Sulin Chen

Evaluation on residual stresses of silicon-doped CVD diamond films using X-ray diffraction and Raman spectroscopy

The effect of silicon doping on the residual stress of CVD diamond films is examined using both X-ray diffraction (XRD) analysis and Raman spectroscopy measurements. The examined Si-doped diamond films are deposited on WC−Co substrates in a home-made bias-enhanced HFCVD apparatus. Ethyl silicate (Si(OC2H5)4) is dissolved in acetone to obtain various Si/C mole ratio ranging from 0.1% to 1.4% in the reaction gas. Characterizations with SEM and XRD indicate increasing silicon concentration may result in grain size decreasing and diamond (110) texture becoming dominant. The residual stress values of as-deposited Si-doped diamond films are evaluated by both sin 2 ψ method, which measures the (220) diamond Bragg diffraction peaks using XRD, with ψ-values ranging from 0° to 45°, and Raman spectroscopy, which detects the diamond Raman peak shift from the natural diamond line at 1332 cm −1 . The residual stress evolution on the silicon doping level estimated from the above two methods presents rather good agreements, exhibiting that all deposited Si-doped diamond films present compressive stress and the sample with Si/C mole ratio of 0.1% possesses the largest residual stress of ~1.75 GPa (Raman) or ~2.3 GPa (XRD). As the silicon doping level is up further, the residual stress reduces to a relative stable value around 1.3 GPa.

Tribological behaviors of diamond films and their applications in metal drawing production in water-lubricating condition

The frictional and wear performance of the microcrystalline diamond (MCD), nanocrystalline diamond (NCD) and MCD/diamond-like carbon (DLC) bi-layered film are comparatively investigated under water-lubricating conditions. A full factorial experimental plan is conducted with four sliding velocities ranging from 0.126 to 0.503 m/s and four normal loads from 2 to 8 N. The duration of each sliding process is 24 h. The results show that although the MCD/DLC film shows the lowest stable coefficient of friction ranging from 0.025 to 0.12, the top-layered DLC film does not show beneficial effect on enhancing the sliding stability of single-layered diamond films. Comparatively, the MCD film exhibits the superior sliding stability and wear resistance, which are regarded as the more critical characters that determinate the lifetime and performance of a metal drawing die. Therefore, a drawing production of aluminum pipes is conducted in the water-lubricating condition using the MCD-coated drawing die. A novel drawing device is designed for the water-lubricating metal drawing production. It is capable of providing a fully immersed drawing environment for the metal pipe and thus guarantee the lubrication and cooling effects. The result of the drawing production shows that utilizing the diamond-coated drawing die in the water-lubricating condition could elevate the production of a drawing die by over 100 times compared with using the tungsten carbide drawing dies in oil-lubricating condition. Moreover, good surface quality and geometrical precision could also be obtained by the diamond-coated drawing dies in the water-lubricating drawing.

Investigation on the long-duration tribological performance of bilayered diamond/diamond-like carbon films:

The long-duration tribological performance of two bilayered diamond/diamond-like carbon films, namely microcrystalline diamond/diamond-like carbon and nanocrystalline diamond/diamond-like carbon films, is evaluated in 24-h dry sliding against the silicon nitride counterpart ball, in terms of the stable coefficient of friction, the sliding stability, and the specific wear rate of both the film and counterpart surface. Moreover, their sliding behaviors are illustrated from the aspect of sliding interface evolution. Characterization results indicate that the top-layered diamond-like carbon films significantly reduce the surface roughness of single-layered diamond film and determine the surface property of composite film. For the tribological performance, the top-layered diamond-like carbon coating significantly reduces the stable friction coefficient of the microcrystalline diamond film by 10–34%, but does not exhibit a similar effect on the nanocrystalline diamond film due to the severe delamination of diamond-like carbon coating. Either, the top-layered diamond-like carbon film does not show noticeable benefit on enhancing the sliding stability or wear resistance of single-layered diamond coating. The delamination of top-layered diamond-like carbon coating is supposed to play a critical role on such phenomenon. Furthermore, an enhanced tribomap is developed that is capable of providing a visualized and highly customized evaluation on tribological performance of sliding contacts according to the specific requirements a variety of applications. According to this tribomap, nanocrystalline diamond film always shows the superior performance to others.

