Polycrystalline Diamond Layer Research Articles

Morphological, cathodoluminescence and thermoluminescence studies of defects in diamond films grown by HF CVD technique

A set of diamond layers was synthesized by Hot Filament Chemical Vapor deposition (HF CVD) from methanol-hydrogen (CH3OH–H2) gas mixture. By changing feed gas composition the obtained diamond films show different morphologies and microcrystal sizes as indicate the Scanning Electron Microscopy (SEM) pictures.The obtained diamond films were analysed by Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), cathodoluminescence (CL) and thermoluminescence (TL) spectroscopy.The diamond's microcrystallities preferred orientation in diamond layers was assessed from X-ray diffraction measurements and described by texture coefficient TC(hkl). The cathodoluminescence study clearly shows that relative intensities of the band at 430 nm and at 605 nm changes as the preferred orientation changes from <400> to <220>. It was also shown that the morphology and the microcrystallities preferred orientation has an essential influence on the thermoluminescence properties of polycrystalline diamond layers.

On the route to produce conductive Ni-related color centers in CVD-grown diamond

This paper focuses on the development of procedures able to provide multifunctional optical and electrical properties to polycrystalline diamond layers synthetized on silicon substrates. By exploiting the HF-CVD technique and the Si-Ni chemistry promoted by the presence of Ni during diamond growth, Si and Si-Ni defects acting as both color centers and free charge carriers were inserted into diamond lattice. To clarify the role played by the metal in modulating photoluminescence (PL) and charge transport, the Ni source is supplied either by drop-casting of NiCl2 solutions or by sputtering of Ni targets. A deep investigation of structure and emitting features of the produced samples is achieved by SEM, Raman spectroscopy, XPS, XRD and PL analyses, while the electrical behavior is pointed out by I-V and Hall effect measurements. The study allows for optimizing the state and amount of the Ni source able to give reliable functional features to the final materials, whereas preserving the structural integrity of the hosting diamond lattice. The collected results are evidence that the proposed synthesis approach enables the production of diamond-based systems showing a PL characterized by multiple emission lines and a significant conductivity suitable for assembling multifunctional devices working at room temperature.

Atmospheric-Pressure Microwave Plasma Torch for CVD Technology of Diamond Synthesis

An electrodeless microwave jet plasma source is considered, and its various applications in the technology of chemical vapor deposition of diamond films and dimension increasing of small diamond single crystals synthesized at high pressures and temperatures are discussed. The plasma jet is ignited in an atmospheric-pressure gas (argon) flow with hydrogen and methane additives. The operation of the microwave jet reactor is described, and the plasma characteristics measured using emission spectroscopy are presented. The brightly glowing atmospheric-pressure plasma jet is ignited and stably burns at a microwave power of ≤1 kW supplied from a microwave oven magnetron. The specific microwave power density absorbed by the compact plasma jet (≤104 W/cm3) is comparable with that absorbed by a dc arc. The growth rate of the polycrystalline diamond layer amounts to 40 µm/h. The process of film deposition on the substrate can be controlled by scanning the substrate surface with the jet.

A study on structural evolution of metamorphic layer on the surface of PCD in electrical discharge machining

Polycrystalline diamond (PCD) is one of the most important wear-resistant materials. Electrical discharge machining (EDM) is often used to remove pieces with different sizes, shapes, and surface qualities from polycrystalline diamond compacts (PDC) in order to satisfy the demands of PCD tools. However, as the special properties of the PCD, the structure transformation and removal process of the PCD materials are different to that of the cemented carbide substrate. This paper proposes a removal mechanism for PCD considering the morphology of discharge craters and the micro-structural evolution of diamonds. It was observed that the tensile residual stress was contributed to a large diameter value and irregular shapes of discharge craters in cobalt-rich areas. The graphene was obtained through the peeling process of graphite under the action of tensile residual stress when the thickness of metamorphic layer was in a small level. The degree of graphitization and number of crystal defects increased with increasing the thickness value of metamorphic layer according to the Raman shifted analysis. A metamorphic layer containing graphite, cobalt, and cobalt carbide was observed by the XRD analysis, which increased in thickness with increasing discharge energy. The hardness of PCD layer was reduced due to the action of the metamorphic layer, which damaged the performance of PCD tools. The electrical conductivity of the PCD layer was increased with increasing the thickness value of metamorphic layer, which contributed to the high machining efficiency with enlarging the discharge areas. Meanwhile, the diamonds were removed in the graphite form due to the graphitization of diamonds were the mainly removal mechanism in electrical discharge machining.

