Diamond Grains Research Articles

On the question of nitride ceramics processing with soft abrasive grain

Development of modern mechanical engineering relates to development and introduction of new materials and progressive technological processes of their treatment. Unique properties of ceramics make it possible to use ceramics in different areas of engineering that is as cutting instruments, details of machines, devices, radioelectronic apparatus. Due to high hardness machining of ceramics is possible only with the use of synthetic diamonds, but synthetic diamonds are rather expensive that is why it is necessary to use cheaper materials instead of expensive synthetic diamonds. To improve the treatment of ceramics it is necessary to study the rules and regularities of the complicated and multivariable polishing process. Productivity, surface quality, wear and firmness of instrument result from the properties of ceramics, diamond instrument performance, modes and from the technological features of the equipment. Providing high quality of accurate ceramic details surface is a difficult technical task. Besides roughness that results from the conditions under which the instruments are used it is highly necessary to have neither microcracks nor microcavities. As ceramic materials are fragile, all ceramic ware has a tendency to shearing off under load at cutting and polishing. Point loading at low yield makes the ceramics crumble out under strong mechanical and thermal loading of the diamond grains. And there forms a ditch, its width exceeding the area of the diamond grain and the material collision. The article describes an attempt to treat nitride ceramics with a soft abrasive, the ceramics being much harder than the abrasive. It has been supposed that a soft abrasive wearing out does not stop removing material from the ceramic surface. It has been proved graphically and mathematically, that removing material does not stop

Theoretical analysis of technological possibilities of reducing surface roughness at abrasive processing

The paper presents analytical dependences for determining the parameters of surface roughness at abrasive machining with cutting grains positioned spherically at the same height on the working surface of the tool and overlapping mutually in the process of forming the roughness of the processed surface. The calculations established that, provided that the cutting grains are located at the same height on the working surface of the tool as applied to the free abrasive finishing process, it becomes possible to significantly reduce the surface roughness parameters Ra and Rmax. The calculations also stated that these parameters are the less, the less the granularity of the abrasive or diamond grains is at processing. With an increase in the grain size of the diamond powder, in order to achieve the specified values of the surface roughness parameters Ra and Rmax, it is necessary to increase the number of the cutting grains forming the surface roughness. However, this leads to reduced processing performance. It has also been shown that it is not possible to put into practice the processing conditions under which the surface roughness parameters Ra and Rmax, established theoretically, become very small due to the difficulty of providing the same arrangement of abrasive grains on the working surface of the tool. Based on the theoretical solutions obtained, specific practical recommendations have been developed to reduce the surface roughness. So, an effective method of internal grinding has been developed using a soft felt (felt) wheel with a glued layer of abrasive powder 63C. This method can significantly reduce the surface roughness parameter Ra without increasing the complexity and reducing processing productivity. For its practical implementation, it is proposed to use abrasive powder with granularity F80-F150. In doing so, it is effective to grind by setting the axis of rotation of the grinding wheel with an individual drive perpendicular to the axis of rotation of the hole being machined. As a result, the number of simultaneously working abrasive grains significantly increases as compared to conventional internal grinding due to an increase in the contact area of the grinding wheel with the workpiece. Also, a certain different-height arrangement of cutting grains on the working surface of the tool, which takes place under real processing conditions, also decreases, which results in a decrease in the roughness of the machined surface

Determination of Oxides Intended for the Surface Modification of Diamond Grains by their Functional Characteristics

We determined the restrictions on the selection of oxides, which can be effective in modifying the surface of the grains of synthetic diamond grinding powders with heat-resistant oxides. The first group of oxides that are most efficiently used for modifying the surface of grains of diamond grinding powders includes B2O3, TiO2, SiO2, and Al2O3, and the second group consists of less effective ZnO, BaO, and CaO. To achieve an additional positive effect of the modification, one can consider a double modification of the surface of diamond grains with a mixture of oxides of the first group and chlorides (CaCl2, NaCl).

