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 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 A three-dimensional model of free-surface flows with heat transfer, including solidification, was used to model the build-up of a coating layer in a thermal spray process. The impact of several nickel particles on a stainless steel plate in different scenarios was considered. Particles diameter ranged from 40 to 80 µm and their impact velocity ranged from 40 to 80 m/s. Particles were initially super-heated; their temperature ranged from 1600 to 2000°C. Fast growth of solidification was found to be one cause of particle splashing in thermal spray coatings. Different splat morphologies obtained from the numerical model were comparable with those obtained from the experiments. Simulation of the sequential impact of two nickel particles showed side-flow jetting and particle splashing observed in experiments. The numerical model proved to be capable of simulating different impact scenarios that occur in a thermal spray; this was demonstrated by simulating nine consecutive particles during their impact on the substrate. Several characteristics of a coating layer build-up such as particle splashing and formation of small satellite droplets and rings around the splat could be seen in the numerical results. Particle splashing is one possible cause of porosity formation in thermal spray coatings.

Abstract A model for oxidation of molybdenum particles during plasma spray deposition is developed. The diffusion of metal an-ions or oxygen cat-ions through a thin oxidized film, chemical reactions on the surface, and diffusion of oxidant in gas phase are considered as possible rate-controlling mechanisms with controlling parameters as the temperature of the particle surface, and local oxygen concentration and flow field surrounding the particle. The deposition of molten particle and its rapid solidification and deformation is treated using a Madejski-type model, in which the mechanical energy conservation equation is solved to determine the splat deformation and one-dimensional heat conduction equation with phase change is solved to predict the solidification and temperature evolution. Calculations are performed for a single molybdenum particle sprayed under the Sulzer Metco-9MB spraying conditions. Results show that the mechanism that controls the oxidation of this droplet is the diffusion of metal/oxygen ions through a very thin oxide film. A higher substrate temperature results in a larger rate of oxidation at the splat surface, and hence, a larger oxygen content in the coating layer. Compared to the oxidation of droplet during m-flight, the oxidation during deposition is not weak and can become dominant at high substrate temperatures.

Abstract The crystal structure of the coating depends on the splashing and solidification of the sprayed particles. The splashing of the particles is simulated by commercial software taking temperature dependent parameters like melt viscosity and surface tension into account. This article provides the theoretical background and the experimental verification. The steps of simulation solidification, splashing, and heat and momentum transfer help to develop novel coating systems with defined microstructure and shorten the time to market. Experimental investigations were carried out using Ni and Ni/Cr 80/20 spray powders with a mean particle diameter of about 15 μm. The theoretical investigations to describe the flattening process of a droplet show the influence of particle velocity and temperature. It was shown that different material parameters, as latent heat or surface energy, have a significant influence on the simulation results.

Abstract Some applications of thermally sprayed coatings need a metallurgical bonding of substrate and coating. This can be reached by laser remelting of a thermally sprayed coating, which causes, on the other hand, a certain dilution of the substrate elements into the coating. This article discusses the influence of reaction enthalpies on the microstructure formation in the alloying systems Ni-Al and Ti-Al. Experimental work and simulation were done to examine the time constants of solidification influenced by laser dwell time and reaction enthalpy. It was observed that, for short dwell times, the reaction heat dominates the solidification process and the microstructure formation.

Abstract Individual splats are the building blocks of any thermal spray coating. Near the coating-substrate interface, they affect coating properties like adhesion strength. This article examines the effect of substrate heating on droplet splashing. Nickel powder was plasma-sprayed onto a polished stainless steel substrate at various temperatures and the resulting splats were analyzed. Droplet splashing was observed experimentally for three different cases: low substrate temperature, high substrate temperature, and droplet-splat interaction. Mechanisms for splashing were explained with the help of computer-generated nickel droplet impacts. The article proposes that the jetting of molten metal is not triggered by the formation of a central splat but rather a solidified ring on the periphery of the splat. It was observed that, on substrates below 350 deg C, splashing is triggered by solidification at the edge of the spreading droplet. Interactions with previously deposited splats also cause droplets to splash.

