Diamond electroplated coating with protection by diamond micron powder in a dressing tool. Part 1. Coating deposition conditions

The authors put forward a method of protection of diamond electroplated coating in a dressing tool made by the electroforming process. The method consists in using diamond micron powder in the surface layer of the nickel bond in order to protect the bond surface against hydroabrasive wear. The paper provides a procedure of calculation of nickel (the basis for the diamond electroplated coating) deposition conditions, taking into account the change in the free surface area of deposition due to cross-sectional area of diamond grains of the working fraction and the micron powder. The procedure permits computing the refill-deposition rate and duration, the final thickness of the coating and its protective sublayer.     

Development of diamond coated tool and its performance in machining Al-11% Si alloy

An attempt has been made to deposit CVD diamond coating on conventional carbide tool using hot filament CVD process. ISO grade K10 turning inserts with SPGN 120308 geometry were used to deposit diamond coating. This diamond coating well covering the rake surface, cutting edges and flank surfaces could be successfully deposited. The coatings were characterized by SEM, XRD and Raman spectroscopy for coating quality, morphology etc. Performance of diamond coated tool relative to that of uncoated carbide tool was evaluated in turning Al-11% Si alloy under dry environment. The diamond coated tool outperformed the uncoated carbide tool which severely suffered from sizeable built-up edge formation leading not only to escalation of cutting forces but also poorer surface finish. In contrast, the diamond coated tool, owing to chemical inertness of diamond coating towards the work material, did not show any trace of edge built-up even in dry environment and could maintain low level of cutting forces and remarkably improved surface finish. It has been further revealed that success of the diamond coated tool depends primarily on adhesion of the diamond coating with the carbide substrate and this is strongly influenced by the pre-treatment of the carbide substrate surface before coating.     

Molecular dynamics simulation model for the quantitative assessment of tool wear during single point diamond turning of cubic silicon carbide

Silicon carbide (SiC) is a material of great technological interest for engineering applications concerning hostile environments where silicon-based components cannot work (beyond 623 K). Single point diamond turning (SPDT) has remained a superior and viable method to harness process efficiency and freeform shapes on this harder material. However, it is extremely difficult to machine this ceramic consistently in the ductile regime due to sudden and rapid tool wear. It thus becomes non trivial to develop an accurate understanding of tool wear mechanism during SPDT of SiC in order to identify measures to suppress wear to minimize operational cost.In this paper, molecular dynamics (MD) simulation has been deployed with a realistic analytical bond order potential (ABOP) formalism based potential energy function to understand tool wear mechanism during single point diamond turning of SiC. The most significant result was obtained using the radial distribution function which suggests graphitization of diamond tool during the machining process. This phenomenon occurs due to the abrasive processes between these two ultra hard materials. The abrasive action results in locally high temperature which compounds with the massive cutting forces leading to sp³-sp² order-disorder transition of diamond tool. This represents the root cause of tool wear during SPDT operation of cubic SiC. Further testing led to the development of a novel method for quantitative assessment of the progression of diamond tool wear from MD simulations.     

Improving wear resistance of drilling tools fitted with synthetic diamond

The paper summarizes the findings of investigation of the influence of physicochemical and physical-mechanical properties of synthetic diamond grits on the drilling tool performance. Wear resistance of drilling tools is demonstrated to increase when they are fitted with synthetic diamond grits that are highly uniform in strength. The use of diamond grits with a better developed surface is found to provide a stronger grit retention in the bond and reduced drilling tool wear.     

Evolution of the working surface of diamond dressing sticks prior to its fracture

The paper addresses working surface wear of electroplated diamond dressing sticks at a stage prior to fracture during the dressing of abrasive wheels. The fracture is found to occur through drastic breakoff of diamond grits from the bond, the grit size and the nickel bond type being the main factors that dictate the fracture onset time. The authors provide a more precise definition of the relationship between the bond hydroabrasive wear parameters, kinematic characteristics of dressing, and physical-mechanical characteristics of the bond. The difference in the wear rate between diamond grits and the bond in a single-layer dressing tool is demonstrated to introduce large errors into the measurement of specific diamond consumption by weighing. A procedure is put forward which allows a variation in the mass wear ratio between diamond grits and a bond to improve the measurement accuracy.     

