Study On Deposition Of Interlayers For CVD Diamond Coated Co-cemented Tungsten Carbide Tools

CVD diamond thin films are superhard and multifunctional materials with many excellent characteristics such as high hardness, high elastic modulus, high thermal conductivity, high chemical inertness, low friction coefficient and low thermal expansion coefficient, which make CVD diamond thin films ideal materials applied to cutting tools and wear resistant parts. Cutting tools coated with CVD diamond thin films have wide application potential in the cutting and machining of hard-to-cut non-ferrous materials such as titanium alloy, silicon-content aluminum alloys, nonferrous metals, ceramics and composite material. However, a key problem of insufficient adhesion exists between diamond film and cemented carbide substrate restricting the practical application of diamond film. In this thesis, we attempt to prepare diamond-silicon carbide-cobalt silicide films and nanocrystalline silicon carbide films as interlayers for deposition of adherent diamond coatings on Co-cemented tungsten carbide (YG6X) substrates by bias-enhanced hot filament CVD using hydrogen, methane and tetramethylsilane (TMS) as the reactive gas source. Surface morphology, composition, structure, surface roughness and adhesion of the deposited films were characterized by scanning electron microscopy, electron probe microanalysis, X-ray diffraction, Raman scattering, infrared spectroscopy, metallographic microscopy, surface profile, Rockwell-C indentation test, friction and wear test and cutting test. The main research results in the thesis can be concluded as follows:(1) During the deposition of diamond-silicon carbide-cobalt silicide films, diamond, silicon carbide and cobalt silicide grow competitively with the changes of bias currnet and tetramethylsilane concentration. As the bias increases, the content of diamond whose grain size become more and more larger is gradually increased, while the contents of silicon carbide phase and cobalt silicide phase are both gradually reduced. As tetramethylsilane concentration increases, the content of silicon carbide phase are increased by degrees, while diamond and cobalt silicon phase are both gradually reduced. The interlayers containing of diamond phase, silicon carbide phase and cobalt silicon phase are proved to improve the adhesion of diamond coating on WC-Co substrates by Rockwell-C indentation test, together with friction and wear test. (2) During the deposition of nanocrystalline silicon carbide films, with the increase of tetramethylsilane concentration in the reactive gas source, the crystal structure of silicon carbide whose grain size decreases changes from hexagonal to cubic and the particle shape changes from spherical to cauliflower-like. Rockwell-C indentation tests show that nanocrystalline silicon carbide interlayer deposited with0.1vol.%TMS evidently improve the adhesion of diamond coating on WC-Co substrates.(3) Cutting tests results indicate that compared to tool without diamond coating and diamond coated tool with composite films as an interlayer, diamond coated tool with the pretreatment of sharping, Murakami reagent30min, Caro acid60s and with composite films as an interlayer deposited with0.075vol.%TMS has better cutting performances such as better adhesion strength, lower wear rate and lower surface roughness of workpiece.