Adhesion/cohesion testing represents one of the most common methods for benchmarking and optimization of thermal spray coatings. However, due to the inhomogeneous coating microstructure, such testing may be quite troublesome. In this study, adhesion/cohesion strength of representative metallic and ceramic coatings deposited by Water Stabilized Plasma (WSP) spraying was evaluated by different methods, namely Tensile Adhesion Test (TAT), newly utilized pin test and Tubular Coating Tensile (TCT) test. Combination of various methods enabled the evaluation of the splat bonding quality in different loading modes. Limitations and benefits of each method for testing of WSP coatings are demonstrated. Dominating failure micromechanisms were determined by supplementary fractographic analysis.

Mechanical and thermal properties of thermally sprayed coatings, especially ceramics, are strongly influenced by cracks and pores that are present in the coating microstructure. In the recent past, there have been efforts to find an analytical model describing the coating properties based on the microstructural characteristics. Various analytical models were developed and published in the literature. In this study, several major models were applied to ceramic and metal coatings to describe their elastic modulus and thermal conductivity. The sensitivity of the models to the variations in the microstructure and relevancy of their use in specific cases were examined. The results were compared with those obtained by FEM modeling and experimentally measured values.

The effect of grit blasting exposure time on the adhesion of plasma sprayed Al 2 O 3 and 316L stainless steel coatings was studied in the present work. The steel substrates were grit blasted prior to the coating deposition. Two sets of substrates with exposure time of 1 and 4 seconds were prepared. Both types of coatings were deposited using Water Stabilized Plasma (WSP) torch. Adhesion strength was evaluated using standardized pulloff test. The obtained results showed a slight improvement in the adhesion strength for the blasting time of 4 s. Failure processes taking place in the coatings during the pull-off tests were described based on the detailed fractographic analysis.

A hybrid DC arc plasma torch, combining water and gas stabilization, offers a high flexibility in plasma characteristics. These can be controlled in a wide range by the torch operational parameters, such as arc current and secondary gas flow rate. In this study, their influence on plasma spraying of tungsten and copper was investigated. To suppress the in-flight oxidation of the metals, inert gas shrouding was applied. In-flight particle diagnostics, analysis of free-flight particles and coatings was performed for spraying experiments in the open atmosphere and with argon shrouding. Both in-flight particle behavior and coating properties were found to be sensitive to the torch parameters. The application of shrouding was found to affect particle in-flight parameters, reduce the oxide content in the coatings and generally improve their properties, such as thermal conductivity. However, different degree of these effects was observed for copper and tungsten.

This study investigates the mechanical response of plasma-sprayed ceramic coatings to different levels of mechanical and thermal loading. Test samples were subjected to four-point bending and thermal cycling loads. Nonlinear behavior and significant hysteresis were observed, indicative of inelastic phenomena. Previous tests were complemented by structural examinations and bonded-interface testing. Relevant structural features and possible mechanisms underlying this behavior are discussed.

Tungsten-copper composites and FGMs can find applications in various thermal management systems. One example is plasma facing components for nuclear fusion devices, where tungsten provides the heat-resistant plasma facing armor, copper provides the highly conductive heat sink, while the composite or FGM can reduce the stress concentration at the interface. In this study, W+Cu composites of various compositions were produced by water-stabilized plasma spraying. With the help of in-flight particle and plume diagnostics, the powder injection was optimized for each material, and the feed rates were adjusted to account for different deposition efficiencies. The composition, structure, thermal and mechanical properties of the coatings were characterized.

In modern jet engines, the efficiency of the compressor stages is highly dependent upon the clearance between housing and rotating compressor blades. To control the over-tip leakage, abradable coating systems are applied on the housing. In the high pressure compressor they typically consist of a thermal sprayed multiphase material, comprising a metal matrix combined with a dislocator and/or a solid lubricant as well as a defined level of porosity. In this study, novel material systems have been sprayed via the plasma and flame spray process and compared to reference materials. Resulting microstructures have been analyzed as well as important coating characteristics evaluated, including coating hardness and erosion resistance. Furthermore rig tests were performed to analyse the coatings abradability behaviour under different operation conditions of the compressor.

