Papers by Author: Zhi Ming Zhang

Abstract: Diamond-coated drawing dies are considered as ideal drawing dies for their unique characteristics, such as high hardness, wear resistance and low friction. In order to utilize the superior characteristics of diamond coatings towards improving the drawing performance, the nonlinear FEM simulation is used to simulate the whole stainless steel tube hollow sinking process, with 2D axi-symmetric elastic-plastic element. Based on the simulation results, the distributions of the axial stress and radial stress are analyzed, the influence of parameters of drawing dies on the diameter shrinkage is investigated. Optimal die parameters are obtained.

Abstract: The tribological performance of conventional microcrystalline diamond (MCD) film and diamond-like carbon (DLC) film is investigated comparatively under water lubricating condition. The MCD and DLC film are deposited on cobalt cemented tungsten carbide (WC-Co) substrate using the hot filament chemical vapor deposition (HFCVD) method and the vacuum arc discharge with a graphite cathode respectively. Scanning electron microscopy (SEM), white light interferometer, and Raman spectra are employed to characterize as-deposited MCD and DLC samples. The friction tests are carried out on a ball-on-plate reciprocating friction tester, where the sliding process is conducted under water lubricating condition. Silicon nitride, tungsten carbide, ball-bearing steel and copper are used as counterpart materials. The results indicate that DLC film always exhibits lower friction coefficient than MCD film under water lubricating condition, except the case of sliding against the silicon nitride, in which DLC film is worn out very rapidly and thus leads to the high friction coefficient. The wear resistance of DLC film under water lubricating condition is significantly poorer than that of MCD film. While sliding against silicon nitride, tungsten carbide, ball-bearing steel and copper, its wear rate is calculated as 3.67´10^-7 mm³N^-1m^-1, 9.31´10^-9 mm³N^-1m^-1, 3.54´10^-7 mm³N^-1m^-1, and 4.97´10^-8 mm³N^-1m^-1 respectively. Comparatively, no measurable wear track can be found on the worn surface of MCD films.

Abstract: Microcrystalline diamond (MCD) and fine-grained diamond (FGD) films are deposited on silicon nitride (Si3N4) inserts using the hot filament chemical vapor deposition (HFCVD) method. Scanning electron microscope (SEM), X –ray diffraction (XRD) and Raman spectrum are employed to characterize these as-deposited diamond films. Cutting performance of as-fabricated CVD diamond coated Si3N4 inserts is examined in dry turning glass fiber reinforced plastics (GFRP) material, comparing with the uncoated Si3N4 inserts. The results indicate that the tool failure is mainly attributed to its severe flank wear, which is caused by continuous friction and impact brought by many hard SiO2 particles distributed in the GFPR work piece. The lifetime of Si3N4 inserts can be prolonged by depositing MCD or FGD films on them and the FGD coated insert shows better wear resistance than the MCD coated one.

Abstract: The diamond and diamond-like carbon (DLC) films were deposited on the cobalt cemented tungsten carbide (WC-Co) cutting tools respectively adopting the hot filament chemical vapor deposition (HFCVD) technique and the vacuum arc discharge with a graphite cathode. The scanning electron microscope (SEM), X-ray diffraction spectroscopy (XRD) and Raman spectroscopy were used to characterize the as-deposited diamond and DLC films. To evaluate their cutting performance, comparative turning tests were conducted using the uncoated WC-Co and as-fabricated CVD diamond and DLC coated inserts, with glass fiber reinforced plastics (GFRP) composite materials as the workpiece. The research results exhibited that diamond and DLC coated inserts had great advantages in cutting tests compared to uncoated insert. The flank wear of the CVD diamond coated insert maintained a very low value about 50μm before the cutting tool failure occurred. For the DLC coated insert, its flank wear always maintained a nearly constant value of 70~200μm during whole 45 minutes turning process. The flank wear of CVD diamond coated insert was lower than that of DLC coated insert before diamond films peeling off.

Abstract: Diamond thin films doped with various boron concentrations were grown on WC-Co cemented carbide tools by hot-filament-assisted chemical vapor deposition (HFCVD). The trimethyl borate dissolved in acetone solution was used as the boron resource (B/C=0%, 0.1%, 0.3%, 0.5%). The surface morphology of diamond films with different boron contents was investigated by Scanning electron microscopy, the adhesive strength was calculated by means of indentation test under a load of 1500N. A real cutting performance was carried out on Al metal matrix composites material (20vol%SiC, 15μm), and the insert flank wear was examined by measuring the scars that appeared on the cutting edge with tool microscope. The research results shown the surface morphology and structure of the diamond films changed owing to boron doping. As the doping levels increased, the average grain size of the films decreased from 10 to 2μm. A significant improvement in adhesion and cutting performance were observed as the boron contents increased from 0% to 0.5%. The adhesion and cutting performance were best when the boron concentration was 0.3%. Adequate boron can effectively suppress the cobalt diffusion to the substrate surface and avoid the catalytic effect of cobalt at the high temperature. It is of great significance for improvement of the adhesive strength and cutting performance of diamond-coated tools using above method.

