Diamond Machining Research Articles

Advanced imaging of dentin microstructure

This study aimed to demonstrate the feasibility of using optical coherence tomography (OCT) for locating the sectioning site of a specimen before characterizing the ultrastructural features of dentin surfaces as well as the inner wall of the dentinal tubules (DT) using a field emission scanning electron microscope (FESEM). Eight sound human molar teeth were extracted, examined via cross-polarization optical coherence tomography (CP-OCT), embedded, and hemisectioned using a low-speed diamond sawing machine. Next, each sectioned surface was further trimmed, polished, and examined under a confocal laser scanning microscope (CLSM) to locate the target area on the superficial dentin. Subsequently, each section was gold-coated and examined using FESEM. Backscattered reflection from the dentin layer was less than that from the enamel under CP-OCT. Distinct reflections from certain enamel and dentin microstructures were observed before sectioning the specimens. Areas with enamel cracks and dentin defects were identified and avoided during sectioning. At the micron level, the CLSM images exhibited a homogenous distribution of the DT orifices. Low magnification FESEM images showed intertubular dentin as a loosely condensed globular layer with shallow grooves in between, whereas peritubular dentin exhibited more organized condensation of apatite crystals surrounding the DT orifices. High magnification of the DT revealed a cross-linking layer of mineralized collagen network extending in the peri-intratubular lumen, with scattered globules of matrix vesicles. CP-OCT enabled the realization of rapid initial scanning and image acquisition with high contrast at the micron scale before profound insights into dentin ultrastructures at the nano scale were provided by FESEM. The variations in structural densities of the dental tissues significantly affected the image contrast and helped identify underlying structures.

Efficient microspark erosion-assisted machining of a monocrystalline microdiamond stylus using a heat-avoidance path

Monocrystalline diamond possesses covalent bonding making diamond extremely hard and difficult to machine. In this study, a microdiamond stylus typically used in measuring surface roughness is machined to exemplify the proposed ‘microspark erosion-assisted machining with heat-avoidance path’ technique. Based on the high thermal conductivity and weak electrical conductivity of boron-doped monocrystalline diamond, high-frequency pulsed discharge plasma is employed to efficiently perform microspark erosion machining on an extremely hard monocrystalline diamond blank. It was found that the pulse-on time and servo voltage respectively affect erosion plasma length and the erosion gap during diamond machining. Also, the safety distance and safety height of the erosion path dominate heat transfer to filler metal. These factors all affect the firmness of the brazed diamond blank on the substrate. Three mechanisms for removing carbon atoms from the diamond blank surface were observed. They are vaporization, melting, and graphitization of carbon atoms. This graphitized carbon atoms have weak electrical conductivity, which is conducive to inducing the wire-electrode to generate a greater electric field and secondary discharging, facilitating removal of additional carbon atoms. Experimental results indicate that a microdiamond stylus prototype with a tip of 10 μm can be safely formed using a ‘microspark erosion-assisted machining with heat-avoidance path’ technique, creating 93.7% repeatability of the minimum residual stylus diameter. The tangential micro-grinding facilitates the stylus tip to receive grinding from the grinding wheel's maximum tangential speed and create the precision microdiamond stylus with 1 μm in tip-radius. The applied microspark erosion-assisted machining had a diamond material removal rate that was 54% more efficient than conventional grinding of a commercial microdiamond stylus. The formed microdiamond stylus was inspected by Raman spectroscopy and verified by the surface roughness standard gauge to be up to industry standards.

Microstructural influence on damage-induced zirconia surface asperities produced by conventional and ultrasonic vibration-assisted diamond machining

