Improved Tribological Properties of Diamondlike Carbon/Metal Nanocomposites
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Improved Tribological Properties of Diamondlike Carbon/Metal Nanocomposites Roger J. Narayan and Dirk Scholvin School of Materials Science and Engineering Georgia Institute of Technology Atlanta, GA 30332-0245 USA ABSTRACT One of the many forms of carbon, diamondlike carbon (DLC), consists mainly of sp3bonded carbon atoms. DLC coatings possess properties close to diamond in terms of hardness, atomic bonding, and chemical inertness. Unfortunately, DLC exhibits poor adhesion to metals and polymers. The adhesion of the DLC film is determined by internal stresses in the film and by interfacial bonding. This work involves processing, characterization and modeling of diamondlike composite films on metal and polymer substrates to improve adhesion and wear properties. A novel target design was adopted to incorporate metal (silver or titanium) atoms into DLC films during pulsed laser deposition. STEM of the DLC/metal nanocomposites has shown that the metals that do not form carbides (e.g., silver) form 2-10 nm inclusions within the DLC matrix. Wear resistance measurements made on these samples have demonstrated that DLC/metal nanocomposites possess exceptional wear resistance. Diamond-like carbon nanocomposites also exhibit significantly enhanced adhesion. Careful analysis of the Raman data also indicated a significant shift to shorter wavelength in DLC nanocomposite films, indicating a reduction in compressive stress within these films. The sp3 content of these films was studied using electron energy loss spectroscopy (EELS). By varying the metal concentration as a function of distance from the interface, we have created functionally gradient DLC nanocomposites. These DLC/metal nanocomposite coatings have multiple biomedical applications. INTRODUCTION Biomedical researchers have over the past three decades created advanced materials by selecting commonly used bulk materials for their mechanical properties, and then modifying the surface to fit the biological environment.1 The rationale for biomaterial surface modification is straightforward: to retain the key physical properties (mechanical properties, durability, and functionality) of the bulk biomaterial, but improve the biocompatibility of the surface. For example, carbon- coated implants will behave mechanically as the metal or polymer bulk material, but chemically as the carbon coating. The term diamondlike carbon (DLC) is used to describe hydrogen-free amorphous hard carbon solids with atomic number densities greater than 0.19 g-atom/cm3. These materials possess a cross-linked, amorphous network of sp2- and sp3- hybridized carbon.2 Films of DLC demonstrate outstanding hardness (~80 GPa). The friction coefficient (~0.1) and wear coefficients of diamondlike carbon against a variety of tribological materials are among the lowest recorded. Unlike polycrystalline films, diamondlike carbon does not offer open corrosion paths. Besides their exceptional mechanical and tribological properties, DLC offers transparency to light ranging from deep UV through visible to far
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