Abstract: Nanocrystalline diamond films (NDFs) were deposited on mirror-polished silicon (100) substrates using a graphite-grid assisted hot filament chemical vapor deposition (HFCVD) technique. The evidence of nanocrystallinity, smoothness and purity was obtained by characterizing the sample with various advanced analyses. A graphite-grid was used as the DC electrode, which was pre-coated by diamond films and could emit electron when a negative bias was applied to the under electrode. The results show that the film consists of nanocrystalline diamond grains with sizes of about 7-15nm. The Raman spectroscopy, XRD pattern, HR-TEM image and SAED pattern of the films indicate the presence of nanocrystalline diamond. Surface roughness is measured as Ra<20nm by the profilometry scans. A large quantity of electrons emission from the graphite-grid and positive ions bombardment to the graphite-grid results in an enormous enhancement of the generation of diamond nuclei. Density of a diamond nuclei as high as 1010~1011/cm2 can be attained with this method. NDFs can be deposited on mirror-polished Si substrate surfaces without damaging surface pretreatment for nucleation enhancement. These ultra-smooth films will display excellent performances, which make them the best candidates for semiconductor and MEMS applications.

Abstract: The bearing support implement is the key component in the precision process of bearings. Diamond films are deposited on Co-cemented carbide bearing support implements using the acetone and hydrogen as the gas source by the bias–enhanced hot filament chemical vapor deposition (HFCVD) technique. Diamond-coated bearing support implements are fabricated and used in the precision grinding of bearings. The research results show that the appropriate pretreatment methods and CVD process can effectively control the morphology, chemical quality, surface roughness, and adhesion of diamond films. As compared with the cemented carbide bearing support implements, the diamond-coated bearing support implements have obviously better wear resistant properties and working performance. It is of great significance for improvement of the bearing processing quality and raising of the bearing processing level.

Abstract: Improving adhesion and surface roughness of diamond films on WC–Co substrate is the key factor of the widespread application of diamond coated tools. A new pretreatment method has been performed for smooth Co-cemented carbide inserts in order to lower the surface roughness of diamond films under the premise of good adhesion between diamond films and substrates. The effect of the new pretreatment on the adhesion of the diamond films is investigated. Research results show that the boronization pretreatment can effectively suppress cobalt diffusion to the surface and avoid catalytic effect of Co at high temperature. This new pretreatment can avoid the surface roughening of inserts and ensure the deposition of smooth diamond films. Investigation shows that the optimum boronization compounding is a powder mixture of 70%B4C+15.5%KBF4+1.5% La2O3+13%Na2CO3. Adhesion between substrates and diamond films is evaluated by Rockwell A indentation tests and the cutting performance of the diamond-coated tools is investigated by the cutting tests. Diamond films on smooth cemented carbide inserts with cobalt boride interlayer have high adhesive strength and low surface roughness. Diamond-coated tools with boronization pretreatment have a 5-fold increase in tool life compared with untreated ones.

Abstract: Nanocrystalline diamond films were deposited on Co-cemented carbide substrates using CH4/H2/Ar gas mixture by hot filament chemical vapor deposition (HFCVD) technique. The evidence of nanocrystallinity, smoothness and purity was obtained by characterizing the sample with scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, atomic force microscopy (AFM), high-resolution transmission electron microscopy (HR-TEM) and selected-area electron diffraction (SAED). A new process was used to deposit composite diamond films by a two-step chemical vapor deposition procedure including first the deposition of the rough polycrystalline diamond and then the smooth fine-grained nanocrystalline diamond. The results show that the film consists of nanocrystalline diamond grains with sizes range from 20 to 80 nm. The Raman spectroscopy, XRD pattern, HR-TEM image and SAED pattern of the films indicate the presence of nanocrystalline diamond. Surface roughness is measured as Ra<100nm by AFM. Smooth nanocrystalline diamond layers can be deposited on conventional microcrystalline diamond layers using a two-step chemical vapor deposition by regulating the deposition parameters. These composite diamond films with the multiplayer (nanocrystalline/microcrystalline) structure have low surface roughness and high adhesive strength on WC-Co substrates. The diamond-coated tools and drawing dies with these composite coatings display excellent performances in the practical application.