Polycrystalline Diamond Coatings Research Articles

Wear of Carbide Plates with Diamond-like and Micro-Nano Polycrystalline Diamond Coatings during Interrupted Cutting of Composite Alloy Al/SiC

The complexity of milling metal matrix composite alloys based on aluminum like Al/SiC is due to their low melting point and high abrasive ability, which causes increased wear of carbide tools. One of the effective ways to improve its reliability and service life is to modify the surface by plasma chemical deposition of carbon-based multilayer functional layers from vapor (CVD) with high hardness and thermal conductivity: diamond-like (DLC) or polycrystalline diamond (PCD) coatings. Experiments on an indexable mill with CoroMill 200 inserts have shown that initial tool life increases up to 100% for cases with DLC and up to 300% for multilayered MCD/NCD films at a cutting speed of 800 m/min. The primary mechanism of wear of a carbide tool in this cutting mode was soft abrasion, when wear on both the rake and flank surfaces occurred due to the extrusion of cobalt binder between tungsten carbide grains, followed by their loss. Analysis of the wear pattern of plates with DLC and MCD/NCD coatings showed that abrasive wear begins to prevail against the background of soft abrasion. Adhesive wear is also present to a lesser extent, but there is no chipping of the base material from the cutting edge.

Synthesis of Polycrystalline Diamond Films in Microwave Plasma at Ultrahigh Concentrations of Methane

Polycrystalline diamond (PCD) films are usually grown by chemical vapor deposition (CVD) in hydrogen–methane mixtures. The synthesis conditions determine the structure and quality of the grown material. Here, we report the complex effect of the microwave plasma CVD conditions on the morphology, growth rate and phase composition of the resulting PCD films. Specifically, we focus on the factors of (i) increased methane concentrations (νc) that are varied over a wide range of 4%–100% (i.e., pure methane gas) and (ii) substrate temperatures (Ts) varied between 700–1050 °C. Using scanning electron microscopy, X-ray diffraction and Raman spectroscopy, we show that diamond growth is possible even at ultrahigh methane concentrations, including νc = 100%, which requires relatively low synthesis temperatures of Ts < 800 °C. In general, lower substrate temperatures tend to facilitate the formation of higher-quality PCD films; however, this comes at the cost of lower growth rates. The growth rate of PCD coatings has a non-linear trend: for samples grown at Ts = 800 °C, the growth rate increases from 0.6 µm/h at νc = 4% to 3.4 µm/h at νc = 20% and then falls to 0.6 µm/h at νc = 100%. This research is a step toward control over the nature of the CVD-grown PCD material, which is essential for the precise and flexible production of diamond for various applications.

Polycrystalline diamond and magnetron sputtered chromium as a double coating for accident-tolerant nuclear fuel tubes

Controlling the hot steam corrosion of nuclear fuel tubes is crucial for power plant efficiency and safety. We demonstrate an effective method of protecting ZIRLO ® (ZIRLO) fuel tubes against corrosion at accident temperatures in a water-cooled nuclear reactor environment. The tubes surfaces were coated by double layers consisting of the polycrystalline diamond (PCD) and magnetron-sputtered Cr. Coated and bare ZIRLO tubes were subjected to hot steam tests for 30 min at 900 °C followed by 40 min at 1000 °C. Surprisingly, Cr and PCD double layers ability to protect ZIRLO fuel tubes against hot steam corrosion strongly depends on the order of the layers in coatings. ZIRLO coated with Cr as the bottom layer and PCD as the top layer exhibited the best ability to slow down the oxidation process. This double layer coating reduced ZIRLO tube corrosion by 47% compared to unprotected ZIRLO. On the contrary, the double layer coating with PCD as bottom and Cr as the top layer reduced the ZIRLO tube corrosion by 20–25% compared to unprotected ZIRLO. To explain the specific roles of Cr and PCD coatings experimental and theoretical procedures were performed. Our results pointed to two basic parameters, significantly affecting the corrosion of coated ZIRLO tubes: 1. the amount of Cr carbides in the surface part of the coated ZIRLO fuel tubes and 2. adhesion of the coatings to the ZIRLO substrate.

Technology Features of Diamond Coating Deposition on a Carbide Tool

The production of carbide tools with polycrystalline diamond coatings, which are used for processing modern carbon composite materials, includes a number of technological techniques that ensure reliable adhesion of the coating to the substrate. This review examines these features of substrate-surface pretreatment to improve adhesion, which includes chemical etching, mechanical hardening, modification by ion beams, plasma treatment and application of buffer layers between the substrate and the coating. This review also discusses the advantages and disadvantages of the most common methods for obtaining polycrystalline diamond coatings using hot filament and deposition of coatings from microwave plasma.

Polycrystalline Diamond Coating on Orthopedic Implants: Realization and Role of Surface Topology and Chemistry in Adsorption of Proteins and Cell Proliferation.

