Polished Polycrystalline Diamond Research Articles

Influence of Abrasive Grain Protrusion on High-Quality Machining of Cemented Carbide Using PCD Ball End Mills

In this study, cemented carbide was machined to a high quality using polycrystalline diamond (PCD) ball end mills characterized by various surface textures. The effect of the surface texture of the tools on the machining characteristics was studied using two types of PCD tools featuring abrasive diamond grains at various protrusion heights. In addition, a single crystal diamond tool with the same shape as that of the PCD tool was fabricated, and the experiment was repeated to study the differences in machining characteristics. The polished PCD tool yielded a high-quality machined surface with an average surface roughness of 1 nm. The polished PCD tool yielded superior sample surface roughness compared to the PCD tool for feed rates of 10–500 mm/min. The use of a polished PCD tool enables the efficient elimination of material through plastic flow, leading to the attainment of a high-quality machined surface while preventing the adhesion of materials on the tool surface. A single crystal diamond tool can also be used for machining cemented carbide within a feed rate range of 10–200 mm/min; however, its performance is inferior to that of a polished PCD tool. Experiments confirmed that the polished PCD tool was the most effective among the tested tools for the precision machining of cemented carbide.

Effect of grain boundary on polycrystalline diamond polishing by high-speed dynamic friction

In-situ micro-area analysis of precisely polished polycrystalline diamond (PCD) film by high-speed three-dimensional dynamic friction polishing (3DM-DFP) is carried out in this work, continuing from the previous works focusing on the subsurface cleavage and polishing mechanism. Multi-orientational grains with variable hardness anisotropy are generally considered the reason for the surface irregularity of PCD after smoothing. The unreached surface ultra-smoothness in local region after polishing is mainly resulted from the height difference at grain boundary (GB) area (e.g., a selected GB micro-region is of around Ra roughness of 5.7 nm and altitude intercept of 6.6 nm). However, different individual grains nearby showing (100), (110), (111) orientations are all of roughness <0.5 nm, demonstrating these places are contiguously polished. The observed local roughening at GBs region after mechanical sliding accompanying with friction heat is the major factor to cause the surface irregularity, which is affected by the inhomogeneity of crystal thermal expansion and contraction associated with the defects in multi-orientational crystals generated by mechanical process. Among multi-crystals the low-position (110) region has a Ra roughness of 0.1 nm but with great tensile stress and relatively weak distortion compared with other orientational areas. An insight of the inhomogeneous volume change associated with thermal expansion and contraction affected by defects condition was proposed to straightforwardly illuminate the rationale of GB effects on the ultra-smoothing of PCD by 3DM-DFP.

Experimental study on abrasive water-jet polishing of cemented carbide and polycrystalline diamond tools

This research paper studies the material removal mechanism, influence of water jet pressure and jet impact angle on the polishing effect of super-hard tools surfaces and edges experimentally. The results showed that the water jet pressure and jet impact angle have great influence on the materials removal efficiency. The jet angle of 15° and pressure of 10 MPa resulted in better-polished surface of the cemented carbide tools. The rake face of polycrystalline diamond cutting tools experienced no significant change under low pressure of 5 Mpa. Nevertheless, at a jet impact angle of 75° and a pressure of 20 MPa, the edge of the polished polycrystalline diamond cutting tool was bright, smooth, and rounded with 'ductile erosion micro material removal mechanism' without any fracture. The results demonstrated that the built abrasive water jet polishing technology system can be used in the super-hard tools industry to improve on the tool's services life.

Experimental study on abrasive water-jet polishing of cemented carbide and polycrystalline diamond tools

This research paper studies the material removal mechanism, influence of water jet pressure and jet impact angle on the polishing effect of super-hard tools surfaces and edges experimentally. The results showed that the water jet pressure and jet impact angle have great influence on the materials removal efficiency. The jet angle of 15° and pressure of 10 MPa resulted in better-polished surface of the cemented carbide tools. The rake face of polycrystalline diamond cutting tools experienced no significant change under low pressure of 5 Mpa. Nevertheless, at a jet impact angle of 75° and a pressure of 20 MPa, the edge of the polished polycrystalline diamond cutting tool was bright, smooth, and rounded with 'ductile erosion micro material removal mechanism' without any fracture. The results demonstrated that the built abrasive water jet polishing technology system can be used in the super-hard tools industry to improve on the tool's services life.

