| In the1970s, with the development of zirconia ceramic, the application of zirconia innew ceramic materials is expanded greatly. As an outstanding material in structuralceramics and functional ceramics, zirconia ceramics with high temperature resistance andcold, wear resistance, corrosion resistance and high strength and other advantages, iswidely used in national defense, energy, automotive, aerospace and other industrial fields.Milling is the most frequently machining operation in the industrial application of zirconiaceramic. However, due to the hardness of zirconia ceramic, low toughness and poor plasticdeformation ability, it makes machining process difficult and tools wear and failure easily,ultimately affects the machining efficiency and quality. Chemical vapor deposition (CVD)diamond films have many excellent properties, such as high hardness, good thermalconductivity, high wear resistant, low friction coefficient together with chemical inertness,which made them as the ideal materials for tool coatings. CVD diamond coated tools havegreat potential in solving the problems of milling zirconia ceramic for a variety of reasons:extremely high hardness and wear resistant, which helps reduce tool wear; lowcoefficients of friction, which decreases cutting and thrust forces. This dissertation aims atfabricating suitable CVD diamond coated tools for milling zirconia ceramic, depositingdiamond films with multiple structures on Co-cemented tungsten carbide (WC-Co) mills.Experiments in milling zirconia ceramic using diamond coated mills are conducted toinvestigate the cutting performance of the coated tools and obtain the appropriate cuttingparameters for milling zirconia ceramic with diamond coated mills. The main innovativeaccomplishments in this dissertation can be concluded as follows:1. Use hot filament CVD method to deposite microcrystalline diamond (MCD),nanocrystalline diamond (NCD) and microcrystalline and nanocrystalline composite diamond (MNCD) films on WC-Co ball end mills, and deposite films with differerntthickness, which is3μm,4.5μm and9.5μm respectly. The diamond films are characterizedwith scanning electron microscope (SEM) and Raman spectrum. a lot of diamond grainsof~2-3μm in size and many small diamond crystallites can be observed distributed on thesurface of MCD film; NCD and MNCD films present shape of cluster with tiny diamondcrystallites whose sizes can’t measured by SEM and a relatively flat surface without sharpshape.2. The frictional properties of the micro-crystalline diamond films (MCD), nano-crystalline diamond films (NCD), micro and nano-cryatalline composite diamond films(MCD/NCD) are comparably examined on UMT-2ball-on-plate friction tester, using thezirconia ceramic balls as counterpart materials. The friction performance between WC-Coinserts and zirconia ceramic balls is also examined for comparison. The frictioncoefficients of WC-Co and three types of diamond films are0.553，0.205，0.181and0.138. The experimental results show that the film morphology has great effect on thefriction properties. The MNCD films show the smallest coefficient due to their smoothsurfaces and lower hardness compared with MCD. Due to the sp2non-diamond structurein the NCD, the wear rate of NCD is higher than MCD. For the MNCD, it diaplays thehigher wear rate for the interlayer of MCD between substrate and NCD.3. Use hot filament CVD method to deposite films with differernt thickness, which is3μm,4.5μm and9.5μm respectly. The cutting performance of this three diamond coatedmills in milling zirconia ceramic are carried out, test results show that the diamond coatinghas good protection effect of the tools on the cutting process, the wear of the coating isvery small, and no shedding phenomenon. The wear of three diamond coating is12μm,19μm and20μm, showed that the thickness of the cutting tool wear is more serious, atthe same time, the cutting force is larger.4. Use hot filament CVD method to deposite microcrystalline diamond (MCD),nanocrystalline diamond (NCD) and microcrystalline and nanocrystalline compositediamond (MNCD) films on WC-Co ball end mills. The cutting performance of uncoatedMCD, NCD and MNCD diamond coated mills in milling zirconia ceramic are carried out.Uncoated tool has obvious wear in40min, has entered the severe wear stage in80min, flank wear more than0.05mm. After cutting in320min, the wear of MCD film, NCD filmand MNCD film coated tools is12μm,8.5μm and8μm. The cutting experiment resultsshow that the wear of diamond coated tools are greatly reduced than uncoated tools, andgreatly extend the life of cutting tools. Compared with MCD film, surface roughness ofNCD film and MNCD film is small, so wear is less and the cutting force is smaller. |
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