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oumei Baba Applied Technology Division, Industrial Technology Center of Nagasaki, Omura, Nagasaki 856-0026, Japan (Received 15 July 2011; accepted 13 September 2011)

Unbalanced magnetron sputtering (UBMS) is suitable for the preparation of hard and hydrogenfree diamond-like carbon (DLC) films. Since those films generally suffer from internal stresses and bad adhesion, the addition of a methane source offers two advantages: (i) the control of the film properties by variation of the hydrogen content and (ii) a pretreatment of methane plasma source ion implantation (PSII), which results in a gradient carbon layer within the substrates, ensuring the adhesion of the subsequently deposited DLC films. PSII and UBMS were combined to prepare DLC films on stainless steel substrates and silicon wafers. Different amounts of methane were added to the working gas, argon, to investigate the effect of the hydrogen content on the film properties, i.e., hardness, adhesion, and friction coefficient. Composition and chemical structure of the films were investigated by depth profiling (secondary-ion mass spectrometry) and Raman spectroscopy. Smooth adhesive films could be obtained with the lowest friction coefficient for small additions of methane as a hydrogen source during the sputtering process.

I. INTRODUCTION

Diamond-like carbon (DLC) films can be prepared by a variety of methods, usually plasma- or ion-based, with the properties of the DLC films depending on the preparation technique and the specific preparation parameters.1 Hardness, friction coefficient, and adhesion, for example, depend on the structure of the films and the existence of internal stress within the film.2,3 This can be influenced by the hydrogen content of the films; usually, a hydrogen-free DLC film exhibits a higher hardness, but suffers from internal stress, whereas hydrogen-containing DLC films have a smoother surface, less internal stress, lower friction coefficients but also lower hardness.4–7 One way to control the hydrogen content is to add a hydrogen source to a method, such as unbalanced magnetron sputtering (UBMS), that is used to prepare nominally hydrogenfree layers. The latter technique has been used to prepare DLC films, with the resulting hardness and tribological properties depending on parameters such as the substrate bias voltage and ion-current density.8–11 The adhesion of DLC films on certain types of substrates is problematic, however, and to improve adhesion one or several interlayers can be added.1,10,12 Another approach is the pretreatment of the substrate by implantation a)

Address all correspondence to this author. e-mail: fl[email protected] DOI: 10.1557/jmr.2011.341 J. Mater. Res., Vol. 27, No. 5, Mar 14, 2012

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of carbon-containing species.13 Because of the resulting gradient layer of carbon and hydrogen within the substrate surface, a better adhesion can be ensured.14 In the experimental setup discussed here, the possibility of an added flow of a hydrocarbon gas can be exploited for the pretreatm