Functional Gradient Ceramics, and Thermal Barriers

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Authors: Marc Anglada
Abstract: The fracture toughness and strength of ceramics can be improved with respect to monolithic ceramics by developing graded materials as laminates composed of periodic alternating layers of one material separated by layers of a second material. The second layer must contain residual compressive stresses which are induced during densification because of differential thermal contraction of the layers. The overall residual stresses increase the apparent fracture toughness of the laminate. However, most deleterious natural flaws and most of the damage induced in service by the environment, contact loading, wear, etc, are small cracks on the surface of the outer layer, so that the effect of the laminate residual stresses on these cracks should be rationalised to understand their behaviour. This work presents an analysis of the influence of the gradient residual stresses on the behaviour of surface cracks under bending and indentation in materials with outer layers either with tensile or compressive residual stresses.
Authors: Emilio Jiménez-Piqué, Yves Gaillard, Marc Anglada
Abstract: This paper gives a short review of the instrumented indentation technique when applied to ceramic layered materials. The main causes of error are commented, together with the effect of the substrate, the microstructure and the possible fracture events.
Authors: Jakob Kübler, Gurdial Blugan, M. Lugovy, V. Slyunyayev, N. Orlovskaya, Richard Dobedoe
Abstract: Recent developments have shown that producing multi-layer ceramic laminates with alternative layers under compressive and tensile stress can lead to significant improvements in toughness at a low cost. However, in many cases the improvements in fracture toughness is associated with the presence of surface “edge cracks” in the compressive layers or the use of porous interfaces between the layers. At the same time such effects can limit the performance of ceramics when used in harsh environments. This review covers the development of silicon nitride based laminate structures and characterisation of these multi-layer structures. The work presents the results of macro-layered laminates with layers greater than 150 μm thickness. The apparent fracture toughness of different designs is measured and the conditions for failure tolerant effects, including crack deflection, bifurcation and edge cracking, are shown and discussed. The structural and processing limitations of the macro-layered laminates are also presented. The development of a weight function analysis as an effective design tool for developing micro-layered laminates with layers of approximately 50 μm thickness is discussed along with the apparent fracture toughness results from these micro-laminates. The failure tolerant behaviour as well as the ease of producing micro-layered laminates with a toughness of 2-3 times higher than that of silicon nitride is shown.
Authors: Giuseppe Pezzotti
Abstract: The piezo-spectroscopic (PS) effect, which may be defined as the shift in wavelength of a spectroscopic transition in a solid in response to an applied strain or stress, may occur both in crystalline and in amorphous structures, regardless of the particular spectroscopic transition involved (e.g., luminescence or Raman spectrum), and independent of the specific mechanism of luminescence emission (i.e., including spectra generated from substitutional impurities, optically active point defects, etc.). The PS effect can be monitored on electro-stimulated spectra when the scale on which the needed characterization lie is of a nanometer length. This effect, being a physical property of the studied material, should be calibrated case by case. The high scanning speed (and computer control) of the electron beam, which can be easily obtained with scan coils, is unsurpassed. Since the most recently developed optoelectronic devices have active areas of submicron dimensions and many of them less than 100 nm, no obvious choice is left but urgently developing an electro-stimulated probe for nano-scale residual stress assessments. In this paper, we show the feasibility of nano-scale stress assessments in the neighborhood of the tip of a crack propagating in GaN, selected as a paradigm semiconductor material.
Authors: Bilge Saruhan, Uwe Schulz, Marion Bartsch
Abstract: Partially Yttria Stabilized Zirconia (PYSZ) based Thermal Barrier Coatings (TBC) manufactured by EB-PVD process are a crucial part of a system, which protects the turbine blades situated at the high pressure sector of aero engines and stationary gas turbines under severe service conditions. These materials show a high strain tolerance relying on their unique coating morphology, which is represented by weakly bonded columns. The porosity present in ceramic top coats affects the thermal conductivity by reducing the cross sectional area through which the heat flows. The increase in thermal conductivity after heat-treatment relates to the alteration of the shape of the pores rather than the reduction of their surface-area at the cross section. The studies carried out by the authors demonstrate that the variation of the parameters during the EB-PVD processing of PYSZ based top-coats alters the columnar morphology of the coatings. Consequently, these morphological changes affect primarily the thermal conductivity and eventually the Young’ Modulus which are the key physical properties of this material group. New ceramic compositions covering zirconia coatings stabilized with alternative oxides, pyrochlores and hexaluminates are addressed. Failures occurring in ceramic top coats mark the lifetime of TBC system and therefore, it is necessary that their performance should go beyond that of the-state-of-the-art materials. This context summarizes the research and developments devoted to future generation ceramic top coats of EB-PVD TBCs.