Reducing the Metal Content in PCD Polycrystalline Diamond Layer by Chemical and Electrochemical Etching

This article is devoted to investigating polycrystalline diamond compacts (PDCs), which find broad application in drilling, tool-and-die, and building branches of industry. They are a complex composition of diamond and cermet phases. The diamond phase consists of diamond grains of various granulometric compositions and shapes and forms a strong hard skeleton. A cermet phase serves as a binder. The presence of catalyst metals in a diamond layer of bilayer PDC composite materials lowers their operational properties, because the difference in thermal expansion coefficients between diamond grains and catalyst can lead to material cracking in the cutting process, while a high temperature when fabricating the diamond tool and its operation in the cutting zone can lead to the reverse diamond–graphite phase transition. In order to increase wear-resistance characteristics of diamond PCD composites formed using catalyst metals (cobalt and tungsten), etching of metals from the surface of the tool working zone is performed by two methods, notably, electrochemical and chemical. Electrochemical etching is performed in sulfuric acid with various current modes and concentration, and chemical etching is performed in a mixture of hydrochloric and nitric acids and a mixture of fluoric and nitric acids. The distribution of the chemical composition over the depth of PCD samples after etching is performed using scanning electron microscopy. It is established that electrochemical etching is more active kinetically, while chemical etching is promising for industrial application. Abrasive tests of PCD samples before and after etching show the absence of a noticeable effect of both electrochemical and chemical etching of their abrasive ability.

Effect of quenching processes on microstructures and tribological behaviors of polycrystalline diamond compact (PCD/WC-Co) in annealing treatment

Enhanced tribological behaviors are urgently desired for scientists and engineers to improve the service life of polycrystalline diamond compact (PDC) instruments. Here, PDC samples, involved sintering a polycrystalline diamond (PCD) layer to a cemented carbide (WC-16wt%Co) substrate under high temperature/high pressure (HT/HP) condition, have been annealed at 500°C in ambient air to achieve perfect wear resistances and low friction coefficients. The various quenching processes, such as air-quenching (AQ), water-quenching (WQ) and furnace-quenching (FQ), control the morphologies and chemical conversions of PDC surfaces, and eventually affect their tribological behaviors. The WQ process with the highest cooling rate can effectively remove the tiny diamond grains (TD grains, sizes of 0–5μm) on PDC surface before sliding test, which can greatly enhance the wear resistance and slightly increase the friction coefficient of PDC (<0.1). Oppositely, the FQ process with the slowest cooling rate leads to decreasing of friction coefficient and deterioration of wear resistance of PDC. It is noted that the TD grain acts as a double-edged sword. On the one hand, it induces the formation of transfer film to decrease the friction coefficient of annealed PDC. On the other hand, it causes the extraction of medium size diamond grains (MD grains, sizes of 5–15μm) and eventually results in the wear of PDC. These results may give significant instructions for improving tribological behaviors of annealed PDC with selectable quenching processes.

Exploiting the Properties of Ti‐Doped CVD‐Grown Diamonds for the Assembling of Electrodes

A hybrid chemical vapor deposition (CVD)‐powder flowing technique specifically developed in lab has been employed to produce high‐quality polycrystalline diamond layers containing Ti inclusions. Morphology, structural features, and surface composition of nanocomposite diamond‐based samples produced by different growth times have been analyzed by scanning electron microscopy, Raman and Auger spectroscopy, respectively. The CVD methodology adopted for the Ti incorporation in the diamond lattice does not perturb the crystalline quality of the diamond matrix, therefore maintaining the outstanding properties of the C‐sp3 phase. The functional properties of the nanocomposite layers have been tested by nanoindentation and I–V measurements. The electrochemical performance of the diamond/Ti electrodes is evaluated by performing cyclic voltammetry in different media, namely, acidic, neutral, and basic aqueous solutions, and by estimating the rate constant of heterogeneous electron transfer to diamond surface for the ferro/ferricyanide redox couple. The rather good electrochemical performances, the mechanical strength, and the chemical inertness of the Ti‐doped diamond electrodes produced by the CVD approach, comply with the whole set of technological requirements, such as robustness, long durability, and biocompatibility, required for use in hostile environments or in biological systems.