Polycrystalline diamond has the potential to improve the osseointegration of orthopedic implants compared to conventional materials such as titanium. However, despite the excellent biocompatibility and superior mechanical properties, the major challenge of using diamond for implants, such as those used for hip arthroplasty, is the limitation of microwave plasma chemical vapor deposition (CVD) techniques to synthesize diamond on complex-shaped objects. Here, for the first time, we demonstrate diamond growth on titanium acetabular shells using the surface wave plasma CVD method. Polycrystalline diamond coatings were synthesized at low temperatures (∼400 °C) on three types of acetabular shells with different surface structures and porosities. We achieved the growth of diamond on highly porous surfaces designed to mimic the structure of the trabecular bone and improve osseointegration. Biocompatibility was investigated on nanocrystalline diamond (NCD) and ultrananocrystalline diamond (UNCD) coatings terminated either with hydrogen or oxygen. To understand the role of diamond surface topology and chemistry in the attachment and proliferation of mammalian cells, we investigated the adsorption of extracellular matrix proteins and monitored the metabolic activity of fibroblasts, osteoblasts, and bone-marrow-derived mesenchymal stem cells (MSCs). The interaction of bovine serum albumin and type I collagen with the diamond surfaces was investigated by confocal fluorescence lifetime imaging microscopy (FLIM). We found that the proliferation of osteogenic cells was better on hydrogen-terminated UNCD than on the oxygen-terminated counterpart. These findings correlated with the behavior of collagen on diamond substrates observed by FLIM. Hydrogen-terminated UNCD provided better adhesion and proliferation of osteogenic cells, compared to titanium, while the growth of fibroblasts was poorest on hydrogen-terminated NCD and MSCs behaved similarly on all tested surfaces. These results open new opportunities for application of diamond coatings on orthopedic implants to further improve bone fixation and osseointegration.

Super-Hard DLC Coatings as an Alternative to Polycrystalline Diamond for Cutting Tools: Predictive Analysis of Aluminium Alloy Surface Quality

The use of lightweight materials, such as ultra-high-strength aluminium alloys, is in high demand in the automotive and aerospace industries where weight savings are critical. The tool materials used for high-speed cutting of these aluminium alloys are subjected to severe conditions that promote premature failure of cutting tools. The application of polycrystalline diamond (PCD) coatings provides cutting tools with increased mechanical and thermal fatigue resistance and improved tribological performance. Despite these good properties, their high cost remains a major limitation in this sector. Super-hard Diamond-Like Carbon (DLC) coatings offer a technologically and economically feasible alternative to PCD-coated tools for cutting and machining non-ferrous materials. In this paper, the machining performance of coated and un-coated hard metal inserts in the turning of 7075 aluminium alloy has been explored. The surface quality of machined parts, the cutting tool wear resistance and the vibrations generated during turning of un-coated, PCD and super-hard thin DLC coatings on tungsten carbide inserts were compared. The results obtained demonstrate that DLC coatings are a potentially interesting alternative to PCD coatings for machining ultra-high-strength aluminium alloys, where surface component finish is a key factor.

Osteoblast Cell Response on Polycrystalline Diamond-Coated Additively Manufactured Scaffolds.

Additive manufacturing of metals using selective laser melting can create customized parts with various degrees of complexity and geometry for medical implants. However, challenges remain in accepting orthopedic implants due to the bio-inert surface of metal scaffolds, resulting in a lack of osseointegration. Here, we show that polycrystalline diamond (PCD) coatings on selective laser melted titanium (SLM-Ti) scaffolds can improve the cell-to-material interaction of osteoblasts. The results show that by controlling the uniformity of the diamond coatings, we can mediate the biological response of osteoblasts, such as cell adhesion, proliferation, and spreading. The osteoblasts show favorable cell adhesion and spreading on non-planar PCD-coated scaffolds compared to the un-coated SLM-Ti scaffold. This study plays an important role in understanding the key physicochemical behavior of bone cell growth on customized orthopedic implant materials.

Polycrystalline diamond coating on 3D printed titanium scaffolds: Surface characterisation and foreign body response

