A new multiple chemical pretreatment for fabrication of high quality CVD diamond-coated drills

摘要

Because of its extreme hardness, wear resistance, good thermal conductivity and low thermal expansion coefficients, a diamond tool exhibits a range of properties which makes it a key and unique component in many fields of mechanical engineering. In comparison with the other types of diamond tools, the thin film diamond-coated cutting inserts have great potentialities in the commercial application because of its low costs for fabrication equipment and high performance. Thus, they are on the verge of becoming the next generation of tools used for high speed machining non-ferrous metals and alloys, metal-compound materials, and hard brittle non-metals. However, the poor adhesion of the diamond film on the substrate becomes the main technical barriers for the successful development and commercialization of diamond-coated tools. In this paper, diamond thin films were deposited on WC-Co cemented carbide substrates by the electron aided hot filament chemical vapor deposition (EACVD), using a mixture of hydrogen and acetone as source gases. A new multiple chemical pre-treatment including microwave oxidation, reaction in alkaline solution and cleaning by ultrasonic treatment in acid solution has been performed for free shape cemented WC-Co drills in order to increase the diamond nucleation and to enhance the coating adhesion. And also, another new pretreatment method using Ar-H{sub}2 gas etching decarburization by microwave plasma is performed for the WC-Co substrate surface. The influences of this pretreatment technique on the adhesion strength, diamond nucleation and surface morphology of the diamond thin film are investigated. Scanning electron microscope (SEM) was used to view the substrate morphology, and to examine the cobalt removal. After pre-treatment, the surface of the WC-Co substrate becomes slightly rough, but its composition and structure shows no changes identified by X-ray diffraction (XRD). Scanning electron microscope (SEM) indicates a distribution of uniform micro-roughness WC grains on substrate surface. And also the research results show that the pretreatment using Ar-H{sub}2 etching decarburization by microwave plasma is an effective method to enhance adhesive strength. The appropriate CVD processes were studied for the fabrication of tools with complex geometries. An adequate amount of additive can effectively prevent the overflow of Co from inner to surface of the substrate and avoid the catalytic effect of Co during CVD. The smooth diamond film with low surface roughness can be deposited by increasing the carbon source concentration and reducing the reactive pressure at the later stage of CVD process. The diamond morphology quality and homogeneity of coatings were characterized by SEM and Raman spectroscopy. The cutting performances of diamond-coated tools were studied by the cutting tests of difficult-to-machine materials. The research results show that the high quality diamond coating has been deposited on complex shape substrates for WC-Co cemented carbide twist drills. The cutting performances of diamond-coated drills are improved further by the new pretreatment and CVD process. It is of great significance for the fabrication and application of the diamond-coated tools with complex geometries.