Features of the Formation of an Interface Zone Structure during Thermal Diffusion Metallization of Diamond by Transition Metals

The features of the morphology and chemical and structural-phase compositions of the diamond–metal interface zone formed in the process of thermal diffusion metallization of diamond by chromium, titanium, iron, nickel, and cobalt powders under the same temperature–time mode of the vacuum furnace operation that corresponds to the mode of sintering of diamond-containing WC–Co matrix with copper impregnation are studied. In the process of thermal diffusion metallization by chromium and titanium, a metalized coating is formed on the diamond surface, consisting of a phase mixture of carbides, metals, and graphite of variable composition. The small content of graphite formations and their discontinuous character of location in the diamond–metal interface zone ensure a strong adhesion of the metalized coating to the diamond through the carbides of the corresponding metals. During thermal diffusion metallization by iron, the intermediate layer strongly adhering to the diamond is also formed in the diamond–metal interface zone. The intermediate layer has a complex structural-phase composition comprising a mixture of iron phases, a solid solution of carbon in iron, and graphite of variable composition. An assumption is made that the intermediate layer could be formed on the surface of diamond grains by solidification of the liquid phase with eutectic composition, resulting from the eutectic melting of the diamond–iron contact pairs. However, the confirmation of this assumption requires performing special experiments using highly sensitive methods of research. Under the heating conditions specified in the experiment, the nickel–cobalt and cobalt–diamond interaction causes intense catalytic graphitization of diamond followed by the formation of numerous traces of erosion on its surface. The observed weak adhesive interaction of these metals with diamond is likely to be determined by the high melting temperatures of the Ni–C and Co–C eutectics, which does not allow the metals to react with diamond actively under the specified experimental conditions.

A comparative study of the mechanical and tribological properties of intermittently and continuously grown multilayer diamond films on RB-SiC

Multilayer diamond films were deposited on a reaction bonded silicon carbide (RB-SiC) substrate by a single-step continuous process as well as a three-step intermittent process in a hot filament chemical vapour deposition (HF-CVD) equipment. The recipes of HF-CVD during the single-step multilayer film (C-MC) were selected by varying the CH4/H2 concentration and chamber pressure such that the initially deposited diamond grains were microcrystalline, which became progressively finer with further deposition. In the case of the intermittently grown films (Ix-MC), a one, two and three-layered structure, each layer having a gradual decrease in grain size was deposited with the same recipes as that of the C-MC. All the films were characterised using SEM, AFM and Raman spectroscopy. Fine grains in the range of 0.2–0.7 μm were identified in the C-MC and sharply faceted diamond crystallites having sizes between 0.5- 2 μm were seen in the intermittently deposited coatings. The hardness values were 71 GPa, 56 GPa and 46 GPa for the intermittent coatings. In contrast, a lower hardness of 29 GPa was obtained in C-MC. The friction coefficients of all the diamond films were measured up to 106 sliding cycles (530 m) using a ball-on-disc universal tribometer. In the initial stages of sliding, the continuously grown C-MC exhibited a friction coefficient ~0.17, while the intermittently coated three-layered film (I3-MC) showed a friction coefficient of 0.28. However, beyond 40,000 sliding cycles, the friction coefficient of I3-MC showed a value of ~0.05, which was slightly lower than C-MC (~0.06).

Grain size dependent tribological behaviors of 700 °C annealed polycrystalline diamond

The role of original diamond grain size in the tribological behaviors of the 700 °C annealed polycrystalline diamond (PCD) is discussed in view of experimental studies using complementary analytical techniques (SEM&EDS, TGA-DSC, XRD and Raman). The original diamond grain size has a strong effect on the tribological behaviors of the annealed PCD, which can be accounted for by the various cobalt content and distribution shape. The annealed PCD with the original diamond grain size of 2 μm possesses the lowest cobalt content and presents severe abrasive wear, which is due to the plowing action of exfoliated diamond grains. During the annealing treatment of PCD with large original diamond grain, the large amount of leaf-like distributed cobalt facilitates the diamond graphitization by the catalytic action, which accelerates the formation of friction reducing carbonaceous transfer film, thus yields to a low friction coefficient. The fully oxidation filling into the spalling inhibits the serious grain exfoliation, thus further prevents the annealed PCD wearing substantially. Overall, the cobalt-catalytic diamond graphitization without an excessive wear resistance sacrifice can be exploited to achieve excellent tribological properties of PCD after 700 °C annealing treatment.