THE EFFECT OF THE GAS INLET ON THE FLUID FIELD DURING FABRICATING HFCVD DIAMOND-COATED CUTTING TOOLS

In the present study, the fluid field in a process of fabricating diamond coated cutting tools using the hot filament chemical vapor deposition (HFCVD) method is investigated using the finite volume method (FVM), in which the effects of the inlet height, gas initial velocity, inlet radius and arrangement are illustrated in terms of the gas velocity magnitude and vector distribution near the filaments and the flute surface of cutting tools. In the simulations, the coupling effect of the temperature and the gas field is also considered by simultaneously calculating the temperature distribution. The simulation results suggest that either shortening the distance between the gas inlet and filaments, or increasing the gas initial velocity is helpful for the reactive gas arriving at filaments surface and being dissociated. Furthermore, increasing the inlet area is able to significantly increase the velocity of gas field around the filaments, as well as produce a much more uniform gas velocity field. Based on this conclusion, two novel multi-inlets setups are proposed to further improve the generated gas field and the simulation results show that the most superior gas field can be achieved with the one including 8 larger central inlets and 24 smaller outskirt inlets. Finally, an actual deposition experiment is carried out and its result indicates that adopting the optimized such inlet arrangement could generate a highly uniform and homogeneous growth environment on whole deposition area.

THE EFFECT OF THE DOUBLE-DECK FILAMENT SETUP ON ENHANCING THE UNIFORMITY OF TEMPERATURE FIELD ON LONG-FLUTE CUTTING TOOLS

In the present study, a double-deck filament setup is proposed for the hot filament chemical vapor deposition (HFCVD) method and an optimization method is presented to determine its optimal geometry that is able to produce a highly uniform temperature field on the whole flute surface of long-flute cutting tools. The optimization method is based on the finite volume method (FVM) simulation and the Taguchi method. The simulation results show that this double-deck filament setup always produce a highly uniform temperature distribution along the filament direction. Comparatively, for the temperature uniformity along the drill axis, the heights of the two filament decks present virtually significant influence, while the separations between the two filaments in either deck exhibit a relative weak effect. An optimized setup is obtained that can produce a highly uniform temperature field with an average temperature of 834°C, a standard deviation (σ) of 2.59°C and a temperature range (R) of 11.75°C. Finally, the precision of the proposed simulation method is verified by an additional temperature measurement. The measured temperature results show that a highly uniform temperature fields with σ/R = 9.6/35.2°C can be generated by the optimized setup and the deviation of the simulated results from the measured actual temperatures are within 0.5–3.5%, which justifies the correctness of the simulation method proposed in present study.

Study on the deposition and tribological properties of diamond/DLC composite coating on WC-Co substrate

Two types of diamond/DLC composite films, namely MCD/DLC and NCD/DLC, are fabricated on the WC-Co substrate by depositing a layer of DLC film on the surface of conventional MCD and NCD film. Characterisations on their surface morphology suggest that the diamond/DLC composite films exhibit a rather similar surface topography, but much lower surface roughness, comparing with the single-layered MCD or NCD film. The atomic-bonding state on their surface is largely determined by the top-layered DLC film, which is mainly composed of amorphous carbon phase. The results of friction tests indicate that the self-lubricating effect of top-layered DLC film significantly suppresses the initial friction peak and shortens the run-in period. With silicon nitride as the counterpart material, the average friction coefficients of MCD/DLC and NCD/DLC composite films during stable sliding period are respectively 0.07 and 0.10 in dry sliding, and 0.03 and 0.07 under water-lubricating condition.