Improved HVOF spraying with a gas shroud has been developed to fabricate environmental barrier coatings of corrosion resistant alloys such as HastelloyC. For such coatings, control of oxidation of the powder material during spraying is very important and the gas shroud has been effective to lower oxygen content to 0.19mass%. In the present study, further reduction of oxygen content to 0.063mass% was achieved by changing the composition of combustion gas by introducing nitrogen into the combustion chamber. This value is almost comparable to the oxygen content 0.042mass% of the feedstock powder but the porosity of the coating increased. Introduction of nitrogen to the combustion chamber lowered the temperature of the spray particles in flight while maintaining their high velocity. Another coating with 0.14mass% was obtained with open porosity below 0.1vol% by changing the mixing ratio of nitrogen, which exhibited improved environmental barrier property in artificial seawater.

In this paper describes the design of the arc spray gun with secondary atomization, which using the mixture of propane and air as the main gas and the compressed air as the gas for secondary atomization. The test shows that in the high velocity airflow produced by the combustion of propane – air mixture, the atomized particle size is less than 20 micron. In the second- combustion thermal airflow, the high temperature area of the arc beam is obviously lengthened at the same time the combustion gas environment restrains the oxidation of the spraying materials. Because the speeds of particles are improved greatly, the coatings obtained are denser. Through practical application it shows that this system can obtain stable deposition efficiency, high bond strength, and low roughness and greatly reduce the cost of machining. Abstract only; no full-text paper available.

Based on the principle of the liquid-fueled rocket engine, a combustion model is proposed for the HVOF/HVAF system. The combustion gas components and temperature for different mixture fractions were analyzed. At lower oxygen content condition (under-stoichiometry), the combustion temperature is lower and the solid carbon content is higher. The whole fluent flow mode was proposed for the supersonic spray, which consists of the gas combustion, the accelerating process, the cooling process and the decelerating process in the atmosphere. The velocity and temperature distributions were calculated according to this model; the results fitted well with experiments. The combustion gas parameter distributions are almost identical in the barrel, but differ significantly in the atmosphere. For HVOF system, the under-expanded gas will expand in the atmosphere, while HVAF system exhibits an opposite behavior. At the gun exit, the combustion gas reaches supersonic velocities both for the HVOF and HVAF condition.

The effect of hardware on operating parameters and the resultant coating are qualitatively known; however, the quantitative effects have not been well defined. This study quantitatively characterizes particle temperature and velocity for the Sulzer-Metco 6P oxy-acetylene torch with 3 different nozzles and 3 air caps and also, the Alamo PG-550 then relates those data to particle diagnostics, deposition efficiency and coating microstructure. Both torches were evaluated using statistically designed experiments where the process inputs of oxy-fuel ratio, total combustible gas flow, and standoff distance were varied. Both torches can access similar regions of particle temperature - particle velocity space. Increasing total combustible gas flow increased particle velocity with little effect on particle temperature. Increasing oxy-fuel ratio decreased particle temperature with little effect on particle velocity. Higher particle velocity and particle temperature conditions yielded denser, less porous coatings. Flame cooling air caps increase the particle speed while decreasing particle temperature. Nozzles which inject powder directly into the flame jets significantly increase particle temperature as compared to nozzles which do not. Deposition efficiency is shown to not only be affected by particle temperature and particle velocity where hotter and faster usually increase efficiency, but is also dependent on the distribution of particles within the plume.

The warm spray gun was developed to make a coating of temperature-sensitive material, such as titanium, on a substrate. The gun has a combustion chamber followed by a mixing chamber, in which the combustion gas is mixed with the nitrogen gas at room temperature. The temperature in the gun can be controlled in the range of about 1500 - 2500 K by adjusting the mass flow rate of nitrogen gas. The mixed gas is accelerated to supersonic speed through a converging-diverging nozzle followed by a straight passage. In this paper, the performance of the warm spray gun is investigated by the simulation program in order to deeply understand the performance of the warm spray gun. The gas flow as well as the velocity and temperature of titanium particle inside and outside the gun are predicted by the numerical simulation.

