Suspension plasma spraying (SPS) is able to process a stabilized suspension of nanometer-sized feedstock particles to form thin (from 20 to 100 µm) coatings with unique microstructures. The void network architecture of these ceramic coatings is a challenge to be characterized and quantified using commonly used techniques due to small sizes involved. Nevertheless, the discrimination of these pore architectures in terms of size and shape distribution, anisotropy, specific surface area, etc., is critical for the understanding of processing, microstructure, and properties relationships. USAXS (Ultra-Small Angle X-Rays Scattering) appeared as a suitable measurement technique allowing discriminating the void size distribution over a large range (up to four orders of magnitude). Results indicate that as-sprayed SPS coatings exhibit unusual porous architecture: 1) average void size is about the same than the feedstock one; i.e., nanometer sizes with multimodal void size distribution; 2) about 80% of the voids exhibit characteristic dimensions smaller than 30 nm; 3) the total void content varies between 13 to 20% depending upon considered operating parameters. In-situ annealing measurements were performed as they proved to deliver more relevant results compared to ex-situ measurements: even at temperatures as low as 800°C, the microstructure transforms - while the total void content does not change significantly. Indeed, it has been demonstrated that the smallest voids (equivalent diameters smaller than 50 nm) coalescence was the predominant mechanism and that it was more sensitive to temperature than time.

This paper reviews some of the tools used to characterize the pore microstructure of thermally sprayed thermal barrier coatings. It describes the capabilities of optical and SEM analysis, intrusion porosimetry, high-flux X-ray and neutron imaging, small-angle scattering, and X-ray microtomography. It explains why it is often necessary to combine multiple techniques and provides examples showing where it has been done and what it can achieve. It also reviews commonly used porosity descriptors.

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.

The pore microstructure of thermal spray coatings can be described as a combination of interlamellar pores, intralamellar cracks, and volumetric globular pores. This complex pore morphology can strongly influence the properties of the deposited layer. In this paper, the authors use a multiple small-angle neutron scattering (MSANS) technique to characterize the pore system in NiCrAlY layers. It is shown that the inter-lamella system is of utmost importance. Paper includes a German-language abstract.

This paper evaluates an electrochemical mapping method for determining the corrosion resistance and structural integrity of thermally sprayed coatings. In the test setup, a potentiostat is suspended over the test sample, forming an electrochemical cell. The circuit is completed through an electrolyte-containing porous tip. Capillary forces keep the electrolyte on the surface of the tip, preventing transfer to the substrate surface. In the investigation, electrochemical, spatially resolved measurements are carried out on flame and vacuum plasma sprayed nickel-base coatings and compared with the results of salt spray testing. It is observed that the new method offers many advantages being faster, nondestructive, and quantitative in nature. Paper includes a German-language abstract.

Relationships between the properties of thermally sprayed nickel based alloys and their microstructure (density, porosity, oxide phase content) are investigated. Cross-property- correlation of physical properties such as electrical conductivity and elasticity were examined. The experimental results of the structures and properties of the different coatings are discussed with respect to their pore surface area obtained by small angle neutron scattering (SANS) measurements. As wide as possible range of thermally sprayed microstructures of NiCr and NiCrAlY was produced by vacuum - and atmospheric plasma spraying, flame spraying, HVOF and water stabilized plasma spraying. Commercially available powders with process specific grain size distributions were used as feedstock materials resulting in a wide range of microstructures of the coatings depending on the spraying technique and, to much less extent, on the variation of the process parameters. In this work the examination of the pore structure was carried out by optical microscopy on metallographic cross sections. Phase composition and distribution were investigated by hot gas extraction for oxygen and nitrogen determination and by Scanning Auger Microscopy on polished cross sections and fracture surfaces. The properties of the coatings were characterized by their wear (ASTM G75) behavior, reflecting application-oriented properties. Significant and varying amount of anisotropy of the coating properties - electrical conductivity and elastic modulus - was found in the sprayed microstructures. This anisotropy could be directly linked to microstructure anisotropy as characterized by Small-Angle Neutron Scattering.

