Papers by Author: Bin Shen

Abstract: This paper presents a study of the influence of cutting conditions (cutting velocity, feed, cutting depth and lubrication) on turning TC11 (Ti-6.5Al-3.5Mo-1.5Zr-0.3Si) titanium alloy. Taguchi methodology design was adopt for carrying out experiments. Turning process parameters such as cutting speed, feed rate and depth of cut were varied to study their effect on process responses such as cutting force (Ft), surface roughness (Ra) and temperature on cutting zones (T). Minimum quantity lubrication (MQL) technology was adopt to increase the lubricating and cooling effect. Meanwhile, CVD diamond coating was deposited on the cemented carbide insert to reduce its friction with workpiece and increase its wear resistance. From the analysis of orthogonal tests, depth of cut contributes the most for the main cutting force and cutting temperature, while feed rate had the most significant effect on surface roughness on the workpiece. MQL can reduce the cutting temperature at the cutting zones, especially for the uncoated cutting inserts whose temperature decreases by an average of 60~80°C. The cutting force, surface roughness and cutting temperature of CVD diamond coated inserts were all higher than those of uncoated tools, especially with MQL lubrication. Considering the cutting efficiency and cost, the optimal parameters in the turning process of TC11 for minimizing the cutting force, surface roughness and cutting temperature are obtained as Vc=115m/min, f=0.08mm, a_p=0.5mm under MQL lubricating with uncoated cemented carbide as the cutting tool.

Abstract: The present study reports the influence of graphene layers on the tribological performance of CVD diamond films when they are used as the solid lubricants. Friction tests are conducted on a ball-on-plate friction tester, where the stainless steel is used as the counterpart material. The CVD diamond film sample is a typical microcrystalline diamond (MCD) coating which is deposited on a flat tungsten carbide substrate using the hot filament chemical vapor deposition method (HFCVD). Besides the MCD sample, a polished MCD film (pMCD) and a polished tungsten carbide (pWC) are also adopted in frictional tests, aiming at illustrating the influence of the surface morphology, as well as the physical property, of the sample on the lubricative effect of graphene layers. The experimental results show that graphene layers can effectively reduce the coefficient of friction (COF), regardless of the samples. The MCD sample presents the lowest stable COF, which is 0.13, in dry sliding period when the graphene flakes are sparyed on the sliding interface; while the pMCD and pWC samples exhibit slightly higher COFs, which are 0.16 and 0.18, respectively. Comparatively, the COFs of these three samples obtained in dry sliding process without graphene are 0.20, 0.25 and 0.64. In additon, the MCD sample exhibits a much longer stable dry slidng process which is more than 5000 cycles. Comparatively, the other two tribo-pairs only exhibit a stable low-COF dry sliding period for around 2000 cycles. The reduction of COF could be attributed to the graphene flakes adhered on the sliding interface. It forms a layer of solid lubricative film with extremely low shear strength and significantly decreases the interactions between two contacted surfaces. The rugged surface of the MCD film provides sufficient clogging locations for graphene flakes, which allows the generated lubricative film enduring a long sliding duration. It can be arrived from this study that the tribological properties of the MCD film could be enhanced by simply adoping graphene layers as a solid lubricant. Furthermore, an improved performance of a variety of MCD coated cutting tools or mechanical components could be expected when they are utilized with graphene layers.

