Research Interests
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Chemical Vapor Deposition (CVD) of diamond and other hard forms of carbon continues to be a very active area of research, particularly in the field of tribology where coatings of high hardness and low friction are required. The superlative properties of diamond, which include the highest known hardness and room temperature thermal conductivity, as well as the lowest compressibility of any material make it an ideal candidate for many applications. Much interest has been generated in the synthesis of hard carbon films exhibiting a wide range of structural forms and properties. The computer hard-disc, cutting tool, and biomedical implant industries are only a few examples in which hard carbon films (with names including “diamond”, “diamond-like carbon”, “nanostructured diamond”, and “tetrahedral amorphous carbon” in order to distinguish between different structural forms) have been investigated. |
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High phase purity diamond films can be grown using CVD techniques with feedgas mixtures containing hydrogen (H2) and methane (CH4). While these highly crystalline diamond films are exceptionally hard, they are typically very rough and very brittle. For applications requiring smooth surfaces (for low friction), high hardness (for high wear resistance), and high toughness (for preventing catastrophic brittle fracture), a more viable solution is needed. This is especially true for deposition on metals since large thermally-induced residual stresses can be present in the film, providing a driving force for delamination. Ideally, the film must be well bonded to the substrate and be able to undergo some plastic deformation without hard elastic-to-brittle fracture occurring. One promising material is that of nanostructured diamond, generally defined as containing small diamond grains (from about 5-100 nm) imbedded in an amorphous carbon phase that will typically include both sp2 (graphite-like) and sp3 (diamond-like) carbon bonds. These films can still be quite hard (up to 80% that of natural diamond) but can also offer the advantage of much lower surface roughness and much higher fracture toughness. Several processing routes for these nanostructured films exist, depending on deposition technique and/or feedgas mixture We have developed a patented process for making hard, smooth, and well-adhered nanostructured diamond films suitable for some metals. My research interests include an investigation of the processing, structure, and mechanical property characterization of diamond films grown using a variety of deposition conditions and substrate materials. So far, most of my focus has been on the titanium alloy (Ti-6Al-4V) as a substrate material. This material is used extensively in the aerospace and biomedical implant industries. Diamond deposition (particulary nanostructured diamond) has proven to be very successful on this alloy in terms of film/substrate adhesion. Other substrate metals of current interest but of more significant challenge for producing adhered films include steel and cobalt chrome alloy. Eventually, we hope to coat more complicated substrate geometries other than flat surfaces. Instrumentation in our laboratory used for characterization of our CVD diamond coatings include micro-Raman spectroscopy, surface profilometry, glancing-angle x-ray diffraction (XRD), optical microscopy, nanoindentation, and atomic force microscopy (AFM). We also make use of scanning electron microscopy (SEM) as shown below:
SEM images of a high phase purity polycrystalline diamond film (left) and a nanostructured diamond film (right). The film on the right is an order of magnitude lower in surface roughness than the film on the right. |
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Homoepitaxial Diamond Growth and Polishing for High Pressure
Electrical Transport Measurements Electrical transport experiments are among the most challenging of ultra-high pressure experiments with diamond anvil cells (DAC’s). These experiments require very careful and painstaking preparation. Standard high pressure electrical transport techniques involve the use of thin metal wires or foils which must be carefully maneuvered into place on top of a small sample in a specially prepared alumina gasket, which makes precise placement of the probes difficult. In addition, due to the very large amount of plastic flow of the gasket which accompanies pressurization of the sample, electrical contact to the sample may be lost, or the wires or foils may short-circuit to each other or to the surrounding metal gasket. These difficulties have limited the number of ultra-high pressure techniques that can be performed. For example, at the present time, no nuclear magnetic resonance (NMR), Hall effect, or internal ohmic heating experiments have been performed at Mbar pressures. We have collaborative research efforts with Lawrence Livermore National Laboratory (LLNL) to study electrical conductivity of materials at ultra-high pressures using a novel diamond anvil cell technique. This technique makes use of recent advances in epitaxial CVD diamond deposition and advanced lithography techniques to fabricate custom-designed diamond anvils especially suited for electrical transport measurements at Mbar pressures. We refer to these custom-designed diamond anvils as “Designer Anvils”. The fabrication process involves lithographically preparing thin metal probes to the tips of beveled diamond anvils, epitaxial CVD diamond deposition of the probe-anvil in order to mechanically and electrically insulate the probes, and final polishing/shaping of the CVD layer for ultra-high pressure experiments in a DAC. This new technology has the potential of greatly advancing the pressure range of a number of existing high-pressure diagnostic techniques, and for expanding the capabilities of DAC’s into new directions. Reflection Transmission
A schematic (above) of the designer anvil. The designer anvil features a set of thin-film metal microprobes and a protective CVD diamond layer encasing the microprobes. Optical micrographs of a completed designer anvil are shown in both reflection (above left) and transmission (above right) light. |
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