Technological properties of thermally sprayed deposits are to a great extent related to the underlying microstructure. The present project aims to relate macroscopic properties of metallic coatings to their microstructure. For this purpose, thermally sprayed deposits of nickel based alloys (NiCr, NiCrAlY) were manufactured by various spraying techniques - atmospheric and vacuum plasma spraying, flame spraying, high velocity oxygen fuel and water-stabilized plasma spraying. One of the key microstructural features is the void system. This system is usually characterized by the total volume of voids, the so called porosity. An additional characteristic parameter of the void system is the specific surface area. The method of anisotropic Small Angle Neutron Scattering (SANS) in the "Porod Regime" allows the determination of the anisotropic specific surface area of the complex void system that consists of intralamellar cracks and interlamellar pores. In contrast to optical microscopy, the SANS technique is capable of resolving the pore structure down to the nanometer scale, and the measured specific surface area represents a statistically relevant average value for the whole illuminated sample volume which is usually a few mm 3 . Besides the presence of voids and cracks the performance of thermally sprayed coatings is also significantly influenced by residual stresses. In the present work residual strains were determined by the technique of neutron diffraction as well as by bending tests, i.e. laser profilometry of the substrate before and after the spraying process. The specific surface area and the residual stresses are discussed with respect to total porosity, the presence of secondary phases like oxides and wear behavior. Special attention is drawn to the anisotropy of the apparent surface area, which is discussed with respect to the anisotropy of macroscopic properties like electrical resistance.

Phase stability of the thermal barrier deposits made from yttria-partially-stabilized zirconia (Y-PSZ) is a requirement for extended service lifetime. The response of Y-PSZ plasma-sprayed deposits to annealing at 1000 °C, 1200 °C, and 1400 °C with times from 1 to 1000 hours has been evaluated using Rietveld analysis of neutron diffraction data. Results show that yttria concentration of the as-sprayed tetragonal zirconia component generally decreased with increasing annealing temperature and time. As the yttria content in the tetragonal phase approached a limiting concentration, the tetragonal phase transformed into monoclinic phase on cooling. An increase in monoclinic phase content was clearly observable after annealing 24 hours at 1400 °C and was nearly 35 % after 100 hours at 1400 °C. A similar trend was observed at 1200 °C for longer annealing times, with monoclinic phase formation beginning after 400 hours. At 1000 °C experimental times were not sufficient for monoclinic phase to form although a decrease in the yttria concentration in the tetragonal phase was observed. Keywords: neutron scattering, yttria-stabilized zirconia, phase composition, Rietveld analysis

Improved understanding of microstructure-property relationship can help to shift from experiment-based to science-based development of thermally spray deposits. This should result in shorter and less expensive development as well as in higher functionality and reliability of the deposits. Significant amount of work has been done, however, nearly always studying deposits manufactured by only one of the thermal spray techniques. Results are therefore often spray technique specific. A broad study with samples manufactured by a number of different thermal spray techniques seems to be missing yet. Relationships valid across different techniques should provide better understanding of the generic relationships. This research employs number of different techniques - flame, HVOF, plasma (APS, VPS, WSP), to generate a wide range of microstructures. Various Ni-based alloys are studied starting from a simple chemistry (Ni) and ending with complex NiCrAlY alloys. Presented results were obtained with NiCr (80% Ni, 20% Cr) feedstock. Microstructures are characterized by various techniques-OM, SEM, XRD, small-angle neutron scattering (SANS) and others - to obtain the most comprehensive set of macro to micro structural parameters available today. The wear and corrosion properties of these deposits are measured together with internal coating stresses and the most generic microstructure-property relationships are sought.

In this paper, a simple cutting method is used to examine the changes in wear resistance of four thermally sprayed coatings, two yttrium stabilized zirconia, and two aluminum oxides using a heat treatment. These changes were correlated with the changes in the microstructure of the application layers, which were characterized using intrusion porosimetry, scanning electron microscopy, and small-angle neutron scattering. Analysis of the results shows that changes in wear resistance are believed to have been caused by changes in the shape and surface area of the voids caused by sintering as well as crystallographic phase changes. Paper includes a German-language abstract.

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.

Silicates represent a broad group of industrially important ceramic materials. The only silicate that is widely used for plasma spraying is zirconium silicate. Other silicates are generally not used, although they can offer interesting application properties. This paper presents results with other silicates: synthetic wollastonite, stoichiometric mullite, cordierite, and steatite. The input materials were produced in selected sizes from ceramic in industrial quality and applied with the water-stabilized plasma spray system PAL160 to form free-standing panels. The morphology of the microstructure, the thermal expansion, the bulk and filling densities, the open and closed porosity, the phase, and the chemical changes were evaluated. 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.