Polishing of Black and White CVD Grown Polycrystalline Diamond Coatings

Microwave plasma CVD growth can produce black and white varieties of polycrystalline diamond (PCD), depending on their as-grown purity. These two types of PCDs have been polished by mechanical and chemo-mechanical polishing respectively. It has been observed that initial roughness of 2.21μm for white PCD can be brought down to 175 nm after 70 hours of combined polishing, whereas, 85 hours of combined polishing could bring down the high initial roughness of 11.2μm for black PCD down to 546 nm at the end. Although, the material that was removed during polishing was higher for the black variety of PCD but it had lower polishing rate of 4nm/hr than white PCD (13nm/hr) during chemo-mechanical polishing. Such differential polishing rate was due to harder top polished surface of the black diamond than the white diamond. The nanoindentation study on the polished PCD surfaces revealed that the black PCD has a final nanohardness of 32.58±1 GPa whereas the white variety PCD had a polished surface nanohardness of 28.5±2 GPa. More conversion of diamond surface into harder amorphous sp3 than softer graphite during polishing action may have resulted such slow rate of anisotropic polishing for black diamond than white diamond.

Friction and wear performance of different carbon coatings for use in dry aluminium forming processes

Lubricants are commonly used in metal forming processes to reduce the friction between the workpiece and the forming tool, to protect semi-finished products and goods against corrosion and to reduce the load on the tool. One aim of environmentally friendly production technologies is to achieve dry forming without the use of lubricants. An oscillating ball-on-plate tribometer (load: 10 N, frequency: 2.5 Hz, ball diameter: 10 mm, 99,900 cycles) was used to compare the suitability of different carbon coatings for dry contact against the aluminium alloy EN AW 5083. As candidates a graded and multilayered hydrogenated amorphous carbon coating (a-C:H) system by physical vapour deposition (PVD) as well as a polycrystalline diamond coating by chemical vapour deposition (CVD) were studied. The CVD diamond coatings were tested in two surface conditions, a rough as-deposited (roughness Sa 1.19 μm) and a subsequent polished (Sa 0.01 μm) variant. An uncoated, hardened steel plate of ledeburitic cold working steel (X153CrMoV12-1) was used as reference material. Worn surfaces were examined using laser scanning and atomic force microscopy (AFM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy to determine their wear rates and to characterize the degree of adhesion, the change in nano-topography and the type and quantitative change of chemical bindings. The tests showed that polishing of CVD-diamond coatings is mandatory for the use in regards to dry forming. Sliding against the hydrogenated amorphous carbon (a-C:H) coating and polished polycrystalline CVD-diamond coating (pCVDD) resulted in the lowest sliding friction coefficients (COF) of 0.13 respectively 0.12. The polished polycrystalline diamond coating showed the lowest wear rates at the round-ended aluminium pin of 4 · 10−9 mm3/Nm and at the coating of 1 · 10−8 mm3/Nm. Cycle-dependent friction coefficient transitions were exhibited by the a-C:H-coating. For the pCVDD-coating as well as for the a-C:H-coating the formation of a transfer film was detected, which explains the small COFs. Due to the nearly negligible wear rate the pCVDD-coating is the most promising one of the tested carbon coatings to enable dry aluminium forming with a long lifetime of the coated tool.

Influence of power density on high purity 63 mm diameter polycrystalline diamond deposition inside a 2.45 GHz MPCVD reactor

63 mm diameter polycrystalline diamond (PCD) films were synthesized via a microwave plasma chemical vapor deposition (MPCVD) reactor in 99% H2–1% CH4 atmosphere. Two different conditions, i.e. the typical condition (input power of 5 kW and gas pressure of 13 kPa) and the high power density condition (input power of 10 kW and gas pressure of 18 kPa), were employed for diamond depositions. The color changes of the plasma under the two proposed conditions with and without methane were observed by photographs. Likewise, the concentrations of hydrogen atoms and carbon active chemical species in plasma were analyzed by optical emission spectroscopy (OES). The morphologies and purity of the PCD films were investigated by scanning electron microscopy (SEM) and Raman spectroscopy, respectively. Finally, the transmission spectrum of the polished PCD plates was characterized by a UV–Vis–NIR spectrometer. Experimental results showed that both the concentrations of hydrogen atoms and carbon radicals increased obviously, with the boost input power and higher pressure. The films synthesized under the high power density condition displayed higher purity and more uniform thickness. The growth rates in 10 kW and 18 kPa reached ~7.7 µm h−1, approximately 6.5 times as much as that occurred in the typical process. Moreover, the polished plates synthesized under the high power density condition possessed a relatively high optical transmittance (~69%), approaching the theoretical values of approximately 71.4% in IR. These results indicate that the purity and growth rate of big-area PCD films could be simultaneously increased with power density.