Abstract Many studies have been devoted to particle flattening and resulting splat cooling. However, if recent models allow to compute the particle flattening time evolution, very few experiments in spraying conditions have been achieved to back such calculations. The aim of this paper is to describe an imaging device allowing the visualization of particle impacts on cold and hot surfaces. This technique makes it possible : • to investigate the "impact mode" : splashing, deposition or rebound, • to link the particle parameters at impact and the substrate parameters to the observed impact mode, • and therefore, to have a better understanding of coating formation. It consists in a controlled atmosphere chamber where is followed the impact of a single particle on a substrate which can be inclined. The particle parameters prior to its impact are measured : its surface temperature by fast (100 ns) two-color pyrometry, its velocity and diameter by Phase Doppler Anenometry (PDA). The particle image during flattening, splashing or rebounding is given by a fast camera (exposure/delay time 100ns to 1ms) with possible multi exposures. The camera is triggered by the PDA and/or the pyrometer. It is then possible to calculate for each molten particle its Sommerfeld parameter characterizing its impact mode (rebounding, deposition or splashing) when no solidification occurs during flattening. The substrate are made of stainless steel 304L rapidly covered by alumina splats resulting in a Ra~5-6µm. They are kept at 300°C, temperature at which splats are disk shaped on smooth substrate (Ra<0.05µm). The very preliminary results obtained show that unmolten or partially molten particles rebound in all directions but not elastically : the rebounding particle velocity is 3 to 5 times lower than that of the impacting one. For fully molten particles, splashing occurs in all cases even for low Sommerfeld numbers. It thus seems that the substrate roughness plays a key role in splashing.

Abstract Stoichiometric cordierite (2MgO-2Al2O3-5SiO2) with additions of titania have been investigated for use as coatings on low thermal expansion refractory concretes. These concretes have coefficients of thermal expansion on the order of 2 ppm/°C. Titania additions of up to 8 mole percent were investigated and the effect of titania in the crystallization of cordierite was examined. Cordierite coatings were air plasma sprayed and both glass and crystalline coatings were produced. The crystalline structure of the coating was found to be dependent upon the preheat temperature of the substrate. Preheats greater than 700 °C produced a mixture of a quartz solid solution and indialite whereas glass coatings were produced at preheats less than 700 °C. Coefficients of thermal expansion for the cordierite materials were dependent upon titania addition and generally increased with addition of titania. In the glass state, the thermal expansion was modestly increased (4.6 to 4.9 ppm/°C) with titania additions, but the quartz and indialite forms of the cordierite increased from 1.2 to 4.7 ppm/°C as the titania addition increased from 0 to 8 mole percent.

Abstract A brief feasibility study was performed to produce thermal spray coatings using gas atomized powders of Cu47Ti34-xZr11Ni8Six, where x=0 and 1. These alloys have previously been shown to be capable of forming metallic glasses having thick (1-2 cm) cross sections because they can be cooled from the melt at relatively low cooling rates (e.g., 100-102Ks-1). The properties of these metallic glasses include high strength, high elasticity and high fracture toughness. Amorphous plasma arc sprayed coatings were produced which were close in composition to the starting powders, and exhibited comparable glass transition and crystallization behavior. The amorphous structure of the as-sprayed coatings was used as a source for forming a range of partially devitrified and fully crystallized structures. The average hardness of the coatings increased from around 6 GPa to near 10 GPa as the degree of crystallization increased.

Abstract Layers of high-purity copper and iron produced by cold gas-dynamic spraying have been thermally processed to induce recrystallization and grain growth. In the case of copper deposits, the as-sprayed structure could be "pinned" by arrays of Cu2O particles present on the surfaces of the feedstock powder, however copper powders of higher purity and sphericity yielded sprayed structures which could be annealed to induce recrystallization and grain growth. The higher purity copper compacts exhibited a morphological change in fracture from a brittle, intraparticle mode in the as-deposited condition, to a ductile, "cup-and-cone" morphology in the annealed condition. For compacts produced from water atomized iron, annealing at sub-critical temperatures produced recrystallization and grain growth as found with copper, and thermal processing in the austenitic region resulted in altogether new and coarser grain structures upon cooling. Ease of thermal processing of cold-sprayed materials may offer additional processing routes for engineered surfaces and functional devices produced in this manner.

