Físico-química de adesão de filmes de DLC sobre aço AISI 4140 utilizando intercamadas contendo silício

Abstract

Diamond-like carbon (DLC) is a thin film material that has attracted much attention due to its properties as high wear resistance and super-low friction coefficient. Although silicon-based intermediate layers are employed to enhance the adherence of DLC films on ferrous alloys, the role of such buffer layers is not yet understood in chemical terms. In this context, the aim of this study is to clearly understand the physical-chemical phenomena and chemical bonding guaranteeing the DLC adhesion on ferrous alloys using Si-based interlayers deposited by different process conditions. The films were deposited on AISI 4140 steel by pulsed direct current plasma enhanced chemical vapor deposition assisted by electrostatic confinement. The interlayers were grown from tetramethylsilane at different deposition temperatures and times and the DLC thin film was grown from acetylene with the same experimental conditions for all samples. The physical-chemical structure of the bi-layers produced was evaluated by scanning electron microscopy, energy-dispersive X-ray spectroscopy, glow discharge optical emission spectroscopy and Raman spectroscopy. The local chemical bonds at the interfaces of the steel/interlayer/DLC structure were evaluated by X-ray photoelectron spectroscopy. Finally, the critical loads for thin films delamination were measured by nanoscratch testing. The results show that the interlayer thickness and structure are dependent on both deposition time and temperature, but the temperature was established as being the key parameter controlling the reaction kinetic and the interlayer growth. The interlayer is structured as a hydrogenated amorphous silicon carbide alloy with enhanced formation of Si–C bonds at deposition temperatures ≥ 300°C. At such transition temperature of interlayer deposition, adhesion of DLC is reached, with no spontaneous delamination. The improved adhesion is associated with the nature of chemical bonds formed in the interfaces. Whereas C–C and C–Si bonds are formed on the interlayer/DLC interface, the steel/interlayer interface is constituted by Si–Fe bonds. The presence of hydrogen and residual oxygen in the interlayer structure degrades the adhesion of DLC thin films.