Advances in the functional properties of thermal barrier coating (TBC) deposits are important for increasing the efficiency of, and reducing emissions from both stationary and aircraft turbine engines. Computer modeling is the preferred method for developing new materials with minimum cost and development time. However, modeling of TBCs is complex and must take into account interactions among the layers and with the substrate, in-service phase changes, oxidation, and stress development. Understanding the microstructure of the ceramic layer is important for building these models, as it strongly influences the properties responsible for the basic TBC function – thermal resistance. As is well known the ceramic microstructure changes in service, potentially leading to coating and engine failure. A major challenge is ensuring that the model reliably describes the actual material. Thus, it is important to develop representative models, which can be related to real practical coating systems. We present such a model. It has been developed to interpret small-angle X-ray scattering data that characterize TBC ceramic deposit microstructures. This model is also suitable for incorporation into computer algorithms such as are used in finite-element analysis. Quantitative parameters that describe the microstructure changes occurring under service conditions are readily obtainable for current systems, and these can then be re-measured for future materials of interest.

In many empirical studies on the structure and properties of thermally sprayed coatings, a set of two predefined parameters (e.g. porosity and elastic modulus) is correlated over a narrow range of structural variation assuming a continuous correlation function. Such a data evaluation assumes the existence of physical correlation’s between material behavior and microstructure. The experimental approach, undertaken in this study, comprises a maximum range of morphologies for starting materials with nearly identical chemical compositions to reveal the influence of microstructural changes of diverse defect species on different coating properties. The large matrix of structural and physical data is statistically correlated without any preconceived assumptions concerning the mathematical functions or the physicochemical nature of the property-microstructure-correlation’s. The divergent morphologies are realized by using different coating processes such as vacuum (VPS) and atmospheric (APS) plasma spraying, water stabilized plasma spraying (WSP), wire arc (WAS)- and flame spraying (FS), including variation of process specific parameters. The microstructure is systematically analyzed along length scales starting from defects in the micrometer down to the nanometer range. The microstructure and its anisotropy is quantified by small angle neutron scattering (SANS). The phenomenological coating behavior is successively investigated starting from basic properties such as electrical and thermal conductivity, elastic constants, residual stresses up to application oriented properties such as wear resistance. Property combinations presuming high sensitivity to microstructural changes are preferentially characterized and statistically correlated.

HVOF-sprayed alumina appears to be well suited for applications in semiconductor devices. This paper investigates the influence of HVOF spraying parameters on the electrical properties of alumina layers. Diagnostic tests show that small changes in gas ratios and flow rates can significantly alter particle and splat characteristics as well as the dielectric breakdown strength of the coatings. A large number of parameters are changed in order to assess the extent to which electrical properties can be controlled. Paper includes a German-language abstract.

This is the second paper of a two part series based on an integrated study carried out at the State University of New York at Stony Brook and Sandia National Laboratories. The goal of the study is the fundamental understanding of the plasma-particle interaction, droplet/substrate interaction, deposit formation dynamics and microstructure development as well as the deposit properties. The outcome is science-based relationships, which can be used to link processing to performance. Molybdenum splats and coatings produced at three plasma conditions and three substrate temperatures were characterized. It was found that there is a strong mechanical /thermal interaction between droplet and substrate, which builds up the coating/substrate adhesion. Hardness, thermal conductivity increase, oxygen content and porosity decreases with increase of particle velocity. Increasing deposition temperature resulted in dramatic improvement in coating thermal conductivity and hardness as well as increase in coating oxygen content. Indentation reveals improved fracture resistance for the coatings prepared at higher deposition temperature. Residual stress was significantly affected by deposition temperature, although not to a great extent by particle conditions within the investigated parameter range. Coatings prepared at high deposition temperature with high-energy particles suffered considerably less damage in a wear test. The mechanism behind these changes is discussed within the context relational maps which is under development.