Tungsten particles were sprayed by a novel plasma torch with hybrid water-gas stabilization (WSP®-H). Several spraying parameters were varied – arc current, argon flow rate, carrier gas flow rate and spraying distance. The temperature and velocity of the individual particles were monitored by the DPV 2000 optical sensor. Individual splats were collected on polished stainless steel substrates and analyzed by SEM to assess their melting, flattening and/or fragmentation. These features were correlated with the basic in-flight particle characteristics and conditions for production of dense coatings were sought for. Significant dependence of the splats morphology on spraying parameters was found, and important improvement of particle melting at WSP-H over conventional water stabilized plasma torch (WSP) was registered.

Plasma sprayed tungsten and tungsten-copper coatings are being developed for potential application as plasma facing materials for fusion reactors. Initial spray tests indicated difficulties in tungsten melting and in-flight oxidation. Numerical modeling was performed to help explain these issues. A complex study of the process and its products was performed, including: in-flight diagnostics, characterization of isolated splats, and structure, composition, thermal and mechanical properties of the coatings. Based on these results, the process was optimized, with respect to powder size and various spraying parameters, to improve melting of the particles, reduce oxidation and increase the deposition efficiency.

An investigation was conducted to assess the potential of water-stabilized plasma (WSP) spraying for applying protective boron carbide coatings to fusion reactor components. This paper describes how test samples were produced and how coating quality was determined. The authors sprayed boron carbide powder onto steel and stainless steel substrates using different powder feeding and spraying distances, substrate preheat temperatures, and carrier gases. They also investigated methods for optimizing the plasma jet and improving coating adhesion. The boron carbide coatings were characterized based on phase composition, porosity, oxygen content, and flexural strength. Paper includes a German-language abstract.

In this paper, a four-point bending test is developed that makes it possible to load layers with defined tensile stresses. In the experiments, high-density stainless steel is applied using HVOF and ceramic layers are produced using water-stabilized plasma spraying. Both coating materials are then subjected to bend testing. The steel layers show a volume of elasticity twice that of the surface elasticity module, while in the ceramic layers, no surface tension was observed. This can be explained by the opening and closing of crack structures in the material. Paper includes a German-language abstract.

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.

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.

Properties of thermally sprayed coatings, including residual stress, are controlled by various parameters of the spraying process. This study is focused on three thermal spraying techniques with significantly different particle temperatures and velocities. These are plasma spraying, twin wire arc spraying and high velocity oxy-fuel spraying. For each method, in-flight particle diagnostics was performed. Through-thickness residual stress profiles in Ni+5%A1 coatings on steel substrates were determined nondestructively by neutron diffraction. The stresses range from high tensile in the plasma sprayed coating to compressive in the HVOF one. Various stress generation mechanisms, including splat quenching, peening, and thermal mismatch, are discussed with respect to process parameters and material properties.

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.

Processing induced residual stresses play an important role in the performance of thermally sprayed coatings. Their precise determination is a key to influence the coating properties by modification of process variables and to understand the processing-property relationship. Among various methods for residual stress measurement, x-ray diffraction holds a specific position by being nondestructive, phase distinctive, localized and applicable for real parts. The sin 2 ψ method is commonly applied for bulk materials as well as coatings. However, the results are often reported without sufficient experimental details and the method is used in its simplified form without justification of certain assumptions. In this investigation, the sin 2 ψ x-ray diffraction method was used to measure residual macrostress in plasma sprayed Ni, NiCrAlY and ZrO 2 +8%Y 2 O 3 coatings. Reproducibility of the method was tested and the assumptions allowing its use are discussed and experimentally verified. For Ni coatings, a comparison with blind hole and neutron diffraction measurements is presented. The results are discussed with respect to processing, structure and properties of the coatings.

Neutron diffraction is a promising tool for the investigation of residual stresses in thermally sprayed coatings. In principle, the neutron diffraction method has several distinct advantages over other methods. 1) It is possible to perform triaxial stress analysis throughout the thickness of both the coating and the substrate without material removal. 2) The stress can be determined in all phases of a multi-phase coating. 3) Repeated measurements can be performed on mechanically or thermally fatigued specimens. 4) Stress concentrations and shape/edge effects in actual parts can be located. In this paper, these unique capabilities will be reviewed first. In the second part of the paper it will be shown how the analysis of these coatings differs from experimental analysis of bulk materials. Finally, the analysis of the stress distribution in plasma sprayed NiCrAlY and ZrO 2 +Y 2 O 3 thermal barrier coatings in as-sprayed and annealed states will be presented and discussed.

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.