The formation of the microstructure is influenced by a number of parameters. The spray angle is considered to be one of the most important and difficult to control parameters, especially for substrates with complex shapes. In this paper, the influence of the spray angle on the microstructure of gray aluminum oxide and yttrium oxide stabilized (8% by weight) zirconium oxide applications is investigated. Plots are made from each material at four spray angles (90 degree, 70 degree, 55 degree, and 45 degree). Their microstructure is characterized by means of intrusion porosimetry and SEM. The results show clear differences between the two materials. The aluminum oxide microstructure is significantly influenced by the spray angle, while the YSZ microstructure is almost independent of the spray angle. Paper includes a German-language abstract.

This paper examines ways to control the porosity of thermally sprayed deposits. All spraying was done with a water-stabilized plasma system using different combinations of alumina, zircon, Ni, and Al powders. Sandwiched structures with alternating ceramic and metal layers were sprayed as were thick deposits consisting of metal and ceramic mixtures. Porosity was characterized by methods such as gas permeability, water immersion, MIP, SEM, and SANS. In addition, several post-processing methods were tested to determine their effect on porosity volume. For example, removing metallic phases by leaching or by annealing at temperatures above the melting point was found to effectively increase porosity, while the use of sealing materials proved effective at reducing porosity. Another method tested was calcination, which resulted in an increase or decrease of porosity depending on the deposit's chemistry and annealing conditions.

The evolution of the void microstructure of yttria-stabilized zirconia (YSZ) plasma-sprayed deposits (PSD) was studied as a function of heating in air from room temperature to 1400°C , and during a constant temperature hold at 1100°C for 19 hours. The samples were studied using the terminal slope (Porod scattering) of small-angle neutron scattering (SANS), modified for the analysis of anisotropic structures. The experiment was done in-situ using a special furnace built for use on the small-angle scattering instrument. SANS Porod scattering can distinguish between the two major void systems - interlamellar pores and intralamellar cracks - that are present in the PSD microstructure. Thus, changes in the void surfaces of the cracks and the pores could be followed separately as a function of temperature. The surface area attributable to the interlamellar cracks significantly decreased at temperatures below 1000°C, whereas the surface area of the interlamellar pores only began to decrease at temperatures above 1000°C. This suggests that there are significant differences in the sintering of these two void systems, probably associated with differences in their sizes and shapes. The first noticeable changes in the void surfaces were observed at temperatures just above 600°C, which is a very low temperature for YSZ. During annealing at 1100°C for 19 hours, there was a decrease in the interlamellar surfaces of about 10%.

The phase composition of plasma sprayed (8wt%) YSZ was studied using neutron and X-ray scattering. Comparison shows that neutron scattering is superior for analysis of the phase composition as well as for the analysis of the yttria content of the tetragonal phase. The presence of large amounts of the cubic phase are probably often neglected in standard XRD analysis due to scattering-related limitations and the inherent difficulty of the analysis. The importance of this fact needs to be addressed in future studies. The amount of monoclinic, tetragonal, and cubic phases was determined for as-sprayed deposits and for samples annealed at various temperatures. The as-sprayed deposit was composed of metastable phases, and the phase composition on annealing did not change significantly until 1400 °C for one hour. This indicates the relative stability of the phase composition. The influence of the spray distance is not clear, but the resultant phase composition may be related to the deposition temperature.

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.

A technique has been developed to characterize the elastic modulus of zirconium oxide - 8 % yttrium oxide plasma sprayed deposits. A commercial hardness indenter has been modified to record load - displacement as a spherical ball is elastically loaded onto the surface of the material to be measured. The resulting data are used to calculate the elastic modulus. Since the loads used are in the elastic region, the technique is, in theory, nondestructive. Relatively small areas of the material, approximately 50 μm in diameter, are sampled by the indenter, allowing local mapping of elastic modulus variations throughout the deposit. Using this technique, elastic modulus variations have been measured through the thickness of the deposit. Also, different moduli were measured in the cross-section and through the thickness and these differences are correlated with the microstructure. Finally, significant increases in elastic modulus have been found in samples annealed for 2.5 h at 1100°C. These changes have been correlated with small angle neutron scattering measurements of void surface area.

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.