In plasma spraying, the dynamics of droplet deformation and cooling on the substrate determines the geometry of the resulting splat, the porosity formation and the quality of contact between the splat and the underlying layer, thereby controlling the properties of coatings. The process of splat formation depends on the droplet velocity, size, molten state, chemistry and angle of impact, and on the substrate material, roughness, temperature and surface chemistry. This paper concentrates on investigating the way the droplets impact onto the substrate. A fast-imaging camera is used to observe the impact of plasma-sprayed alumina droplets on cold and hot substrates. Three modes of impact are observed: splashing, deposition or rebounding. A droplet is said to splash when it disintegrates totally or partially into secondary droplets after impacting onto the surface while it is said to deposit if the liquid material spreads and solidifies on the surface with a minimum splashing. The results are expressed in dimensionless form as a correlation between the impact mode and the Sommerfeld parameter K. The latter quantifies the importance of the particle parameters at impact (temperature, velocity and size) and viscous and surface tension forces. It is found that, when K is higher than 57.7, the splashing mode prevails as already found in a variety of different fields for droplets impacting on surfaces.

Investigations have been carried out to study the influence of Atmospheric Plasma Spray process (APS) on the in-flight oxidation of pure iron particles. After collecting the molten droplets in-flight, XRD, SEM and Mössbauer spectroscopy are used to determine the amount and distribution of formed oxides. The results indicate that the Wüstite, Fe 0.95 O, is the only oxide formed during the APS. In the jet core and at the beginning of its plume, oxidation is controlled by convection within molten droplets and then for the downstream of the plasma by diffusion where the solubility of oxygen through the external oxide layer governs the growth of Wüstite. Calculations have shown that the convective movement is due to the drastic velocity difference between the plasma jet and particles. Wüstite granules can be distinguished within the particles due to the immiscibility between Fe and FeO in liquid phase. This oxide phase represents about 13 wt% of the collected particles at 100 mm stand-off distance in an Ar-H 2 plasma jet (50:10 SLM) with 18 kW effective power. The amount of oxide decreases when the H 2 volume percentage of the plasma, the internal diameter of the anode nozzle and the effective power increase. Sessile drop studies of molten iron on ceramic substrates are carried out to simulate the wetting of oxidized iron particles during coating formation. It is found that the oxidation state of iron particles during APS has a significant effect on the observed contact angles. A strong decrease of the contact angle is observed in the case of oxidized iron particles. In-flight oxidation of iron particles allows a better splat covering.

As underlined in 1981 by Mc Pherson, thermo-mechanical properties of plasma-sprayed coatings depend not only on the way particles flatten and resulting splats solidify and cool down, but also on the thermal history of particle layering at the same location. To illustrate what is our present knowledge in that field, plasma-sprayed alumina coatings will be considered through modelings and measurements. The first part of this paper discusses the phenomena linked with particle impact and splat formation: splashing, spreading, solidification and grain growth, angle of impact in conjunction with particle parameters at impact and substrate surface parameters (chemistry, phase structure and roughness, temperature). The second part examines splats layering. It addresses the influence of plasma jet heat flux, relative velocity torch-substrate, powder flow rate and deposition efficiency on splat time-temperature evolution and resulting quenching stress, coating adhesion/cohesion and microstructure. The shadow effect when spraying off normal angle is also discussed. The last part deals with the effect of the successive cooling and reheating of passes on coating properties, and condensation of the vapor issued from evaporating particles.

Aerosol Flame Spraying (A.F.S) combines the atomization of a colloidal suspension with the lateral injection of the aerosol in a flame. The aerosol droplets are partially dried when crossing the flame and deposited as a coating onto a substrate. The coating is then consolidated by a heat treatment. In this paper, a modeling of the trajectories, acceleration and vaporization of the droplets is presented. This model supposes that the droplets dry at a constant rate until they impact onto the substrate. It predicts the size and water content of droplets at impact. From these data and hydrodynamic properties (viscosity, surface tension, contact angle) of the suspensions, the morphology and size of the lamella deposited on the substrate can be determined by using classical impact models. A colloidal monoclinic zirconia suspension with a 60-nm particle diameter is used in this study. In spite of the complexity of the mixing of suspension spray and flame, and the diversity of the thermal histories of the droplets, the observation of the latter after impact shows that the results of the model are quite consistent with measurements.

This paper presents a numerical simulation of the plasma spraying of alumina particles using a three-dimensional commercial fluid dynamics code ESTET 3.4 . The objective of this study is to investigate the effect of (i) turbulence model and turbulence radial profiles at the torch exit on plasma flow and particle behavior, (ii) particle injection conditions on particle trajectories and heating and (iii) plasma jet fluctuations on temperature and velocity flow fields. The comparison of predictions with experimental measurements of gas and particle velocity and temperature, makes it possible to determine the influential parameters and set them to pertinent values.

This paper is devoted to thermal spraying and presents the state of our current knowledge, as well as the research or development needs are in; spraying heat sources i.e., flame, HVOF, D-Gun, plasma torches; particle heat and momentum transfer (measurements and modeling), process on-line control, powder morphologies and injection within the hot jet and reactions with environment; coating formation i.e., particle flattening and solidification, splat layering, residual stresses, coating microstructure and properties; reliability and reproducibility of coatings.