Study On The Fabrication And Application Of Diamond Films On Micro Cutting Tools

The micro part system belongs to the high-tech products, which are formed via mechanical, electrical and hydraulic technologies in the small scale, widely used in the green energy, information electronics, aerospace, industrial production and instrument industries. Micro cutting is a processing technology of directly machining workpiece in a mesoscopic scale, which has excellent 3D machining ability, the universality of cutting material, high machining efficiency and low cost, is an important part of micro parts processing technology system. In the process of micro cutting, the high cutting speed between cutting tool and the workpiece results in their intense mechanical friction interaction, putting forward higher requirements of wear resistance and anti workpiece adhesion performance of micro cutting tools compared with conventional ones. However, ordinary carbide micro cutting tools easily encounter rapid severe wear, broken, workpiece adhesion, low cutting efficiency and large consumption in high speed micro cutting processes. Therefore, there is an urgent need of new micro cutting tools with better wear resistance and longer working life in micro cutting industry to meet its demand of development.Chemical vapor deposition(CVD) diamond films exhibit many excellent mechanical and tribological properties, such as high hardness, low friction coefficient, good wear resistance, high surface chemical inertness and low adhesion against many materials. Depositing diamond films by hot filament CVD(HFCVD) technique have several advantages, such as easily controlled equipment, high deposition efficiency, low cost and unlimited substrate shapes, which have become the main method to fabricating diamond coated cutting tools. Consequently, depositing diamond films on micro cutting tools by HFCVD method can effectively slow down the wear of cutting edges, improve durability of micro cutting tools, which may break through the technical bottleneck of micro cutting.Based on the HFCVD technique, this dissertation aims at solving four key problems inhibiting the fabrication and application of high performance HFCVD diamond coated micro cutting tools, they are: fabrication and wear properties of CVD diamond films, the simulation of temperature distribution on micro cutting tools when diamond films are deposited on massive micro cutting tools by HFCVD technique, the optimized fabrication of high performance diamond coated micro cutting tools and the fabrication and application of high performance diamond coated micro cutting tools which are suitable for machining of graphite, printed circuit board(PCB) and 7075 aluminum alloy. The main works accomplished in this dissertation can be concluded as follows:1. The research on fabrication and wear behavior of CVD diamond films. The fundamental properties of CVD diamond films have direct impact on its application properties on micro cutting tools. Firstly, the difference of micro structure, surface roughness, grain orientation, diamond purity as well as film adhesion between micro crystalline diamond(MCD) and nano crystalline diamond(NCD) films on cemented carbides are comparatively studied. Afterwards, a new tribological experiment method involving self-mated and mutual mated contacts between MCD and NCD films is proposed to quantitatively compare the wear resistance of MCD and NCD films. The results show that the main wear mechanism of these working pairs is found to be abrasive wear mode induced by self-polishing. The wear rate of NCD film is twice as MCD film, the main reason is that the MCD grains with higher hardness can be easily insert the NCD film, resulting in rapid wear of NCD film. In conclusion, MCD film exhibit better wear resistance than NCD film.2. The optimization of simulated temperature distribution when depositing diamond films on massive micro cutting tools by HFCVD method. The three dimensional simulated model of fabricating diamond coated micro cutting tools by HFCVD method is established in computational fluid dynamics software based on finite volume method. The temperature distribution in the vacuum chamber is simulated in the software coupled with the three hear exchange modes which are heat convection, thermal radiation and heat transfer. Firstly, the effectiveness of the complicated three dimensional simulated model is verified by temperature measuring experiment. Then, the orthogonal experiment method is adopted to study the influence of the diameter(d); vertical height(H); horizontal spacing distance(D) and length of hot filaments on the temperature distribution on micro cutting tools and to obtain the optimized strategy. When depositing diamond films on massive micro cutting tools by HFCVD method, the uniformity of the temperature field distribution significantly increased under the optimized arrangement way of hot filaments(d=0.65 mm, H=12 mm, D=27 mm, L=160 mm). At last, the actual diamond deposition experiments show that the grain size and thickness of deposited diamond films on micro cutting tools at different positions under the optimized hot filament arrangement strategy are similar.3. The optimizing fabrication methods of