Abstract In this work, a novel liquid fuel HVOF process fueled with ethanol was used to prepare 75wt%Cr 3 C 2 –25wt%NiCr coatings on AISI304 stainless steel substrate. Taguchi method was employed to optimize the spray parameters (ethanol flow rate, oxygen flow rate, powder feed rate and standoff distance) to achieve better erosion resistance at 90° impact angle. The results indicated that ethanol flow rate and oxygen flow rate were identified as the highly contributing parameters on the erosion wear loss. The important sequence of the spray parameter is ethanol flow rate > oxygen flow rate > standoff distance > powder feed rate. The optimal spray parameter (OSP) for minimum erosion wear loss was obtained under ethanol flow rate of 28slph, oxygen flow rate of 420slpm, powder feed rate of 76.7 g/min and standoff distance of 300mm. The phase composition, microstructure, hardness, porosities, and the erosion wear behaviors of the coatings have been studied in detail. Besides, erosion wear testing of the optimized coating was conducted at 30°, 60° and 90° impact angle using air jet erosion testing machine. The SEM images of the erodent samples were taken to analyze the erosion mechanism.

Abstract In this paper, the phenomenological behaviour of gas flow and particles motion during cold spraying has been studied. Observations of particles behaviour show two features: a uniform jet over a short distance ahead of the nozzle exit and then, a progressive dispersion. These behaviours are explained using a computational analysis based on a direct numerical simulation of the gas flow and the kinematic interactions with the particles. The CFD computation demonstrates that the gas stream starts to be unstable inside the nozzle with more turbulence as it moves towards the exit of the nozzle. The flow is self-oscillated along the flow direction and drives the motion of the Cu particles outside the nozzle. The zone of gas flow instability does correspond to the zone of experimental particle dispersion. Outside the nozzle, the particles form a straight jet over a certain distance that corresponds to the zone of the experimental uniform particles jet. Then, they are deviated and become more and more dispersed towards a very sparse jet along the flow direction. This phenomenon is explained by a Magnus lift force that deviates the particles trajectory when the gas flow becomes highly turbulent while developing a vorticity shedding.

Abstract The denser structure of HVOF-sprayed aluminum layers compared to APS-sprayed ones opens up high-quality application possibilities, especially in the form of electrical layers in semiconductor applications. This paper presents the effect of fuel gas/oxygen ratio on melting level of the particle, total gas flow, and particle flux in terms of coating thickness per pass on the electrical properties of the coatings. A large number of spray parameters are changed for more detailed investigations. Splats are created to investigate the relationship between diagnostic data, melting behavior, and the interactions between droplets and substrate in the case of aluminum particles. Paper includes a German-language abstract.

Abstract This paper aims to check the informative value of two fluid dynamic computer programs (CFD) for predicting correct trends with regard to different particle speeds and temperatures under changing operating conditions. The variety of numerical models demands a theoretical understanding of the atmospheric plasma spraying process as well as an experience of CFD. An investigation was carried out according to a simple test plan with two different operating parameters, current strength and gas flow rate. The result from the two calculations show a different degree of accuracy for both calculation models but allow a sufficient prediction of tendencies and can be used to optimize and control atmospheric plasma spray processes in industrial applications. The results were compared with experimental data and specifically the more advanced model was found capable of predicting the variation in particle characteristics when operating conditions where varied. Paper includes a German-language abstract.

Abstract In this paper, effect of flow regime on particle velocity is examined and experimental validations are performed accordingly. The flow rate of two-phase particle streams (gaseous and solid) in the HVOF process is analyzed. The dispersing flow and the gas flow are taken into account in the Euler equation, whereby most of the physical aspects of the gas particle flow are included. The simulation is carried out for typical operating conditions in which the powder load was high and the two-phase flow near the injection point was not weakened. The simulation showed large scattering of the flow in regions with the greatest number of particles. The tests carried out for confirmation showed that, when the flow changes due to a change in the relative Mach number, significant changes occur in the air resistance of the particles, and a corresponding decrease in the application rate is associated with this. Paper includes a German-language abstract.

