Effect of deposition temperature and hydrogen as a process gas on mechanical properties and specific electrical resistivity of thick a-C:H obtained by means of PACVD

Abstract

DLC coatings are widely used for their protective properties such as high wear resistance, low friction coefficient as well as chemical inertness. However, their electrical resistance is usually very high, which limits their utilization in electrotechnical applications. To improve electrical conductivity, DLC films are typically doped with nitrogen or metals. This study, however, investigates the mechanical and electrical properties of un-doped, hydrogenated DLC films deposited using temperatures above 450 °C. To further enhance the coating's properties, hydrogen gas was added during deposition.The DLC coatings were produced by means of PA-CVD using a pulsed DC discharge. Temperatures of 450 °C, 500 °C and 550 °C were used to deposit a-C:H films on steel substrate. The process gas consisted of a mixture of argon and acetylene. Additionally, coatings were deposited with hydrogen added to the gas mixture. A silicon-based interlayer served as an electrical insulator between substrate and coating and was deposited with HMDSO as a precursor. To measure the specific electrical resistivity of the films, the van der Pauw method was performed. The mechanical properties of the coatings were determined through nanoindentation. Raman spectroscopy was performed to analyze the structure of the DLC coatings.The films showed a significant decrease in specific electrical resistivity with increasing deposition temperature. Values dropped to <104 μΩ cm at 550 °C, attaining levels close to graphite. Hardness and Young's modulus increased up to 147 % with rising deposition temperature. The addition of 18 % hydrogen gas during deposition resulted in at least 60 % further reduction in specific electrical resistivity, while also slightly raising coating hardness for deposition temperatures above 450 °C. With this new distinct deposition method, electrically conductive a-C:H coatings with improved mechanical properties can be produced only by increasing the deposition temperature and the utilization of hydrogen as process gas.