An effort in planarising microwave plasma CVD grown polycrystalline diamond (PCD) coated 4 in. Si wafers

Large area CVD grown diamond coatings should have very smooth surface in many of its applications, like microwave power transmission windows etc. Combination of mechanical and chemical forces during polishing helps to achieve desired surface roughness of the polycrystalline diamond (PCD) coatings. Authors report the variation of pressure, slurry feeding rate, addition of chemicals and the time of polishing to observe the efficiency of chemo-mechanical polishing (CMP) technique in planarising CVD diamond material grown over 100mm diameter silicon wafers. PCD were found to be polished by bringing down the as-grown surface roughness from 1.62µm to 46nm at some points on large areas. One coherence scanning interferometer was used to check the roughness at different stages of CMP. Raman spectroscopy was used to evaluate the polished PCD samples in terms of their quality, internal stress at different positions on the same wafer surface after CMP process. It was found that the well polished regions were of better quality than the less polished regions on the same wafer surface. But due to coating non-uniformity of the deposited PCD grown by microwave plasma CVD over large area, CMP could not produce uniform surface roughness over the entire 100mm diameter wafer surface. We concluded that CMP could effectively but differentially polish large area PCD surfaces, and further process improvements were needed.

Effect of atomic hydrogen bombardment on the surface conductivity of polycrystalline diamond films

The surface conductivity of polished polycrystalline diamond films after atomic hydrogen bombardment for different time was compared and the carrier transport characteristic beneath the H-terminated diamond film surface was investigated correspondingly. It is found that, as the bombardment time increases, the surface roughness of diamond films first decreases due to disappearance of scratches, and then increases because of appearance of protrusions produced by plasma preferentially etching. Meanwhile the total CH bonding concentration on the diamond surface increases gradually until it is saturated when the bombardment time is up to 30min. The almost invariable carrier density is obtained for all samples treated for different time, which indicates the monohydride (CH) mode responsible for surface conductivity forms when hydrogen treatment starts. While for the carrier mobility, it shows the inverse trend with change of the surface roughness. A model based on the surface roughness scattering was proposed to explain the relationship between the surface conductivity and roughness. Combining with the carrier mobility and density, the lowest square resistance of 13.85kΩ is obtained for polycrystalline diamond film when it is treated for 30min. It is speculated that the surface conductivity can be further enhanced by reducing the surface roughness.

Analysis of polished polycrystalline diamond using dual beam focused ion beam microscopy

This paper presents a study on polycrystalline diamond (PCD) polished by dynamic friction polishing (DFP) with the aid of advanced dual beam FIB (focused ion beam) microscopy. After disclosing a variety of wear tracks by DFP using electron imaging in combination with the ion channelling effect, a dual beam FIB was successfully employed at wear track sites to specifically create both the large cross-sectional specimen for microanalysis and thin foil for nanoanalysis. The study concluded that the polished PCD subsurface was free from microscale cracking. However, the attached debris layer on the top surface contained metal oxides and non-diamond carbon phase with inhomogeneous distributions of C, Fe, Cr, Ni, Si and O across the layer. An attached layer directly above a diamond grain was composed of essentially amorphous carbon, suggesting that a direct phase transformation from diamond crystalline to amorphous occurred during DFP.

Fabrication of submicron scale vertically aligned diamond rods by mask-free oxygen plasma etching

A mask-free plasma etching process is described to fabricate 6 μm long submicron diamond rods (SDRs) in long conical shape. Polished polycrystalline diamond is etched in oxygen plasma ignited at a pressure of 10 mTorr by radio-frequency power of 100 W at 13.56 MHz. Each SDR is a bi-crystal, consisting of two diamond crystallites of micron size. The SDR is coated with a Fe 2O 3 layer, as characterized by Auger electron spectroscopy, X-ray photoemission microscopy, and transmission electron microscopy. We propose that a “self-forming” mask of Fe 2O 3 is generated during the etching process in which iron atoms sputtered from the substrate holder are deposited and oxidized on the diamond surface forming “micromask” that protects the underlying diamond and promotes the formation of SDRs.

Development of partly polished polycrystalline diamond films on steel substrates for slider applications

CVD diamond films are expected to become maintenance-free solid lubricant coatings. However, substrates on which diamond film can be deposited have been limited to mater ials such as silicon and tungsten carbides. If diamond films can be deposited onto steel substrates, it will increase the number of possible applications. Diamond film was deposited on a steel substrate with an interlayer and was partly polished. Tungsten and titanium were selected as materials for the interlayer between diamond film and AISI 440C steel substrate. Low and st able friction coefficients were obtained between diamond fil m and AISI D2 pin, with friction coefficient 0.1 in the steady-s tate regime.