Zirconia surface asperities associated with machining-induced damage and deformation jeopardize the quality of zirconia products. Conventional and emerging ultrasonic vibration-assisted machining processes are used to shape zirconia materials. However, a deep understanding of how zirconia microstructures and ultrasonic vibration amplitudes affect material removal mechanisms and surface quality in these processes is missing, rendering the proper mechanical process selection challenging. This paper reports on the 3D characterization of damage-induced surface asperities and the investigation of material removal mechanisms of pre-sintered porous and sintered dense zirconia materials in conventional and ultrasonic vibration-assisted diamond machining processes. 3D white light profilometry was used to measure surface asperities in terms of texture parameters, together with scanning electron microscopy (SEM) for imaging damage and deformation morphologies. The results show that removal mechanisms and damage-induced zirconia surface asperities depended on material microstructures and ultrasonic vibration amplitudes. Both porous and dense zirconia materials had a brittle-ductile mixed removal mode in conventional and ultrasonic vibration-assisted diamond machining processes. However, brittle fracture was dominant for the porous state and ductile deformation was presiding for the dense state. Thus, there were significantly higher fracture damage area ratios with much higher average and maximum roughness values, and maximum peak and valley heights on machined porous surfaces than dense ones. Ultrasonic assistance at an optimal vibration amplitude promoted brittle-ductile transitions on both porous and dense zirconia surfaces, resulting in reduced brittle fracture damage areas with reduced surface asperities. This microstructure-process-surface quality relation provides insights into manufacturing processes for zirconia products.

Zirconia responses to edge chipping damage induced in conventional and ultrasonic vibration-assisted diamond machining

Abstract Machining-induced edge chipping damage represents a common challenge in ceramic applications. This paper reports on responses of zirconia materials with porous and dense microstructures to edge chipping damage induced in conventional and ultrasonic vibration-assisted diamond machining. The machining-induced damage was evaluated using optical and scanning electron microscopies. The results show that edge chipping damage produced in these processes was associated with brittle fracture and depends on the material microstructure and the vibration amplitude. Pre-sintered porous zirconia with a high brittleness index yielded significantly larger edge chipping damage than sintered dense zirconia with a low index in these processes. Ultrasonic machining at an optimal vibration amplitude minimized the scale of brittle fracture at the micro level, and thus significantly diminished edge chipping damage in zirconia materials with distinct microstructures. The investigation underpins the transition from conventional to ultrasonic vibration-assisted machining for manufacturing of ceramics to achieve better product quality.

Tension analysis and fluctuation control of diamond multi-wire sawing machine

During the use of diamond multi-wire sawing machine, tension of diamond wire has an important impact on the quality of silicon wafer. In this paper, ADAMS is used to simulate the tension of diamond wire during the operation of machine, and results are verified by experiments. The general law of tension fluctuation of diamond wire in the operation of diamond multi-wire sawing machine is obtained, and the methods of controlling tension fluctuation are put forward. The proposed method is verified by simulation and experiments, and the conclusion of controlling tension fluctuation is obtained. It is helpful to reduce the broken probability of diamond wire, prolong the service life of diamond wire, and improve the use efficiency of diamond wire.

Freeform based hYperspectral imager for MOisture Sensing (FYMOS).

We present FYMOS, an all-aluminum, robust, light weight, freeform based, near infrared hYperspectral imager for MOisture Sensing. FYMOS was designed and built to remotely measure moisture content using spectral features from 0.7-1.7µm integrating an InGaAs sensor. The imaging system, operating at F/2.8, is based on the three-concentric-mirror (Offner) spectrograph configuration providing a spectral resolution of 8 nm optimized for broad spectral coverage with sufficient resolution to make assessments of water levels. To optimize the optical performance, whilst minimizing weight and size, the design incorporates a bespoke freeform blazed grating machined on a commercial 5 axis ultra precision diamond machine. We achieve a 30% improvement on the RMS wavefront error in the spatial and spectral fields compared to a conventional Offner-Chrisp design with similar aperture and the monolithic Primary/Tertiary mirror eases the manufacturing assembly whilst minimizing weight. We demonstrate the performance of FYMOS by measuring the evaporation rate of water on a soil sample and results are processed with a physical multilayer radiative transfer model (MARMIT) to estimate the mean water thickness.

