Abstract Three different coatings were deposited using the Detonation Gun Spraying (DGS) technology from steel powders alone, and steel powers mixed with Fe3C and SiC particles, respectively. The microstructural characteristics of these coatings were examined and the hardness of each type of coating was studied. The morphology and structure of the feedstock powders were affected by the exposure to high temperature during the spraying process and rapid solidification of steel powders that resulted in the formation of an amorphous structure. The unreinforced steel coating had the highest hardness among the three types of coatings, possibly due to a higher degree of amorphization in the coating compared to the other two samples. The microstructural observation confirmed the formation of dense coatings with a layered structure with good connectivity between layers with minimum defects and porosities in the interfacial regions.

Abstract The effect of martensitic phase transformation on cavitation erosion resistance for a deposited layer prepared from a Fe-8Cr- C-1.5Al-Ti flux-cored wire of metastable steel was studied. A reference material of AISI 316L stainless steel was used as a substrate. Cavitation tests were performed using a modified ultrasonic tester. X-ray diffraction was used to examine the phase transformation before and after cavitation tests. Also, the eroded surfaces of specimens were investigated by optical microscope (OM), scanning electron microscope (SEM), and 3D optical profilometer. The cavitation results revealed that the deposited layer exhibited a resistance to cavitation erosion approximately 10 times higher than the AISI 316L steel due to the martensitic phase transformation occurring during the cavitation process. The phase transformation plays a main role to minimize the cavitation damage of specimen. This is due to the fact that it contributes to obstructing movement of dislocations and increasing the hardness as a result of the increased hardening on the surface.

Abstract The thermal phase stability of plasma sprayed TBC is presented and discussed. TBC phase transformation after various isothermal heat treatment processes was studied via Xray diffraction (XRD) and Raman Spectroscopy methods. The YSZ-Gd-Yb and 20YSZ TBCs demonstrated superior thermal phase stability as compared with the high and standard purity 8YSZ TBCs, although higher purity also helps delay the tetragonal to monoclinic phase transformation. The phase transformation appears to be suppressed by cooling at higher rates. This data presents a qualitative phase stability comparison between the various coatings. However, cooling rate has to be taken into consideration in determining the extent of phase instability during the coating design of aero turbine applications.

Abstract Tungsten carbide coatings are often applied to improve surface properties such as wear, high temperature degradation, and corrosion resistance. Zirconia coatings have also been used extensively in various industries due to their excellent tribological and insulation properties combined with high stiffness. It is speculated that adding zirconia to tungsten carbide may result in a coating with combination of excellent thermal and mechanical properties of constituents. In the current study, a powder mixture of 50 wt. % WC-Ni and 50 wt. % ZrO2-Y2O3 deposited on a low carbon steel substrate using atmospheric plasma spray technique. The microstructural evolution of deposited sample was investigated. Splat boundaries, micro cracks, pore morphology conversion, and grain growth mechanism were elucidated comprehensively. Results indicated a good adhesion between two different major components. No porosity formed due to mismatch between zirconia and tungsten carbide. This study pays special attention to the dependency of the microstructural characteristics to the phase distribution within the coating.

Abstract The mist cooling technique was developed and applied to various thermal spraying guns. For example, aluminum-magnesium coatings prepared using a gas flame thermal spraying gun with mist cooling had superior anticorrosion characteristics. Stellite coating thermally sprayed with mist cooling had higher anti-cavitation-erosion characteristics. Next, we endeavored to develop high velocity oxygen fuel guns with mist gas cooling to improve high-temperature toughness.

Abstract The effect of substrate template effect on the crystalline structure of plasma sprayed 8YSZ (8mol%Y2O3) splats was investigated by high resolution transmission electron microscopy (HR-TEM) examination of FIB-processed splat samples. 8YSZ splats were deposited by the atmospheric plasma spraying (APS) on the polished sintered tetragonal structure substrate (3YSZ) and cubic structure substrate (8YSZ) at different preheating temperatures. The focused ion beam (FIB) was utilized to prepared TEM cross-sectional sample of splats. The crystalline structures of both the splat and the underlying substrate were examined by HRTEM. Results showed that the 8YSZ splats deposited on the polished sintered cubic structure 8YSZ substrate at a substrate surface temperature of 900°C exhibited cubic structure and the epitaxial grain growth was confirmed between the crystalline of splat grain and immediately underlying cubic crystalline substrate grain. Moreover, epitaxial grain growth was confirmed between the crystalline of splat grain and the tetragonal structure substrate when substrate surface temperature was increased to 1200°C. The present results suggest that the crystalline structure formation of 8YSZ splats produced by plasma spraying was affected by the substrate template effect.

