Deposition Techniques And Dry Machining Properties Of CVD Diamond Coated Cutting Tool On WC-Co Cemented Carbide

CVD diamond coated cutting tool is an outstanding candidate for machining difficult-to-machine material due to its unique properties such as low coefficient of friction, high thermal conductivity, high-temperature hardness and chemical stability. A wide spread application in the case of CVD diamond cutting tools is used for machining abrasive and hard materials, such as MMC's, aluminum silicon alloys and tungsten carbide. It will find its wider and wider application in the field of automobile, aeronautics and astronautics. However poor adhesion of diamond coating is the main technical barrier for commercialization of diamond-coated tools. The thesis aims at some key technological issues regarding on improving the adhesion strength of diamond coated cutting tool and its application in machining. Those key technological issues include decreasing the negative effect of cracks in the diamond coating, improving the adhesive strength between the diamond film and the substrate, and evaluating the cutting performance.The main research contents of the thesis are as following:1. A theoretical model which describes the microstructural aspects of short original cracks was developed firstly based on fracture mechanics to provide a rational explanation for the phenomenon of interface strength between CVD diamond coating and its substrate. The expression of stress field, displacement field and stress intensity factor (SIF) k at interface crack tip were obtained. The interface crack extension, the coating crack growth and the coating fracture process were studied using the original crack model. The coalescence conditions were carried out to develop the adhesion between the CVD diamond coating and its substrate.2. A gas flow field model of HFCVD system was set up. Relationship between the gas flow and CVD diamond coating on the rake face and flank of cutting tool was studied. Finite element analysis method was used to investigate the gas flow field around the cutting tool substrate. Parameters such as distance between cutting tool substrates dt-t, distance between hot-filament and cutting tool substrate df-t, distance between gas inlet and substrate upper surface din-t and gas inlet number nin, which affect on the system gas flow field was discussed in detail. Experiments have been done to prove the simulation results. Excellent agreement between simulation and experiment was obtained by depositing of CVD diamond coated cutting tools using a hollow substrate holder.3. Relationship between the substrate temperature and the quality of CVD diamond coating was studied. A hollow substrate worktable on which the substrate temperature increases rapidly during the heating period and reduces slowly during the cooling period was designed based on the HFCVD system energy transfer. The hollow substrate worktable can be used for large area diamond coating deposition, which can get high efficient productivity. Parameters such as hot-filament number nf, distance between hot-filament and cutting tool substrate df-t and hot-filament radius rf, which affect on the system substrate temperature field was studied using a substrate temperature field finite element analysis model. Optimized parameters to deposite high quality CVD diamond coatings using a hollow substrate holder were obtained.4. Relationship between the substrate/diamond-coating adhesion strength and interface properties such as substrate material, substrate surface state, and substrate diamond seed implanting was discussed. The substrate pretreatment process of etching WC and etching Co was studied. A gradient substrate etching pretreatment process (GSEPP) was developed firstly. The substrate treated by GSEPP shows good properties for CVD diamond nucleation and growth, and which appears to result good substrate/diamond-coating adhesion strength. GSEPP is a high efficient, easy-to-operate and low cost substrate pretreatment for WC-Co cemented carbide substrate to deposite CVD diamond coated tools.5. The elastic modulus and hardness of CVD diamond coatings on WC- 6wt%Co cemented carbide were measured by a nano-indenter system. Stress in diamond coatings was evaluated by XRD and Raman spectroscopy. Relationship between deposition parameters and stress in diamond coatings deposited on both water-cooled substrate holder and hollow substrate holder was studied. Indentation method was developed to evaluate the substrate/diamond-coating adhesion strength.6. Cemented carbide cutting tools with and without CVD diamond coatings were used in continuous and dry turning operations of Al-20wt%Si. Comparing with diamond coated inserts, the uncoated inserts tend adhere at the tool tip and cutting edge which leads to built up edges. The analys is of frank wear and tool tip wear revealed that the tool life of diamond coated inserts is longer than that of the uncoated inserts. The diamond coated inserts are fit for aluminum silicon alloy engine piston operation.