Microstructural responses of Zirconia materials to in-situ SEM nanoindentation

Development of optimal shaping processes for pre-sintered and sintered zirconia materials requires a fundamental understanding of damage and deformation mechanisms at small-scale contacts with diamond tools. This paper reports on responses of zirconia materials with distinct microstructures to nanoindentation associated with diamond machining using a Berkovich diamond indenter. In-situ nanoindentation was performed in a scanning electron microscope (SEM) and in-process filmed to record small contact events. Indentation morphology was SEM-mapped at high-magnifications. Although both pre-sintered porous and sintered dense zirconia materials mechanically revealed the quasi-plastic behavior in indentation, there were distinct responses of the two materials to quasi-plasticity at the microstructural level. For pre-sintered porous zirconia, the quasi-plasticity was attributed to shear faults resulting from breaking pore networks as microstructurally discrete interfaces, to lead to compression, fragmentation, pulverization and microcracking of zirconia crystals in indentation imprints. In contrast, sintered dense zirconia had shear band-induced quasi-plastic deformation, accompanied with localized tensile microfracture. A material index associated with the mechanical properties ranked the lower quasi-plasticity for pre-sintered porous zirconia than its sintered dense state, predicting more machining-induced damage in the former than the latter. Significantly higher indentation imprint volumes induced in indented pre-sintered porous zirconia than sintered dense state previses higher machining efficiency for the former than the latter. The microstructure-dependent indentation mechanisms provide the fundamental knowledge into micromechanics of abrasive machining of zirconia materials and may lead to a new microstructural design for zirconia materials to achieve a balanced machining efficiency and damage control.

Micro-Injection Molding of Diffractive Structured Surfaces

In recent years, the use of highly functional optical elements has made its way into our everyday life. Its applications range from use in utility items such as cell phone cameras up to security elements on banknotes or production goods. For this purpose, the Leibniz Institute for Materials Engineering (IWT) has been developing a cutting process for the fast and cost-effective production of hologram-based diffractive optical elements. In contrast to established non-mechanical manufacturing processes, such as laser lithography or chemical etching, which are able to produce optics in large quantities and with high accuracy, the diamond turning approach is extending these properties by offering several degrees of freedom. This allows for an almost unlimited geometric complexity and a structured area of considerable size (several tenth square millimeters), achieved in a single process step. In order to introduce diffractive security features to the mass market and to actual production goods, a high-performance replication process is required as the consecutive development step. Micro injection molding represents a feasible and promising option here. In particular, diamond machining enables the integration of safety features directly into the mold insert. Not only does this make additional assembly obsolete, but the safety feature can also be placed inconspicuously in the final product. In this paper, the potential of micro-injection molding as a replication process for diffractive structured surfaces will be investigated and demonstrated. Furthermore, the optical functionality after replication will be verified and evaluated.

Optimization of a continuous hot embossing process for fabrication of micropyramid structures in thermoplastic sheets

Reported is the manufacture and optimization of inverted micropyramid cavity structures into thermoplastic sheets using roll-to-roll (R2R) embossing. To manufacture the master, an ultraprecision diamond machining method was applied to create seamless surface structures into a copper-coated hot embossing roller. Using the hot embossing process, the roller features were successfully transferred to 2 mm thick polymethyl methacrylate (PMMA) sheets. Optimization of the R2R control process variables was conducted using Taguchi's numerical methods, which showed the importance of the roller temperature for a successful pattern transfer. The work presents a novel fabrication technique that allows microstructures to be manufactured into thick PMMA sheets in a continuous process.

Ultra-Precision Machining: Cutting With Diamond Tools

Abstract This article is written as a tribute to Professor Frederick Fongsun Ling 1927–2014. Single-point diamond machining, a subset of a broader class of processes characterized as ultraprecision machining, is used for the creation of surfaces and components with nanometer scale surface roughnesses, and submicrometer scale geometrical form accuracies. Its initial development centered mainly on the machining of optics for energy and defense related needs. Today, diamond machining has broad applications that include the manufacture of precision freeform optics for defense and commercial applications, the structuring of surfaces for functional performance, and the creation of molds used for the replication of a broad range of components in plastic or glass. The present work focuses on a brief review of the technology. First addressed is the state of current understanding of the mechanics that govern the process including the resulting forces, energies and the size effect, forces when cutting single crystals, and resulting cutting temperatures. Efforts to model the process are then described. The workpiece material response when cutting ductile and brittle materials is also included. Then the present state of the art in machine tools, diamond tools and tool development, various cutting configurations used, and some examples of diamond machined surfaces and components are presented. A discussion on the measurement of surface topography, geometrical form, and subsurface damage of diamond machined surfaces is also included.