Plasma Processing of Boron-Doped Nano-Crystalline Diamond Thin Film Fabricated on Poly-Crystalline Diamond Thick Film

Plasma treatments of boron-doped nano-crystalline diamond (NCD) thin films were carried out in order to improve their electrical properties of the films. Boron-doped NCD thin films were fabricated on well polished poly-crystalline diamond (PCD) thick films in a microwave plasma enhanced chemical vapor deposition (MPCVD) reactor, then they were processed in methane, argon, hydrogen and B2H6 (0.1% diluted by H2) plasmas, respectively. Scanning electron microscopy (SEM) and atomic force microscope (AFM) results show that the surface morphology changed little during the 10 min treatment. Secondary ion mass spectroscopy (SIMS) results indicate that B2H6 plasma was efficient for increasing boron concentration in NCD films, while the carrier analyses demonstrates that CH4 plasma processing was effective to activate the dopants and resulted in good electrical properties.

鋼材基板に作製した研磨多結晶ダイヤモンド膜の摩擦特性評価(機械要素,潤滑,設計,生産加工,生産システムなど)

鋼材基板に作製した研磨多結晶ダイヤモンド膜の摩擦特性評価(機械要素,潤滑,設計,生産加工,生産システムなど)

Tribological properties of undoped and boron-doped nanocrystalline diamond films

Undoped and boron-doped nanocrystalline (NCD) diamond films were deposited on mirror polished Ti–6Al–4V substrates in a Microwave Plasma Assisted Chemical Vapor Deposition system. Sliding wear tests were conducted in ambient air with a nanotribometer. A systematic study of the tribological properties for both undoped and boron-doped NCD films were carried out. It was found for diamond/diamond sliding, coefficient of friction decreases with increasing normal loads. It was also found that the wear rate of boron-doped NCD films is about 10 times higher than that of undoped films. A wear rate of ~ 5.2 × 10 − 9 mm 3/Nm was found for undoped NCD films. This value is comparable to the best known value of that of polished polycrystalline diamond films. Although no surface deformation, film delamination or micro-cracking were observed for undoped films, boron-doped NCD film undergoes a critical failure at a normal stress of 2.2 GPa, above which surface deformation is evident. Combined with high hardness and modulus, tunable conductivity and improved open air thermal stability, boron-doped nanocrystalline diamond film has tremendous potentials for applications such as Atomic Force Microscope probes, Micro-Electro-Mechanical System devices and biomedical sensors.

Characterizations of nano-crystalline diamond coating cutting tools

Diamond coatings made by processes such as chemical vapor deposition (CVD) have been increasingly explored for cutting tools applications, in particular, for machining lightweight high-strength materials. However, most literature to date reported that the wear resistance of CVD diamond tools is still distant to the polycrystalline diamond (PCD) counterparts. Recently, a microwave plasma-assisted CVD technology was developed to increase the diamond growth rate, and by adding nitrogen gas, this process can produce nano-structured coatings which consist of nano-diamond crystals embedded into a hard amorphous diamond-like carbon matrix and have high hardness and low surface roughness. In this study, the nano-crystalline diamond (NCD) coating tools were characterized, compared to the conventional microcrystalline diamond coating (MCD) and PCD tools, in surface topography, grain sizes, carbon bonds, and mechanical properties. Moreover, the diamond tools were evaluated in machining Al-matrix composite and high-strength Al alloy. The results show that the NCD tools have smoother surfaces than the MCD tools, but rougher than the polished PCD tools. As to the diamond crystals, the NCD tools have ultrafine grains and the PCD tools have the largest grains. For the cutting edges, the PCD tools have the sharpest edge resulted from polishing and the NCD tools have larger edge rounding than the MCD counterparts. In addition to the nature-diamond peak identified in the Raman spectra, the NCD and MCD tools have additional Raman shifts associated with graphite and other disordered carbon bonds. The NCD tools have a much higher hardness, but a lower elasticity, than both the MCD and PCD tools. In machining testing, the NCD tools substantially outperform the MCD tools and are comparable to the PCD tools. For both the NCD and MCD tools, coating delamination is the major tool wear mechanism that leads to catastrophic tool failures.

