The mechanical and biocompatibility properties of DLC-Si films prepared by pulsed DC plasma activated chemical vapor deposition

Abstract

Films of diamond-like carbon containing up to 22 at.% silicon (DLC-Si) were deposited on to silicon substrates by low-frequency pulsed DC plasma activated chemical vapor deposition (PACVD). The influence of silicon doping on deposition rate, composition, bonding structure, hardness, stress, surface roughness and biocompatibility was investigated and correlated with silicon content. A mixture of methane and tetramethylsilane (TMS) was used for the deposition of DLC-Si films at a pressure of 200 Pa. The deposition rate increased with increasing TMS flow. The addition of silicon into the DLC film leads to an increase of sp³ bonding, as measured by Raman spectroscopy, and also resulted in lower stress and hardness values. The RMS surface roughness of the films was measured by atomic force microscopy and increased from 0.35 nm for DLC to 6.7 nm for DLC-Si (14 at.% Si) due to the surface etching by the H atoms. Biocompatibility tests were performed using MG-63 osteoblast-like cell cultures that were left to grow for 3 days and their proliferations were assessed by scanning electron microscopy. The results indicated a homogeneous and optimal tissue integration for both the DLC and the DLC-Si surfaces. This pulsed PACVD technique has been shown to produce biocompatible DLC and DLC-Si coating with potential for large area applications.

Introduction

Diamond-like carbon (DLC) has been the focus of extensive research in recent years due to its potential application in many technological areas. The combination of low friction, wear resistance, high hardness, biocompatibility and chemical inertness makes it suitable for a number of applications ranging from the coating of stents, heart valves, orthopaedic and prostheses in the biomedical industry [1] to the coating of magnetic storage disks [2] in the semiconductor industry. There is also a growing market for the application of DLC in wear-resistant applications, for example coatings on cutting tools and automotive components [3].

A number of different deposition technologies have been described for synthesising DLC films [4]. The most common deposition technique used is RF based (13.56 MHz) plasma-assisted chemical vapor deposition (PACVD). This technique allows high quality DLC coatings to be deposited onto complex shaped items at low temperature, although it is difficult to scale up the process to industrial size. In recent times, there have been various studies to find alternatives to the standard RF processes. Bipolar-pulsed direct current (DC) discharge was identified as a technique that is both suitable for scale up and also enables the deposition of thick DLC coatings because surface charge up during growth of the coatings can be avoided [5]. Michler et al. [5] reported that pulsed DC discharges lead to enhanced film properties and more stable processes. Pulsed DC PACVD is also preferred over RF PACVD because inherently the plasma sheath is thinner, which enables closer packing of the items to be coated and a deeper penetration of the plasma into holes or around edges, than in the case of RF activated CVD, which allows for a higher throughput of coated items. Pulsed DC of large areas and three dimensional deposition of DLC by direct current (DC) were also reported by Nakanishi et al. [6], indicating that further up scaling of this technology is possible.

There are commonly two different types of DLC: (i) amorphous hydrogenated carbon (a-C:H) films and (ii) hydrogen-free tetrahedrally coordinated carbon (ta-C) films. In this paper only the properties and deposition of a-C:H (subsequently referred to as DLC in the present work) will be discussed. Excellent overviews of the deposition methods and properties of DLC have been written by Robertson [4] and Grill [7].

There are a number of limitations regarding the use of DLC for the growth of hydrogenated amorphous carbon. Firstly, the material has high internal stress, which leads to poor adhesion on some practical substrates (i.e. steels) and also restricts the thickness of the films that can be deposited. Secondly, the low coefficient of friction only occurs at low humidity, and is therefore strongly dependent on the environment. Thirdly, the thermal stability is very poor. A number of researchers have reported that the introduction of additional elements such as silicon, nitrogen and various metals improves the properties of DLC [4].

The incorporation of silicon into DLC stabilises the sp³ bonding [8], [9], reduces the stress, makes the friction coefficient highly insensitive to changes in humidity and improves the thermal stability [10]. The addition of Si can promote sp³ bonding by chemical means, as it inhibits sp² hybridisation, and thus reduces the need for ion bombardment to develop sp³ bonding [4]. There are contradictory reports in the literature with regard to the influence of silicon on the mechanical properties, such as hardness and elastic modulus. Some reports show increases in hardness [11], others report decreases [12] and some report no change in the hardness at all [8]. In the case of artificial heart valves, the properties of DLC can be improved by the incorporation of silicon. The silicon in the films renders the surface antithrombogenic by inhibiting the fibrinogen activation [1].

In this work, silicon is added to the DLC films using the pulsed DC PACVD technique. The influence of silicon addition on the bonding structure, mechanical properties, surface roughness and biocompatibility of the films with MG-63 osteoblast-like cells is investigated and discussed.