Abstract Different directions characterize the advances being made in thermal spraying today. On the one hand, spray processes are becoming colder, on the other hand, high-performance systems are being designed which enable a higher powder throughput, thus making production faster and more efficient. The current spectrum of substrate materials is much broader and even more versatile, as can be seen with new materials which prevent thermal stresses from arising during the production process. More and more applications require the use of special cooling methods to increase the cooling efficiency and, in turn, optimize the process. An ongoing objective of the gas industry is to offer the user hardware which not only exploits all the advantages of CO2, for example, but which is also suited to new applications. The Need for Cooling

Abstract Cavitation erosion is a mass loss process that occurs in a component subject to a liquid pressure variation. The mass loss phenomena occurred by extensive microstructure deformation with combination of shock loading and fatigue caused by the impact and collapse of the bubbles at surface. Many studies have been done to evaluate the cavitation resistance of the thermally sprayed coatings. Oxide formation, microstructure and tensile adhesion are important characteristics for coatings against cavitation. In this work some Fe-Mn-Cr-Si alloys, that is a class of steel with strain induced phase transformation, were used to evaluate the chemical composition influence on the microstructure, oxide formation, chemical composition and tensile adhesion of HVOF coatings. It was observed a significant reduction on the porosity, from 3% to 0,1%, and area fraction oxide from 15% to 7%, with Boron and Nickel addition. A combination of Nickel and Boron addition improve the wettability of the splats with increase on the tensile adhesion of the coatings, up to 60MPa, otherwise, higher levels of Boron content reduce the adhesion of the coatings.

Abstract La0.58Sr0.4Co0.2Fe0.8O3-δ (LSCF), deposited on a metallic porous support by means of plasma spray-physical vapor deposition (PS-PVD) is a promising candidate for oxygen-permeation membranes. However, after O2 permeation tests, membranes show vertical cracks leading to leakage during these tests. In this work, a feature leading to crack formation has been identified. More specifically; Membrane residual stress changes during thermal loading have been found to be related to a phase transformation in the support. In order to improve the performance of the membranes, the metallic support has been optimized by applying an appropriate heat treatment. Additionally, it has been found that coatings deposited at lower oxygen partial pressures consist of 70% cubic and 26% rhombohedral perovskite phases. This increases the non-stoichiometry, which drives the formation of non-perovskite phases during annealing, affecting the membrane stability and the ionic conductivity. The amount of oxygen added during spraying can be used to suppress the cubic to tetragonal phase transformation.

Abstract Two directions characterize the advances being made in thermal spraying today. On the one hand, the spray processes are becoming colder, as with cold spraying which facilitates the production of oxide-free coatings in the atmosphere. On the other hand, high-performance systems are being developed which enable a higher powder throughput, thus making production faster and more efficient. This development has made it more important than ever to maintain this time-saving advantage and not lose it as a result of cooling intervals. More and more applications require the use of CO 2 as a cooling medium to increase the cooling efficiency and, in turn, optimize the process. An ongoing objective of the gas industry is to offer the user hardware which not only exploits all the advantages of CO 2 , but which is also suited to new applications.

Abstract The phase transformation and reaction of ZrO 2 -CaO- ZrSiO 4 and ZrO 2 -Y 2 O 3 -ZrSiO 4 coatings with manganese oxide at 1273 K were investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). SiO 2 phase formed in the coatings, which was from the ZrSiO 4 decomposed and easy react with manganese oxide or CaO. SiO 2 has precedence over react with CaO than manganese oxides for ZrO 2 -CaO-ZrSiO 4 coatings, and which result in to promote t-m phase transformation. On the contrary, the reaction between SiO 2 and MnO is primary for the ZrO 2 -Y 2 O 3 -ZrSiO 4 coatings and result in the damage or exfoliation on the surface of the coatings.

