandwich specimens with glass ceramic between the interconnect steel were used to determine the fracture energies for different glass ceramic–steel interfaces. Different combinations of ferritic steel and glass ceramic were tested. The fracture path developed partly along the interface and partly in the glass ceramic, which did not influence the fracture energy. However, a significant improvement of the fracture energy with annealing time was found.
The reliability of coatings that are used in industrial applications critically depends on their mechanical properties. Nanoindentation and scratch testing are well-established techniques to measure some of these properties, namely the elastic modulus and hardness of coatings. In this paper, we investigate the possibility of also assessing the coating fracture toughness and the energy of adhesion between the coating and the substrate using indentation and scratch testing. Various existing and new methods are discussed, and they are illustrated by measurements on particle-filled sol-gel coatings on glass. All methods are based on the occurrence of cracking, and they are therefore only applicable to coating systems that act like brittle materials and exhibit cracking during indentation and scratching. The methods for determining the fracture toughness give comparable results, but the values still differ to within about 50%. The values of the adhesion energy obtained from different measurements are consistent, but it remains uncertain to which extent the obtained values are quantitatively correct. The results show that the methods used are promising, but more research is needed to obtain reliable quantitative results.
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