IMPROVING SELECTED PLASTIC MATERIALS WITH DIAMOND-LIKE CARBON (DLC) COATINGS

Christian B. Fischer 1, Magdalena Rohrbeck 1, Stefan Wehner 1, Matthias Richter 2 and Dieter Schmeißer 2

1Department of Physics, University Koblenz-Landau,

56070 Koblenz, Germany

2Department of Applied Physics and Sensors, Brandenburg University of Technology, 03046 Cottbus, Germany

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IMPROVING SELECTED PLASTIC MATERIALS WITH DIAMOND-LIKE CARBON (DLC) COATINGS

Christian B. Fischer 1, Magdalena Rohrbeck 1,

Stefan Wehner 1, Matthias Richter 2 and Dieter Schmeißer 2

1Department of Physics, University Koblenz-Landau, 56070 Koblenz, Germany

2Department of Applied Physics and Sensors, Brandenburg University of Technology, 03046 Cottbus, Germany

Introduction

The engineering of plastic materials and their wide range of application in industry and daily life is accompanied by essential requirements. Besides the basic performance of these materials such as flexibility, elasticity, fracture strength and chemical resistance, further determinants are long term stability, durability or biocompatibility. Concerning materials features according to its specified exposure mainly the surface is in intensive interaction with the environment. To avoid incompatibilities in surface characteristics of the optimal parent plastic these materials can be customized for their intended use by coating. The main limiting factor for such modifications is the preservation of basic material performance in addition to benefits like changed adhesive, improved repellent or antibacterial behavior. Especially surface modifications with amorphous carbon coatings are consistent with required characteristics of high hardness combined with enhanced scratch and abrasion resistance extending materials lifetime and period of use [1]. In addition to these mechanical advantages amorphous diamond-like carbon (DLC) coatings show reduced adhesion behavior [2-4] and biocompatibility [5]. Protective coatings with the objective to preserve main characteristics of selected plastic materials have been realized by a few layers of DLC. The assembly of soft plastic material and robust carbon layers is important for the resulting material characteristics. A detailed understanding of the interphasing between these unequal materials and the connection of carbon centers in a chemically different neighborhood is the perspective of the presented work.

Experimental Amorphous diamond-like carbon coatings were deposited

on industrial available polyethylene (PE) and medical relevant polyurethane (PU) samples as plastic materials using radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD) with acetylene as the main precursor gas. A detailed description of the RF-driven (13.56 MHz) commercially available plasma beam source [6,7] and of the coating procedure is given elsewhere [8]. Two types of DLC films one forming more flexible (on PU and PE) and one resulting in more robust (on PE) coatings were examined. The specific surface structure of raw and coated material of different layer thickness on PE-material were compared by SEM (Philips SEM515) and AFM (Omicron) microscopic analysis. The

near-edge X-ray absorption fine structure (NEXAFS) spectra were performed on beam line U49/2-PGM-2 at the facilities of electron storage ring BESSY II (Berlin, Germany).

Motivation The composition of DLC layer is a network consisting of

sp2- and sp3-carbon centers. Depending on the allocation and fraction of each carbon state the material exhibits different properties ranging from medium-hard to stiff. Two types of nanometer-scaled DLC films forming more flexible and robust coatings are investigated. Due to the fact that even flexible DLC coating of basic PU material was not sufficiently stable due to manufacturing specific contents over a period of several weeks the following examination concentrates on plain PE samples. Different DLC compositions and increasing layer thickness have been examined for PE material by the use of microscopic analysis and synchrotron radiation.

During deposition different plasma generated carbon species (ions and radicals) are in charge to react with the polymer surface. Even though PE and DLC are composed of carbon and hydrogen atoms there are several possible ways of interaction between carbon from the plasma and plastic material (Fig. 1). Depending on energy and ion flux fraction more or less subplantation processes generating a more robust network by crosslinking carbon centers take place [9].

Fig. 1 Proposed ways of interaction for the plasma generated carbon with plastic material.

Figure 1 shows possible ways of DLC coatings on plastic materials. In scheme A an only deposition process based on adhesive interaction with no crosslinking between both materials at the interface is shown. Figure 1 C illustrates an equilibrium effect of material erosion and DLC-film growth. It is accompanied by the fact of permanently losing basic material resulting in thin DLC-films composed of own carbonization products. The most likely process is presented in scheme 1 B consisting of a distinct multilayer interphasing

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between basic material and DLC coating. The rearrangement of the surface and subsurface of plastics opens suitable conditions for the growth of DLC coatings. The interphasing possibly ranges from a monolayer (D) to a multilayer (B) form, but it may also include an initial erosion process (E).

Results and Discussion In Figure 2 a comparison of raw and robust DLC-coated

PE-material by SEM (on the left side) and AFM analysis (on the right side) is shown. SEM images of uncoated raw material (A) show (Fig. 2) recurrent surface characteristics such as indentations and protrusions mainly caused by the manufacturing process. AFM measurements (B) indicate the raw material to be soft, resulting in poor contrast images (topography). Indentations and protrusions are still observable on SEM images for the coated samples. The exemplarily shown more robust DLC films seem to cover small indentations completely (C,E). Films of 10 nm coating (C) show charging effects probably caused by incomplete layer formation and in AFM study higher contrast (D) compared to the raw material. Coatings of 100 nm thickness clearly show deposited grains of DLC (F). Coatings with more flexible DLC exhibit similar behavior but smaller grain size of 100 nm film.

Fig. 2 Comparison of raw and robust DLC-coated PE-material by SEM (left A,C,E) and AFM analysis (right B,D,F).

Near-edge X-ray absorption fine structure (NEXAFS) spectra (Fig. 3) of uncoated raw material are in good agreement to results reported in the literature [10]. Additional peaks are observable around 285 eV which could be caused by a minor content of additives in the industrial material. Samples coated with robust DLC show a smaller amount of sp2-hybridized carbon than samples with more flexible DLC coatings at 291 eV (not shown). Spectra of PE samples with 10 nm film thickness indicate coating-specific characteristics in the fingerprint region above 291 eV which are less pronounced for samples with deposited films of 100 nm thickness.

Fig. 3 NEXAFS spectra of uncoated PE raw material and with 10 nm / 100 nm robust DLC film coated PE samples.

The deposition of 10 nm layer is supposed to indicate an interphasing showing electronically different carbon centers. The characteristic shape of the fingerprint region tends to fade more and more with layer thickness as can be seen in the spectra for the 100 nm DLC film. Further research with pure instead of industrial available PE material to eliminate distracting additives is currently under investigation.

Conclusions Microscopic and spectroscopic measurements were

performed on selected plastic materials (PU and PE) with different types of DLC coatings. Coating specific and thickness dependent characteristics were observed. Further investigation of various layer thicknesses and further analysis techniques are necessary to provide an enhanced understanding of interface formation between the basic plastic material and the DLC coating. Results will help to optimize parameters for the surface coating to achieve improved product characteristics. Collected information will be transferable to other plastic materials with diverse needs, e.g. energy storage devices, water treatments and coatings for everyday commodities or industrial parts.

Acknowledgments. We gratefully thank Barbara Kessler

(RheinAhrCampus, Remagen, Germany) for the use of SEM and equipment. We also thank Heinz Busch, David Seidlitz and Falk Bernsmann, NTTF Coatings GmbH (Rheinbreitbach, Germany), for coating of various samples.

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