Diamond Coatings on Surface-Modified Carbide Tools Using KrF Pulsed Laser
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ABSTRACT The surface of a cemented carbide was modified using a KrF pulsed laser process to achieve a microrough structure, leading to stronger adherence of diamond films to cemented carbide substrates. The surface morphology and roughness were investigated with laser conditions. After the surface modification with the laser and the etching of the modified surface, heat treatment was performed prior to deposition of diamond film in order to observe changes in surface morphology and adhesion. Diamond films were grown by chemical vapor deposition process. The results indicated that the heat treatment of the modified cemented carbide improved the adhesion of diamond films by the recrystallization of tungsten carbides into the fine and long grains. As laser energy for the modification of a cemented carbide increased, the surface roughness increased and tungsten carbide (WC) was transformed to WC1 ~x(X=0-0.3) and then to W 2C. INTRODUCTION Diamond has attracted a lot of interests over past decade due to the combination of its extreme properties. Especially the highest hardness, thermal conductivity, chemical inertness of diamond combined with a low coefficient of friction made it ideal for a tool material. Diamond coated (DC) tools have some advantages over conventional polycrystalline diamond (PCD) tools which are sintered polycrystalline diamond tips bonded with cobalt. DC tools have a flexibility in designing tool shapes such as chip breaker geometries and a usability of multiple tips with low manufacturing
cost [1, 2]. However, there is a serious problem to solve for the commercialization ofdiamond tools. During tooling operations, the diamond film flakes off easily due to poor interfacial adhesion with a cemented carbide substrate. The poor adhesion of diamond films to cemented carbides primarily results from the large differences in thermal expansion coefficient and the interaction of carbon obtained from hydrocarbon gas plasma with cobalt binder, leading to the formation of a thin graphite film on the cemented carbide surface [3-6]. There have been several approaches to improve the weak adhesion of diamond film to the cemented carbide substrate. One method is the removal of cobalt, which is used as a binder for tungsten carbide particles, from the surface of cemented carbide by chemical etching agents before diamond deposition [7-9]. Cobalt has a high solubility and diffusivity for carbon at temperature of diamond deposition [10]. Cobalt is helpful in maintaining toughness of a cemented carbide, but is harmful for diamond coating because of causing weak adhesion. Other method is the deposition of interlayer to prevent cobalt interaction with carbon and to reduce thermal stress arising from thermal mismatch between diamond and cemented carbide [11-13]. However, both methods are still not sufficient for tools to withstand severe machining condition. In this paper, a novel approach has been investigated to improve the adhesion of diamond films on cemented carbide substrates. This method involves the creation of mic
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