Development of models for estimating specific energy and specific wear rate of circular diamond saw blades based on properties of carbonate rocks

This paper presents an experimental study to assess the effects of physical and mechanical properties of carbonate rocks and diamond/metal matrix characteristics on performance and tool life of segmented circular sawblades. For this purpose, pertinent rock properties including strength, hardness, abrasivity, water absorption, porosity and unit weight were measured by laboratory tests and compiled into a database for subsequent analysis. The testing program including using six commercially available sawblades with different segment compositions in full scale tests. The related characteristics of the sawblades were diamond concentration, Vickers hardness of the hard facing matrix, mean diamond grain size, cobalt, tin, copper, and ferrous content. Thirteen different carbonate rocks including marble, limestone, and travertine samples from various sites in Turkey were used in the experimental program. The experimental data was analysed by multiple linear and nonlinear regression analysis to develop models for estimation of specific energy (SE) and specific wear (SW) rates using rock properties and sawblades segment parameters. The proposed models were validated by comparing the results of their prediction with those of actual test results and showed good agreements. The concept of optimum total (cutting) cost (OTC) estimation for natural stone cutting is also discussed in the context of estimating sawblade wear life.

Geological Evidence does not Support a Shallow Origin for Diamonds in Ophiolite

Geological Evidence does not Support a Shallow Origin for Diamonds in Ophiolite

Diamond-bearing ceramic material for precision abrasive micromachining

The paper describes an energy-efficient technology for producing a diamond-bearing composite material on the surface of a sintered diamond-aluminum part by microarc oxidation. The material formation time and energy consumption cost is reduced through replacing 10–25 vol% aluminum powder with white corundum powder. The corundum powder grit should be 5–6 times less than the diamond grain size. The material has high wear resistance and volumetric cutting ability. Frictional characteristics are improved due to an increasing matrix hardness through increasing α modification of Al2O3 in its composition. The new material might be a tool for precision abrasive microprocessing of hard materials.

Experimental research on the biological in-process dressing (BID) of Cu–Co matrix diamond tools

Biomachining, a recently developed non-traditional machining method, uses extreme microorganisms for the controlled material removal from material surfaces. The present paper proposed a new dressing method—biological in-process dressing (BID)—for metal-bonded abrasive tools. The cell-free culture supernatant filtered from cultured microorganisms was continuously sprayed on the surface of a Cu–Co bonded diamond tool to conduct the dressing process. BID experiments were conducted on a series of fixed diamond pads for the lapping of silica glass and sapphire. The material removal rate (MRR), machined surface roughness (Sa), tool surface topography, and thickness loss of the diamond tool were measured during lapping experiments. It was found that BID effectively exposed all diamond grains and improved the machining quality and efficiency. The dressing mechanism of BID was schematically illustrated.

Stability evaluation of cutting edges in operation of composite materials for instrumental purposes based on a metal matrix

The authors analyzed the existing methods for assessing the stability of cutting grains in the article. The most optimal method for assessing the performance of abrasive grains was selected based on the results of the analysis. This method was used to calculate the concentration of diamond grains on the working surfaces of samples of a composite material made on the basis of bronze. The results of the studies showed that when testing the samples for friction and wear, the number of active abrasive grains and the relative solids content of natural diamond grinding powders show the stability of the cutting edges.

