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U7.8.1

Structure and Mechanical Properties of Functionally-Graded Nanostructured Metalloceramic Coatings Shane A. Catledge, Yogesh K. Vohra, Shanna Woodard1, and Ramakrishna Venugopalan1 Department of Physics, University of Alabama at Birmingham Birmingham, AL 35294-1170, U.S.A. 1 Department of Biomedical Engineering, University of Alabama at Birmingham Birmingham, AL 35294-4440, U.S.A. ABSTRACT A functionally graded nanocrystalline metalloceramic coating on cobalt-chrome alloy was investigated using thin film x-ray diffraction (XRD), cross-sectional transmission electron microscopy (TEM), nanoindentation, and scratch adhesion testing. The gradual transition in bonding from metallic (Cr/CrTi) near the interface to predominantly covalent (CrTiN) near the surface provides a combination of high toughness and high surface hardness. XRD analysis of the (CrTiN) coating suggests a cubic sodium chloride type phase structure with lattice parameter a = 4.2169±0.0035 Å. The surface layer structure is described as a tertiary Ti-N-Cr disordered solid solution that is predominantly cubic TiN, but with some Cr atoms substituted for Ti. TEM shows a transition from equiaxed 20-40 nm-sized grains at the surface to larger, elongated columnar grains below the surface. Nanoindentation measurements of the coating result in a hardness of 27 GPa and Young’s modulus of 320 GPa. In addition, the high plasticity of 55% observed for this coating represents an increase in toughness over other ceramic coatings having similar hardness. The unique, functionally graded, smooth nanocrystalline metalloceramic coating structure provides an opportunity to reduce wear and increase longevity of total hip joint replacements. INTRODUCTION Functionally graded, ultra-smooth nanocrystalline metalloceramic coatings [1, 2] are designed to exhibit a transition of chemical bonding such that excellent interfacial adhesion is attained (metal-metal bonds) while insuring a high surface hardness consistent with ceramic materials (predominantly covalent/ionic bonds). Such coatings may potentially reduce ultra-high molecular weight polyethylene (UHMWPE) wear in orthopedic devices [3-8] without the common adhesion problems evident in coating applications. The functionally-graded coating described in this work is deposited by ion-beam assisted deposition (IBAD), has metallic initial layers and a hard, ceramic outer coating (Cr/CrTi/CrTiN). A (Cr/CrTi) metallic layer at the interface will increase adhesion to the cobalt chrome substrate, while the surface layer (CrTiN) is predominantly covalent in nature and will enhance scratch and wear resistance of the coating. The chromium metal layer increases coating adhesion and increases strength of the coating by serving as a barrier to crack formation and propagation. These coatings have three primary advantages over conventionally deposited ceramic films: nanocrystalline structure with average grain size of 12 nm, functionally-graded layer whose bonding gradually changes from metallic to covalent with increasing ceramic materia