Review of hybrid methods and advanced technologies for in-process metrology in ultra-high-precision single-point diamond turning

Single-point diamond turning (SPDT) is a leading machining technology for advanced manufacturing of high-accuracy optical components characterized by their nanometric surface characteristics. By employing SPDT, optical surfaces with roughness down to 1 nm could be machined. However, in a purely mechanical ordinary SPDT procedure, different factors may influence the outcome of the process and significantly affect the quality of the diamond-turned optical surface. In the last few years, non-conventional machining technologies have been successfully implemented on the basis of traditional mechanical SPDT for optical surface generation. These non-conventional methods assist the purely mechanical SPDT process to improve the diamond machining performance and machinability of various optical materials. They attempt to eliminate the process disturbances that occur during mechanical interactions between the tool and workpiece in order to further reduce the surface roughness indicator. Recently, the application of hybrid SPDT methods in optical surface generation has emerged, in which more than one non-conventional machining technique impact the workpiece surface during the mechanical interaction with the diamond tool cutting edge. Compared with conventional SPDT solutions, in a hybrid SPDT method, superior results in terms of optical surface quality could be obtained. In addition, development of intelligent SPDT solutions for achieving the best possible results in optical surface generation is a novel approach. In an intelligent SPDT solution, in addition to the hybrid machining methods, automatic control systems and advanced on-machine metrology systems need to be implemented on the diamond turning machine platform. This may help further improve the quality of optical surface by continuously performing in-process error diagnosing and consequently conducting in-process tuning of the diamond cutting parameters and optimizing the machining conditions. In this review paper, recent advances in ultra-precision SPDT based on hybrid methods, associated metrology systems, and their process control systems are surveyed. In addition, the effect of developed various hybrid SPDT methods and influencing factors that may affect optical surface generation have been discussed. At the end, guidelines for further developments of intelligent SPDT platforms by using hybrid SPDT methods, automatic process control, and in-process metrology solutions are proposed.

Effects of pressure and velocity on the interface friction behavior of diamond utilizing ReaxFF simulations

The effects of pressure and sliding velocity on the interface friction behavior during the chemical mechanical polishing process of diamond were investigated utilizing ReaxFF molecular dynamics, with a focus on the subsurface damage, friction force, and atom removal. Simulation results indicate that in the initial stage, the friction force depends on the number of interfacial C-C bonds and C-O-C bonds and shows a positive correlation with the pressure and sliding velocity. Later on, the friction force relies on the number of amorphous carbon atoms, and exhibits a negative correlation with the pressure and sliding velocity. Under low pressure, the carbon atoms are mainly removed along with the formation of C-C single bonds. In contrast, with increasing pressure, the carbon atoms are removed together with the formation of more C-C single and multiple bonds. This accounts for more extensive atom removal, followed by the more severe wear, as well as deeper subsurface damage. This study systematically evaluates the underlying influence mechanism of pressure and sliding velocity on the interface friction behavior from atomistic scale, thus elucidating technological parameters for ultra-precision and low-damage machining of diamond.

Wetting behaviours and interfacial characteristics of Co-binder sintered polycrystalline diamond by Sn[sbnd]Ti active solder

The reactive wetting and interfacial characteristics of the Sn2Ti active solder were experimentally investigated on sintered polycrystalline diamond (PCD) substrates containing 8–18 wt.% Co binders. The initial spreading temperature of Sn2Ti on PCD was determined by the continuously elevating temperatures wetting experiment. The Sn2Ti started to spread on PCD at 500–630 °C. The higher the cobalt content, the lower the initial spreading temperature. Meanwhile, the spreading kinetics analysis through isothermal wetting experiment indicated the spreading process of Sn2Ti on PCD can be well described by the classic chemical reaction-controlled model at 650–900 °C. Both rod-like TiC carbide and faceted CoSnTi intermetallics (IMCs) were observed at the wetting triple line, with the formation morphologies strongly depending on wetting temperatures. The results obtained in this work are believed to be of importance for the optimization of brazing technique for sintered PCD, and hence the development of PCD diamond machining tools with minimized thermal damages.

