ICMCTF2003 Session TP: Symposium TS Poster Session

Thin wear protecting films, like TiN, CrN or diamond-like carbon (DLC), are potential coatings on metallic medical implants [1-3]. However, the results from tribological and corrosion tests are contradictory. Some studies reveal superiority of DLC coatings in comparison with uncoated metallic substrates, while other give no evidence of an improved performance. This outcome stresses the importance of paying attention to the deposition method as well as parameters and the correlating nano- and microstructural features, the kind of substrates and the coatings adherence. These features directly influence the mechanical, electrochemical and tribological behaviour, but also effect the biological interaction at the surface, and with this indirectly the coatings wear and corrosion properties.

In the following study, diamond-like carbon coatings (0.8 micrometer, 2.7 micrometer) were deposited on polished Co28Cr6Mo-samples by a plasma deposition method (direct current discharge) using gaseous hydrocarbon as precursor. The electrochemical behaviour of DLC is investigated in 0.89 wt.% NaCl solution and compared with the behaviour of the uncoated medical grade CoCrMo. For 500 h the potential was measured against time under open circuit conditions. In addition, potentiodynamic polarisations were carried out starting from the rest potential into the anodic direction, and subsequently, into the cathodic direction. The polarization curves are evaluated and correlated to the nano- and microstructure of the coatings (Raman spectroscopy, Scanning electron microscopy). Additional information is given by hardness measurements (load 4 mN) and characterisation of the surface topography by atomic force microscopy (AFM).

[1] Bolton, J., X. Hu, In vitro corrosion testing of PVD coatings applied to a surgical grade Co-Cr-Mo alloy, J. Mater. Sci.: Mater. Med. 13 (2002) 567-574.

[2]Platon, F., P. Fournier, S. Rouxel, Tribological behaviour of DLC coatings compared to different materials used in hip joint prostheses, Wear 250 (2001) 227-236.

[3] Xu, T., L.Pruitt, Diamond-like carbon coatings for orthopaedic applications: an evaluation of tribologcal performance, J. Mater. Sci.: Mater.Med. 10 (1999) 83-90.

Scientists are pursuing high-level hemocompatible materials with all possible technology in these years. Specific surface modifications of materials for good blood compatibility are developed by biomaterials researchers. As an effective method, thin film deposition has been applied to modify metallic biomaterials such as stainless steel and Titanium, which have been applied as substrates of stents and mechanical heart valves etc, for improving blood compatibility. In this paper, Titanium oxide films doped with Ta (Ti(Ta)O₂ film) were fabricated using magnetron sputtering and plasma immersion ion implantation and deposition (PIII-D) respectively. The microstructure of the Ti(Ta)O₂ films was investigated using X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The blood compatibility of the films was evaluated using in vitro platelet adhesion investigation. The surface energy of the film was calculated by Young function according to the contact angle measurement. The interfacial energy between the film surface and stimulant body fluids such as Hank's liquids and Ringer's liquid was further calculated. In addition, the investigation of endothelial cells growth in vitro on the films was also performed. The different characteristics and properties of the Ti(Ta)O₂ films synthesized by different deposition methods were described. The blood compatibility mechanism of Ti(Ta)O₂ film was also discussed.

Acknowledgement: Financially supported by the State key basic research G1999064705 and 2001AA320604.

Die casting is a high temperature process that generates extreme cyclic variation of stresses and strains in the H13 steel within a shot cycle. Finite element thermo-mechanical simulation of the die casting process is an important in the development of the engineering design of thin films for specific die casting process applications. For example, this tool can design an optimal coating system with spatial varying compositional architecture tailored for a varying thermal-mechanical environment in different parts of the die cavity.

In this paper a two-step non-linear thermo-mechanical finite element analysis is developed and applied to study these cyclic conditions within a die cast coating system. The first step is a global thermo-mechanical analysis of the complete die. These results determine the critical points on the interior surface of the die. This information is illustrated in stress and strain path plots along the interior of the die surface. Note, critical points are based on the von Mises yield criterion and the deviation of the surface strain from the free thermal expansion strain.

The second step is a local finite element analysis model that analyzes the behavior of a die coating system at the previously identified critical interior die surface locations. The boundary conditions of the local analysis are matched to the detailed surface strain data and the transient temperature results obtained previously from the global analysis. From the results of the local modeling various coating architectures can be studied and optimized.