Tightly adhering diamond-like carbon films on copper substrates by oxygen pre-implantation
Introduction
Copper is combined with diamond-like carbon (DLC) films mainly in the form of copper containing DLC films. The copper reduces the stress of the films [1] and adds antibacterial properties [2], [3]. A combination of separate copper and DLC layers can be used to improve optical and electrical properties [4], e.g. the field emission behavior of the films [5]. DLC coatings on copper-based materials can be advantageous, e.g. when a lower friction coefficient or higher hardness is required [6]. It has recently been suggested to use DLC coatings on copper-based archaeological artefacts to protect them against corrosion [7]. The adhesion of DLC films on copper and on other metallic substrates, such as nickel, steel and gold, is low, however. This happens when two factors are combined: a weak or even non-existent tendency to form carbides and a high linear coefficient of thermal expansion of the substrate. Strategies to improve the adhesion, apart from an optimization of the deposition parameters, are the removal of surface contaminants, the increase of surface roughness to enhance mechanical interlocking, and an improved chemical bonding of the layer by the use of surface modification via implantation or of interlayers [8]. Copper and carbon do not form bonds. In fact, in sputter deposition it has been found that self-assembled nanoperiod multilayered carbon‑copper films tend to form [9].
For copper it has been shown that argon sputtering as a pretreatment is not sufficient to increase the adhesion of a DLC film distinctly. A treatment with − 10 kV argon ions, generated by a high voltage pulse of 10 μs length and 2 kHz repetition rate, for 30 min at a pressure of 0.7 Pa resulted in an adhesive strength of 12.7 MPa for a DLC film with a thickness of 150 nm on copper [10]. The generation of a gradient carbon layer by implanting a hydrocarbon into the copper substrate is also not effective [11]. Interlayers of a different material, usually metallic or silicon-based, are commonly used to increase film adhesion. However, they require not only the use of additional components, e.g. a metal source, but most times also of sample manipulation to achieve a uniform coating. The latter point depends on the shape of the sample; coating from sources such as magnetron sputter guns is mostly unidirectional, thus samples that are complexly shaped cannot be treated from all sides without sample manipulation. A more effective approach in this case is the use of plasma source ion implantation and deposition (PSII&D) wherein positive ions from a plasma are accelerated towards a sample by a negative high voltage [12].
The use of oxygen implantation is reported to increase the adhesion of DLC films on copper. Tonosaki et al. coated copper with adhering DLC films after oxygen implantation [6]. They used voltages in the range of − 18 to − 20 kV, providing neither the full experimental details nor a quantitative value for the adhesive strength. A survey of different implantation gases indicated that best results could be achieved with oxygen [10]. The absolute values of adhesive strength of the DLC films did not exceed 20 MPa, though. So far, there is no detailed investigation that is able to provide the reasons of the necessity of high energy oxygen ions for an increased adhesion.
When using polycrystalline copper, an additional complication arises because of the severe dependence of its sputter yield on the crystal orientation [13]. This leads to different concentrations of the implanted species, depending on the crystallographic orientation of the copper grain [14]. So, there is still uncertainty about the best pretreatment conditions when using oxygen as well as about the changes in the surface that are responsible for the increased adhesion. Furthermore, there is no detailed investigation so far on the reasons of the increased adhesion. Here, both topics are studied. To avoid high voltages such as − 20 kV the experimental parameters have to be changed. In order to achieve a good adhesion with a lower voltage, the effect of argon addition to the oxygen plasma was investigated.
Section snippets
Experimental
The substrates were cut from a polycrystalline copper plate (99.95% purity, 1 mm thickness) into pieces of 10 × 10 mm2. After polishing them to a mirror-like finish, they were cleaned in ethanol with ultrasonic agitation. The samples were fixed with small screws near the center of a sample holder with a diameter of 92 mm. The sample holder features a large open area and thereby increases the deposition or implantation up to several times, depending on the voltage, as compared to a normal bulk holder
Results and discussion
Annealing of the copper substrates in air as well as oxygen implantation generated an oxide layer. In Fig. 1 the oxygen profiles from the SIMS measurements are shown for several samples. The thickness of the oxide layer depends on the treatment conditions. The implantation depth of oxygen in copper at an acceleration voltage of − 15 kV is 17.4 nm (O+) and 9.9 nm (O2+), respectively, according to SRIM simulations (version 2013.00). The surface of the sample annealed at 200 °C consists mostly of Cu2O;
Conclusion
Adhering DLC films on copper can be prepared when the copper substrate is implanted beforehand by oxygen using a pulse voltage of − 18 kV. The adhesive strength of the films surpasses the one of the epoxy resin in the pull test, i.e. 90 MPa. This is the result of two factors: an increased surface roughness up to about 125 nm due to the sputtering and the presence of oxygen within the surface. The thickness of the oxide layer and its roughness increase with the pulse voltage. The addition of argon
Acknowledgement
T.H. Ferber's help with the XPS measurement is gratefully acknowledged.
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