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.

Thermal spray processing involves a multitude of interdependent process variables resulting in a complex layered microstructure with each layer composed of several splats separated by splat-splat interfaces. Each splat contains columnar/equiaxed grains with multiple orientations. A typical microstructure also contains defects such as globular pores, and interlamellar pores. This gamut of features encompasses a range of macro, micro and nano length scales. This study involves a thorough microstructural characterization across length scales of Ni-5wt%Al coatings to understand the dependence of microstructural variability on processing conditions and its profound influence on coating properties. Studied here are coatings produced by four different spray methods, namely twin wire-arc, air plasma, high velocity oxyfuel and cold spraying with distinct variations w.r.t. nature of feedstock, method of material injection, amount of melting, and process parameters such as particle velocity and temperature in flight. Macro-scale, Micro-scale and Nanoscale microstructural characterization of these Ni5Al coatings elucidates the variations in oxide content, splat structure, grain structure and phases obtained, based on differences in processing parameters. Through characterization of these bond-coats, using Optical microscopy, Image analysis, Scanning and Transmission electron microscopy techniques, variations in coating properties such as porosity and thermal conductivity can be explained with respect to coating microstructure and process characteristics.

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.

The current study was undertaken as part of an ongoing effort at CTSR towards development of 'Process Maps' for thermal spray coatings of molybdenum. Extensive study of the spray stream of molybdenum powders in a plasma jet was carried out. The study was undertaken for three distinct nozzle (anode) geometries and different spray conditions for each nozzle. The results demonstrate that the particle velocities and temperatures vary in an interdependent manner over a narrow range for any single nozzle. When nozzles with different geometeries are used the velocity and temperature range can be extended considerably. By estimating the residence time of particles in the plasma jet and defining a 'melting index' for the particles, a cross comparison of nozzles provides insight into the particle state achieved in each case.

Molybdenum splats were produced at three plasma conditions on steel substrates preheated to three temperatures. Morphology of splats and corresponding craters formed on substrates were observed; dimensions of splats and craters were measured with an optical non-contact interferometer. It is found that substrate is significantly melted and deformed upon impact of the droplet, which leads to the formation of flower like splats and craters. On average, only about 36 to 53 % of the areas covered by splats were in good metallurgical/mechanical contact with substrate. Normalized crater volume increases with droplet size and the contact is improved for the high particle energy/high substrate temperature condition as compared with low particle energy/medium substrate energy condition. Splat morphology and crater formation is explained based on impinging jet heat transfer model.

Thermal barrier coatings (TBCs) are used on heat engine parts to impart protection to components against failure under excessive heat loads, to increase inlet temperatures with consequent improvements in efficiency, and to reduce requirements for cooling. Control of thermal conductivity is addressed since low thermal conductivity depends not only on the nature of the yttria stabilized zirconia (YSZ) layer, but also on the morphology of pores and cracks, which are closely linked to process parameters. This paper will present the influence of feedstock characteristics (particle size distribution and powder morphology) and thermal cycling on porosity content and thermal conductivity of zirconia coatings. The results show increased porosity with particle size, due to an increase in the degree of particle fragmentation and unmelted particles, leading to lower thermal conductivity. Coatings sprayed with powders of different powder morphology yielded changes in porosity and interlamellar contact, thus, influenced thermal conductivity. Sintering effects during thermal cycling resulted in reduced porosity and increased thermal conductivity.

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.

BaTiO3 has been successfully sprayed by HVOF to produce dense 25-150 µm thick deposits for use as dielectric and capacitive layers within prototype multilayer conformal electronics. Parameter optimization has been shown to play a critical role in the effective spraying of these materials as thin structurally homogeneous deposits. The affect of standoff distance and combustion chamber size on the phase structure of the coatings have been studied and related to the dielectric properties of the layer. The proportion of crystalline to amorphous phase was found to be critically dependent upon the degree of melting of the particles in the flame and the rate of cooling of the deposits. The crystalline/amorphous ratio is directly related to the dielectric properties of the layer with greater crystallinity giving higher values of dielectric constant. Microcracks and splat/splat interfaces are also believed to adversely affect the dielectric properties. The maximum dielectric constant (K) values achieved using the HVOF method for deposition have been in the range 70-115.

