Abstract High power supersonic plasma guns operating in excess of 200 kW can produce molten particles with 3 to 4 times the impact velocity of conventional plasma sprays. With this increased range of particle velocity it is important to understand the relationship between the torch input parameters and the sprayed particle velocity, temperature, pattern and size. Stainless steel particle velocity, temperature, size and relative number are measured for a high power plasma spray system operating at 110 kW. At the same torch operating conditions the plasma and particle flow fields are simulated with a newly developed computational model. It was found that the injection geometry plays an important role in the particle entrainment, heating and acceleration. In spite of the complexity of the system, i.e. supersonic plasma velocity with a high swirl component, the simulation produced reasonalble particle trajectories resulting in good agreement between the calculated and measured particle velocity, temperature and size distributions.

Abstract In plasma spraying, deposition efficiency depends, to a great extent, on powder losses at the injection point, vaporization phenomena and particle rebound at impact. This paper discusses an attempt to determine the mass balance in metal spraying by studying the injection and vaporization of a metal powder in a plasma jet and correlating the results to the mass deposition efficiency of the process. The study of particle vaporization consisted of measuring the metal atoms concentration by absorption spectroscopy while injection was examined by a 2-D imaging technique.

Abstract The quality of a plasma sprayed coating is influenced by the plasma jet stability; entrainment of cold air through large scale turbulence can lead to variations in particle heating and trajectories resulting in increased unmelt densities, reduced deposition efficiencies, and oxidation of metal particles. The jet instabilities are in part caused by the swirl flow of the plasma gas. With two modifications to an atmospheric pressure plasma spray torch, we have investigated the influence of reduced swirl flow on jet stability, particle trajectories, and coating quality. The modifications are (1) addition of a shroud consisting of a porous ring surrounding the anode nozzle while simultaneously injecting part of the shroud gas inside the nozzle with a swirl component in the direction opposing the plasma gas vortex, and (2) an injector ring with which part of the plasma gas is injected radially and part tangentially producing reduced vortex flow for the same plasma gas flow rate. Jet stability and particle trajectories are determined using a LaserStrobe system combined with image analysis, and coatings have been evaluated by determining porosity and unmelt density. Results indicate that deposition efficiency is most affected by reduced vortex flow, while the shroud addition reduces unmelt density and porosity.

Abstract This paper is devoted to the presentation of a modeling approach of DC arc plasma flows by implementation of the CFD PHOENICS code. The equations of mass, momentum and energy, of elliptic type, are discretized using a finite/control volumes method. The closure of the equations set is obtained with the standard k-ε turbulence model. Finally, the algebraic equations set is solved by means of the SIMPLE algorithm. 3D calculations are performed from the point of the cold gas injection into the torch to the plasma ejection into the surrounding atmosphere, taking into account some currently used spray parameters (i.e., plasma gas flow rate, power, etc.). Results are discussed and compared with experimental values taken from the literature.

Abstract This paper presents a mathematical model of the plasma-spray coating formation process that allows one to estimate bond strength energy, a parameter related to coating quality. Bond strength energy is defined on the basis of particle-substrate or system balance. Unknown quantities in the energy equation are obtained from nonstationary Navier-Stokes equations for velocity field and pressure and from thermoelasticity equations for temperature and stress. Complexities associated with particle spreading and nonlinear hydrodynamics have made it necessary to develop a stable numerical technique.

Abstract Electric arc spraying with dual wires is an economical coating process finding diverse applications. Turbulence and velocity of an atomizing gas exert strong effects on the droplet formation and therefore on the coating properties. Turbulence intensity of an atomizing gas flow can be estimated by analysis of the waveforms of arc voltage fluctuations, and the velocity can be estimated by the frequency and the amplitude of these waveforms. Higher gas velocities result in higher frequencies and smaller amplitudes of the voltage fluctuations, and in smaller molten droplets leading to coatings with lower porosity but higher oxidation levels. Lower turbulence levels at the electrode tips result in more periodic waveforms with less high frequency content, and in lower oxidation of the coatings. Nozzle configurations such as a converging-diverging nozzle provide higher gas velocities with less turbulence leading to coatings with lower oxidation and lower porosity.

Abstract Investigation of gas dynamics of detonation coating process is conducted. One dimensional model which allows calculation of particle exit parameters based on Chapman-Jouquet picture of detonation wave is developed. Kinetics of particle acceleration by a detonation wave show several novel features such as nonmonotonic dependence of velocity upon coordinate and inverse acceleration. Obtained results indicate that final particle velocity can be manipulated in a broad range by changing the gas mixture composition and the powder loading distance into the barrel.

Abstract Maximizing dissociated species transport in plasma assisted chemical vapor deposition (CVD), is important in many low pressure plasma jet processes. To deposit high quality diamond by low pressure plasma assisted CVD, it is important to maximize the atomic hydrogen transport to the substrate. One route to process improvement is to explore ways in which unstable species transport can be maximized. A two-dimensional computational model of a supersonic contoured nozzle attached to a dc torch will be described for examining the chemical non-equilibrium of the flow. If the fluid dynamic time scales of interest are faster than the kinetic time scales of interest, it is believed that unstable precursor transport can be controlled, improved and optimized. This paper will examine an implicit formulation for the numerical simulation of a multi-component reacting Ar-H 2 plasma. It is found that dissociation, ionization and charge exchange reactions must all be included in a reaction model. The ionic species significantly alter the temperature profiles upstream of nozzle choking. However, to increase the number of hydrogen atoms at the nozzle exit, the arc attachment should be positioned as close as possible to the converging-diverging nozzle throat.

Abstract This paper presents a preliminary study of mathematical modeling and numerical simulation of laser engraving process. The 1-D simulation concerned CO 2 c. w. pulsed laser engraving of plasma sprayed Cr 2 O 3 coatings as used in the manufacture of anilox rolls. The thickness of the evaporated material was calculated as a function of laser processing parameters viz. pulse length and power density. The actual thermophysical coefficients of plasma sprayed chromia were used if available and the results of calculated thickness of evaporated layer are compared with the experimentally determined depth of engraved cells to estimate unknown coefficients of liquid chromia. Finally, the experimental effect of overflow was related to the calculated thickness of liquid layer. The experimental procedure of minimization of this effect is proposed.

Abstract In this paper the influence on the coating generation of substrate preheating during spraying, relative movements torch to substrate and cooling conditions is systematically studied for partially stabilized zirconia and alumina coatings. Beads and coatings are sprayed with Ar-H 2 dc plasma jets in air. The obtained structure, resulting hardness and adhesion/cohesion for each spraying condition is discussed.