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.

This paper presents a simulation of the simultaneous spraying of a metal and a ceramic powder with different configurations for the injection of the powder into the plasma jet. The plasma jet and the behavior of the injected particles were modeled with a commercially available computational model of the dynamics of liquid bodies. The particles are modeled as discrete Lagrangian objects. Three series of numerical tests were carried out: simultaneous spraying of the powder in a three-dimensional plasma jet in a stable state; simulation of the 3-D plasma flow, assuming that it fluctuates at the same frequency as the arc voltage; and simulation of the effect of the current fluctuation on particle behavior. A pre-calculation with an analytical model made it possible to determine the suitable gas flow rate so that the "average" trajectories of the metal or ceramic powders coincide at the same point on the surface. Paper includes a German-language abstract.

The conditions of particle injection into the side of plasma jets play an important role in determining the microstructure and properties of sprayed deposits. However, few investigations have been carried out on this topic. The current work presents the results of an experimental and computational study of the influence of injector geometry and gas mass flow rate on particle dynamics at injector exit and in the plasma jet. Two injector geometries were tested: a straight tube and a curved tube with various radii of curvature. Zirconia powders with different particle size range and morphology were used. A possible size segregation effect in the injector was analyzed from the space distribution of particles collected on a stick tape. The spray pattern in the plasma jet was monitored from the thermal radiation emitted by particles. An analysis of the particle behavior in the injector and mixing of the carrier-gas flow with the plasma jet was carried out using a 3-D computational fluids dynamics code.

When spraying is conducted in the ambient atmosphere, the entrainment of air cools down the plasma jet and affects its expansion. It may also cause the oxidation or the chemical decomposition of the sprayed materials. Inert Plasma Spraying (IPS), generally conducted in argon atmospheres, prevents these phenomena. However, the main drawbacks of IPS in comparison with air plasma spraying are the capital and apparating costs. To reduce the latter by 25 to 30%, nitrogen atmospheres may be used as a substitute for the conventional argon atmosphere. This paper presents a study in which titanium carbide and niobium powders were sprayed in argon and nitrogen atmospheres. Cryogenic cooling of the substrate was used during the spray process. This helps to maintain a low temperature in the chamber, produces thick coatings and allows the use of substrate materials that are sensitive to heat. The adhesion, roughness and microstructure of the coatings produced in both atmospheres are compared as well as their nitrogen content.

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.