Study On The Deposition, Tribological Properities And Applications Of Ultra-smooth Composite Diamond Films

Chemical vapor deposition (CVD) diamond film has attracted intensively investigations as coating materials on a wide variety of Co cemented tungsten carbide (WC-Co) cutting tools, especially on complicate-geometry tools, and drawing dies, due to its many excellent properties, such as extremely high hardness, wear resistance, chemical inertness, together with very low friction coefficient. Depositing CVD diamond films on the WC-Co cutting tools and drawing dies not only yields impressive lifetime elongation and performance enhancement, but also attributes to the machining technique promotion, resource saving and sustainable economy development. This dissertation aims at solving three key problems inhibiting the industrial application of CVD diamond films, i.e. (1) depositing adhesive and smooth CVD diamond films on WC-Co substrate, (2) a deep insight into the tribological properies of CVD diamond films deposited on WC-Co substrate and (3) applications of CVD diamond films on the complicate-shaped cutting tools and drawing dies with large aperture. The main innovative accomplishments in this dissertation can be concluded as follows:1. Study on the ultra smooth composite diamond (USCD) film deposition technique. A novel pre-treatment method and a dynamic boron doping method are proposed with the aim of improving the adhesive strength between diamond films and susbtrate. Furthermore, a surface refinement process is developed based on the HFCVD method by applying negative bias voltage between hot filaments and substrate; moreover, a USCD film deposition method is proposed through alternative performing the MCD/NCD deposition and surface polishing processes multiple times. The USCD film, which consists of a layer of MCD film and several layers of NCD films, has strong adhesive strength and its surface roughness is measured as Ra / Rq ~ 8.854 / 11.3 nm (AFM) and Ra / Rq ~ 88.8 / 118.1 nm (surface profilometer). 2. Study on the tribological properties of CVD diamond films on WC-Co substrate. The frictional properties of the microcrystalline diamond film (MCD), fine-grained diamond film (FGD) and USCD film are comparablely examined on a ball-on-plate reciprocate friction tester, using ball-bearing steel and copper as counterpart materials. The results demonstrate the significant influence of the surface morphology of CVD diamond films on its friction behaviors. The sharp tips of diamond grains on the diamond film surface will cause significant plough effect on the mating surface and lead to much higher friction coefficient and wear. The generated wear debris will adhere on the diamond film surface, increase the adhesion between the sliding interfaces and thus increase the friction coefficient. Water lubricating can decrease the friction coefficient significantly. This decresing effect can be attbributed to the low adhesive strength of physisorbed water layer formed on the sliding interface to a large extent. Additionally, another important attribution is supposed to be the fact that fluidity of water removes a large amount of wear debris adhered on the diamond film surface and thus decreases the adhesion on the sliding interface. The friction coefficient of USCD films sliding against ball-bearing steel and copper are 0.13 and 0.17 respectively in dry sliding, and reduce to 0.05 and 0.14 with water lubricating.3. Study on the atomic-scale tribological mechanism of CVD diamond films. The atomic-scincofapilnpeie t ref(r 0ficl at0ito -1fnl)a tsb ucerohfnaavtcaeico tar sra en odifn t vhdeeisa stmiignoagntleded- a (us1ps 1ienr 1git )ym aconoldne ct aduciltaa.mr T dohynend sa(im0m i0ucl s1a ts)iio msnuu rrlfeaastcuioel tn sis n,e xw chioitbnhit tab ctohtt ah wt ttihhthee dependence of the atomic-scale friction behaviors on the atomic-scale surface roughness, surface atomic structure, surface hydrogenation, load and sliding velocity. The friction behavior of diamond is anisotropic, and the surface hydrogenation can eliminate such anisotropy to some extent. With increasing load, the atomic-scale wear, lattice dislocation and deformation appears and accordingly the friction coefficient increases significantly. The atomic-scale surface roughness of diamond lattice causes the scale effect of the continuum mechanics, and such scale effect depends on the the tip crystallographic surface, tip radius and load.4. Study on the application of complicate-geometry CVD composite diamond coated cutting tools. Boron-doped CVD diamond films are deposited on the WC-Co cutiing tools and drills, and the cutting performance of as-fabricated diamond coated tools are examined using SiC particulate reinforced Al-metal matrix composite (Al/SiC-MMC) materials as the workpiece. The cutting test results show that the optimum boron concerntration is B/C = 3000 ppm while fabricating CVD diamond coated cutting tools. Furthermore, a revised HFCVD apparatus with vertical spiral hot filament arrangement was developed for depositing adhesive and smooth CVD composite diamond films on the complicate-geometry printed circuit board (PCB) milling tools. CVD composite diamond films are deposited on the cutting tools and PCB milling tools, and the cutting performance of as-fabricated diamond coated tools are examined using glass fibre reinforced composite materials as the workpiece. The cutting test results demonstrate that the lifetime of diamond coated cutting tools is as 8 times as that of the WC-Co cutting tools, and the surface roughness of workpiece is enhanced largely. For the diamond coated PCB milling tool, its flank wear on circle and end cutting edge after 40 min milling are only about one-fifth and a quarter of that of uncoated WC–Co PCB milling tool. Additionally, the thrust forces acted on the diamond coated PCB milling tool during the milling process are much smaller than that on the WC-Co milling tool.5. Study on the applications of USCD coated drawing dies. The revised HFCVD apparatuses with twisted and squirrel-caged filament arrangement are developed for depositing adhesive and smooth USCD films on the inner-hole surface of various drawing dies used in producing coaxial cables, copper tubes and low-carbon steel pipes, especially for the drawing dies with large aperture. The surface roughness on the entry cone, working cone and sizing cone are measured as Ra ~ 25.7 nm, 23.3 nm and 25.5 nm respectively. In the coaxial cable production, the lifetime of USCD coated welding dies is as more than 100 times as that of nylon welding dies, and as 10 times as that of WC-Co welding dies. In the copper tubes production, the lifetime of USCD coated drawing is as more than 30 times as that of WC-Co drawing dies in the drawing process with hollow sinking, 7 times in the fixed plug drawing and 10 times in the floating drawing. In the low-carbon steel pipes production, the lifetime of USCD coated drawing dies is as 10 times as that of WC-Co drawing dies. The application of USCD coated drawing dies not only yields impressive lifetime elongation, efficiency promotion and resource saving, but also produces pipes with smooth surface and constant diameter, which saves a large amout of raw materials of copper and aluminium. Furthermore, the water lubricating system can be adopted while producing coaxial cable and low-carbon steel pipes using USCD coated drawing dies, which is beneficial for reducing the environmental pollution and implementing the green manufacturing process.