Abstract Recently it has been suggested that the carrier gas jet interaction with the plasma can have a large effect on the resulting particle temperature. The postulated interaction is through deflection of the main plasma jet and by delaying the heating of particles by the formation of a "cold" gas bubble. We have examined the effect of the gas jet itself on the temperature of the particles by attempting to artificially form a cold gas bubble using a separate, closely oriented gas jet. The effect of the "twin" co-flowing jet was evaluated by measuring its effect on the mean and standard deviation of the particle injection velocity and the resulting spray pattern and particle temperature. Additionally we have used alternative carrier gases with similar density but with specific heats that are higher than argon by a factor of two. A measurable but minor effect on particle temperature is observed.

Abstract Conventional DC plasma torch designs lead to a circular cross-section of the emanating plasma jet. Consequently in surface treatment applications the plasma jet hits the substrate within a limited circular working area. Large scale workpieces therefore have to be scanned resulting in a time-consuming procedure. The innovative DC plasma torch system LARGE is characterized by the arrangement of the anode and the cathode opposite to each other on a common axis with a variable distance. The central body of the torch between the electrodes is divided into electrically insulated cascade plates. The plasma gas is injected perpendicular to the torch axis. Passing through the arc, the gas is transferred to the plasma state and leaves the torch laterally through a slit as a plasma jet with extended stripe width. The plasma torch LARGE is investigated by electrical, optical and enthalpy probe diagnostics. Shrouding the electrodes with an inert gas and feeding reactive gas mixtures as main plasma gas allow the torch to be used for plasma chemical reactions, too. Preliminary applications focus on preheating, surface modification of paper and plastic materials as well as on sterilization of nutrition packaging. The capability of plasma enhanced CVD is examined experimentally.

Abstract Spectroscopic and electrostatic probe measurements were carried out to understand the plasma feature inside and outside a 10-kW-class direct-current arc plasma jet generator with a supersonic expansion nozzle. Ammonia and a mixture of nitrogen and hydrogen were used as the working gas. The NH3 and N2+3H2 plasmas in the throat were expected to be nearly in a temperature-equilibrium condition, although the plasmas in the expansion nozzle and in the downstream plume without substrate plates were in thermodynamical nonequilibrium states. As a result, the H-atom excitation temperature and the N2 rotational excitation temperature decreased from 7000-11000 K in the throat to about 4000 K and to 1000-1500 K, respectively, on the nozzle exit at 0.1-0.2 g/s, although the NH rotational temperature did not show an axial decrease even in the nozzle. On the other hand, each temperature was almost kept a small range in the downstream plume without substrate plates under an ambient pressure of 130 Pa except for the NH rotational temperature for NH, working gas, although in the case with a titanium plate the nonequilibrium plasma came to a temperature-equilibrium one as approaching the plate.

Abstract The industrial flame spraying process has been analyzed by three-dimensional Computational Fluid Dynamics (CFD) simulation. The actual process is employed at the Volvo Aero Corporation for coating of fan and compressor housings. It involves the Metco 6P gun where the fuel, a mixture of acetylene and oxygen, flows through a ring of 16 orifices, while the coating material, a powder of nickel-covered bentonite, is sprayed through the flame with a stream of argon as a carrier gas by a central orifice. The gas flow was simulated as a multi-component chemically reacting incompressible flow. The standard, two equations, k-e turbulence model was employed for the turbulent flow field. The reaction rates appeared as source terms in the species transport equations. They were computed from the contributions of the Arrhenius rate expressions and the Magnussen and Hjertager eddy dissipation model. The particles were modeled using a Lagrangian particle spray model. In spite of the complexity of the system, the complex geometry and the numerous chemical reactions, the simulations produced fairly good agreement with experimental measurements. The powder size distribution was found to play a critical role in the amount of unmelted fraction of particles. The modeling approach seems to give a realistic description of the physical phenomena involved in flame spraying, albeit some model refinement is needed.