On-machine surface defect detection using light scattering and deep learning.

This paper presents an on-machine surface defect detection system using light scattering and deep learning. A supervised deep learning model is used to mine the information related to defects from light scattering patterns. A convolutional neural network is trained on a large dataset of scattering patterns that are predicted by a rigorous forward scattering model. The model is valid for any surface topography with homogeneous materials and has been verified by comparing with experimental data. Once the neural network is trained, it allows for fast, accurate, and robust defect detection. The system capability is validated on microstructured surfaces produced by ultraprecision diamond machining.

Comparison of work environment air quality and application of geostatistic technique in the spatial distribution of PM2.5 and PM10 in a number of industrial workshops

Introduction: Air quality in industrial workshops is affected by various harmful chemical agents. Purpose of this study was to compare work environment air quality in a number of industrial workshops based on the mass concentration and number of suspended particles in air and application of Kriging geostatistical method in industries. Materials and Methods: This observational study was carried out in four workshops including cuprous waste separation works, rolling and cutting, casting and copper smelting, manufacturing aluminum products and double-glazed glass production. Using particle mass counter sampler of Tes 5200 model, mass concentration and number concentration of suspended particles in row of different diameters were measured. The spatial distribution map of PM2.5 and PM10 in each of workshops was prepared using Kriging geostatistic method in GIS environment Version 10.5. Results: The highest mass concentration (8.42 mg/m3) and number concentration (445058 Particle/lit in differential mode and 885446 Particle/lit in cumulative mode) were measured in the manufacturing aluminum products workshop. The highest mass concentration and number concentration of suspended particles were obtained for row diameters of 10 µm and 0.5 µm, respectively. Interpretation of Kriging maps showed that various areas of the industries such as radiation furnace, scrap press, aluminum smelter, sieve, diamond machine, glass furnace and ingot molds had the highest frequency of PM2.5 and PM10. Conclusion: Engineering control actions such as installation of ventilation systems in halls and design and installation of a local hood in the entrance sections of the aluminum smelting furnace and sieve in manufacture of aluminum products workshop and copper ingot molds in casting and smelting workshop is essential for improved air quality.

Effect of bur selection on machining damage mechanisms of polymer-infiltrated ceramic network material for CAD/CAM dental restorations

The surface and edge quality of polymer-infiltrated ceramic network material (PICN) in CAD/CAM milling and clinical adjusting is crucial to restorative successes. The paper reports on PICN responses to diamond and tungsten carbide machining with respect to machining forces and frictions, material removal mechanisms, surface quality and edge integrity. A force sensor together with a high-speed data acquisition system was used to measure machining forces. Scanning electron microscopy was applied to reveal bur wear and debris adhesion, and machining-induced surface and edge chipping damage. The results show two distinct removal mechanisms of PICN in which tungsten carbide machining was dominated by friction-induced ductile deformation and normal pressing of the material while diamond machining was controlled by the penetration-induced fracture. These mechanisms were reflected by significantly higher tangential (or frictional) forces and coefficients of friction in tungsten carbide machining than in diamond machining (p < 0.05). The ductile removal in tungsten carbide machining yielded more smoother areas and much less edge chipping damage in PICN. The fracture mode in diamond machining broke the ceramic and polymer networks in PICN, resulting in more fractured and pulverized areas with 6–7 times higher roughness and much more severe edge chipping damage in PICN. Thus, for better surface and edge quality, tungsten carbide burs are superior than diamond burs and recommended for surface finishing and adjusting while diamond burs are the good choice for rapid material removal in both CAD/CAM milling and clinical adjusting. However, high material removal rate-induced high tangential (or frictional) forces in tungsten carbide machining limited the bur usage for rapid oral adjusting. This paper also compared the influence of PICN microstructure on the machinability with CAD/CAM glass ceramics.

