Otimização da adesão de filmes de DLC sobre aço SAE 4140 utilizando intercamadas contendo silício depositadas a partir de tetrametilsilano

Abstract

Diamond-like carbon (DLC) films have been gaining space in the automotive, aeronautical, biomedical and decorative industries due to their low coefficient of frictionand high wear resistance. However, the low adhesion between the DLC film and ferrous alloys represents an obstacle to its application. The use of Si-containing interlayers aims to improve the adhesion of the films. However, the need for high temperatures in the deposition of the interlayers makes technological applications unfeasible. Therefore, hydrogen etching is considered an alternative to improve the overall adhesion of the films, through a chemical cleaning of the interlayer surface with hydrogen plasma. Hydrogen etching is applied after the deposition of the Si-containing interlayer. This work aims to evaluate the effect of hydrogen etching on the adhesion of DLC films to SAE 4140 steel, in systems with Si-containing interlayers, obtained from tetramethylsilane (TMS) and deposited at low temperatures (starting at 85 °C). Two sets of samples were deposited, called (1) series with hydrogen etching and (2) series without hydrogen etching, which differ from each other only by the presence of hydrogen etching. The interlayer's deposition temperature was varied in the two series. The scratch test showed adhesion of the DLC films for all deposition temperatures of the series without hydrogen etching, with critical loads ranging from 4 N to 6 N. The series with hydrogen etching showed partial delamination in some deposition conditions. According to the results of glow-discharge optical emission spectroscopy (GDOES), the hydrogen etching removed considerable amounts of elements such as C, Si and O from the interface between the DLC film and the Si-containing interlayer. This contributed to the reduction of the interlayer's thickness. Raman spectroscopy and Fourier-transform infrared spectroscopy (FTIR) characterized the DLC films and the Si-containing interlayers as hydrogenated amorphous carbon (a-C:H) and hydrogenated amorphous silicon carbide (a-SiCx:H), respectively. Oxygen contents below ~5% were measured by X-ray photoelectron spectroscopy (XPS) at the interfaces and at the interlayer of the series without hydrogen etching. It was concluded that the good adhesion of the DLC films of the samples of the series without hydrogen etching (at temperatures as low as 85 °C) is a consequence of the low oxygen content present in the structure and the use of a low background pressure (~0.8 Pa) in the deposition system. This work observed that (1) the application of hydrogen etching is not necessary to ensure adhesion of DLC films in processes with low deposition temperatures of Si-containing interlayers, formed from TMS and deposited in systems with low background pressure as ~0.8 Pa; (2) it is possible to deposit well-adhered DLC films in systems with Si-containing interlayers (formed from TMS) deposited at temperatures as low as 85 °C. [resumo fornecido pelo autor]