Mechanisms of films and coatings formation from gaseous and liquid precursors with low pressure plasma spray equipment

Plasma spraying is a well developed and widely used technology, successfully applied for ceramic and metal coatings in many fields of applications such as aeronautics, gas turbine, automotive or medical. The coatings obtained are usually intentionally porous and thick (more than 100 μm). Presently, thin films (< µm) are deposited using various physical vapour deposition (PVD) or chemical vapour deposition (CVD) processes with low deposition rates. In this paper, we make use of the high enthalpy and high ionisation degree of the plasma jet of conventional plasma spraying guns operated at low pressure (mbar) to obtain dense coatings by CVD from gaseous and/or liquid precursors. The advantages of such thermal plasma CVD processes are the high deposition rates to obtain dense and thin layers, and the possibility of combining these thin films with thermally sprayed coatings using the same equipment.An efficient injection and mixing of the liquid and gaseous precursors in the plasma jet, which is especially challenging for liquids in low-pressure processes, has been obtained by extensive developments and proper equipment design. Results of several different coatings based on liquid and gaseous precursors are presented. In particular, SiOx thin films from HMDSO (Hexamethyldisiloxane, C6H18OSi2) precursor and oxygen can be deposited over large areas (50 cm diameter) at typical deposition rates of 35 nm/s, with a precursor-to-film conversion efficiency exceeding 50%. For the case of amorphous carbon deposited from CH4 or C2H4, deposition rates exceeding 25 nm/s are obtained. Results from mass spectrometry of the gas sampled in the plasma jet by an enthalpy probe show that the depletion of hydrocarbon precursors can reach 95% and that higher hydrocarbon species are formed by secondary reactions. In the case of carbon-containing precursors, results from mapping of the optical emission intensity throughout the plasma jet volume are presented. The formation and transport of excited precursor-based species, such as CH, C, C2, and H are addressed. These results show, in particular, that the very high dissociation efficiency of the precursors takes place through (dissociative) charge exchange from Ar+ ions and subsequent dissociative recombination with low energy electrons. The peculiarities of plasma chemistry taking place in the low-pressure plasma jet compared to conventional low-density non-equilibrium plasmas are outlined.