Diamond-EuF3 nanocomposites with bright orange photoluminescence

We report the manufacturing of a novel diamond – rare-earth (RE) composite material with EuF3 nanoparticles (NP) embedded in the synthesized microcrystalline diamond films that show strong photoluminescence in the orange part of the visible spectrum. Synthesis of the aforementioned composite includes placement of EuF3 NP on the diamond substrate and subsequent coating of them with an additional polycrystalline diamond layer grown by microwave plasma chemical vapor deposition (CVD). The produced composite films exhibit high intensity localized photoluminescence (PL) at 612nm with the decay time of 0.34ms, which is generated by the EuF3 particles buried within a very stable transparent diamond matrix. The proposed synthetic approach is quite versatile, as it allows preparation of the luminescent diamond - RE particles nanocomposites of different sizes and natures which perform well over a broad range of the visible spectrum.

Wear Mechanism of pcd tools of different grain sizes manufactured by conventionally abrasive grinding and electrical discharge grinding

With the development of the aviation industry, high-performance tools, which can overcome high temperature and severe abrasion, are in large demand in order to save manufacturing cost and increase machining efficiency in cutting hard-to-machine materials such as titanium alloys, composites and carbon fiber reinforced plastic. Poly crystalline diamond (PCD) tools have been widely applied in cutting difficult-to-machine materials in industry. However, the severe abrasion and high cutting temperature cause significant tool wear and reduce machining efficiency. In this paper, the processes of using ground PCD tools (the tools machined by conventional abrasive grinding) and eroded PCD tools (the tools electrical discharge grinding(EDG)) to cut tungsten carbide were investigated. By analyzing the morphology of worn surface and the change of cutting force, it was found that the wear and wear mechanism of PCD tools machined by EDG and abrasive grinding processes were different. In ground PCD tools, compressive residual stress within the PCD layer combined with external compressive stress exerted by work piece broke the diamond-diamond bonding, and led to the fracture of PCD structure around the tool tip. In contrast, the excessive heat in EDG process generated tensile residual stress within PCD structure and weakened the C-C bonding. The forming-removing cycle of build-up layer (BUL) and the breakage of C-C bonding during the cutting process caused a steady adhesive-abrasive wear process, which was the main wear mechanism of eroded PCD tools.

Cutting of polycrystalline superhard materials by jet methods

The authors study the intensity of cutting workpieces of hardmetals, polycrystalline cBN-based material and polycrystalline diamond-on-hardmetal compacts in the following operations: waterjet cutting, laser cutting, liquid-cooled laser cutting, and laser–jet cutting. Special features of cutting two-layered composites that comprise a polycrystalline diamond layer on a hardmetal substrate are analyzed. The paper demonstrates the efficiency of the hybrid machining process that combines the laser–jet and waterjet technologies and provides an effective cutting of workpieces to a required profile.

The wear analysis model and rock-breaking mechanism of a new embedded polycrystalline diamond compact

Polycrystalline diamond layer peel-off is a hot topic in oil and gas drilling engineering. When an integrated polycrystalline diamond compact cutter suffers overload, there is a rapid decline in its rock-breaking performance and drilling rate of penetration. To modify the comprehensive performance of polycrystalline diamond compact, we innovatively propose a new embedded polycrystalline diamond compact. According to geometric analysis theory combined with the linear combination rule, the three typical embedded design schemes – the imitation palm-shaped, imitation Z-shaped and ring-embedded designs – are discussed. The influences of the number, size, location and combination of the embedded polycrystalline diamond layer on the polycrystalline diamond compact wear mechanism and rock-breaking performance are analysed. The results show that the embedded element and carbide matrix are combined by brazing welding, which not only exerts high abrasion resistance on the polycrystalline diamond layer but also combines with the good impact performance of carbide matrix. Compared with ordinary polycrystalline diamond compact, the intake amount of a single-embedded polycrystalline diamond compact is smaller, and it wears more evenly during the rock-breaking process. Comparing the results from before and after drilling, it effectively prevents the ordinary polycrystalline diamond compact from easily peeling off when suffering overload. The unique wear analysis model can be applied to other types of polycrystalline diamond compact by adjusting the embedding method. The research conclusions provide useful insights into the study of the polycrystalline diamond compact wear mechanism and rock-breaking performance.