Investigation on diamond tool wear in ultrasonic vibration-assisted turning die steels

This paper presents comprehensive theoretical analyses and experimental investigations for evaluating the ultrasonic vibration-assisted turning (UVAT) of die steels with single-crystal diamond tools. The diamond tool wear was found to rely heavily on the feed rate and the cutting speed while being insensitive to the depth of cut and the tool relief angle under the cutting conditions used in the tests. The tool wear characteristics were further studied based on the observation of wear zone using Raman spectral analysis and energy-dispersive X-ray (EDX) analysis. The detection results of the tool worn topography, the phase transformation and the carbon diffusion of diamond crystals revealed that tool wear mainly occurred on the tool flank face due to the graphitization and the diffusion of the diamond tool. Analytical results of the function mechanisms of the ultrasonic turning indicated that the friction force between the tool flank face and the machined surface, which depended mainly on the ratio of the cutting speed and the vibration speed, could be effectively reduced in ultrasonic turning process. The analytical and experimental results indicated that compared with conventional turning (CT), the cutting performance, in terms of the tool life, was markedly improved by applying ultrasonic vibration to the cutting tool.     

Analysis of direct transformation from graphite to diamond crystal

The lattice parameters of cubic diamond and rhombohedral graphite under the probable direct transformation synthesis conditions have been obtained by means of linear expansion coefficient and elastic constant. Based on the empirical electron theory in solid and molecules, the valence electron structures (VESs) of graphite and diamond, the covalent electron densities (CEDs), and the relative electron density differences (REDDs) of the diamond growth interfaces have been calculated. It has been found that the REEDs of graphite/diamond interfaces were awfully large and the CEDs were discontinuous at the first order approximation. Not any meaningful atomic state of graphite structure, which satisfied the bond length difference formula, existed on the detonation synthetic conditions. Accordingly, it was considered that the direct transformation from graphite to diamond could not come true from the perspective of VES. In addition, the mechanism of synthesis diamond by explosive detonation was discussed based on the VESs of graphite and diamond.     

Stress-strain state in the grit-matrix system of a diamond tool under force disturbances

The stress-strain state in the grit-matrix system of carbide-bonded diamond tools has been studied by numerical simulation. A transition layer in contact with a diamond grit is shown to be the most heavily loaded portion of the bond. The behavior of stresses in this region has been clarified for variable cutting force directions, properties of the matrix, and the transition layer. Stresses have been analyzed for various extents of the grit embedding in the matrix; critical loading parameters have been determined for various grit shapes and Poisson ratios of the bond.     

Theoretical analysis of a carbon-carbon dimer placement tool for diamond mechanosynthesis

Density functional theory is used with Gaussian 98 to analyze a new family of proposed mechanosynthetic tools that could be employed for the placement of two carbon atoms--a carbon-carbon (CC) dimer--on a growing diamond surface at a specific site. The analysis focuses on specific group IV-substituted biadamantane tool tip structures and evaluates their stability and the strength of the bond they make with the CC dimer. These tools should be stable in a vacuum and should be able to hold and position a CC dimer in a manner suitable for positionally controlled diamond mechanosynthesis at room temperature.     

Doping Fe-based diamond tool matrix composites with a rare-earth element

The paper presents the experiments on adding rare-earth element cerium to diamond matrix composites. Based on the doping of rare earth in metal powders including tungsten carbide, a small amount of nickel, iron entirely replacing cobalt in diamond matrix and the process route of rare earth doping is indicated. The performance of matrix composites with and without rare-earth elements has been assessed. The results obtained show that the flexural strength, the hardness, and the impact ductility of matrix composites with rare-earth elements have been improved. The flexural strength and the impact ductility increased correspondently by 10–62% and about 5, as compared to composites free of rare-earth elements. Rare-earth diamond tool matrix composites where Co was r’eplaced with Fe, which provides a good practical service performance and a low price, have been successfully studied, corresponding diamond bits and saw blades have been manufactured.     