Abstract Today, powder particles diameter used for thermal spraying is generally comprised between 5 and 100µm with a preferred range around 40µm for APS applications. Actually, the future trends in plasma spraying are directed to the use of fine or ultrafine powders and the reduction of the steps between raw materials and coatings. So, the present paper investigates the way to use directly spray dried ceramic powders in suppressing the sintering stage. AI2O3 based powders were obtained by the spray drying process. By optimizing the parameters (slurry composition and injection as well as drying characteristics), a narrow grain size distribution was achieved. Chemical composition and shape of synthesized powders were analyzed by scanning electron microscopy (SEM) equipped with energy dispersive spectrometry (EDS). The crystallographic structure was identified by X-ray diffraction (XRD). Demonstration was made that it is possible to obtain coatings using directly spray dried ceramic powders. The plasma spray process parameters (such as current intensity, gas flow rate, powder feed rate and injection mode, cooling stage,...) have to be managed to achieve cohesive coatings. The structure and chemical composition of these coatings were studied. In this way, the direct use of spray dried powders appears as a promising way to realize ceramic coatings.

Abstract The quality of plasma sprayed coatings depends strongly on substrate surface preparation, especially roughness, grit residue, and oxidation stage; particle spray jet position and size relative to the plasma jet; impacting particle distribution; particle velocity, temperature, and size prior to impact; substrate temperature; and pass thickness. A simple and low-cost spray and deposit control system developed in our laboratory allows to monitor on-line the position, shape, and centroid of the hot particle spray jet. Such a tool has proved to be very sensitive to any drift in powder injection conditions and torch input parameters. Although it gives no direct information on particle velocity and temperature, this system can be easily implemented in an industrial environment and help to maintain constant the particle parameters during spraying. A CCD camera is used in conjunction with a pyrometer making it possible to measure simultaneously substrate temperature. The system can monitor coating parameters such as deposition efficiency and residual stresses. This paper describes how the system can be used to set the tolerance range of process input parameters to obtain coating parameters within given specifications.

Abstract This paper provides a summary of conventional thermal spray powder feeders, examines the influence of carrier gas flow, powder tube diameter, powder tube type, and gas flow path on the feed precision and continuity of pressure-type powder feeders, and discusses the cause of pulsation associated with the injection of super fine ceramic powders.

Abstract In dc plasma spray guns, the properties of the plasma forming gas largely control the characteristics of the plasma jet and the momentum and heat of the particles injected into the flow. This paper examines the effect of Ar-He-H2 mixtures on the dynamic and static behavior of plasma jets expressed in terms of arc voltage and gas velocity. Correlations between these parameters and operating variables (arc current, gas flow rate, volume composition) were established from a dimensional analysis and supported by the calculation of the thermodynamic and transport properties of the gas mixtures used in the study.

Abstract Plasma spraying of metals and metallic alloys performed in controlled atmosphere or soft vacuum results in coatings with a low oxidation level and excellent thermomechanical properties. Unfortunately, the spraying cost is drastically increased by one or two orders of magnitude compared to air plasma spraying (APS). Thus the minimisation of oxidation during APS is a key issue for the development of such coatings. Oxygen concentrations sucked into plasma jets have been measured by an enthalpy probe linked to a mass spectrometer. This technique allows to determine simultaneously plasma composition, temperature and velocity distributions within the plasma plume. Results have been compared to those obtained with a two-dimensional turbulent flow model. The obtained results have shown that surrounding air entrainment is reduced when using adequate Ar/H2/He mixtures which viscosity is higher than that of Ar/H, mixtures, limiting the turbulence in the jet fringes and pumping of the surrounding atmosphere.