Tower-Like ZnO Nanorod Bundles Grown on Freestanding Diamond Wafers for Electron Field Emission Improvement

Tower-like ZnO nanorod (ZNR) bundles were grown on freestanding diamond (FSD) wafers by a low-temperature hydrothermal method without any catalysts, and the corresponding electron field emission property was investigated. The structural characterizations disclosed that these tower-like bundles were composed of single crystalline ZNRs with hexagonal wurtzite structure. They grew perpendicularly to each crystal planes of the diamond grains. Different hydrothermal reaction times (e.g., 1.5, 3, 6, 8 h) were carried out to investigate the morphology features of ZNRs grown on the growth surface of FSD wafers. The ZNR bundles with a growth time of 6 h showed the highest crystallinity and aspect ratio. Such ZNR bundles/FSD hybrid exhibited a high emission current density of 0.2 mA at an applied field of 11.8 V/μm and a reduced turn-on field of 6.8 V/μm. The field enhancement factor β was calculated to be ~4979 based on the Fowler-Nordheim theory. The geometry and structure of polycrystalline diamond were responsible for the formation of ZNR bundles and the high β-value.

Wear of electroplated diamond tools in lap-grinding of Al2O3 ceramic materials

Current development of modern products, together with ever-increasing demands for their operation and usage, necessitate the search for new processing methods. Abrasive machining is widely used in many industrial areas, especially for processing difficult-to-machine materials such as advanced ceramics. Grinding with lapping kinematics, also called lap-grinding, is still one of the innovative methods of abrasive processing being under continuous development. It combines the advantages of grinding and lapping, allowing for meeting high design requirements and, at the same time, ensuring efficient and safe machining. This paper presents experimental and modelling results on the application of electroplated wheels with diamond grains (D107 and D64) in the lap-grinding of Al2O3 ceramic materials in a single-disc lapping machine configuration. The grain size along with the thickness of the nickel bond influenced the material removal rate and surface roughness obtained. The relationship between the material removal and the processing time was approximated by asymptotic mathematical functions. Moreover, the empirical models were modified according to the Preston's equation when the unit pressure increased. The linear and nonlinear regression models enabled accurate curve fitting to the surface roughness data. Microscopic images of the active surface of electroplated diamond tools were analyzed in the aspects of the resultant tool wear and technological effects. In addition, the results were referred to the analysis of microscopic images of the suspension taken from the active surface of the tool after subsequent tests. The suspension consisted of particles of fragmented abrasive grains as well as chips removed from the machined surface.

Laser dressing of arc-shaped resin-bonded diamond grinding wheels

Laser dressing technology is expected to solve the dressing problem of superabrasive forming grinding wheels. However, this technology is still in the initial stage of research, and is facing bottlenecks such as poor profiling accuracy due to the difficulty of tool setting of laser beam, uncontrolled sharpening topography due to laser focus characteristics, and reduced grain performance due to ablation thermal damage. In this paper, the research on laser dressing of arc-shaped resin-bonded diamond grinding wheel with a grain size of about 180 μm was systematically carried out. Acoustic emission monitoring technology was first introduced into laser dressing, which solved the problem of low accuracy of tool setting of laser beam. The laser tangential profiling method based on deep cutting and intermittent feeding was improved to achieve efficient and precise profiling of arc-shaped grinding wheels. It took about 4.5 h to profile a parallel grinding wheel into an arc-shaped grinding wheel. Limited by the accuracy of the experimental device, the surface contour accuracy of the arc-shaped grinding wheel after profiling was currently only about 20 μm. It was found that the grains in the middle area of the surface of the grinding wheel were only slightly graphitized, and the damage degree of the grains in the edge area was more serious. A new method of laser sharpening based on layered scanning was proposed, which effectively reduced the difficulty of controlling the laser scanning trajectory, and achieved the uniform removal of the bond and the precise control of the grain protrusion height everywhere on the surface of the grinding wheel. It was found that the laser beam did not damage the diamond grains during the sharpening process. Compared with that before sharpening, the proportion of C and O elements in the resin bond after sharpening increased by about 5.7 % and decreased by about 5.0 %, respectively.