This paper describes the technique of anisotropic multiple small-angle neutron scattering (MSANS). Anisotropic MSANS, when combined with anisotropic Porod scattering, electron microscopy, and measurements of elastic modulus and density, has made possible the determination of the porosities, surface areas, mean opening dimensions, mean diameters, and approximate orientation distributions, of the intra-splat cracks and interlamellar pores, as well as the porosity, surface area, and mean diameter, of the globular pores. The changes in these parameters, as a function of annealing, are studied. Paper includes a German-language abstract.

This paper presents novel results of a series of experiments intended to study the role of the size of the feedstock powder on the microstructure of the deposits. For this purpose, Metco and the feedstock powder, yttria-stabilized (8% wt) zirconia, with number-weighted mean particle sizes of 32, 47, 56, and, 88 micrometer, are used. Small-angle neutron scattering and multiple small-angle neutron scattering (MSANS) methods are applied to determine the microstructure of the four deposits. Companion indentation measurements are performed to determine the elastic moduli of the deposits. The paper also discusses the MSANS 3-void model in relation to the anisotropic elastic properties. Paper includes a German-language abstract.

Plasma sprayed deposits consist of multitude of flattened lamellar particles - 'splats' - which as basic building elements form their structure and determine the deposit properties. Therefore, knowledge of the mechanism of their formation and characteristics is important for understanding the processing property relationships. Although extensive studies have been done on splat formation, there is a lack of correlation to macroscopic deposit properties. Among factors influencing the deposit properties and performance is residual stress, originating from splat quenching and thermal mismatch between the substrate and coating. This phenomenon has been studied mostly on macroscopic level. In the present study, an attempt is made to establish a connection between these two approaches. In the focus of this study is the effect of selected processing variables on splat characteristics, deposit properties and residual stress in a single-splat layer. The processing variables of primary interest were deposition temperature and substrate material. Molybdenum as a representative material of practical interest was used throughout this study. The correlation between stresses and processing conditions is discussed with regards to microstructure and relevant coating properties.

Conventional corrosion protection of steel structures has usually involved the application and reapplication of lead-based paint (LBP), a material now known to be highly toxic and likely to find its way into the environment. LBP is no longer used in the field, but repair crews, nearby communities, and the environment may be exposed to unacceptably high levels of lead as the substrates of older structures are prepared for repainting during routine M&R operations. Conventional dust-containment enclosures used onsite during surface preparation (abrasive blasting) are often inadequate. The most effective containment technologies, on the other hand, tend to be expensive and cumbersome. All of these factors make surface preparation and recoating slow, technically difficult, physically demanding, and hazardous to the worker and the environment. Automated technologies have the potential to address all aspects of these interrelated infrastructure M&R problems. An example of such a technology is the Automated Thermal Spray System (ATSS). The ATSS utilizes a triaxial array of linear motion actuators to form a robot capable of performing preprogrammed sequences. The demonstration proved that the ATSS can successfully remove deteriorated lead-based paint from a steel bridge and then apply a protective coating to the exposed surface.

Two sets of plasma spray processing conditions were utilized in the investigation of graded layers, consisting of NiCrAlY and PSZ. Following the optimization of the plasma spray parameters and particle characteristics, the deposition efficiencies (DEs) of various powder species was examined. Selection of the best suited powders for coating production were selected based on the DE results. The base DEs were corrected by conducting the analyses using pre-deposited substrates as targets. Individual mixed coating layers were prepared and their compositions confirmed by image analysis. The effects of standoff distance and substrate temperature were also seen to have an effect on the DE and thus the coating formation. It is suggested that two-feeder, single injector plasma processing may not be the optimal method for the formation of FGMs.

The microstructures of as-sprayed and thermally-cycled freestanding and on-substrate deposits of yttria-stabilized zirconia were studied using small-angle neutron scattering (SANS). The SANS analysis allows the interlamellar pores and the intralamellar cracks, which are the two dominant void systems in the microstructure, to be characterized separately. Whereas up to 20% of the void surface area in the as-sprayed deposits was found to be in the cracks, the thermally-cycled deposits contained only a negligible quantity of cracks. At the same time, changes in the pore surface areas between the lamellae (i.e., the interlamellar pores) were much smaller. As a result, the microstructure of the thermally-cycled deposits was much more anisotropic than the microstructure of the as-sprayed deposits. Varying the cooling and the heating rates did not significantly change the microstructure but varying the total time that the deposits were at high temperature did affect the evolution of the surface area. The presence or absence of a bond coat and substrate also did not measurably influence the results.