Titanium has an excellent corrosion property in chloride containing environments such as seawater. A modified HVOF spray process was developed by introducing a mixing chamber between the combustion chamber and the powder feed port. Nitrogen gas was fed into the mixing chamber to control the temperature of the combustion gas generated in the combustion chamber. By controlling the flow rate of nitrogen, various Ti coatings with different degree of oxidation and porosity could be fabricated. The densest coating produced by this process with surface polishing treatment maintained excellent corrosion protection over a steel substrate in artificial seawater in a laboratory test over 1 month.

Effect of nozzle geometry (such as throat diameter of a barrel nozzle, exit diameter and exit divergence angle of a divergent nozzle) on HVOF thermal spraying process (thermodynamical behavior of combustion gas and spray particles) was investigated by numerical simulation and experiments with Jet Kote II system. The process changes inside the nozzle as obtained by numerical simulation studies were related to the coating properties. A NiCrAlY alloy powder was used for the experimental studies. While the throat diameter of the barrel nozzle was found to have only a slight effect on the microstructure, hardness, oxygen content and deposition efficiency of the coatings, the change in divergent section length (rather than exit diameter and exit divergence angle) had a significant effect. With increase in divergent section length of the nozzle, the amount of oxide content of the NiCrAlY coatings decreased and the deposition efficiency increased significantly. Also, with increase in the exit diameter of the divergent nozzle, the gas temperature and the degree of melting of the particle decreased. On the other hand the calculated particle velocity showed a slight increase while the gas velocity increased significantly.

Nozzle geometry has a profound effect on HVOF spraying, influencing combustion gas dynamics as well as particle behavior. Nozzle dimensions are also important in cold gas-dynamic spraying (CGDS), particularly the length of the nozzle which affects gas flow temperature and speed. In this study, numerical simulations and experiments were conducted to determine how the length of the entrance convergent section of gun nozzles affects HVOF spraying. Process changes that occur inside the nozzle (as predicted by simulation) were correlated with coating properties. An Al2O3-TiO2 powder was used for the experimental studies. Changes in nozzle length had a significant impact on deposition efficiency, microstructure, hardness, and particle velocity. These relationships (as measured and calculated) were then applied to the nozzle design for the CGDS method.

Thermal spraying of dense titanium coatings in the air atmosphere was achieved by using a two-stage high velocity oxy-fuel process (HVOF) called the Warm Spray Process. In the process nitrogen gas is mixed with the combustion gas to lower the gas temperature. Gas dynamics modeling of the flow field of the gas in the spray apparatus as well as the acceleration and heating of titanium powder injected from the powder feed ports were conducted. Based on the obtained temperature history of a titanium powder particle, its oxidation during flight was also predicted by using a Wagner-type oxidation model. These results were compared with measured velocity and temperature of sprayed particles by DPV2000 and the properties of deposited coatings. Significant discrepancy in the temperature of sprayed particles was found between the calculation and measurement whereas the measured velocity was closer to the model calculation. The model prediction of oxygen content was in a good agreement with the analysis of actual coatings. Furthermore, properties of the sprayed coatings such as porosity, oxygen content were correlated with the particle velocities and temperatures. Nitrogen gas was highly effective in lowering the oxygen content, but excessive nitrogen addition caused the coating porosity to increase due to insufficient particle temperatures.

In wire arc spray, atomizing gas is one of the most important parameters. The atomizing gas in wire arc spray is improved by using super sonic cold gas (non combustion gas) jet for better coating characteristics. To generate the super sonic cold gas jet, one is an optimization of nozzle shape with conventional compressed gas and another is that the pressed gas heated at 500-800K is supply to converging-diverging nozzle. Namely, coating deposition mechanism of cold spray, which is high particle impact velocity to substrate, is applied to wire arc spray. In this study, gas dynamics investigated the effect of pressure and temperature of supplied nitrogen gas, nozzle geometry on wire arc spray process (thermodynamical behavior of atomizing gas and coating properties).