Laser-dressing topography and quality of resin-bonded diamond grinding wheels

Laser dressing technology has shown extremely outstanding advantages and broad application prospects in dressing of superabrasive grinding wheels. However, this technology is currently facing bottlenecks that the generation mechanism of the metamorphic layer on the surface of the grinding wheel is unclear, and the influence rule of the heat accumulation effect on the removal threshold of the grain is not mastered. In this paper, the laser cutting experiment of single diamond grain, and the laser ablation experiment and dressing experiment of resin-bonded diamond grinding wheels were systematically carried out, and the effects of process parameters on the topography and quality of nanosecond laser dressing were comprehensively explored by means of SEM, Raman, EDS, XRD and other detection methods. The mechanism of generation and removal of the graphite layer on the surface of diamond grains during profiling was revealed. It was found for the first time that there was a clear interface between the graphite layer and diamond, and the increase of the finishing time was beneficial to reduce the degree of graphitization of profiled diamond grains. The influence rule of the heat accumulation effect on the removal energy threshold of diamond grain during the sharpening process was first clarified. It was found that the removal energy threshold of diamond grain decreased first and then stabilized with the increase of the spot overlap ratio. The pyrolysis process and phase transformation of resin bond during laser dressing were innovatively explored. It was found that the proportion of C element increased and the proportion of O element decreased in the bond, while the composition and content of the auxiliary materials in the bond changed little. Finally, the comprehensive optimization of laser dressing parameters was completed, and low damage and ultra-precision dressing of resin-bonded diamond grinding wheels were achieved.

Effect of Diamond Surface Pretreatment and Content on the Microstructure and Mechanical and Oxidation Behaviour of NiAl/Fe-Based Alloys.

The effect of diamond surface pretreatment and content on the microstructure and mechanical properties of NiAl/Fe–x diamond (x = 0, 5, 10, 15, and 20 wt.%) alloys was investigated after mechanical alloying with subsequent hot-pressing sintering. The results showed that after the surface pretreatment, a complete transition layer containing W existed on the outer surface of the diamond grains, which improved the interfacial bonding strength of the diamond grains and NiAl/Fe matrix to an excellent level. As the diamond content increased, the compressive strength of the NiAl/Fe-based alloys declined, but the alloy with 10 wt.% diamond had a higher value than that of the other NiAl/Fe-based alloys. Short cracks and transgranular fracture were observed in the fracture surface of all materials. For the material with 20 wt.% diamond, intergranular fracture was obvious, and many diamond particles appeared along the fracture direction, which caused the compressive strength to be the lowest of the samples considered in this study. After the addition of diamond, the oxidation resistance of NiAl/Fe-based alloys decreased due to a loose oxidation layer and diamond graphitization. The thermal conductivity of the alloy first increased and then decreased with increasing diamond content. A NiAl/Fe-based alloy with 15 wt.% diamond demonstrated the maximum thermal conductivity of 53.2 W/(m·k) at 600°C among the samples in this study.

Impedance spectroscopy analysis of n-type (nitrogen-doped) ultrananocrystalline diamond/p-type Si heterojunction diodes

Heterojunction diodes are constructed by growing n-type (nitrogen-doped) ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) composite films onto p-type Si substrates in nitrogen and hydrogen mixed-gases by coaxial arc plasma deposition. The fabricated heterojunctions are analyzed regarding their film morphology and electrical properties. The complex structure of UNCD/a-C:H films, which consists of nano-sized-sp3 diamond grains and sp2-grain boundaries (GBs), makes it difficult to separate their contribution to electrical conductivity of the films using conventional characterization methods. In this paper we characterize the n-type UNCD/p-type Si heterojunction diode by employing impedance spectroscopy method, which can isolate the differing components that contribute to the overall conductivity of the film. The impedance spectroscopy is measured in the frequency range of 100 Hz to 2 MHz, with AC small signal superimpose on DC bias voltage in the range of 0–1 V. The influence of the bias on UNCD grains and GBs contribution to resistance and capacitance of UNCD/a-C:H film and n-UNCD/p-Si heterojunction is investigated by equivalent circuit model using fitting of the impedance data. The results revealed that the electrical conductivity is mainly controlled by the GBs rather than the UNCD grains. Furthermore, the extracted value of dielectric constant of N-doped UNCD/a-C:H film is found to be comparable with that of microcrystalline diamond, which indicates the capacitance contribution in the device is mainly originated from the UNCD grains. This study demonstrates capability of impedance spectroscopy to provide an obvious separated contribution of sp3 and sp2 bonded carbons to the electrical conductivity in their coexistence materials.