Structural, optical and mechanical properties of thin diamond and silicon carbide layers grown by low pressure microwave linear antenna plasma enhanced chemical vapour deposition

In this work, we detail the properties of thin silicon carbide and polycrystalline diamond layers grown by microwave plasma enhanced chemical vapour deposition with linear antenna delivery. Structural, mechanical and optical properties are compared for their potential use as transparent hard coatings. Silicon carbide layers exhibit mechanical properties comparable to thin diamond layers but with a significantly higher adhesion and lower optical absorption coefficient over a wide spectral range.

Layer Protecting the Surface of Zirconium Used in Nuclear Reactors.

Zirconium alloys have very useful properties for nuclear facilities applications having low absorption cross-section of thermal electrons, high ductility, hardness and corrosion resistance. However, there is also a significant disadvantage: it reacts with water steam and during this (oxidative) reaction it releases hydrogen gas, which partly diffuses into the alloy forming zirconium hydrides. A new strategy for surface protection of zirconium alloys against undesirable oxidation in nuclear reactors by polycrystalline diamond film has been patented- Czech patent 305059: Layer protecting the surface of zirconium alloys used in nuclear reactors and PCT patent: Layer for protecting surface of zirconium alloys (Patent Number: WO2015039636-A1). The zirconium alloy surface was covered by polycrystalline diamond layer grown in plasma enhanced chemical vapor deposition apparatus with linear antenna delivery system. Substantial progress in the description and understanding of the polycrystalline diamond/ zirconium alloys interface and material properties under standard and nuclear reactors conditions (irradiation, hot steam oxidation experiments and heating-quenching cycles) was made. In addition, process technology for the deposition of protective polycrystalline diamond films onto the surface of zirconium alloys was optimized. Zircaloy2 nuclear fuel pins were covered by 300 nm thick protective polycrystalline diamond layer (PCD) using plasma enhanced chemical vapor deposition apparatus with linear antenna delivery system. The polycrystalline diamond layer protects the zirconium alloy surface against undesirable oxidation and consolidates its chemical stability while preserving its functionality. PCD covered Zircaloy2 and standard Zircaloy2 pins were for 30 min. oxidized in 1100°C hot steam. Under these conditions α phase of zirconium changes to β phase (more opened for oxygen/hydrogen diffusion). PCD anticorrosion protection of Zircaloy nuclear fuel assemblies can significantly prolong lifetime of Zirconium alloy in nuclear reactors even above Zirconium phase transition temperatures. Even after ion beam irradiation (10 dpa, 3 MeV Fe(2+)) the diamond film still shows satisfactory structural integrity with both sp(3) and sp(2) carbon phases. Zircaloy2 under the carbon-based protective layer after hot steam oxidation test differed from the original Zircaloy2 material composition only very slightly, proving that the diamond coating increases the material resistance to high temperature oxidation. Zirconium alloys nuclear fuel pins' surfaces were covered by compact and homogeneous polycrystalline diamond layers consisting of sp(3) and sp(2) carbon phases with a high crystalline diamond content and low roughness. Diamond withstands very high temperatures, has excellent thermal conductivity and low chemical reactivity, it does not degrade over time and (important for the nuclear fuel cladding) being pure carbon, it has perfect neutron cross-section properties. Moreover, polycrystalline diamond layers consisting of crystalline (sp(3)) and amorphous (sp(2)) carbon phases could have suitable thermal expansion. Zirconium alloys coated with polycrystalline diamond film are protected against undesirable changes and processes. Further, the polycrystalline diamond layer prevents the reaction between the alloy surface and water vapor. During such reaction, water molecules dissociate and initiate formation of zirconium dioxide and hydrogen, accompanied by the release of large amount of heat. Thus the protective layer prevents the formation of hydrogen and the release of reaction heat. Few relevant patents to the topic have been reviewed and cited.

