On the Use of Low-Pressure Plasma Spraying Equipment to Produce Thin Films and Thick Coatings Using Liquid and Gaseous Precursors

Plasma spraying is successfully applied for ceramic and metal coatings in many industrial applications. Conventional thermal spray coatings are thick (more than 100 µm) and intentionally porous. However, there are applications where dense and thin coatings are needed. A dedicated Plasma Spraying Process called LPPS-Thin Film was developed a few years ago that can deposit dense layers at low pressure with thicknesses as low as ~ 10-20 µm with high deposition rates. Thinner films (< µm) are conventionally obtained by Physical Vapour Deposition (PVD) or Chemical Vapour Deposition (CVD) processes, with limited deposition rates (< 10 nm/s). In this paper we demonstrate the possibility to deposit thin and dense layers from liquid or gaseous precursors at high deposition rates using Low Pressure Plasma Spraying equipment. Both techniques could be combined to get multilayer deposits having special properties. Extensive developments on F4-VB nozzles were required to achieve an efficient injection and mixing of the liquid and gaseous precursors in the plasma jet. The gas precursors are also injected through a perforated gas ring at various axial positions along the jet flow. Several coatings from liquid and gaseous precursors were obtained. In particular, good quality SiOx thin films from HMDSO precursor and oxygen were deposited over large areas (50 cm diameter) at deposition rates exceeding 30 nm/s with precursor conversion efficiency better than 50%. The effect of process parameters, such as the location of precursor injection, torch current, chamber pressure, and substrate standoff distance, on the deposition rate, uniformity and quality of the coatings will be discussed. Optical emission spectroscopy and mass spectrometry have been applied to investigate the vaporisation of liquids, the dissociation and transport of the precursors and the production of reactive species. Analysis of the 2-D optical emission intensity profiles over the whole plasma jet will be presented.