Adhesion and cohesion properties of diamond-like-carbon coatings deposited on biomaterials by saddle field neutral fast atom beam source: measurement and modelling

摘要

Diamond-like-carbon (DLC) has been shown to be strategically important in respect to biomedical applications due to its biocompatibility. Despite decades of work on film deposition there is an insufficient understanding in respect of the film’s adhesion characteristics, particularly on biomaterial substrates. The central aim of this study is two pronged. A programme of work has been undertaken to set-up, study, understand and optimise the production technique for DLC deposition, while on the other hand diamond like carbon films have been characterised to investigate the strength of adhesion and cohesive strength with particular reference to biomedical applications.ududDLC films have been deposited on to substrates of 316L stainless steel, cobalt chrome (CoCr) and Ti6AI4V alloy using a saddle field neutral beam deposition system (Microvac 1200DB, Ion Tech Ltd) with acetylene and acetyleneargon mixture as the process gas. It is noted that numerous parameters influence coating adhesion including the stress in the film, contamination and chemical bonding between the film and the substrate, and the physical properties and roughness of the substrate. Discharge current vs. discharge voltage characteristics were investigated with different pressure and process gas. Uv absorption spectra were used to measure the photon energy and optical band gap of the films. The optical band gap was found in the range of -0.85 and 0.85 -0.97 eV for lower and higher deposition current respectively. The adhesion of the films has been measured as a function of the duration of in-situ etching by a neutral argon beam and also as a function of source current, system pressure and process gas (pure C2H2 and C2H2+Ar gas mixture). The studies were performed on DLC films with thickness -0 .4 |im. The adhesion of the film was measured using pull-off and Rockwell C tests whereas four point bend (FPB) test was used to measure the cohesive strength of the films. Argon pre etching for 15 minutes is recommended to guarantee an optimal adhesion. The etching process also influenced the film structure in terms of the sp3/sp2 ratio and stress. It was also found that this optimisation of the adhesion is correlated with a change in the structure and thickness of the native oxide layer on the steel surface of the substrates. Substrate surface temperature during etching and deposition also influenced film structure and adhesion. Correlation between the residual stress and the adhesion of the films has also been established which helped to identify optimum process parameters for substrate-film adhesion properties. No significant change with deposition pressure has been observed but high anode currents may lead to higher sp3 content. The adhesion strength has been found to be inversely proportional to residual stress and to increase at low deposition pressures. At source anode current of 0.6A, the adhesion is a monotonic function of pressure in the range examined where as with 1.0A source current the behaviour is more complex. The relationship between the stress and the sp3 content of the films measured by analysis of Raman signature has also been investigated.ududThe experimental work of FPB has been used to support and develop a numerical (Finite Element) model for the determination and prediction of the film's cohesive strength. The model takes into account the film hardness, Young’s modulus and thickness and has been shown to be capable of predicting the film’s cohesive strength when combined with a theoretical formulation for brittle fracture. It has been observed that maximum stress developed at the outer surface of film during the bend test, which influenced the initiation of cracks at the outer surface of the film and their propagation through the film-substrate interface. This result has only been valid for films with higher Young's modulus compared with the substrate.