Abstract The nanostructured TiO 2 deposits can be synthesized through flame spraying with liquid feedstock based on our previous result. The deposit consisted mainly of anatase TiO 2 with a small fraction of rutile TiO 2 . But the crystalline size of rutile TiO 2 was larger than that of anatase TiO 2 . To clarify the phase formation during the deposition, the nanostructured TiO 2 deposits formed by flame spraying with liquid feedstock were annealed at different temperatures from 200 to 800 °C. The microstructure of the as-deposited and annealed TiO 2 deposits was characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and thermal analysis. The transformation of the anatase crystalline to rutile crystalline in the nanostructured TiO 2 was also examined through investigating photocatalytic performance of the TiO 2 coatings to degrade the acetaldehyde under the ultraviolet illumination. XRD analysis showed that the significant transformation of anatase crystalline to rutile crystalline occured at the temperature higher than 600 °C. It was found that the grain size of the rutile phase was larger than that of the anatase phase in the deposits annealed at different temperatures. The deposits annealed at temperature lower than 450 °C presented a good photocatalytic activity. However, although the deposit annealed at temperature of 500 °C contained about 95% anatase crystalline, it became photocatalytically inactive. Despite the anatase contents from 95% to 0% in the deposits annealed at temperatures higher than 500 °C, those deposits presented no significant difference in the photocatalytic activity. On the other hand, thermal analysis suggested that the phase transformation of anatase to rutile occurred at 400 to 500 °C. Based on the experimental findings, a model for the phase formation in flame-sprayed nano-TiO 2 deposit with liquid feedstock and transformation of the nanostructured TiO 2 is proposed.

Abstract It has been said that plasma-sprayed ceramics particles are often supercooled before the impact on substrate. Some numerical models of the droplet impact actually included the supercooling effects. However, there is no report that has experimentally confirmed the effects on splat morphology. Therefore, in this research, we have mainly investigated the supercooling effects on splat morphology as well as splat microstructure. To achieve this, we developed an in-situ measurement technique utilizing radiation from a melt particle to monitor the impact of single particle successively under plasma spraying. The system was able to identify each single particle, which enabled us to correlate the splat morphology with impact velocity and thermal history of each particle during the impact. Yttria-stabilized zirconia powders were sprayed onto quartz glass substrate by the argon-hydrogen dc-rf hybrid plasma under atmospheric pressure. Waveforms of emissions and thermal history obtained during the impact were precisely analyzed. Especially, we closely examined thermal history during particle spreading to find the recalescence. In addition, splat morphologies were examined statistically in relation to their thermal histories. Based on the measurement, we also evaluated the viscosity of zirconia, cooling rate, and thermal contact resistance experimentally.

Abstract It is usually an advantage if the particle temperature during arc spraying is very high in order to achieve adequate adhesion. In the capacitor industry, however, the material to be coated is very sensitive and damage occurs at temperatures between 120 deg C and 180 deg C, that is, well below the melting point of the materials to be sprayed. Freshly developed arc spraying systems enable spraying materials to be processed at the lowest possible temperatures, which is presented in this article. This discussion attempts to strengthen the readers' background on the production steps used for capacitor end spray. Paper includes a German-language abstract.

Abstract In this paper, a quantitative model for determining the viscous behavior of two-phase particles when they hit the substrate is derived and used for the production of silicon-nitride composite layers. Plasma spray deposition is carried out using a Sulzer-Metco unit and powder feed system in which the powder is injected into the jet external to the nozzle. Silicon nitride-YAS layers with low melting points are developed so that the YAS matrix successfully protects the silicon nitride from decay. The model is developed from the observed layer behavior and provides a basis for the further development of ceramic-matrix composite layers. Paper includes a German-language abstract.

Abstract Ni and Ti aluminides show a high potential for lightweight applications at elevated temperatures. The strength of TiAl can be increased, if directionally solidified structures are producable. Recent developments of superalloys show that the laser rapid prototyping can be used to generate directionally solidified parts. To generate directionally solidified TiAl parts, first the problem of crack formation has to be solved. The solution was developed by the use of Ti48Al2Cr alloy and an adjusted cooling of the sample. Second the process parameters have to be chosen in a way, that directional solidification occurs. For this a macroscopic process simulation including the modeling of the feedstock material and a microstructure simulation is build up. The correlation of experiments and simulation will lead to process guidelines for laser rapid prototyping of directionally solidified TiAl parts. Paper text in German.

