Atmospheric pressure PECVD based on a linearly extended DC arc for adhesion promotion applications

In the aircraft industry, an increasing demand for adhesion promotion layers for titanium based materials is observed to replace polluting and unhealthy substances. Plasma enhanced chemical vapour deposition (PECVD) at atmospheric pressure (AP) seems to be favourable for the deposition of thin silica-like layers. In this work, a linearly extended and scalable DC arc plasma source is used for PECVD of silica layers on Ti6Al4V. The influence of the precursor chemistry is evaluated for HMDSO and TEOS precursors. Using HMDSO, hydrophobic almost stoichiometric SiO2 coatings with a carbon content of 3 at.-% to 5 at.-% are deposited on the titanium alloy substrate. For TEOS, hydrophilic carbon-free stoichiometric SiO2 coatings are deposited. The dynamic deposition rate for TEOS is 3 times lower t han that for HMDSO (up to 640 nmm/min). For the intended coating of slightly curved substrates, the film properties and deposition rates are studied for different working distances from 20 mm up to 60 mm. For 60 mm the deposition rate is reduced by a factor 6 compared to 20 mm, also the index of refraction of the film decreases from 1.45 at 20 mm distance to 1.39 at 60 mm. With increasing distance, the particle size and concentration on the surface increases and leads to less dense structures than in the case of shorter distances. The wedge test is used to evaluate the adhesion properties of the layer. The HMDSO-based coatings exhibit a good long-term durability and a high bonding strength comparable to that of the currently used standard pre-treatment, but without the use of harmful subs tances.