Functionally graded materials (FGMs) offer solutions to such engineering problems involving multi-layer systems with large differences in CTE, i.e. thermal barrier coatings, by allowing for a continuous change in the properties over a defined distance, thus minimizing sharp interface effects. By its nature, plasma spraying is well suited to the fabrication of FGMs. However, in order to achieve optimal performance from the material it is necessary to ensure the FGM is uniform in its compositional variation. The deposition efficiency of the particulate species as well as their trajectories will determine the degree of homogeneity of the FGM. It is therefore important that the inter-relationships between the particle size distributions, injection orientation and feed rates are determined. Towards this end, a series of investigations have been carried out to determine the effects of injection orientation on the particle segregation. Analysis of the particle segregation as a function of particle size distribution was then examined in the formation of FGMs. The results indicate that optimal deposition occurs when the various particle species trajectories converge as they approach the substrate.

Iron-based thermal spray deposits have been used for reclamation of machine components and wear resistance. Current thermal spray processes can be used to spray deposits a few millimeters thick at rates of 3 to 10 kg/hr. With a throughput over 100 kg/hr for metals, the Water Stabilized Plasma torch enables cost-effective processing of very thick (>1 cm) deposits at very high rates. However, limited information exists in the literature on the ability of this technology for deposition of oxygen sensitive metals. In this investigation, iron-based coatings were produced with the water stabilized plasma system. The deposits sprayed in air and with an inerting shroud were evaluated using x-ray diffraction, light microscopy, oxygen analysis and microhardness in an effort to understand the processing effects on microstructure and properties.

Reducing the pore size and pore volume can lead to improved mechanical properties and enhanced corrosion resistance of plasma sprayed thermal barrier coatings. In this work, plasma sprayed 8 wt.% yttria stabilized zirconia coatings were removed from the substrate and machined to obtain 25x5x1 mm test specimens. These specimens were vacuum impregnated with alumina sol, calcined at 873 K for an hour and then heat treated at 1273 K for an hour to produce ceramic impregnated specimens. As-sprayed and impregnated specimens were investigated using optical microscopy, XRD, SEM, mercury intrusion porosimetry and electron microprobe analysis. This technique can impregnate the entire thickness of the specimens. Pores in the as-sprayed specimens were impregnated with α alumina grains, resulting in microstructural variations and reduction of the size and volume of the specimen pores.

Joint research work between the University of Limoges and the State University of New York, Stony Brook, has been carried out on the impact and solidification of plasma sprayed zirconia particles. A measurement device, consisting of a phase doppler particle analyser and a pyrometer, was used to correlate the characteristic parameters of splats to those of the substrate and to the size, velocity and temperature of the impacting particles.

This is the second paper of a two part series based on an interdisciplinary research investigation between the University of Limoges, France, and the State University of New York, Stony Brook, USA, aimed at fundamental understanding of the plasma-particle interaction, deposit formation dynamics and microstructure development. In this paper, the microstructure development during plasma spraying of zirconia is investigated from the point of view of deposition parameters and splat formation (part I). Splats and deposits have been produced at Limoges and Stony Brook under controlled conditions of particle parameters and substrate temperatures. The zirconia splat microstructures thus obtained are examined for their shape factors, grain size, crystallographic texture and defects. Further the deposits were analyzed for phases, porosity and mechanical properties in an effort to develop a process-microstructure property relationship. The results suggest a strong role played by the deposition temperature on the microstructure and properties of the deposit.

The porous microstructure of plasma-sprayed deposits prepared from gray-alumina feedstock and from two different yttria-stabilized zirconia (YSZ) feedstocks were studied as a function of spray distance. For each material, the behavior of the two major void systems—intralamellar cracks and interlamellar pores—was investigated. The results offer the first proof that the quantity and the character of the porosity in these materials can be controlled independently by selecting the appropriate processing protocols.