Abstract The spreading and solidification of a molten droplet is governed a number of factors such as fluid inertia, viscosity, surface tension, surface wettability, heat transfer between the droplet and substrate, and surface thermal properties. Most numerical and experimental studies of droplet impact have considered a single molten droplet landing on a flat, solid surface. This paper investigates the sequential deposition of tin droplets with their centers offset using a three-dimensional model of droplet impact and solidification. The paper presents results from simulation of the successive deposition of two tin droplets onto a stainless steel substrate under conditions for which the experimental observations are available. Results from the model are compared with photographs of impacting droplets and found to agree well. Paper includes a German-language abstract.

Abstract This paper presents the results of laser treatment of aluminum oxide + 3wt.% titanium dioxide ceramic coatings deposited by detonation spraying. The solidification process during the laser melting of detonation-sprayed layers made of ceramic powder with a layer thickness of 100 micrometer is investigated. A stainless steel is selected as the substrate, on which a nickel adhesive layer, which oxidized at 923 K in air for one hour, is applied. A carbon dioxide laser is used for melting. Both the sprayed and remelted layers are examined by means of scanning electron microscopy and X-ray diffractometry. The microstructure changes significantly due to the laser remelting. The originally laminar structure is transformed into a fine, pore-free columnar structure in which the grains are oriented perpendicular to the interface between the layer and the substrate. The melted-in zones contain alpha-aluminum oxide as the main phase and are characterized by high micro-hardness. Paper includes a German-language abstract.

Abstract The effect of substrate characteristics on the formation of plasma-sprayed alumina splats was studied using both experiments and numerical simulation. Knowledge of the particle and substrate conditions is critical in understanding coating formation and in validating computational models. The size, velocity and temperature of the alumina particles prior to impact were measured using a particle in-flight diagnostic system. Experiments were performed on two substrate materials: stainless steel and glass. Substrate temperatures were varied in a range of 20-500°C and controlled with an electric heater. For each substrate material, a transition temperature was observed above which there was no fingering/splashing and the splats had a circular disk shape. A 3D computational model of free surface flows with heat transfer and solidification was used to simulate the impact of alumina particles in conditions given by the experiments. The splat shapes from numerical model were comparable to those of the experiments for hot stainless steel substrate. For a cold substrate, the numerical model did not show any fingering/splashing. In the experiments, however, we observed two types of splat shapes: intensive splashing with no central core and circular disk splat. Substrate surface contamination, not considered in the numerical model, was the probable cause of droplet splashing on the cold substrate.

Abstract Fundamental aspects of a plasma sprayed cast iron coating on an aluminum alloy substrate are investigated in the present study: focusing on the effects of preheat substrate temperature (T S ) and chamber pressure (P C ) on the splat morphology, the adhesive strength of splats, the formation of a reaction layer and graphite. Splash-type splats appear at low T S but disk and star-shaped splats arise at high T S . Deformed substrate ridges, mainly due to the slight surface melting, are formed adjacent to the splat periphery at high T S . At low T S , pores are observed at the splat/substrate interface, which cause a decrease in the adhesion of splats. In contrast, a reaction layer composed of iron, aluminum and oxygen is ready to form at high T S . The amount of graphitized carbon increases in cast iron splats with T S . At a low P C of 26.3 kPa, disk-type splats are in the majority at a constant T S of 473 K. As P C increases, star-shaped splats appear along with disk splats. The flattening ratio of disk splats decreases with the increase of P C , because of a decrease in the kinetic energy and temperature of molten droplets. An interfacial oxide layer composed of iron, aluminum and oxygen is ready to form at high P C . The number of pores intensively increases with P C , which leads to a decrease in the adhesive strength of splats. The amount of formed graphite in cast iron splats slightly increases with P C , however, that of a rapidly solidified phase of Fe-Si-C decreases because of lowering of the solidification rate.