In the spraying of functionally graded coatings the particle ensemble delivered to the substrate can vary from a relatively low melting point metallic particle to a significantly higher melting point ceramic particle. At various stages in the spray process the particle ensemble can be either predominantly metallic, ceramic, or an intermediate combination. For co-injected particles the mixtures do not behave as a simple linear superposition of the spray patterns of the individual particle types. The particle temperature, velocity, size distributions, and pattern characteristics of the resulting spray fields is examined for all ceramic particle sprays (ZrO 2 ), all metallic particle sprays (NiCrAlY), and for a 1:1 mixture. The major particle-particle interaction occurs in the injector itself and results in a modified spray pattern which is different from that of either material sprayed alone. The particle velocity distributions generally exhibit a bimodal nature which is dependent on the size and density of the injected particles.

A study of the flattening and cooling of particles plasma-sprayed on a substrate is presented. The characteristic parameters of the splats are linked to the parameters of the impacting particles by using an experimental device consisting of a phase Doppler particle analyzer and a high-speed pyrometer. However, during the long experiments required to get reliable correlations, it was observed that variations in plasma spray operating conditions may alter the particles behavior in the plasma jet. Therefore, a simple and easy-to-use system was developed to control in real time the spray jet. In this paper, the effect of carrier gas flow rate, arc current and powder mass flow rate is investigated. The results on zirconia and alumina powders show the capability of the technique to sense the particle spray position and width.

The thermal spray process melts powder at very high temperatures and propels the molten material to the substrate to produce a coherent deposit. This heating produces a certain amount of vaporization of the feedstock. Upon exiting the plasma plume the fast cooling conditions lead to condensation of the vapor. An electrical low pressure impactor was used to monitor the concentration of ultra-fine particles at various radial and axial distances. Metal, namely iron powder, showed very high concentration levels which increase with distance. Ultra-fine particles from ZrO 2 -8Y 2 O 3 reached a peak concentration at 6 cm. Use of an air barrier during spraying decreases the population of ultra-fine particles facilitating the production of a stronger coating.

Plasma spraying process modeling is useful to understand physical phenomena and to decrease the number of experiments. In this paper, a study of the external plasma jet is proposed: the PHOENICS™ CFD code was used with a 2D axisymmetrical geometry and a standard K-ε turbulence model. In a first step, thermodynamic and transport properties were calculated from chemical equilibrium composition, thermodynamic derivatives and kinetic theory of gases. Local Thermodynamic Equilibrium (LTE) was assumed for both plasma and surrounding gases. The proposed numerical results were computed for comparison with temperature measurements realized by Brossa and Pfender in the case of an argon plasma jet discharging into air, using enthalpy probes. The predictions were found reasonably accurate. The influence of the surrounding gas nature was also verified as the validity of the parabolic assumption.

An analysis of a d.c. plasma jet is presented using a three-dimensional commercial fluid dynamics code, ESTET. This code solves the coupled conservation equations of mass, species, momentum and thermal energy equations for a compressible and turbulent fluid in control volume and finite difference formulation. Computations take into account fluid turbulence using a standard k-s model with the Launder and Sharma correction for the laminar zones, e.g. the plasma core. Two series of spraying conditions differing in the total gas flow rate (30 and 60 slm) and the arc current (300 and 600 A, respectively) are computed. The process parameters are independently varied about the nominal operating conditions. The effect of the variation of primary and secondary gas flow rate, effective power and powder carrier gas flow rate on flow fields characteristics, is discussed.

In plasma spraying temperature and velocity of the sprayed particles are among the most important parameters influencing the microstructure and properties of the deposited coatings. However, the sprayed particle state is influenced by uncontrollable parameters such as the wear state of the electrodes. In order to investigate the influence of the electrode wear state on sprayed particles, a long-term experiment was conducted during which on-line measurements of plasma sprayed yttria-zirconia powder were performed. Results show that even though input parameters were kept constant during the experiment the state of the sprayed particle changed significantly and coatings prepared at different spraying times have different microstructures and can have different properties. However, by changing some input spray parameters it was possible to retrieve the initial sprayed particle state and coating microstructure.

