This is part II of the two part paper based on international collaboration between the University of Limoges, France and the State University of New York, Stony Brook, USA, aimed at fundamental understanding the relationship between processing condition and microstructure development and properties of thermally sprayed materials. In this study, the effects of deposition temperature on the microstructure and properties development of molybdenum coating was investigated. It is found that with the increase of steel substrate temperature, the molybdenum splat morphology changes from fragmented to more contiguous disk-like shape. The splats on molybdenum substrate show predominantly disk shape. With the increase in deposition temperature, the coating exhibits better lamellar structure with less interlayer pores and debris. The fracture characteristics changes gradually from interlamellar to trans-lamellar and, thermal conductivity is enhanced. Higher deposition temperature improves dramatically the adhesion and bonding of the splats, therefore the physical and mechanical properties of coatings.

Coatings have been produced by HVOF spraying of four different WC-Co powders, using two fuel gases and two oxygen contents in the flame, and characterised in terms of microstructure and resistance to abrasive wear. It is concluded that there is a close correlation between high levels of chemical reaction, occurring during spraying (and possibly during powder production), and poor wear resistance. Good wear resistance is favoured by using low porosity powders, which interact with the atmosphere less readily during spraying, and also by using a flame with a relatively low oxygen content. This probably minimises the degree of reaction by ensuring that conditions are reducing. Use of propylene rather than hydrogen gives coatings with slightly better wear resistance, despite the fact that the flame temperatures are higher. It is concluded that, for this relatively small rise in temperature, the positive effect on inter-splat cohesion seems to outweigh the negative effect of increased decarburisation.

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.

New near-net-shape structures of alloy Inconel 718 processed by HVOF spraying require optimum mechanical properties. Dominant factors defining the material quality are the particle properties velocity and temperature adjusted by the HVOF process parameters. Based on theoretical analysis of the HVOF process, experiments were performed with a defined variation of primary process parameters, producing coating samples of alloy 718 and measuring the particle velocities. Microstructural and X-ray analysis shows that in coatings with a high fraction of molten phase and high velocity, mainly divalent and spinell-type oxides are formed during particle impact on the substrate. Due to severe oxidation of the y'/y''- forming elements Ti, Al and Nb, precipitation-hardening effects of In 718 coatings are low. This leads to merely mediocre mechanical properties. The reduction of the molten phase to nearly zero leads to a drastic decrease of the oxide formation. The hardening γ'/γ'' phases are precipitated homogeneously in the Ni-base matrix. Strength values comparable to cast and wrought alloy In718 are attained by spraying with a low molten-phase fraction and high particle velocity. However, extensive intergranular 8-phase precipitation due to too high an Nb content of the powder causes only mediocre fracture elongation. Coatings up to 10 mm thick have been sprayed. The construction effort and hence the costs and weight of combustion chambers for hypersonic propulsion systems are to be reduced through direct thermal spraying of the loadbearing metallic pressure jacket onto the tubular cooling system. As a semifinished product, the selected Inconel 718 alloy exhibits good mechanical properties in the cryogenic temperature range as well as under higher thermal loads, and is commercially available in powder form. Aging serves to increase the strength up to the range of 1,200 N/mm2. For the sprayed In718 version, coating thicknesses in the centimeter range, a porosity < 1% and mechanical properties comparable with those of the cast version are required. The objective of the research work is to optimize spray-process control so that the resultant structural thick layers meet the design as well as the material requirements with respect to combustion-chamber technology. This necessitates elaborating the dominant microstructural parameters influencing the mechanical properties and the effect on them of the spraying process, and correlating them with the particle-condition parameters and the process parameters [1].

The characterization of a thermal barrier coating (TBC) using routine metallographic methods has had investigators uncertain as to the authenticity of the microstructure concerning porosity, cracking and other features. Mounting is a very important step in the process. A greater understanding of relationships between high and low viscosity mounting epoxies exposed to vacuum impregnation, pressure impregnation procedures (PIP), and the combined methods is the goal of this paper. This study will attempt to show how variations in sample mounting procedures will affect the final results in conjunction with varied metallographic procedures.

The purpose of this study is to investigate the microstructure and wear resistance of plasma sprayed WC-Co-Ni coatings. WC-Co-Ni composite powders were prepared by mixing of WC powder, Co powder and a Ni-P alloy powder, followed by sintering and crushing to improve the properties of plasma sprayed WC-Co coatings. In this study, their coatings were deposited by the atmospheric plasma spraying. The evaluation of their coatings were carried out by the observation of microstructure, measuring of microhardness values, adhesion strength values and an abrasive wear test. The abrasive wear resistance of the as-sprayed WC-Co-Ni coatings was comparable with that of WC-Co coatings deposited by HVOF spraying, and besides, the properties of the post-treated WC-Co-Ni coating were comparable with those of cemented carbides.

The paper reports a series of experiments with various HVOF spray systems (Jet Kote, Top Gun, Diamond Jet (DJ) Standard, DJ 2600 and 2700, JP-5000) using different types of WC-Co and WC-Co-Cr powders. The microstructure and phase composition of powders and coatings were analyzed by optical and scanning electron microscopy and X-ray diffraction. Carbon and oxygen content of the coatings were determined in order to study the decarburization and oxidation of the material during the spray process. Coatings were also characterized by their hardness, bond strength, abrasive wear and corrosion resistance. The results demonstrate that the powders exhibit various degrees of phase transformation during the spray process depending on the type of powder, the spray system and the spray parameters. Within a relatively wide range, the extend of phase transformations has only little effect on coating properties. Therefore coatings of high hardness and wear resistance can be produced with all HVOF spray systems when the proper spray powder and process parameters are chosen.