Abstract: Carbon fiber reinforced plastics (CFRP), which are widely used in the aerospace and some other new-tech industries, are considered very difficult to machine due to the material anisotropic and inhomogeneous features. Chemical vapor deposition (CVD) diamond films are suitable as protective coatings on cutting tools for machining advanced composite materials, owing to their extremely high hardness, favorable wear resistance, low friction coefficient and high thermal conductivity. Among different types of diamond films, the fine grained diamond (FGD) film can provide much more favorable environment for machining CFRP due to the small grain size, low surface roughness and the retentivity for the sharpness of the cutting edge. In the present study, aiming at drilling CFRP, FGD films of different thicknesses are deposited on Φ3 mm drills by controlling the growth time, adopting the common-used hot filament CVD (HFCVD) technology. It can be directly proved by deposition experiments that overlong growth time can induce spontaneous film delamination and removal before the cooling stage, probably as a result of the excessive residual stress concentrated on the complicated surfaces. As demonstrated by the cutting tests, with increasing the growth time, the main failure mode of the FGD coated drill changes from film delamination to flank wear/tipping to film delamination, and the maximum tool life exists when the growth time is moderate, because the flimsy film cannot provide sufficient protective effects on the film-substrate interface and even hasn’t totally cover the substrate, while there’s relatively higher residual stress in the film that is too thick, and such the residual stress can significantly deteriorate the film-substrate adhesion. Moreover, during the life cycle of each FGD film, relatively shorter growth time often means the slightly better hole quality, attributed to the retentivity of the initial shape of the uncoated drill that is optimal designed for machining CFRP, especially the weaker passivation of the cutting edge.

Abstract: In the process of HFCVD diamond film growth on the multitudinous micro end mills, the uniformity and stability of the temperature distribution have a vital importance on the quality of film. So a new method by using the finite volume is proposed to analyze the importance of different disposition parameters on the uniformity of substrate temperature field. These parameters are filament diameter (d), filament-substrate distance (H), filament separation (S) and filament length (L). The mono-factor method are used to optimize the best parameter combination. The simulation results show that the optimized parameters are d=0.65mm, H=10mm, S=27mm and L=160mm.

Abstract: Si-doped diamond films are deposited on cobalt cemented tungsten carbide (WC-Co) welding dies using hot filament chemical vapor deposition (HFCVD) method, where tetraethoxysilane (C8H20O4Si) is introduced in the reactive chamber as silicon source by bubbling method. Filed emission scanning electron microscope (FESEM) and Raman spectroscopy are used to characterize the as-deposited diamond films. The results show that silicon doping can reduce the diamond crystal size and residual stress of diamond films, and also increase the FWHM of first order diamond Raman line. The polishing time of diamond coated welding dies also can be shortened by silicon doping. Si-doped diamond coated welding dies possesses comparable practical application performance with conventional diamond coated welding dies. Compared with the nylon and WC-Co welding dies, the working lifetime of diamond coated welding dies increases 200 and 10 times, respectively.

Abstract: The substrate temperature distribution in hot filament chemical vapor deposition (HFCVD) diamond films growth on drill tools in large quantities are simulated by the finite volume method (FVM), adopting a detailed 3-D computational model corresponding with the actual reactor. Firstly, the correctness of the simulation model is verified by comparing the temperature data obtained from the simulation with that measured in an actual depositing process, and the results show that the error between them is less than 3%. Thereafter, the influences of several parameters are studied, including the filament separation (D), the length of the filament (L) and the filament-substrate distance (H). The simulation results show the three parameters have different effects on the distribution of temperature field. The influence of D is the greatest, L is followed and then H. The simulation has important theoretical guidance on both the development of HFCVD deposition equipment using for the diamond coating on tools with complex shapes in large quantities and the research of related production process.

Abstract: The tribo-map of typical CVD diamond film exhibiting the interaction between the wear rate, friction coefficient and friction conditions would help optimize the working parameters of CVD diamond film coated tools and wear-resistance components. The tribological behaviors of CVD diamond films sliding against Si₃N₄ balls were studied by conducting a group of tests on the ball-on-plate type reciprocating friction tester under several sliding speeds and normal loads in the ambient air. The examined MCD films and NCD films were deposited on square flat WC-Co substrates. The worn surfaces on the diamond films were observed by SEM and the wear volumes of diamond films were measured by surface profilometer. The results indicated that the influences of the sliding speeds and normal loads on the friction coefficients for both MCD films and NCD films were obvious. When the load was 6 N, MCD film obtained the lowest friction coefficient of 0.11 at the sliding velocity of 0.2 m/s, while for NCD film the minimum value was 0.07 as the sliding speed was 0.13 m/s. The wear rate of the MCD film decreased as the load improved, while for the NCD film, the tendency was just the opposite. The influence of sliding speed on the wear rate of the MCD films was not distinct, while for the NCD films, the sliding velocity greatly affects their wear rate. The wear rates of most NCD films were around 0.2×10^-7 mm³/Nm, while those of the MCD films fluctuated from 0.6×10^-7~1.6×10^-7 mm³/Nm. To elucidate the effect of operating environment on wear mechanism of diamond/ Si₃N₄ tribo-pair, the tribo-map was developed.

