Reactive thermal plasmas: ultrafine particle synthesis and coating deposition

Thermal plasmas jets produced by d.c. arcs or RF discharges at pressures close to atmospheric pressure are characterized by the high temperatures (between 6000 and 14 000 K) of heavy species and high velocities (between 100 and 2500 m s(-1)) of plasma flow. They can be used either for their physical properties, i.e. acceleration and melting of solid particles (in the diameter range 10-100 mu m) to spray thick coatings (from 0.1 mm to a few millimetres), or for their reactive properties. When reagents are introduced in the plasma stream, they are transformed in highly reactive radicals and/or atoms in excited states. These reactive species can form a coating with high deposition rates (similar to 100 mu m h(-1)) due to the steep gradients in the boundary layer plasma-substrate: this process is called thermal plasma-assisted chemical vapour deposition (PCVD). Without substrate and using adequate quenching, they can also form ultrafine particles, the size of which is controlled (with difficulty) by temperature during the quenching process. These PCVD processes can be used to form new chemical species on the surface of solid and molten particles in flight in the plasma jets, or on the resulting splats and between the successive passes to produce coatings with dispersed hard phases (reactive plasma spraying). It is also possible to agglomerate reactive particles (e.g. carbon and metal) a few micrometres in diameter, to achieve exothermic reactions within the particles in flight and spray carbide coatings in air. This paper presents our knowledge in these fields as follows: (1) the main characteristics of plasma d.c. and RF torches, transferred arcs and the injection of reagents and/or quenching gases; (2) the modelling problems of reactive gas flow injection; boundary layer close to the substrate; ultrafine particle synthesis with the corresponding experimental results obtained for PCVD coatings; thin film deposition by flash evaporation and ultrafine particle synthesis; and (3) the reactive plasma spraying achieved either by spraying agglomerated-particles (in the 10-50 mu m size range) where self-propagating high-temperature synthesis occurs, or by mixing a reactive gas with the plasma jet. (C) 1997 Elsevier Science S.A.