Adhesion Improvement of CVD Diamond Coatings on WC-Co Substrates for Machining Applications
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Adhesion Improvement of CVD Diamond Coatings on WC-Co Substrates for Machining Applications Zhenqing Xu1, 2, Ashok Kumar1, 2, Leonid Lev3, Michael Lukitsch3, and Arun Sikder2 1 Department of Mechanical Engineering, University of South Florida, Tampa, FL, 33620, USA 2 Nanomaterials and Nanomanufacturing Research Center, University of South Florida, Tampa, FL, 33620, USA 3 General Motors Corporation, Warren, MI 48090, USA ABSTRACT In order to improve the performance of WC-Co cutting tools, high quality microcrystalline diamond coatings were produced using microwave plasma enhanced chemical vapor deposition (MPECVD) method. The adhesion of the diamond film deposited on the substrate has been considered to play an important role in the performance of the cutting tools in machining applications. A thin layer of Cr was coated on the WC-Co substrate before the diamond deposition; 75 µm diamond powders were sandblasted on the surface at 40 Psi to increase the nucleation density. Diamond film has been successfully deposited on the substrate at temperature around 750ºC with 1.5 % CH4 in Hydrogen plasma. Scanning electron microscopy (SEM) has been used to study the surface morphology and Raman spectroscopy has been performed to characterize the quality of the diamond films and measure the residual stress. The adhesion of the diamond film has been evaluated by Rockwell indentation test. The results indicated that film grown on the Cr interlayer with diamond powder sandblasting has much better adhesion strength. INTRODUCTION Diamond Coating is an industrial solution to improve the surface properties in a wide variety of wear resistant applications because of its outstanding properties. Along with great wear resistance, the advantages of diamond coating include high surface hardness, high thermal conductivity, reduced friction, better corrosion protection and improved optical properties. Cemented carbide is a material made by "cementing" very hard tungsten monocarbide (WC) grains in a binder matrix of tough cobalt metal by liquid phase sintering. The cemented tungsten carbide cutting tools are widely used all over the word in automotive industry, metal machining, mining and stone cutting industry. However, without a hard coating such as diamond, the cemented WC tools are found to wear rapidly when machining some particular materials such as green ceramics, abrasive composites or high silicon-filled aluminum. Successful development of diamond-coated tools will significantly benefit the performance of WC-Co cutting tools. As a result, diamond coated tools will have reduced down time, better cutting performance and less defective machined surface. However, the poor adhesion of the diamond film on the substrate is the main cause of tool failures and becomes the main technical barriers for commercialization of diamond-coated tools [1-4]. The reason behind is that the deposition of diamond films on cemented carbides is strongly hindered by catalytic effect of cobalt. During the deposition, the Co leaching from the WC-Co substr
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