The high quality of the thermally sprayed tungsten carbide coatings has been attributed to high particle velocity and relatively low particle temperature. Such thermal spray conditions can be obtained with the HVOF spray process. In comparison to the plasma spray process, in the HVOF spray process the high particle velocity and optimum particle temperature have been associated with very high gas velocity (>1000 m/s) and a relatively low gas temperature (< 2700 °C). In this work tungsten carbide coatings (WC-17Co) were sprayed by the HVOF process with a low and a high gas velocity of 1050 and 1560 m/s, respectively. The spray tests were carried out also with different hydrogen/oxygen ratios. The coatings were abrasion tested in order to find out how gas velocity and the fuel/oxygen ratio affect the coating quality and wear rate. Wear rates of the HVOF sprayed coatings were found to decrease with the higher combustion gas velocity. The coating quality and wear rate became also less sensitive to gas parameters with the increasing gas velocity. The coating microhardness and wear rate were also compared to hot isostatic pressed (HIP) reference material from the same spray powder lot. The HIP sintered test piece was found to be less wear resistant than the corresponding thermally sprayed coatings.

The thermal barrier coating (TBC) system is currently a standard technology of gas turbine hot gas path parts to achieve a highly reliable long life operation of engines for a power supply. Mitsubishi Heavy Industries, LTD. (MHI) has applied the TBC on the gas turbine blades, vanes and combustor parts. TBC system consists of MCrAlY as a bondcoat for oxidation protection at elevated temperature and YSZ (yttria stabilized zirconia) as a topcoat for efficient thermal barrier. The conditions where TBC has been in service are at the temperature up to 1500°C and with combustion gas environment. As far as the blades and the vanes are concerned, the MCrAlY bondcoat is coated by low pressure plasma spray (LPPS) or high velocity oxygen fuel (HVOF) system, and the YSZ topcoat is coated by atmospheric plasma spray (APS) system. In the case of the combustor parts, both the MCrAlY bondcoat and the YSZ topcoat are coated by APS. To increase the reliability and prolong the service life of the applied TBC in the gas turbine, it is very important to verify the coating properties, optimize the coating process parameters and control the coating process.

A special HVOF gun is used for aerodynamic research on internal flows of gas and particles in HVOF gun. The gun has rectangular cross-sectional area and has sidewalls of optical glass or transparent acrylic resin. Compressed air is used as process gas instead of combustion gas to visualize internal flow of the gun. The high-speed gas flows including shock waves in the gun are visualized by Schlieren technique. Particle trajectories in the gun are also visualized by high-speed digital video camera. The observation of erosion pattern created by particle collision on the barrel wall helps understand the particle trajectories throughout the barrel.

In this study, metal oxide films were synthesized from an EDTA·Er·H complex by flame spraying. The erbium oxide (Er 2 O 3 ) layers were deposited on stainless steel using N 2 , air, or O 2 as the carrier gas and a H 2 -O 2 mixture as the combustion gas. Test results indicate that the carrier gas has a significant effect on film thickness and porosity and that O 2 is the ideal carrier gas for producing dense metal oxide films.

The unusual effects of plasma sprayed coating on the fire-side of evaporator tubes located in an oil-fired steam generating boiler are discussed. The main heat transfer surfaces are constructed by heat exchanger tubes, evaporator tubes and superheaters. Maintenance to prevent of the boiler failure or the preserve heat exchanger effectiveness is a very important factor in the operation of boiler facilities. In a boiler which employs heavy gravity oil as a fuel, plasma sprayed Ni-Cr alloy has often been applied to boiler tubes for the relief of hot corrosion by combustion gas. However, the circulation of boiler water causes an internal deposit to form on the inner wall of evaporator tubes. The internal deposit generates excess heat load against the tubes. As the overheating of the tubes often causes the evaporator tubes to fail, they are chemically cleaned periodically. In this paper, the influence of Ni-Cr plasma sprayed coating for the heat flux, which dominates the formation of the internal deposit, is investigated. Ni-Cr plasma sprayed coating is substitutionally hot corrosion resistant and is a composite coating into which the fuel ash containing a vanadium or sulfur compound are interstitially penetrated and solidified. It is derived that the existence of the coating on the fire-side of the evaporator tubes normalizes the heat load in their inner walls. Moreover, the suppression of internal deposit formation decreases the frequency of chemical cleaning for tubes. The dual effects of plasma sprayed coating for hot corrosion resistance in the fire side and the suppression of internal deposit on the water side of the tubes are reported.

