Metal incorporation i hydrogenated amorphus carbon films deposited by biased electron cyclotron resonance assisted chemical vapour deposition

Resumen

One of the principal aims of the thesis has been to deepen the understanding of the processes controlling the different carbon structures obtained, such as the role played by the resistivity of the substrate as function of the energy of the ions arriving at the growing surface. Among the results, it is worthy to remark the increase in the negative bias voltage needed for the growth of the fullerene-like structure when a lower resistivity of the substrate is used, which is explained by the stronger loss of ion energy across the thicker sheath when a low resistivity substrate is covered. In parallel, we explored the possibility of introducing metal into the fullerene-like hydrogenated amorphous carbon matrix by Electron Cyclotron Resosnce Chemical Vapour Deposition method (ECR-CVD) in order to improve the tribological properties. As described, two different strategies were implemented to prepare metal containing fullerene-like coatings: i) in-situ methods (Cr and Mo), and ii) ex-situ method (Cr and Ag). The main difference between both is that in one case the synthesis takes place during a single process by means of the introduction of a metal piece (bulk or mesh) within the reactor (in-situ), whereas in the second method the metal in form of nanoparticle is spread out on the substrate surface in a previous stage outside the reactor (ex-situ). The efficiency of the metal incorporation strategy and the structural, tribological and electrical properties of the films have been studied. From the comparison between both methods, it has been shown that the ex-situ method leads to a more uniform metal concentration profile. In addition, it results in a metal content easier controllable by the concentration of nanoparticles in the solution where the substrate is dipped prior to the ECR-CVD growth process. Besides the development of different metal strategies to introduce metal into the carbon matrix and the evaluation of their efficiency, the effect of the metal incorporation on the properties (especially tribological ones) of the deposited coatings has been investigated. The addition of low metal contents to the carbon film produces the increase in the structural order in the coating structure, as detected by Raman spectroscopy that leads to coatings with lower coefficient of friction. Regarding the electrical behaviour of the samples, the incorporation of metal into the carbon matrix gives rise to an evolution in local conductivity from an insulator to an interconnected network of metal or sp2 clusters. The enhancement in the electrical conduction is attributed to either an increase in the density of localized states or to tunnelling processes. One of the most outstanding results for coatings with embedded metallic nanoparticles is the nearly one order of magnitude longer lifetime of the coating with low chromium content (300 ppm ethanol) with respect to the metal-free film. This effect, clearly detected as the samples undergone pin-on-disk tests, has been lastly associated with the presence of metal in the form of nanoparticles. However, for coatings with a large amount of metal nanoparticles (¿ 500 ppm), the contribution of the metal to the wear process produces a fast damage of the coating detected by extremely short lifetimes. Finally, nanocomposite samples underwent some preliminary tests for evaluating the biocompatibility. The results suggest that metal containing nanocomposite thin films with low metal content are a promising material as protective biocompatible layer, due to the synergic combination effect of the good protection properties and the capability as suitable media for mechenchymal stem cell growth and division.