Abstract A high-speed video recording system was developed to measure the velocity and temperature of thermal spray particles in flight. The system consists of three image sensors and a cubic beam-split prism that directs light, based on color or wavelength, to three imaging planes. In this study, one of sensors is used to measure particle velocity while the other two measure particle temperature. The prospect of simultaneously measuring the temperature and velocity of individual particles in flight is discussed along with the insight it provides on the formation of spray coatings.

Abstract The conditions of particle injection into the side of plasma jets play an important role in determining the microstructure and properties of sprayed deposits. However, few investigations have been carried out on this topic. The current work presents the results of an experimental and computational study of the influence of injector geometry and gas mass flow rate on particle dynamics at injector exit and in the plasma jet. Two injector geometries were tested: a straight tube and a curved tube with various radii of curvature. Zirconia powders with different particle size range and morphology were used. A possible size segregation effect in the injector was analyzed from the space distribution of particles collected on a stick tape. The spray pattern in the plasma jet was monitored from the thermal radiation emitted by particles. An analysis of the particle behavior in the injector and mixing of the carrier-gas flow with the plasma jet was carried out using a 3-D computational fluids dynamics code.

Abstract The demand for highly continuous thermal spray deposition processing to gain manufacturing efficiency and enhanced control of deposition microstructure has driven the search for powder feedstocks that exhibit uniform flow behavior in pneumatic feeding devices even at low carrier gas flow ratios. This paper compares the continuous powder feeding characteristics of one type of feeding device for two different powder alloys fabricated by a representative set of powder processing methods, including spray dried, fused (cast) and crushed, and inert gas atomized. Powders were fabricated by both commercial vendors and Ames Laboratory processing equipment. The powder mass flow rate was found to depend directly on the apparent density of a specific powder type instead of the dry powder flowability, as initially expected.

Abstract The given article presents some results of the scientific research devoted to the development of a new class of scale-resistant powder materials of the Si-Ti-Mo-B system for thermal spraying and using these materials for the creation of heat-resistant coatings on the niobium base alloys by means of various methods of thermal spraying. Also under consideration are problems relating to the theory and practice of obtaining reliable protective coatings on high-melting metals and their alloys, niobium ones included, intended for operation in high-enthalpy oxygen-containing gas flows. Hazard in commencing an oxidation reaction of the base material under coating is connected with density of open pores and cracks, and partial pressure of the oxidizer. Powdered multicomponent heterophase materials for gas-thermal spraying of protective coating with a self-healing ability and controlled properties are proposed. Finally the results of some properties of new silicide-type heterophase powders containing silicide and boride phases for a thermal spraying process and some properties of protective coating deposited on the niobium base alloys by means of a thermal spraying technique are presented.

Abstract Current HVOF systems available on the world market are relatively complicated to operate and expensive. At the Bauman State Technical University in Moscow (BMSTU) a new portable HVOF spraying and cutting system MiniJet-30 has been developed. This rugged system offers the ease of operation of conventional gas welding equipment and produces high quality HVOF coatings. In this paper, the results of numerical simulation calculations of supersonic gas/powder flows, with and without a particle mask, are presented and compared with experimental outcomes. Additionally, typical HVOF coatings were sprayed and evaluated for bond strength, porosity, microhardness, abrasive wear, and phase composition (OM, XRD). The results are compared to those of other HVOF systems. Lastly, the cutting efficiencies for stainless steel, cast iron, and non-ferrous metals are discussed.

Abstract his paper examines the influence of nozzle bore diameter and plasma gas flow on the characteristics of plasma torch spraying. Large bore size allows high plasma gas flow rates to be achieved without an excessive increase in voltage, which is apt to damage the cathode. High flow rates (Ar: 1.3x10-3, H2: 1.3x10-4, He: 2.7x10-4 m3/s) are effective in imbuing more power into the plasma without the damage associated with higher chamber pressures. The relationships among key process parameters, including effective plasma power, electrical efficiency, chamber pressure, and plasma gas flow, are expressed qualitatively in the paper based on multiple regression analysis.