An atomic force microscopy study of the effects of surface treatments of diamond films produced by chemical vapor deposition

Diamond films produced by chemical vapor deposition (CVD) technique must be polished and machined in order to be used for many applications (windows, heat spreaders, surface acoustic wave devices, etc). We present an Atomic Force Microscopy study of the effects of mechanical polishing, hydrogen plasma etching, and ion implantation followed by annealing and chemical etching, on the morphology of Microwave Plasma CVD polycrystalline diamond films. Both growth and nucleation sides of 0.3–0.5 mm thick free-standing films were characterized. We found that grain boundaries and anisotropy of mechanical properties (hardness and wear rate) mostly contribute to the overall roughness of Microwave Plasma CVD diamond films, which is below 0.2 nm at the single-grain scale. The further deposition of a diamond overlayer, even of small thickness, on highly polished polycrystalline diamond, results in a variety of growth modes and different growth rates depending on the grain orientation, leading to the roughness increase. Hydrogen plasma etching proved to be effective for inducing morphological and chemical changes on the surface, while an ion implantation/annealing/etching can be used for precise removal of thin diamond layers. The latter approach is promising for diamond lithography.

Electrochemical characteristics for redox systems at nano-honeycomb diamond

Electrochemical characteristics for several redox systems at diamond films with highly ordered nanometer-scale cylindrical pores (‘nano-honeycombs’) were examined with cyclic voltammetry (CV) and electrochemical impedance measurements. The cyclic voltammetric anodic–cathodic peak separations for these nano-honeycomb electrodes were in the same range as those for polished polycrystalline diamond films, indicating that the involvement of the oxygen-terminated surface of the nano-pore walls, which should give rise to large peak separations for certain redox couples was only slight. Moreover, the peak currents in the CV were not enhanced to the extent expected on the basis of the roughness factors of the nano-honeycomb films. Ac impedance plots results indicated the existence of a concentration gradient of the reactant in the nano-pores, which is in agreement with theoretical predictions for charge transfer reactions in porous electrodes. The average concentration of the reactant (Fe 2+/3+) inside the nano-pores was a factor of ca. 80 lower than that in the bulk electrolyte. The results of the impedance analysis also indicated an increase in the reaction resistances with decreasing pore diameters.

COMPLEMENTARY SURFACE INVESTIGATION OF DIAMOND BY SCANNING THERMAL MICROSCOPY AND SCANNING NEAR-FIELD CATHODOLUMINESCENCE

The interest in diamond windows for the application of high power radiation technology is increasing continuously due to its outstanding thermal and opto-electronic properties. In order to analyze the surface properties of very highly polished polycrystalline diamond in the sub-micrometer down to nanometer regime, two different near-field microscopy techniques have been employed. By using a Scanning Thermal Microscope (SThM) it is possible to uncover the varying thermal conductivity of different diamond grains. Dissimilar thermal surface properties within diamond grains have been measured with a resolution in the nm regime. Beside the fact of determining a small surface thermal conductivity compared to the well known data of bulk material, the thermal conductivity of inferior and superior diamond quality is separated quantitatively. Additionally the opto-electronic properties of the polished diamond have been investigated by Scanning Near-Field Cathodoluminescence (NF-CL). This measurement technique allows the detection of the distribution of dislocations within diamond grains with a resolution of less than 100nm. The comparison of these results, obtained by SThM and NF-CL, demonstrates a fine alignment in representing crystal lattice imperfections.

Impedance Characteristics of the Nanoporous Honeycomb Diamond Electrodes for Electrical Double-Layer Capacitor Applications

Nanoporous boron-doped diamond films with highly ordered pore structures and various pore dimensions, fabricated by oxygen plasma etching of polished polycrystalline diamond films through porous alumina masks, were characterized using electrochemical impedance measurements. The porous structures exhibited wide electrochemical potential windows (2.46-2.70 V), although somewhat less than that for the as-deposited films (3.04 V) because of surface damage due to the oxygen plasma treatment. A film of pore diam 400 nm and pore depth 3 μm exhibited a 400-fold increase in the capacitance in comparison to an as-deposited surface film. A film with 30 nm diam pores showed little enhancement in the capacitance at useful frequencies due to the high pore impedance. Impedance measurements carried out at −0.5 V vs. Ag/AgCl indicated a faradaic reaction, most likely hydrogen evolution. Although the dc current was small (ca. 3.0 μA cm−2 real area for the 60 nm honeycomb), it represents a pathway for charge leakage, limiting the negative potential limit for charge storage. However, similar limits were also observed for a representative acetylene black and activated carbon. The as-deposited diamond and nm honeycomb exhibited significantly higher effective potential limits for anodic charging due to the lack of surface oxidation current. This allows more charge to be stored per unit capacitance for diamond materials than for graphitic carbons. Due to the surface damage, the honeycomb sample did not store as much charge per unit capacitance as the as-deposited sample. © 2001 The Electrochemical Society. All rights reserved.