Changing the mechanical and structural properties of diamond-like carbon by Ca-O-incorporation

Abstract

Diamond-like carbon (DLC) is an amorphous carbon coating with partly diamond-like properties which exhibit superior mechanical as well as tribological behavior. Due to their chemical and wear resistance, the low permeability and the promising biocompatibility DLC is a potential candidate for protective thin coatings in various applications [1]. The metastable DLC consists of carbon in different hybridization for example sp3, sp2 or sp1 bonded atoms. DLC is a so called hydrogenated amorphous carbon film. The hydrogen is incorporated into the carbon network or exists in a metastable solid solution, because DLC is far away from equilibrium conditions. Additional elements like nitrogen, silicon and titanium may effect a further adaptation of the DLC properties according very specific demands [1, 2]. In order to enlarge the applicability of DLC for biomedical purposes, the addition of calcium should be able to influence bone formation may be of considerable relevance. The intention of the following study is to gain first the information about the influence of Ca-O-incorporation on the mechanical and structural properties of DLC. The generation of the conventional DLC and the CaO-DLC was carried out by exploiting a direct current discharge for plasma formation. Before coating deposition the titanium-substrates were polished to a 1 μm finish and cleaned ultrasonically in ethanol. After an additional in situ cleaning by bombarding with argon ions the generation of the metastable amorphous coatings took place. The substrate is sufficiently negative biased in relation to the plasma in order to achieve ion bombardment before and during the DLC growth. The CaO was dissolved in water, and the CaO-H2O was evaporated in the plasma chamber. For the generation of Ca-O-DLC, the gaseous precursor (benzene) together with the CaO-H2O vapor is decomposed due to the direct current discharge under a pressure of between 0.005 and 0.05 Pa. For processing conventional DLC, only benzene is supplied into the plasma chamber. The negative bias of the titanium-substrate attracts ions, loaded particles as well as other decomposed fragments and leads to the coating formation. Conventional and Ca-O-modified DLC of about 5 μm thickness were deposited on the titanium. The Ca-O-modified amorphous carbon fractured cross-sections reveal qualitative differences in comparison to conventional DLC. Fig. 1 presents a fractured cross-section of the Ca-O-modified DLC. The Ca-O-modified amorphous carbon exhibits no smooth and featureless fractured surface like that known for DLC [3], but some topography. The damaged Ca-ODLC cross-section exposes grooves, fluting or stretched hillocks which are perpendicularly orientated to the coated surface. The existence of this topography confirms a less brittle fracture behavior of the amorphous Ca-O-DLC coating and also points to changed