Systematic modeling and rapid control for diamond machining of periodical optical surface

Spiral path based fast tool servo diamond turning technique is currently considered to be an efficient method for precision machining of micro-lens arrays. However, due to the inherent structural and methodological drawbacks, such as tracking bandwidth limitation, the system nonlinearity, it is still a great challenge for controllable fabrication of micro-lens arrays especially with complex geometry. To extend both the capability and accuracy, two aspects involving cutting method and control strategy are proposed in this paper. Firstly, the concept of the fast servo-assisted rotary cutting approach is introduced. Due to the unique diamond cutting approach, the fast servo trajectory features the characteristic of quasi-periodicity, and accordingly an advantageous terminal sliding mode repetitive control strategy is designed to realize high closed-loop tracking bandwidth and robustness against the variant system uncertainties and force disturbances during the cutting process. Then, the effectiveness is verified by the experiments on the diamond machine with respect to tracking accuracy, characteristic parameters as well as surface form error. Through comprehensive characterization and comparisons, the proposed diamond machining system with the controller is demonstrated to be able to substantially improve the machining performance of complex periodical optical surfaces.

Insight into the mechanism of low friction and wear during the chemical mechanical polishing process of diamond: A reactive molecular dynamics simulation

Chemical mechanical polishing(CMP) is an important method to achieve ultra-precision machining of diamond. However, the friction mechanism during CMP is not well understood due to a lack of information regarding the interface. Here, reactive molecular dynamics simulation was utilized to elucidate the friction behavior under different pressures and flow fates of polishing slurry. Simulation results indicated that pressure could accelerate the passivation of surface, Pauli repulsion between abrasive and substrate could withstand the applied load and prevent the two surfaces from reaching the C–C interaction range. The number of C atoms removed and the subsurface damage of substrate are the function of flow fate and pressure. This provides a theoretical support for the ultra-precision and low-damage machining of diamond.

Suppression of tool damage in ultraprecision diamond machining of stainless steel by applying electron-beam-excited plasma nitriding

Mirror surface machining of stainless steel with single-crystalline diamond tools is proposed in this study by applying a new nitriding method, called electron-beam-excited-plasma (EBEP) nitriding, to workpiece surfaces as pretreatment. It is well known that mirror surface finish of steel workpieces by conventional diamond cutting is unachievable owing to rapid tool wear. Nitriding of steel workpieces has been one of the several attempts to prevent the rapid tool wear of diamond tools. It has been reported that the rapid tool wear is caused by thermochemical interaction between diamond and steel, and that the wear can be greatly reduced by nitriding of steel. However, hard compounds formed on the outmost surfaces of workpieces by the conventional nitriding methods can cause micro-chippings of cutting tools. The authors has recently developed a new nitriding method called EBEP nitriding, in which a high dissociation rate for nitrogen molecules is achieved using the electron-beam-excited-plasma, and iron-compounds-free nitriding has been realized. Therefore, the EBEP nitriding is applied to a typical mold material, modified AISI 420 stainless steel, aiming at suppressing the micro-chippings as well as the thermochemical tool wear during diamond cutting of the stainless steel. The conventional ion nitriding and the gas nitrocarburizing are also applied to the same stainless steel in comparison. Chemical components of the nitrided workpiece surfaces are analyzed by an electron prove micro-analyzer (EPMA) and an X-ray diffraction (XRD) in advance, and turning experiments are conducted with single-crystalline diamond tools. Subsequently, changes in cutting forces and roughness of finished surfaces and tool damages after the turning experiments are evaluated. Finally, mirror surface machining by using the EBEP nitriding is demonstrated, and its advantages and disadvantages in the diamond cutting of stainless steel are summarized in comparison with the conventional nitriding methods.

Active gap capacitance electrical discharge machining of polycrystalline diamond

This paper introduced a novel active gap capacitance electrical discharge machining (AGC-EDM) method for high efficiency machining of polycrystalline diamond. Graphene was used for the first time in an EDM process to increase the gap capacitance, which would result in enlarged inter-electrode discharge explosive force and higher processing stability. The new principle and machining mechanisms were introduced; an equivalent circuit model was established; and factors that affect the machining performance were analyzed. In order to valid the theory and assumptions and investigate the effectiveness of the new approach, EDM experiments and comparative study were conducted. Test results showed that discharge explosive force in the new AGC-EDM was increased dramatically and the machining process became more stable. The material removal rate in AGC-EDM was found to be more than 10 times higher than that of the normal EDM while the relative electrode wear was reduced by 70 %.