high performance diamond coated micro cutting tools. In order to obtain high performance diamond coated micro cutting tools which exhibit high flexural strength, good uniformity of deposited films and high adhesive strength between films and substrate, the fabrication processes are optimized from two aspects which are pretreatment process and deposition parameters. A novel pretreatment zone is proposed to significantly improve the flexural strength by fracture tests. Then the orthogonal experiment method and indentation experiment are adopted to obtain the optimized alkali treatment time(10 min) and acid treatment time(10 seconds), which do not significantly weaken the flexural strength of micro cutting tool and can effectively coarsen the substrate surface and etch cobalt. Afterwards, the effect of the nucleation reaction pressure, substrate temperature and carbon resource concentration on the nucleation density, growth rate and uniformity of deposited diamond films are studied, and the nucleation reaction pressure, substrate temperature and flow rate ratio between carbon source and hydrogen are optimized to be 1.6 KPa、850 oC and 1:2.75, respectively. Then, based on the facts that boron doped diamond(BDD) film could prevent cobalt overflow from substrate at high temperature and wear resistance of BDD is worse than that of MCD, a novel BDD+MCD composite film in which BDD is used as interlayer is adopted to enhance the adhesion of diamond films. The graphite machining tests show the working life of BDD+MCD coated micro drill is 2-3 times compared with MCD or BDD coated micro drills.4. The study on fabrication and application of high performance diamond coated micro cutting tools suitable for machining of graphite. This part mainly focuses on optimizing cutting performance of CVD diamond coated micro cutting tools by coating type and thickness. The friction tests show that MCD film exhibits higher graphite removing rate. The graphite cutting experiments involving MCD, NCD, DLC and Ti Al N coated micro cutting tools show that the working life of MCD coated micro cutting tool are 1.5, 2, 6 and 9 times of NCD, DLC, Ti Al N coated and uncoated micro cutting tools, which are mainly because of the superior wear resistance of MCD films. Then, MCD films of different thicknesses are deposited on flat samples and micro cutting tools. The MCD film with bigger thickness exhibits higher surface diamond purity, hardness, lower residual compressive stress and lower film-substrate adhesive force. When the coating thickness is 2.5 μm, the MCD coated micro cutting tool show longest working life due to the excellent wear resistance as well as high film-substrate adhesive strength of this coating.5. The optimizing fabrication of CVD diamond coated PCB micro drills. This part studies the tribological properties between MCD, NCD film and cemented carbide against PCBs. The experiment results show that the friction coefficient in NCD-PCB working pair is lowest. The PCB cutting tests involving MCD, NCD, DLC and Ti Al N coated micro cutting tools show that NCD coated micro cutting tool exhibits the longest working life due to the good wear resistance and low friction coefficient of NCD film against PCBs. Then, based on the drilling hole quality, the cutting force and tool wear results, the PCB drilling tests involving NCD coated micro drills of different coating thicknesses show that when the NCD film is 3 μm, the NCD coated micro drill display the longest working life, which is about 5-7 times compared with that of uncoated micro drill.6. The optimization of CVD diamond coated micro cutting tool in aluminum alloy machining. Based on the good adhesiveness and high hardness of aluminum alloy, friction tests are conducted between CVD diamond films and aluminum alloy, which shows that NCD film exhibit lower friction coefficient and wear rate when sliding against aluminum alloy compared with MCD film due to good wear resistance, high chemical inertness and low adhesion of NCD films against aluminum alloy. Thereafter, the aluminum alloy cutting tests involving MCD, NCD, DLC and Ti Al N coated micro cutting tools show that NCD coated micro cutting tool exhibits the longest working life. During the micro cutting process, the cutting edges of NCD coated micro cutting tool maintain their integrity, the cutting force stays at a low value, the drilling holes show clear hole wall and good roundness. The aluminum alloy drilling tests involving NCD coated micro drills of different coating thicknesses display that when the coating thickness is 4.5 μm, the working life of NCD coated micro drill is 4.5 times compared with the uncoated ones, and much longer than that of the other NCD coated micro drills with different coating thicknesses. The proper coating thickness is helpful for protecting the cutting edges as well as maintaining the sharpness of cutting edges.The successful application of high performance CVD diamond coated micro cutting tools in graphite, PCB and aluminum alloy machining would not only improve the fabrication efficiency and reduce the manufacture cost of graphite, PCB and aluminum alloy micro parts, but also play a positive role in upgrading the technology of micro cutting tool industry.