Study of CVD diamond layers with amorphous carbon admixture by Raman scattering spectroscopy

AbstractRaman spectroscopy is a most often used standard technique for characterization of different carbon materials. In this work we present the Raman spectra of polycrystalline diamond layers of different quality, synthesized by Hot Filament Chemical Vapor Deposition method (HF CVD). We show how to use Raman spectroscopy for the analysis of the Raman bands to determine the structure of diamond films as well as the structure of amorphous carbon admixture. Raman spectroscopy has become an important technique for the analysis of CVD diamond films. The first-order diamond Raman peak at ca. 1332 cm−1 is an unambiguous evidence for the presence of diamond phase in the deposited layer. However, the existence of non-diamond carbon components in a CVD diamond layer produces several overlapping peaks in the same wavenumber region as the first order diamond peak. The intensities, wavenumber, full width at half maximum (FWHM) of these bands are dependent on quality of diamond layer which is dependent on the deposition conditions. The aim of the present work is to relate the features of diamond Raman spectra to the features of Raman spectra of non-diamond phase admixture and occurrence of other carbon structures in the obtained diamond thin films.

Thin polycrystalline diamond films protecting zirconium alloys surfaces: From technology to layer analysis and application in nuclear facilities

Zirconium alloys can be effectively protected against corrosion by polycrystalline diamond (PCD) layers grown in microwave plasma enhanced linear antenna chemical vapor deposition apparatus. Standard and hot steam oxidized PCD layers grown on Zircaloy2 surfaces were examined and the specific impact of polycrystalline Zr substrate surface on PCD layer properties was investigated. It was found that the presence of the PCD coating blocks hydrogen diffusion into the Zircaloy2 surface and protects Zircaloy2 material from degradation. PCD anticorrosion protection of Zircaloy2 can significantly prolong life of Zircaloy2 material in nuclear reactors even at temperatures above Zr phase transition temperatures.

A Novel Cutting Concept of CFRP Composites for Extending the Life of Tool

This paper presents a novel concept for extending the life of the tools that are used in the milling process of carbon-fiber-reinforced plastic (CFRP) composites. Three types of tools were compared in milling tests; tool I: an uncoated tungsten carbide (WC-Co) tool, tool II: a polycrystalline diamond (PCD) tool, and tool III: a combination tool comprising a PCD layer surface with a thickness of 100 μm and a WC-Co substrate flank surface. The measurement of changes in cutting forces over cutting distance revealed that tool III had a better (tool) life performance than the other tools. Additionally, observation of changes in the edge profiles of the tools revealed that the effective edge sharpness of tool III remains constant during the milling tests. The difference between the wear rates of the PCD rake and WC-Co flank surfaces in tool III maintained a constant effective sharpness during the milling process, and it extended the tool life. Based on our findings, we concluded that using the difference between the wear rates of rake and flank surfaces is an effective technique for tool life extension in the milling process of CFRP laminates.

Experimental Study of Wheel Rotating Speed Effect on Electrical Discharge Grinding

Electrical discharge grinding is used to machine polycrystalline diamond layer of cutting tools, which can effectively drill composite/titanium stacks in modern aircraft frames. In the machine, an electrode wheel is rotated to accelerate debris flushing to quickly recover the dielectric fluid between a spark gap. The aim of this study was to investigate the effect of the wheel speed on polycrystalline diamond removal rate for high erosion efficiency. Multiple wheel speeds were applied in a electrical discharge grinding machine under various pulse on-times and the volumetric removal rates were calculated by the precise position feedback of this machine over the time, which was obtained by the electrical waveform of discharges. The experimental results show that the removal rate increases with the wheel speed up to a limit at the low range and then declines at the high range. This indicates that the wheel speed needs to be optimized for high erosion efficiency under various discharge conditions such as different materials and pulse on-times.

