Although the HVOF process has shown to be a technological alternative to the many conventional thermal spray processes, it would be very advantageous to design a nozzle that provides improved performance in the areas of deposition efficiency, particle in-flight oxidation, and flexibility to allow coating of ceramic powders. Based on a numerical analysis, a new nozzle was modeled, designed, tested, and used to produce thermal spray coatings according to the industrial needs mentioned above. Performance of the new nozzle was investigated by spraying several coating materials including metallic (Nickel, MCrAlY, Stainless Steel), carbide (WC-Co), and ceramic (Al 2 O 3 ) powders. Particle spatial distribution, velocity, and temperature corresponding to the new nozzle and the standard HVOF gun were compared. The new nozzle provides a superior particle spatial distribution, as well as higher and more uniform particle velocity and temperature.

Numerical modeling of the complex two-phase flow of gas-solid particles in a highly compressible field, as occurs in HVOF spraying, can serve as a tool for optimizing deposition efficiency, nozzle designs, and other parameters. In this paper, the authors develop a numerical model based on Euler’s equation and use it to investigate the effect of flow regime on the velocity of particles in an oxyfuel jet. Simulations based on typical HVOF operating conditions show large variations in the flow regime in regions of high particle concentration. Unlike other modeling approaches, the Eulerian approach predicts that the flow becomes subsonic near the centerline and drag force decreases significantly as do particle velocity and deposition efficiency. The effects of flow regime are also measured experimentally, validating the result. Paper includes a German-language 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.

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.

This paper presents the experimental validation of a curvilinear novel nozzle design. Their effect on the speed and temperature of the particles in flight, on the porosity and adhesive force of the coatings is measured and compared with conventional conical nozzle devices. The nozzle testing is performed using a Miller SG100 dc plasma torch. It is observed that the curvilinear nozzle produced denser and more uniform coatings with lower porosity and higher adhesive force. This could be achieved by increasing the flight temperature of the particles and ensuring more complete melting of the particles. The velocity profile of the particles on the substrate remained unchanged. Paper includes a German-language abstract.

Properties of MCrAlY coatings obtained by High Velocity Oxy-Fuel (HVOF) thermal spray process operated in a standard configuration were compared with those obtained using a gas shroud attachment to the HVOF gun. Our measurements show that the attached gas (nitrogen) shroud nozzle considerably reduces the oxygen content in the coating without an appreciable change in the microstructure. The particle temperatures were decreased by an average of 100 °C at a standoff distance of 0.275 m (11 inches). There was also a large reduction in the particle velocity at this distance. Both these effects were related to the excessive amount of nitrogen used for shrouding.