Elsevier

Diamond and Related Materials

Volume 80, November 2017, Pages 59-63
Diamond and Related Materials

Functional diamond like carbon (DLC) coatings on polymer for improved gas barrier performance

https://doi.org/10.1016/j.diamond.2017.09.001Get rights and content

Highlights

  • Oxygen and carbon dioxide gas barrier performance of functionalized DLC thin films

  • For this study, Raman spectroscopy, X-ray photoelectron spectroscopy and Nano-indentation are used.

  • The gas barrier properties of a-C:H:N's are 5–10 times better than that of uncoated PET substrates.

Abstract

We have studied the optimum deposition conditions for the improvement of oxygen and carbon dioxide gas barrier performance of functional diamond-like carbon (DLC) thin films. The a-C:H: a-C:H:Si, a-C:H:N and ta-C:N thin films with 10–400 nm thickness were deposited on polyethylene terephthalate (PET) substrates by the radio frequency plasma-enhanced chemical vapour deposition method. To study the microstructure of the PET coated films, we have used the Raman spectroscopy, X-ray photoelectron spectroscopy, nano-indentation and surface profilometry. The gas barrier property were analysed and found that the a-C:H:N's are 5–10 times better gas barrier properties than that of uncoated PET substrates. These thin layer PET coated thin films could be use in food packaging and biomedical applications.

Introduction

The coatings of diamonds like carbon (DLC) films are widely used in many applications and technological fields due to its unique physical and chemical properties. Different medical coatings devices made of polymer, ranging from lubrications to anti-microbial liquids to water repellent polymers, and each variety of coatings are use on a multitude of devices for many different applications. Nevertheless, these uses and/or applications are very limited due to the poor mechanical properties. In order to accomplish these applications, the DLC thin films can be deposit on metal, ceramic and/or polymer substrates [1], [2]. Surface modification by a thin-nano-layer of DLC on polymer has tremendous potential to solve the problems when it comes to the use of polymers as higher gas barrier behavior due to their good adhesion, chemically stable in various hazardous atmospheres [3], [4], [5], [6], [7], [8], and wear resistance, mainly for the food packaging applications. The DLC surface modification with polymer is a new and very promising technique due to excellent adhesion, hardness, chemically/thermally stable and transparent behavior in visible light. There is a continuous demand for flexible alternative nano-coatings, which should have high surface barrier quality without affecting any beneficial bulk properties of the polymer. Different physical and chemical techniques are use, for the deposition of DLC films on different substrates. Some of them are suitable for the laboratory studies and some others are preferred for the industrial production. The films deposited by different methods reveal different mechanical and tribological features. Common DLC coating methods include sputtering [1], plasma enhanced chemical vapour deposition (PECVD) [9], direct ion beam deposition (direct IBD), pulsed laser deposition (PLD) and vacuum arc [10]. Each method is consider for a specific application and can have certain advantages and disadvantages. One of the main drawbacks of DLC coatings is their high intrinsic compressive stress, that can be reached to several GPa [10], which alters their adhesion and limits the thickness of coatings, resulting in the peeling off of the coating. Several alternative processes have been studied to reduce the negative impact of internal stress. Doping of DLC with metals, N or Si, post-annealing of the obtained layer or bias-graded deposition [11] are some of them are widely used for these applications. Synthesis of DLC films using atmospheric pressure plasma has wide range of applications in batch processing of substrates. Within the development and design of a high hardness, chemically stable, optically clear nano-coating of DLC on polymers, it is necessary to demonstrate: (i) The role of film thickness, doping with Si, N and nitrogen pre-treatment (pre-heated) on the gas barrier performance of deposited DLC films. (ii) The role of the microstructure and surface morphology on the gas barrier and protective performance of the DLC films. An effective barrier can prevent both losses from the packaged product, and penetration into the package, both of which can affect quality, and shorten product shelf life. Our main goal is to improve the packaged of food products to maintained fresh for longer time.

In the present work, we have prepared different type of DLC thin films (known as a-C:H) and functionalized with Si, N; deposited on polyethylene terephthalate (PET) substrates by the radio frequency plasma-enhanced chemical vapour deposition (PECVD) method. We have studied their microstructural, electronic and mechanical properties for the use of different gas barrier in packaging industries, and for the development of food processing process.

Section snippets

Experimental details

The DLC (a-CH) films were deposited onto 0.5 mm and 0.1 mm untreated polyethylene terephthalate (PET) substrate after cleaning with Ar+ gas flow, using pure acetylene (C2H2) – argon (Ar) plasmas with 2:1 ratio, using RF (13.56 MHz) Plasma Enhanced Chemical Vapour Deposition (PECVD) at different bias voltage. DLC thin films are functionalize with Si (a-CH:Si) and N2 (a-CH:N) using Tetramethylsilane (TMS) vapour as Si source and Nitrogen (N2) gas respectively. To determining the optimum deposition

Results and discussion

Rama spectra of different a-C:H thin films deposited on pristine PET substrate is shown in Fig. 1. Raman spectroscopy has been utilised extensively to study the bonding states of various phases of carbon [14]. The main advantage of this technique is the analysis of different types of carbon atoms, which are Raman active as the carbon – carbon bonds have very strong vibrational modes. In the Raman analysis, the diamond a peak is located at approximately 1332 cm 1. In case of graphite, first order

Conclusion

We have observed the following things regarding this study:

  • (i)

    It is observed that the a-C:H:Si film having ~ 170 nm thick provides a most superior barrier against oxygen and water vapour.

  • (ii)

    There is approximately 10 times improvement in the barrier performance of a-C:H:Si films deposited on PET as compared to pristine PET.

  • (iii)

    Thicker a-C:H film on PET provides a better barrier against oxygen and water vapour. However, after certain film thickness adhesion may be poor and hence gas barrier also become poor.

Acknowledgements

Authors S.C.R. and S.S. gratefully acknowledge the financial support received from the National Research Foundation (NRF), South Africa (Grant No. EQP13091742446 and PD-TWAS150813137166).

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