On theoretical substantiation of the choice of effective grain profile shape in modeling of diamond layer in dressing tools. Part 2. Electroforming

The paper describes, from the statistical standpoint, the mechanism of formation of spherical shape of the averaged cross section of scratches produced by flat faces of diamond grains in abrasive material of grinding wheels during their dressing with diamond rolls manufactured by electroforming. The distribution of parameters of orientation of diamond grain cutting faces has been determined and a brief comparative analysis of characteristics of dressing tools manufactured by electroforming and electroplating has been performed. A notion of the reduced effective diameter has been introduced into the model representation of a diamond cutting grain; a relationship between this diameter and the tool grain size has been found, which is needed for calculating individual and total cross-sections of cuts, dressing forces, and surface roughness of workpieces ground with pre-dressed abrasive wheels.     

Properties of diamond under hydrostatic pressures up to 140 GPa

Diamond is the archetypal covalent material. Each atom in an sp(3) configuration is bonded to four nearest neighbours. Because of its remarkable properties, diamond has been extensively studied. And yet our knowledge of the properties of diamond under very high pressure is still incomplete. Although diamond is known to be the preferred allotrope of carbon at high pressure, the possibility of producing under pressure high-density polymorphs of diamond, including metallic forms, has been discussed. Structural changes have already been reported in diamond under non-hydrostatic pressures around 150 GPa and large deformation. However, measurements of the properties of diamond under hydrostatic pressure have been limited to below 40 GPa. Here, we report accurate measurements of the volume and of the optical phonon frequency of diamond under hydrostatic pressure up to 140 GPa. We show that diamond is more compressible than currently expected. By combining the volume and the frequency pressure shifts, we deduce that diamond remains very stable under pressure: it is a Gruneisen solid up to at least 140 GPa, and the covalent bond is even slightly strengthened under pressure. Finally, the optical phonon frequency versus pressure is calibrated here to be used as a pressure gauge for diamond anvil cell studies in the multi-megabar range.     

Wear resistance of nano-polycrystalline diamond with various hexagonal diamond contents

Wear resistance of nano-polycrystalline diamond (NPD) rods containing various amounts of hexagonal diamond has been tested with a new method for practical evaluation of the wear-resistance rate of superhard ceramics, in addition to the measurements of their Knoop hardness. The wear resistance of NPD has been found to increase with increasing synthesis temperature and accordingly decreasing proportion of hexagonal diamond. A slight increase in Knoop hardness with the synthesis temperature also has been observed for these samples, consistent with the results of the wear-resistance measurements. These results suggest that the presence of hexagonal diamond would not yield any observable increase in both hardness and wear resistance of NPD, contradictory to a recent prediction suggesting that hexagonal diamond is harder than cubic diamond. It is also demonstrated that NPD is superior to single crystal diamond in terms of relatively homogeneous wearing without any significant chipping/cracking.     

Size-related stabilization of diamond nanoparticles

Boundaries of stability regions (of particles sizes) of diamond and graphite at low pressure and various temperatures have been established by modeling and computation of free energies of the crystal phases. The results indicate that diamond is the stable modification of carbon, and graphite is the metastable one at small particle sizes. The diamond charge lattice is presented by ion-electron lattice of negative bond charges and positive ions. The graphite charge lattice consists of hexagonal ion-electron nets and collectivized conduction electrons located between the nets. The consideration of conduction electrons in the graphite model provides the stable graphite structure, since the attraction between the conduction electrons and hexagonal nets compensates the repulsive forces acting between the nets.The intersection of size dependencies of free energies of diamond and graphite indicates the size-related stabilization of small diamond particles. The established boundaries of stability regions of diamond and graphite are: 10.2 nm at room temperature, 6.1 nm at 525 °C, 4.8 nm at 800 °C and 4.3 nm at 1100 °C.     

Effect of cryogenic minimum quantity lubrication on machinability of diamond tool in ultraprecision turning of 3Cr2NiMo steel

This work presents a series of experimental investigations and corresponding theoretical analyses to research on the effect of cryogenic minimum quantity lubrication (MQL) on machinability of diamond tool in ultraprecision turning of typical die steel. The tool wear and machined surface quality were determined as experimental indexes, which were measured using the scanning electron microscope and surface profiler, respectively. Besides, the maximum temperatures of diamond tool surfaces acquired by infrared thermal imager were used to indirectly evaluate the cutting process. The experimental results revealed that cryogenic MQL had obvious advantages in improving diamond tool durability and machined surface quality by comparison with flood cooling, cryogenic gas cooling, and MQL, and its essential function mechanisms were thoroughly understood. On the basis of this, carbon nanofluid was found to achieve optimal tool life in diamond turning compared with polyethylene glycol, castor oil, synthetic ester, and emulsified liquid. Ultimately, the combined machining method of ultrasonic vibration-assisted turning and cryogenic minimal quantity lubrication was proposed in this work. The results showed that this technique could observably improve the machinability of diamond tool and also provide a new direction for exploring a suitable processing method for ultraprecision machining of ferrous materials.     