Wear and breakage behaviors of PCD small-diameter end-mill: a case study on machining 2A12 aluminum alloy

Small-diameter end-mill is a key element for micro-machining technique. It is a new attempt to manufacture a small-diameter tool with polycrystalline diamond (PCD), which is also a hot issue in the research area of micro-manufacturing. PCD end-mills with small-diameter were fabricated and utilized in micro-milling. With a contact simulation model, the stress distribution and potential crack propagation of the PCD tool was revealed via 2D finite element (FE) analysis of 2Al2 aluminum alloy acting on the tool surface. Thereafter, a series of machining experiments on 2A12 micro-part were carried out with the PCD small-diameter end-mill. The wear characteristics of the rake face and flank face were analyzed, and the wear mechanisms of small-diameter end-mills were carefully investigated by using scanning electron microscope (SEM) and energy dispersive X-ray (EDX) spectroscopy. Three typical breakage morphologies of PCD end-mills were demonstrated while machining the aluminum alloy workpiece, i.e., the breakage of the tool tip, the edge chipping, and the peeling off of PCD layer. The consequent experiment results showed that abrasive wear, adhesive wear, and oxidative wear were the predominant characteristics in the damage region of PCD small-diameter end-mill. The effect mechanisms of various wear characteristics were also analyzed and revealed in detail. The experimental results were discussed according to the simulation results and effects on failure behavior of the PCD tool. The research can provide a better understanding for the wear and breakage behaviors of the downsized PCD tool in machining aluminum alloy and underpin the development of micro-milling process with diamond tool.

Sintering of Frustum Shaped Polycrystalline Diamond Compact at Non-Hydrostatic Pressure

This work present the study of the sintering of polycrystalline diamond compact with hyperbolic truncated cone profile using hexagonal boron nitride assembly, rather than salt mold, which is different from conventional assembly and this kind of polycrystalline diamond compact is widely used in percussion rock bits and roller cone rock bits. The well-sintered polycrystalline diamond compact without transition layer is prepared on a WC-10wt% Co substrate at temperature 1450°C for 3 min at non-hydrostatic high pressure of 5.5Gpa.Different analyzing techniques, such as X- ray diffraction, micro-Raman spectroscopy, scanning electron microscopy were applied to characterize the micro-structure, residual stress and sintering behavior. The SEM analysis indicates that diamond-diamond (D-D) direct bonding had formed in the polycrystalline diamond layer. The Raman spectroscopy shows compressive stress in the polycrystalline diamond layer is much higher than that sintered using traditional assembly.

Composite diamond films for short-mm wave and THz traveling wave tube windows

With an increase in frequency, the diamond thickness of the microwave windows for short-mm wave and THz traveling wave tubes (TWTs) approaches 100μm or even tens of μm. This poses problems of mechanical strength and air tightness to the polycrystalline diamond (PCD) window. To overcome these problems, we have studied a composite diamond film that consists of PCD and ultra-nanocrystalline diamond (UNCD). First, SEM was used to examine the early growing process of UNCD on PCD. The 5μm thick UNCD grown on 40μm PCD exhibited a hillock structure with densely packed ≤20nm granules, in contrast to the PCD layer showing randomly packed, micrometer sized grains. Then, the effect of UNCD thickness on fracture strength and thermal conductivity was studied using the test samples with thin layers of UNCD having thicknesses of 1, 2.5, 5, and 10μm on 100μm thick PCD films, respectively. The fracture strengths of all the films are 2–3 times higher than that of the PCD films, which is 350±150MPa. As expected, the thermal conductivity of the samples measured at ~20°C decreases with an increase in UNCD thickness, particularly in the range of 0 to 2.5μm. At a thickness of 10μm, the thermal conductivity was found to be ~10W/cm∗K. Finally, a 100μm sandwich-like structure with a total UNCD thickness of 10μm was fabricated and two 180GHz TWT windows were assembled. RF tests show that for the operating frequency range of 175 to 185GHz, the transmission loss (S21) was found to be ≤1.22 and ≤1.71dB, respectively, indicating an excellent RF performance. Mechanical strength and air tightness of the windows were also found improved and able to meet the requirement of the device. This work provides a novel approach for fabricating relatively thin diamond films for RF applications, such as TWT windows.