The deposition efficiency (DE) of a particular powder for a particular thermal spray process is very important factor when coating economics is being considered. There are many coating applications, however, where it is also important to know how the deposition efficiency changes during a longer coating process. Normally the DE is determined as mass ratio of powder fed into the process and corresponding weight gain of the sample. In this work the deposition efficiency has been determined for aluminum oxide powder in atmospheric plasma spraying using different spray parameters and electrode wear states. The coating process and in-flight particles were monitored using a fast non-intensified CCD-camera. Using digital image analysis the relative hot particle concentrations and velocity distributions were calculated from images. The possibility to use a CCD camera based monitoring system for in-situ measurement of DE is discussed. Additional laser illumination and PTV measurements were performed to verify the cold particle flux unseen by the plain CCD camera.

This paper presents a simulation method in which robot trajectories can be optimised to produce an even coating thickness and how they can be used to predict transient coating temperatures on complex geometries. The coating thickness was simulated by the use of a commercial Offline programming (OLP) system. A robot trajectory was calculated, maintaining a constant spraying distance and normal orientation to the surface. The trajectory was optimised to give a uniform coating thickness while also handling non collision requirements. The plasma was represented as an ideal gas with temperature dependent thermodynamic and transport properties. The governing equations were solved by a developed finite difference elliptic code using a simplified turbulence model. The particles were modelled by a stochastic discrete particle model. The robot trajectory together with the heat transfer model were then used to calculate transient coating and substrate temperatures by the use of the finite element method (FEM). The model predictions were tentatively compared with experimental measurements and reasonable correlations were obtained.

A new measurement technology in plasma spraying has been established at SULZER METCO AG, Switzerland. Based on the principle of C. Moreau, CNRC, it is now possible to measure surface temperature and velocity of melted particles during the flight in a plasma flame. Since temperature and velocity are two of the most important parameters for plasma spraying processes and coating characteristics, the knowledge and control of these parameters offers new methods and assistance in the development of coating solutions. The object of this paper is to outline some important measurement results, to show correlations to coating characteristics and to demonstrate measurement applications in an industrial environment.

An investigation into the dependency of the formation of functionally graded materials (FGMs) on process variables was carried out. The initial stage of the investigation involved a complete analysis of the plasma spray parameters used in the fabrication of an FGM constructed of NiCrAlY and partially stabilized zirconia (PSZ). In flight particle temperature, velocity and trajectory data were gathered for individual powders, as well as mixtures of the particle species, over a range of spray parameters. This data was combined with material specific properties such as flowability, apparent density, particle morphology and size distribution. The end result of the studies allowed for size matching of the particle species so as to ensure both species were molten at the nominal spray distance and possessed coincident impact velocities. Following the initial investigation, two spray conditions were selected for further analysis. Individual layers of specific powder mixture ratios were deposited as well as a complete FGM structure. The resulting structures were then compared based on their deposition efficiencies, porosity levels, compositional homogeneity and microstructures.

Two sets of plasma spray processing conditions were utilized in the investigation of graded layers, consisting of NiCrAlY and PSZ. Following the optimization of the plasma spray parameters and particle characteristics, the deposition efficiencies (DEs) of various powder species was examined. Selection of the best suited powders for coating production were selected based on the DE results. The base DEs were corrected by conducting the analyses using pre-deposited substrates as targets. Individual mixed coating layers were prepared and their compositions confirmed by image analysis. The effects of standoff distance and substrate temperature were also seen to have an effect on the DE and thus the coating formation. It is suggested that two-feeder, single injector plasma processing may not be the optimal method for the formation of FGMs.

This research has focused on characterization of the wire arc spray process with the goal of achieving improved process controls. Arc voltage and current traces have been analyzed on-line using an oscilloscope and a personal computer with LabView software. The characteristic features of the arc voltage fluctuations are correlated with the molten metal droplet formation process using a high speed Laser Strobe video system operating in synchronization with the oscilloscope trigger. Voltage minima occur when larger globules of molten metal leave the wire tip. Analysis of the voltage fluctuations indicate that they are neither random nor periodic, and that they can be described based on chaos theory. This approach may be used for achieving a further understanding of the dynamic nature of the process, and for the development of control algorithms.