Fracture toughness of thermal barrier coatings determined by micro cantilever bending tests

To investigate the local fracture toughness of thin coatings new small scale methods like FIB milling of micro cantilever are used. Webler et al. used this technique for measuring the fracture toughness of NiAl bond [1]. This method can also be used to investigate the local fracture toughness of thermal barrier coatings. The fracture toughness of ceramic coatings can be determined by different indentation techniques [2]. The drawback of these methods is the analysis of the KIc-value without the specific knowledge of the crack front propagation, which can only be determined after the experiment. By using micro-cantilever produced by ion beam milling it is possible to measure the local fracture toughness with freestanding micro-cantilever independent of the substrate. Therefore two yttrium stabilized zirconia (YSZ) top coats with a thickness of 250μm, which were deposited by suspension plasma spraying on a layer of Amdry 9954 bond coat and IN 738 substrate with different standoff distances of about 70 and 100 mm, were investigated. Figure 1. shows the micro-cantilever with the initial crack (a) before testing. Please click Additional Files below to see the full abstract

Correlation of Microstructure and Properties of Cold Gas Sprayed INCONEL 718 Coatings

In the cold gas spray process, deposition of particles takes place through intensive plastic deformation upon impact in a solid state at temperatures well below their melting point. The high particle impact velocities and corresponding peening effects can lead to high compressive residual stresses in cold spray coatings. This can be advantageous with regard to mechanical properties as fatigue life and hence, cold spray is an ideal process for repair applications. In this study, INCONEL 718 particles were cold sprayed by using nitrogen as propellant gas. The deposited coatings with different thicknesses were characterized using electron microscopy techniques to study grain refinement and precipitates in the coating. In addition, depth-resolved residual stress measurements have been performed by the incremental hole drilling method. The residual stress depth profiles in the coatings indicate compressive residual stresses of several hundred MPa which are hardly influenced by the coating thickness. In addition, large compressive stress levels are found in surface- near regions of the substrate due to the grit blasting process. Furthermore, a post-heat treatment analysis was performed to investigate its influence on residual stresses and bonding strength. These findings are used to develop a consistent explanation of the dependence of strength values on thickness

Numerical Investigation into the Effect of Splats and Pores on the Thermal Fracture of Air Plasma-Sprayed Thermal Barrier Coatings

The effect of splat interfaces on the fracture behavior of air plasma-sprayed thermal barrier coatings (APS-TBC) is analyzed using finite element modeling involving cohesive elements. A multiscale approach is adopted in which the explicitly resolved top coat microstructural features are embedded in a larger domain. Within the computational cell, splat interfaces are modeled as being located on a sinusoidal interface in combination with a random distribution of pores. Parametric studies are conducted for different splat interface waviness, spacing, pore volume fraction and fracture properties of the splat interface. The results are quantified in terms of crack nucleation temperature and total microcrack length. It is found that the amount of cracking in TBCs actually decreases with increased porosity up to a critical volume fraction. In contrast, the presence of splats is always detrimental to the TBC performance. This detrimental effect is reduced for the splat interfaces with high waviness and spacing compared to those with low waviness and spacing. The crack initiation temperature was found to be linearly dependent on the normal fracture properties of the splat interface. Insights derived from the numerical results aid in engineering the microstructure of practical TBC systems for improved resistance against thermal fracture

Lanthanum tungstate membranes for H-2 extraction and CO2 utilization: Fabrication strategies based on sequential tape casting and plasma-spray physical vapor deposition

[EN] In the context of energy conversion efficiency and decreasing greenhouse gas emissions from power generation and energy-intensive industries, membrane technologies for H-2 extraction and CO2 capture and utilization become pronouncedly important. Mixed protonic-electronic conducting ceramic membranes are hence attractive for the pre-combustion integrated gasification combined cycle, specifically in the water gas shift and H-2 separation process, and also for designing catalytic membrane reactors. This work presents the fabrication, microstructure and functional properties of Lanthanum tungstates (La28-xW4+xO54+delta, LaWO) asymmetric membranes supported on porous ceramic and porous metallic substrates fabricated by means of the sequential tape casting route and plasma spray-physical vapor deposition (PS-PVD). Pure LaWO and W site substituted LaWO were employed as membrane materials due to the promising combination of properties: appreciable mixed protonic-electronic conductivity at intermediate temperatures and reducing atmospheres, good sinterability and noticeable chemical stability under harsh operating conditions. As substrate materials porous LaWO (non-substituted), MgO and Crofer22APU stainless steel were used to support various LaWO membrane layers. The effect of fabrication parameters and material combinations on the assemblies' microstructure, LaWO phase formation and gas tightness of the functional layers was explored along with the related fabrication challenges for shaping LaWO layers with sufficient quality for further practical application. The two different fabrication strategies used in the present work allow for preparing all-ceramic and ceramic-metallic assemblies with LaWO membrane layers with thicknesses between 25 and 60 mu m and H-2 flux of ca. 0.4 ml/min cm(2) measured at 825 degrees C in 50 vol% H-2 in He dry feed and humid Ar sweep configuration. Such a performance is an exceptional achievement for the LaWO based H-2 separation membranes and it is well comparable with the H-2 flux reported for other newly developed dual phase cer-cer and cer-met membranes.ProtOMem Project under the BMBF grant 03SF0537 is gratefully acknowledged. Furthermore, the authors thank Ralf Laufs for his assistance in operating the PS-PVD facility. Dr. A. 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