The plasma spray deposition of a zirconia thermal barrier coating (TBC) on a gas turbine component has been examined using analytical and experimental techniques. The coating thickness was simulated by the use of commercial off-line programming software. The impinging jet was modelled by means of a finite difference elliptic code using a simplified turbulence model. Powder particle velocity, temperature history and trajectory were calculated using a stochastic discrete particle model. The heat transfer and fluid flow model were then used to calculate transient coating and substrate temperatures using the finite element method. The predicted thickness, temperature and velocity of the particles and the coating temperatures were compared with these measurements and good correlations were obtained. The coating microstructure was evaluated by optical and scanning microscopy techniques. Special attention was paid to the crack structures within the top coating. Finally, the correlation between the modelled parameters and the deposit microstructure was studied.

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.

Spinel powders of different compositions were fabricated for their good properties of chemical resistance. These powders were plasma sprayed on steels and their microstructure was investigated by scanning electron microscopy (SEM), microanalysis, X-ray diffraction and transmission electron microscopy (TEM). Due to the powder fabrication process, coatings were very heterogeneous in composition, but had the spinel structure. TEM observations pointed out that splat solidification occurred with a cooling rate gradient leading to different crystallization inside a lamella. Young's moduli by the coatings were measured by the resonant frequency method and the correlation with coating microstructure was discussed.

Molybdenum based thermal spray coatings are used extensively for wear and friction control. However, the sprayed Mo lamellae have been found to have low breakout resistance leading to decreased wear life of these coatings. An alternate approach is to increase the strength of Mo lamellae by alloying them with Mo-C. This work examines the effect of such alloying on the differences in the friction and wear mechanisms of four Mo and Mo-Mo-C based coatings. It was found that alloying the Mo with Mo-C leads to an improvement in the friction behavior of Mo-Mo-C based coatings.

In this investigation, binary FeAl layers are produced by detonation spraying and examined by means of microscopy and X-ray diffractometry. The examinations show that the layers have a lamellar structure consisting of FeAl and Fe 4 Al 13 phases with a small amount of alumina. It is observed that with increasing amounts of fuel gas and decreasing detonation frequency, the proportion of FeAl phases decreases and variations in microhardness throughout the coating become irregular. Paper includes a German-language abstract.

This paper discusses the optimization of HVOF spraying parameters for applying WC-Co and WC-CoCr powders. Coatings produced using the optimized parameters are evaluated based on microstructure, microhardness, and porosity. The influence of the processing method used to produce the powders is also considered along with in-flight particle characteristics. Paper includes a German-language abstract.

This paper investigates the influence of microstructure on the sintering behavior of thermal barrier coatings. A significant difference is observed between coatings with typical microcracks and those with segmentation cracks. Segmented coatings showed a lower sintering rate and little change in pore size distribution after heat treatment. Conventional coatings, on the other hand, showed an increase in pore diameter and in more porous areas, sintering was more pronounced. All coatings had an elastic modulus in the range of 15-20 GPa, which more than doubled after a 10-h heat treatment at 1200 °C. Paper includes a German-language abstract.

This paper presents a process for manufacturing parts using HVOF spraying techniques. The parts are made from a mixture of chromium carbide and nickel chromium alloy powders and are characterized based on microstructure, hardness, and corrosion and wear performance. Paper includes a German-language abstract.

Twin wire arc spraying is widely used in industry due to its low cost and high rate of material deposition. Much has been learned about the process over the course of its use, most recently in regard to the correlation between spray parameters and coating microstructure. The aim of this study is to investigate the behavior of steel particles in nitrogen and air jets and show how in-flight particle characteristics affect coating properties. In the experiments, an optical sensing system is used to measure the velocity, temperature, and size of carbon steel particles fired from a wire arc spray gun set at 100 A and 30V for gas flow rates of 94, 110, 122, and 144 m 3 /h. The particle data obtained is presented and correlated with coating microstructure, hardness, and oxide content. Paper includes a German-language abstract.

The aim of this study is to find a coating that can improve the wear resistance of bushings and sleeves used in continuous hot-dip galvanizing facilities. In the experiments, a number of wear-resistant alloys and their carbide composites are applied to stainless steel substrates by plasma transferred arc (PTA) surfacing and the resulting deposits are characterized based on XRD and SEM analysis, microhardness measurements, and pin-on-disk sliding wear tests conducted in a molten Zn-Al bath at 470 °C. Changes in microstructure during solidification are discussed and correlations are made with phase composition, hardness, wear behavior, and suspected wear mechanisms. Paper includes a German-language abstract.

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.

In this paper, two long-term experiments are conducted in order to investigate the evolution of the arc root fluctuations and the evolution of the in-flight particle state during plasma spraying. Voltage as well as the acoustic fluctuations measured at three different angles are characterized while particle state was monitored using an optical integrated system, the DPV2000. A detailed study of the evolution of the gun power, in-flight particle state (temperature, velocity, diameter, particle flux) and coating microstructure is was carried out. Results showed that the microstructure of the deposited coating significantly changed during the forty-hour spraying period. Paper includes a German-language abstract.

The aim of this work is to determine how to control the microstructure and tribological properties of HVOF-sprayed TiC composite coatings. The powders used in the study were made by the SHS process and contained a mixture of TiC and either FeCr20Ni10 or FeCr18Ni15Mo3, which serve as a binder and give the sprayed coatings additional corrosion resistance. The composites produced were assessed based on metallographic examination and wear testing. The results show how the structure of the SHS powder changes due to the injection molding process and how the tribological properties of the HVOF layers are influenced by spraying conditions and the formation of mixed carbides. Paper text in German.