Abstract: In the present investigation, both micro-crystalline and nanocrystalline diamond (MCD and NCD) films are fabricated, which are characterized by FESEM (Field Emission Scanning Electron Microscopy), surface profilemeter, Raman spectroscopy and Rockwell hardness tester. Moreover, under the dry environment, the frictional behavior of both the films sliding against commonly-used materials in the metal drawing industry is studied on a ball-on-plate rotational frictional tester, including the stainless steel, low-carbon steel, high-carbon steel and copper, demonstrating that the frictional coefficients between NCD films and all these materials are relatively smaller. Furthermore, the wear rates of both the films, which are hardly measured in the ball-on-plate friction tests, are evaluated using a home-made inner-hole line drawing apparatus, with both the diamond films deposited on the inner-hole surfaces and the low-carbon steel wires as the counterparts. Inversely, the NCD films present higher wear rates than the MCD ones, which can be attributed to the deteriorative film purity and adhesion.

Abstract: In the present investigation, titanium (Ti), silicon carbide (SiC), silicon (Si) and tantalum (Ta) samples with the same geometry are selected as substrates to deposite HFCVD boron-doped diamond films with the same deposition parameters, using trimethyl borate as the dopant. FESEM, EDS, Raman spectroscopy and Rockwell hardness tester are used to characterize as-deposited boron-doped diamond (BDD) films. The FESEM micrographs exhibit that the film deposited on Si substrate presents the best uniformity and that on Ti substrate has smallest grain size and film thickness, with titanium element detected in the EDS spectra. Moreover, it’s speculated by indentation test that the adhesive strength between the BDD films and different substrates can be order as SiC>Ta>Ti for the different thermal expansion coefficient gaps between the substrate and diamond, and the hardness of the BDD coated samples measured using Rockwell hardness tester can also be order as SiC>Ta>Ti due to the different hardness of substrate materials. Finally, similar and representative characterization for BDD films is obtained from the Raman spectra for all the BDD films on different substrates.

Abstract: The CVD diamond/diamond-like carbon composite film is fabricated on the WC-Co substrate by depositing a layer of Diamond-like Carbon film on the surface of conventional Micro- or Nano-crystalline diamond film. The hot filament chemical vapor deposition (HFCVD) method and vacuum arc discharge with a graphite cathode are adopted respectively to deposit the MCD/NCD and DLC films. A variety of characterization techniques, including filed emission scanning electron microscope (FE-SEM) and Raman spectroscopy are employed to investigate the surface morphology and atomic bonding state of as-deposited MCD/DLC and NCD/DLC composite film. The results show that both MCD/DLC and NCD/DLC composite films present similar surface morphology with the MCD and NCD films, except for scattering a considerable amount of small-sized diamond crystallites among the grain boundary area. The atomic-bonding state of as-deposited MCD/DLC and NCD/DLC composite films is determined by the top-layered DLC film, which is mainly consisted of amorphous carbon phase and no discernible sp3 characteristic peak can be observed from their Raman spectrum. Furthermore, the tribological properties of as-deposited MCD/DLC and NCD/DLC composite films is examined using a ball-on-plate reciprocating friction tester under both dry sliding and water-lubricating conditions, comparing with conventional DLC, MCD and NCD films. Silicon nitride balls are used as counterpart materials. For the CVD diamond/DLC composite films, the self-lubricating effect of top-layered DLC film is beneficial for suppressing the initial friction peak, as well as shortening the run-in period. The average friction coefficients of MCD/DLC and NCD/DLC composite films during stable sliding period are 0.07 and 0.10 respectively in dry sliding; while under water-lubricating condition, they further decreases to 0.03 and 0.07.