Abstract This paper investigates the oxidation that occurs during the flight movement of a powder particle and during the spatter solidification in the thermal spray process. The effects of oxidation on droplet flattening, on the mechanical and thermal interactions between spatter and substrate, on spatter morphology, on porosity, and on adhesion are studied. The influence of wetting and oxygen dissolution is analyzed. The experimental results show that during High Velocity Oxy-Fuel spraying of the chromium carbide-nickel-chromium powder, the relative mass of chromium oxide in the coating is about 4.95%. The theoretical results agree well with the experimental observations. Paper includes a German-language abstract.

Abstract In this paper, the flattening and the simultaneous solidification of a liquid particle when it hits a solid surface are described mathematically and numerically simulated in cylindrical coordinates on the basis of the Navier-Stokes equations. The heat transfer in the particle and in the substrate is simulated by solving the 2-D heat conduction problem, whereby hydrodynamic processes in the melted particle as well as pressure forces are taken into account. The particle solidification is investigated using the one-dimensional Stefan problem, taking into account the contact heat conduction at the boundary between particle and substrate. For numerical calculations, computational algorithms were created on the basis of the difference method, which were implemented in the form of an applied program complex. Paper includes a German-language abstract.

Abstract This paper presents a one-dimensional heat transfer model which predicts the solidification and cooling of a plasma-sprayed alumina splat after the flattening process is completed. A heterogeneous nucleation process taking place on the substrate surface was assumed. The density and average size of the formed nuclei were determined from the integration of the nucleation rate calculated from the classical kinetic theory for nucleation. This rate depends on the activation energy required for nucleation which takes into account the effect of the surface via a wetting angle between the growing nucleus and the catalytic surface. This contact angle was estimated from the comparison of the computed grain density with the density observed on splat surface using an atomic force microscope. When 67% of the splat surface in contact with the substrate are covered by grains, a planar solidification front was assumed to move through the melt. The theoretical model accounted also for the selection of the crystalline phase. Calculations were performed for various substrate materials at different initial temperatures. Results are expressed in terms of nucleation temperature, nucleation rate, density and grain size distribution.

Abstract A study was performed to examine the effects of starting powder composition, substrate thermal conductivity, and substrate temperature on the composition and structure of individual Al-Cu-Fe splats formed during thermal spraying. The fraction of quasicrystalline phase which formed was found to depend on the chemistry and solidification history of the splats. Due to evaporative loss of Al during spraying, an initial powder composition higher in Al produced splats closer to the desired composition, which yielded more of the quasicrystalline phase. Deposition onto lower thermal conductivity surfaces resulted in an increase in the quasicrystalline phase, as did solidification onto higher temperature substrates.

Abstract Different mechanisms of splashing of droplets impacting onto the substrate surface during thermal spraying are considered. It is shown that supercooling formed in the flattening droplet consists of the thermal supercooling and that arisen due to the high pressure developed upon the droplet impact. Solidification starts when the supercooling exceeds some critical value. With a "cold" substrate when its temperature is less than a transition temperature the marked contribution to the supercooling is due to its high pressure part. In this case a regular disc-shaped splat will be formed in the central part of the flattening droplet and splashing will occur in the periphery. With a "hot" substrate when its temperature exceeds the transition temperature the thermal supercooling is high enough, no splashing occurs and a regular disc-shaped splat is formed. Theoretical results agree with the experimental observations.

Abstract Some of the recent research progress concerning the structure and properties of thermal sprayed coatings are reviewed. Structures of coatings are classified into three classes of hierarchy, i.e., layer structures, inter-splat structures and intra-splat structures. Important progress in the study and coatings development in each class is described. These include coatings developed to take advantage of the microstructure due to rapid solidification, such as amorphous and extension of solid state solubility, and characterization of porosity and how it is affected by process parameters. Then, stress generation during thermal spray is compared between plasma spray and HVOF spray. Particular attention is given to the importance of thermal and mechanical interactions of sprayed particles with the substrate and coating surface, which determine the nature of interlamellar bonding and that of microscopic stress.

Abstract 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.