The effect of process conditions on flame spraying of titania (TiO 2 ) and magnetite (Fe 3 O 4 ) was investigated. Designed experiments were conducted to determine spraying conditions, specifically total combustible gas flow, stand off distance, and oxygen/acetylene ratio that produce high deposition efficiency (DE) and dense coatings. Along with DE, particle temperature and velocity were determined and correlated with process conditions. Results indicate that for both titania and magnetite, hot and high velocity molten particles result in higher DE and lower porosity coatings. Micrographs of coating cross-sections and surfaces were taken with both field emission scanning electron microscope (FESEM) and optical microscope. X-ray diffraction analysis shows that the titania coating retained its rutile structure while the magnetite coating had small amounts of magnetite (γ-Fe 2 O 3 ) in addition to magnetite.

For deposition of protective coatings different coating techniques are available. Usually, detailed evaluation of various deposit types and materials is necessary for selection of the best suited coating for specific application fields and demands. Subject of this work are thermally sprayed functional coatings applied as wear (and corrosion) protective layers. Examination of different optimized thermal spray coatings, i.e. HVOF sprayed WC/Co(Cr) and Cr 3 C 2 /NiCr coatings, conventional flame sprayed and fused self fluxing alloy coatings reinforced by hardmetal and APS sprayed oxide Al 2 O 3 /TiO 2 and Cr 2 O 3 coatings, is done in comparison to thick hard chromium platings. Two abrasive wear tests featuring wear by lose abrasive particles are carried out. These impart dry wear conditions according to ASTM G65 (Rubber Wheel test) and wear by abrasive suspensions according to ASTM G75 (Miller test). The work also contains evaluation of newly developed HVOF torch components permitting increased combustion gas, and therefore also particle, velocities concerning the benefit in terms of coating properties. Exemplary evaluation of the new components influence on velocity and temperature of spray particles is carried out by comparative SprayWatch analyses. Both the influence on the coatings microstructure and the wear performance are studied. Coating microstructure is evaluated qualitatively by optical and scanning electron microscopy and the micro hardness HV0.3 is measured. Worn surfaces are studied by SEM in order to deduce wear mechanisms.

Though wire flame spraying is a relatively old thermal spray process, modern equipment permits production of high quality coatings featuring outstanding homogeneity, high density and low roughness due to increased particle velocities as a result of increased combustion gas velocity. Typically spray particles are accelerated to velocities exceeding 250 m/s, if the wires are atomized adequately. In order to make a wide spectrum of coating materials available for wire flame spraying use of cored wires needs to be considered. A high speed camera is used to determine the particle velocity depending on process conditions for massive, grooved cored and tube cored AISI 316L wires. Thereby the influence of the wire design without simultaneous influence by the chemical composition is studied. Additionally nickel based carbide reinforced coatings are sprayed and characterized concerning their microstructure and properties in use.

The effect of torch hardware, operating parameters, and powder type on substrate surface heat flux was quantitatively investigated using calorimeters. The Sulzer-Metco 6P oxyacetylene torch with two nozzles and two air caps and the Alamo PG-550 torch were studied using designed experiments to show the effects of total combustible gas flow, oxy-fuel ratio, air flow, and standoff distance on surface heat flux. Air caps which directed cooling air toward the flame produced lower heat flux than air caps providing gun cooling. For the 6P torch, nozzle geometry did not have a significant effect on heat flux. With low air flow rates, both torches exhibited similar heat fluxes. At high air flows, the surface heat flux of the PG-550 was larger than that of the 6P.

In this study, computational methods are used to examine the in-flight dynamics of non-spherical WC-Co particles sprayed from HVOF guns and their impingement on substrates. Two sets of computational models are developed. First, the in-flight particles are simulated in a CFD-based combusting gas flow. Particle information prior to impact is extracted from the CFD results and fed into a FEA model to dynamically track the impingement of particles on the substrate. Particle morphology is examined for both spherical and non-spherical powders to establish critical particle impact parameters for bonding.