Authors: Marion Bartsch, Bernd Baufeld, S. Dalkilic, Iulian Mircea, K. Lambrinou, T. Leist, J. Yan, Anette M. Karlsson
Abstract: Strategies for time-economic lifetime assessment of thermal barrier coatings (TBC) in service are described and discussed on the basis of experimental results, achieved on material systems with coatings applied by electron beam physical vapour deposition. Service cycles for gas turbine blades have been simulated on specimens in thermo-mechanical fatigue tests, accelerating the fatigue processes by an increase of load frequency. Time dependent changes in the material system were imposed by a separate ageing, where the samples were pre-oxidized prior to the fatigue test. Results of thermo-mechanical fatigue tests on pre-aged and as-coated specimens gave evidence of interaction between fatigue and ageing processes. An alternative approach is used, which is focused on the evolution of a failure relevant damage parameter in the TBC system. The interfacial fracture toughness was selected as a damage parameter, since one important failure mode of TBCs is the spallation near the interface between the metal and the ceramic. Fracture mechanical experiments based on indentation methods have been evaluated for monitoring the evolution of the interfacial fracture toughness as a function of ageing time. It was found that the test results were influenced by both changes of the interface (which is critical in service) and changes in the surrounding material.
Authors: Anette M. Karlsson
Abstract: Thermal barrier coatings are commonly used in high temperature parts of gas turbines, to protect the underlying metal substrate from deterioration during high temperature exposure. Unfortunately, the coatings fail prematurely, preventing the design engineers to fully utilize their implementation. Due to the complexity of the coatings, there are many challenges involved with developing failure hypotheses for the failures. This paper reviews some aspects of the current stateof- the-art on modeling failures of thermal barrier coatings, focusing on mechanics based models (such as finite element simulations) where the material physics is incorporated (such as oxidation and diffusion).
Authors: Ján Dusza
Abstract: The present contribution summarizes the recent results in the field of high temperature properties of layered ceramics and thermal barrier coatings (TBC), mainly as regards their thermal shock resistance and creep characteristics. The thermal shock and creep behavior of layered ceramics are discussed with the main focus on the influence of layered composition and interlayer boundary on the creep behavior of the composite. In the last part the high temperature deformation and creep of TBC’s are discussed.
Authors: Rainer Gadow
Abstract: Light weight engineering and composite technologies are key strategies in modern product development in mechanical engineering as well as in biomedical applications, where innovation is driven by novel material concepts and surface functionalities. Designed or customized surface properties by advanced coating technologies are an important discipline in this context. Ceramic, metallurgical and cermet layers can be manufactured in a most appropriate way by high energetic thermokinetic deposition techniques like plasma spraying, electric arc and last not least by supersonic flame spraying (HVOF). These technologies perform high deposition rates, high flexibility to use various materials and their combinations and applications in micro to macro scale products. The final properties of the coatings and layer composites do not just depend on the properties of the combined materials but, as in the case of ceramic coated light metals, are distinctly affected by the occurring residual stresses and their interaction with operational load stresses. With respect to the complex geometries of most components, their dimensional and positional tolerances a further strong influence of the robot kinematics of the plasma or HVOF torches during coating manufacturing is observed. By combining the expertise in materials and manufacturing engineering coatings and composites with high performance and reliability can be achieved. This is shown in the development of functionally coated cylinder liners and crankcases for ultra light weight engines as well as for ceramic coated bioinert and biodegradable substrates in medical surgery. It will be shown that cast engine block bores can be directly coated by using an automated HVOF process, obtaining improved coating results. The internal coating process by hypersonic flame spraying is a superior technological alternative to the APS process for high quality cylinder liner and engine crankcase applications. The applications of such ceramic and cermet coatings are not limited to automotive and biomedical applications, i. e. for wear and friction properties or biomedical compatibility, but can be used for tailored thermophysical, electrophysical or catalytic properties in various technical systems.
Authors: Peter J. Torvik
Abstract: A free layer damping treatment is formed by applying a coating of a high damping material to one or both sides of a structure. If the coating is perfectly bonded to the structure, the coating may be taken as being subjected to the strain at the interface between structure and coating. Consequently, the dominant component of strain is tensile. Free layer treatments may be symmetric (identical coatings on both sides of the structure), or asymmetric (one side only), and applied to beams, plates or shells in bending or in tension. Two classes of treatments are of interest, those employing thin coatings of a relative stiff damping material, and those employing thick coatings of a relatively soft material. As many materials useful as hard coatings are nominally linear but have inherently nonlinear damping characteristics, provision for this must be made.

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