Electrochemical cleaning grinding (ECCG) of aluminum alloy 6061 with electroconductive resin-bonded diamond wheels

A new manufacturing process named Electrochemical Cleaning Grinding (ECCG) is introduced to help reduce wheel loading in grinding of the highly ductile metals which exhibit high adhesion tendency. The key component of the ECCG system is an electroconductive resin-bonded diamond wheel, and its abrasive layer is made of phenolic resin bond and core-shell structured Ni-P alloy coated diamond abrasives. The alloy coating over diamond grains not only helps to increase the bond strength between the resin and the abrasives but improve the electrical conductivity properties of the grinding wheel. During the ECCG process, the abrasives on the wheel surface simultaneously act as micro cutting edges as well as microelectrodes where the anodic dissolution of the adherent metal materials takes place. This dissolution process is defined as “cleaning”. When the rate of accretion of adherent material is balanced by its electrochemical dissolution rate, the sharp abrasives are exposed to carry out efficient grinding, so that better grinding performance can be achieved. The grinding experiments of aluminum alloy 6061 (AA6061) were conducted to verify the effectiveness of the ECCG process. Compared with conventional grinding experiments, the most important changes brought in by the ECCG are the decrease in wheel loading degree and the improvement in machined surface quality. Additionally the electric energy consumption of the cleaning process is relative low and equal from 1.88 × 10−3 J/mm3 to 9.34 × 10-3 J/mm3.

Elastic instability in graphite single crystal under dynamic triaxial compression: Effect of strain-rate on the resulting microstructure

We focus on the behavior of graphite under triaxial loading at a constant strain-rate using large scale molecular dynamics simulations. Buckling patterns (chevrons) in graphite nucleate from an elastic instability strongly related to the material anisotropy and subsequently grow until the first diamond nuclei appear. We show that the phase transition completely inhibits the growth of chevrons in buckled graphite, the diamond grain size being determined by the size of chevrons at the onset of nucleation. Cubic-diamond clusters nucleate within chevrons of buckled graphite and grow until the parent phase is entirely transformed. This phenomenon leads to nano-structured diamond polycrystals, with orientations of interfaces given by those of the buckled material right before the nucleation process. The buckled graphite microstructure is shown to strongly influence the final microstructure/size of nano-diamonds.

Effect of cutting parameters on surface integrity of monocrystalline silicon sawn with an endless diamond wire saw

The cutting of silicon wafers using multi-diamond wire sawing is a critical stage in solar cell manufacturing due to brittleness of silicon. Improving the cutting process output requires an in-depth understanding of phenomena associated with cutting parameters. In order to investigate the influence of diamond wire sawing on surface integrity of monocrystalline silicon, a looped diamond wire was used and cutting parameters wire cutting speed, feed rate and wire tension were varied. The surface morphology was observed by scanning electron microscopy. Surface roughness Sa was measured with a non-contact profilometer. The brittle-ductile transition was identified by presence of residual phases on sawn surface. A bevel-polishing method was employed to determine the microcrack depth. The results show that with higher feed rate the surface presents deeper and wider craters because of deeper penetration of diamond grain. On increasing wire cutting speed, there were more regions formed in ductile mode. The higher Sa values was observed on increasing both feed rate and wire tension, while Sa decreased with an increase in wire cutting speed. The brittle mode was predominant with an increase in feed rate, resulting in Si-I phase in regions formed in fragile mode. Material removal in ductile mode led to appearance of a-Si phase at high wire cutting speed. No significant effect was observed on increasing wire tension. Subsurface microcracks mainly initiating from bottom of grooves generated by cutting mechanism. The most appropriate set of cutting parameters is the lowest feed rate and wire tension and highest wire cutting speed.