Cyclic voltammetric, a.c. and d.c. polarization behavior of boron-doped CVD diamond

Boron-doped polycrystalline diamond films have been deposited over a molybdenum substrate by the microwave plasma CVD process using a methane and hydrogen gas mixture at a pressure of 35.7 Torr. Boron doping of diamond has been achieved in situ by using a solid boron source while growing diamond in the CVD process. The a.c. impedance of boron-doped diamond films in 0.5 M NaCl solution has been determined and compared with the results obtained with a molybdenum substrate. Capacitance, solution resistance, and polarization resistance (corrosion rate) have been determined using the experimental data plotted in Nyquist and Bode formats. D.C. polarization techniques such as linear and Tafel polarization have been used to evaluate the doped diamond coated molybdenum and molybdenum for corrosion resistance characteristics in terms of charge-transfer coefficients and corrosion rate. Cyclic voltammetry has been used to evaluate the molybdenum, platinum, and doped and undoped diamond coated molybdenum materials in 0.5 M NaCl solution. We have observed two time constants with a doped diamond electrode / solution interface. Solution resistance was found to be constant irrespective of the electrode in the same electrolyte solution.     

Strength of diamond powders synthesized in the Mg-Zn-B-C system

The strength of diamond powders of grit size 125/100 μm synthesized in the Mg-Zn-B-C system (reaction mixtures having different boron concentrations) has been investigated. It has been shown that the distribution of strength by fracture load is correctly described by the Weibull function. The strength index of diamond powders and confidence intervals has been defined. It has been found that the strength index dependence on boron concentration in the range from 1 to 40 at % is described by the function having the minimum at 20%.     

Main characteristics of abrasive diamond layer in dressing tools

It is demonstrated that a dressing roller or stick as a single-layer abrasive diamond tool can be characterized, like a multilayer one, by two independent parameters—grit size (it suffices to indicate the upper sieve size only) and conditional concentration. The other characteristics (surface density of a powder batch, specific number of diamond grits) are unambiguously determined from these two parameters by means of theoretically substantiated and experimentally verified relationships. The limiting values of the diamond powder quantity on the surface of single-and multilayer tools have been established which differ from the data reported elsewhere.     

Anodic oxidation with doped diamond electrodes: a new advanced oxidation process

Boron-doped diamond anodes allow to directly produce OH* radicals from water electrolysis with very high current efficiencies. This has been explained by the very high overvoltage for oxygen production and many other anodic electrode processes on diamond anodes. Additionally, the boron-doped diamond electrodes exhibit a high mechanical and chemical stability. Anodic oxidation with diamond anodes is a new advanced oxidation process (AOP) with many advantages compared to other known chemical and photochemical AOPs. The present work reports on the use of diamond anodes for the chemical oxygen demand (COD) removal from several industrial wastewaters and from two synthetic wastewaters with malic acid and ethylenediaminetetraacetic (EDTA) acid. Current efficiencies for the COD removal between 85 and 100% have been found. The formation and subsequent removal of by-products of the COD oxidation has been investigated for the first time. Economical considerations of this new AOP are included.     

On morphometric uniformity of diamond micron powders

The paper presents the findings of characterization of initial diamond powders and products of their flotation separation, analysis of the adequacy of mean values of morphometric characteristics and assessment of powder uniformity by a system-criterion method. The separation is demonstrated to provide a significant improvement of powder uniformity in dimensional characteristics and shape index. The flotation products have been found to feature the best uniformity which explains their higher abrasive ability. The flotation separation method has proved to be a suitable tool for improving morphometric uniformity of diamond micron powders, and the proposed system of computer-aided analytical methods has been demonstrated to offer an efficiency means for quantitative analysis of morhometric characteristics of the powders.