Improvement of the properties and the adherence of DLC coatings deposited using a modified pulsed-DC PECVD technique and an additional cathode

Abstract

Deposition of hard and adherent diamond-like carbon (DLC) coatings employing an asymmetrical bipolar pulsed-DC PECVD system and an active screen that functioned as an additional cathode is presented. This novel system represents a step forward for thin coating growth by using much lower pressure (about 0.1Pa) in an almost collision-less regime, with higher plasma density than the conventional PECVD system. In order to overcome the low adherence of DLC coatings to 316 stainless steel substrates, a thin amorphous silicon interlayer was deposited as an interface. The interlayer was synthesized using silane as precursor gas and varying the applied DC bias voltage during deposition. The DLC coatings were produced using acetylene gas as the precursor. Both the amorphous silicon interlayers and the DLC films were analyzed according to their microstructural, mechanical, and tribological properties as a function of the applied substrate bias voltage. The film's atomic arrangements were estimated by means of Raman spectroscopy, while the hydrogen content was determined via elastic recoil detection analysis (ERDA). The nanoindentation experiments helped determine the films' hardness and other nanomechanical parameters. The friction coefficient was determined using a tribometer in unlubricated sliding friction experiments, while the adhesion of the films was determined through a classic Rockwell C indentation test. The results showed an improvement of the properties and the adherence of the DLC coatings deposited using a modified experimental setup and an amorphous silicon interlayer. The composition, microstructure, and mechanical and tribological properties of the films were dependent on the applied DC bias voltage and, consequently, on the intensity of ion bombardment during coating growth. These results suggest that a combination of a modified pulsed-DC PECVD system, the use of an active screen as an additional cathode, and acetylene as a precursor gas for growing DLC films may represent a new and useful alternative for mechanical and tribological applications.