Jürgen Malzbender

Determination of the Stress-Dependent Stiffness of Plasma-Sprayed Thermal Barrier Coatings using Depth-Sensitive Indentation

The elastic response of atmospheric plasma-sprayed coatings was investigated using Vickers and spherical indenter geometries. In both cases a strong dependency of the stiffness on the applied load (indentation depth) was observed. The stiffness of the coatings decreased with increasing load for a Vickers indenter, whereas it increased for a spherical indenter. This contrary behavior was related to the relative crack density in the deformed volume and to the stress dependence of the stiffness due to crack closure. The effect of annealing on the stiffness was quantified for both tip geometries. The heat treatment yielded additional information on the relationship between the indentation data and the microstructural defects. From the results it was concluded that the stiffness measured using a sharp indenter and small load reflected the elastic behavior of single spraying splats. With the relatively large spherical indenter, the average global stiffness of the thermal barrier coating was measured even at small loads. From the data obtained using the spherical indenter, a compressive stress-strain curve was suggested. Furthermore, values of the apparent crack density and yield strength were determined from the indentation tests.

Mechanical properties of coated materials and multi-layered composites determined using bending methods

Relationships are presented to analyse the mechanical properties of multi-layered composites from experimental data of bending tests permitting a calculation of the modulus of rupture (MOR) for the failure of a particular layer within the composite. The proposed equations can also be used to determine the unknown elastic modulus or thickness for a layer within a multi-layered composite, if the respective properties of all other layers are known. Special consideration is given to the influence of coatings on substrate fracture.

Comment on Hardness Definitions

The different definitions of hardness and elastic modulus as obtained using indentation with conical (also Vickers and Berkovich) or spherical indenters are compared and relationships that permit a conversion and an assessment of the differences are derived. A comparison to experimental data is given.

Determination of the interfacial fracture energies of cathodes and glass ceramic sealants in a planar solid-oxide fuel cell design

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.

Indentation strength method to determine the fracture toughness of La0.58Sr0.4Co0.2Fe0.8O3-δ and Ba0.5Sr0.5Co0.8Fe0.2O3-δ

The temperature-dependent fracture toughness of brittle ceramics can be conveniently assessed from bending tests of specimens with defined cracks introduced by indentation. However, the validity of this indentation strength in bending method (ISM) depends critically on the correct consideration of the residual stress induced by the indentation process. The ISM has been applied to La0.58Sr0.4Co0.2Fe0.8O3-δ (LSCF) and, for comparison, on Ba0.5Sr0.5Co0.2Fe0.8O3-δ (BSCF) perovskite. LSCF with rhombohedral phase exhibits ferro-elastic behavior at ambient temperature, whereas BSCF deforms linear-elastically. Pre-indented specimens of both perovskites were fractured at room temperature in biaxial bending, some of them after an additional annealing step. The fracture toughness values of BSCF match reasonably well when determined with equations which consider the presence or absence of residual indentation stress. Interestingly, annealing has little influence on the apparent toughness results obtained for rhombohedral LSCF, which appears to be related with stress relaxation by ferro-elastic deformation.

Strain analysis of plasma sprayed thermal barrier coatings under mechanical stress

The stiffness of air plasma sprayed (APS) thermal barrier coatings (TBCs) was determined from bending experiments combining strain analysis on a microstructural level with macroscopic mechanical parameters. Tests were performed with freestanding and attached TBCs, the latter either loaded in tension or in compression. Relationships are derived, which describe the TBC stiffness in a multilayer composite (attached TBC) and for a bimodular material that possess a lower stiffness in tension than in compression (stand-alone TBC). The increase of in-plane stiffness with increasing compressive stress emphasizes the importance of the spraying defects for the elastic response of the coating.

Micromechanical testing of glass–ceramic sealants for solid oxide fuel cells

The understanding of the mechanical behavior of sealants is a prerequisite for the improvement of the integrity and reliability of solid oxide fuel cell stacks. The glass–ceramic sealant material in a SOFC stack is usually a thin layer; hence, micromechanical testing methods need to be applied for characterization. Indentation testing is used in the current study to determine elastic modulus and fracture toughness. The properties of sealant materials in stack typical thin layer geometry are compared with sintered bars. In addition to tests of as-joined material, the effect of stack operation on the properties is assessed.

Formation and prevention of fractures in sol–gel-derived thin films

Sol-gel-derived thin films play an important role as the functional coatings for various applications that require crack-free films to fully function. However, the fast drying process of a standard sol-gel coating often induces mechanical stresses, which may fracture the thin films. An experimental study on the crack formation in sol-gel-derived silica and organosilica ultrathin (submicron) films is presented. The relationships among the crack density, inter-crack spacing, and film thickness were investigated by combining direct micrograph analysis with spectroscopic ellipsometry. It is found that silica thin films are more prone to fracturing than organosilica films and have a critical film thickness of 300 nm, above which the film fractures. In contrast, the organosilica films can be formed without cracks in the experimentally explored regime of film thickness up to at least 1250 nm. These results confirm that ultrathin organosilica coatings are a robust silica substitute for a wide range of applications.

Mechanical methods to determine layer compliances within multilayered composites

A comparison of bending and compressive/tensile loading cases of multilayered composites is given. Bending perpendicular to the layer interfaces is suggested as a sensitive and experimentally convenient method for determining the elastic modulus of compliant isotropic layers within a multilayered composite. Different approaches to analyze the strain and compliance behavior of the composite and individual layers are derived. Ranking of the relevant equations with respect to strain sensitivity is made, which favors the bending test perpendicular to the layer interfaces. The derived relationships permit a prediction of the bending behavior and the flexural rigidity of the composite. Furthermore, the properties of a layer within the structure can be determined. Limitations that exist for the bending perpendicular to the layer interfaces in the case of low stiffness of the layers can be overcome using either a compressive or a tensile loading mode. Application of the formulas to buckling and interfacial delamination is considered. Materials, such as plasma-sprayed thermal barrier coatings, that are supposed to behave elastically different in tension and compression are given special consideration. Although determination of elastic moduli is the main interest, in the case of known elastic moduli, the formulas can be used to determine the thickness of individual layers within a composite structure.

Energy dissipated during spherical indentation

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.