Titanium-based implants are the leading material for orthopaedic surgery, due to their strength, versatility, fabrication via additive manufacturing and invoked biological response. However, the interface between the implant and the host tissue requires improvement to better integrate the implant material and mitigate foreign body response. The interface can be manipulated by changing the surface energy, chemistry, and topography of the Titanium-based implant. Recently, polycrystalline diamond (PCD) has emerged as an exciting coating material for 3D printed titanium scaffolds showing enhanced mammalian cell functions while inhibiting bacterial attachment in vitro. In this study, we performed in-depth characterisation of PCD coatings investigating the surface topography, thickness, surface energy, and compared its foreign body response in vivo with uncoated titanium scaffold. Coating PCD onto titanium scaffolds resulted in a similar microscale surface roughness (RMS(PCD-coated) = 24 μm; RMS(SLM-Ti) = 28 μm), increased nanoscale roughness (RMS(PCD-coated) = 35 nm; RMS(SLM-Ti) = 66 nm) and a considerable decrease in surface free energy (E(PCD-coated) = 4 mN m−1; E(SLM-Ti) = 16 mN m−1). These surface property changes were supported by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy as corresponding to observed surface chemistry changes induced by the coating. The underlying mechanism of how the diamond coatings chemical and physical properties changes the wettability of implants was examined. In vivo, the coated scaffolds induced similar level of fibrous encapsulation with uncoated scaffolds. This study thus provides further insight into the physicochemical characteristics of PCD coatings, adding evidence to the promising potential of PCD-coatings of medical implants.

STRUCTURE OF POLYCRYSTALLINE DIAMOND COATINGS DEPOSITED BY СVD METHOD IN THE PLASMA OF GLOW DISCHARGE WITH THE USE OF PULSE POWER SUPPLY

The structure of CVD carbon coatings synthesized in a hydrogen-methane mixture in the plasma of a glow discharge stabilized by a magnetic field using a pulsed power supply was studied by X-ray diffraction analysis and optical microscopy. The range of deposition parameters is determined, which ensure formation of polycrystalline diamond coatings. The coatings consist of diamond crystals with a clearly defined cut and the crystal lattice parameter close to the tabular value for natural diamond. The influence of the methane partial pressure in the gas mixture and the substrate temperature on the size and predominant orientation of diamond crystals in the coatings was determined. It is established that the use of the pulse mode and grounding of the substrate holder helps to improve the quality of diamond coatings.

Comprehensive topography characterization of polycrystalline diamond coatings

The surface topography of diamond coatings strongly affects surface properties such as adhesion, friction, wear, and biocompatibility. However, the understanding of multi-scale topography, and its effect on properties, has been hindered by conventional measurement methods, which capture only a single length scale. Here, four different polycrystalline diamond coatings are characterized using transmission electron microscopy to assess the roughness down to the sub-nanometer scale. Then these measurements are combined, using the power spectral density (PSD), with conventional methods (stylus profilometry and atomic force microscopy) to characterize all scales of topography. The results demonstrate the critical importance of measuring topography across all length scales, especially because their PSDs cross over one another, such that a surface that is rougher at a larger scale may be smoother at a smaller scale and vice versa. Furthermore, these measurements reveal the connection between multi-scale topography and grain size, with characteristic scaling behavior at and slightly below the mean grain size, and self-affine fractal-like roughness at other length scales. At small (subgrain) scales, unpolished surfaces exhibit a common form of residual roughness that is self-affine in nature but difficult to detect with conventional methods. This approach of capturing topography from the atomic- to the macro-scale is termed comprehensive topography characterization, and all of the topography data from these surfaces has been made available for further analysis by experimentalists and theoreticians. Scientifically, this investigation has identified four characteristic regions of topography scaling in polycrystalline diamond materials.

Highly uniform polycrystalline diamond coatings of three-dimensional structures

Diamond is highly desirable as a conformal thin-film coating for three-dimensional (3D) devices due to its extreme hardness, durability and biocompatibility, especially in the case of implantable medical devices and prosthetics. Recent advances in microwave plasma chemical vapour deposition (CVD) have enabled conformal coating of such 3D structures with nanodiamond films. However, these techniques have not been extended to micro-crystalline diamond, due to interactions between the 3D substrates and resonant microwave cavity. Here we demonstrate more uniform growth of CVD polycrystalline diamond (PCD) coatings on complex-shaped structures, with the use of in-cavity Faraday cages, to isolate the 3D sample from the microwave cavity. While this does disrupt the distribution of the microwave plasma, which surrounds the 3D structure, we show that growth of relatively uniform micro-crystalline and faceted PCD films is possible over the entirety of the exposed 3D sample surface. The resultant technique opens possibilities for the coating of complex 3D geometries with a wider range of diamond materials than has been possible to date.

Tribological behavior of PS400-related tribopairs for space exploration

PS400 is a multi-component alloy developed by NASA and intended for high temperature bearing applications. This study reports on four mating surfaces, namely bulk 4130 steel, diamond-like carbon (DLC), polycrystalline diamond, and molybdenum boride cobalt chrome coatings sliding against PS400 coating, and the best performing tribopair at 25 and 500 °C was identified. The temperature range and sliding speeds simulate extreme conditions for low-speed space-related applications, such as bearings on landing probes for future missions to Venus. PS400 vs. DLC showed low friction and excellent wear resistance up to 500 °C. DLC provided good lubrication at 25 °C, whereas at 500 °C oxide glazes transferred on DLC, maintaining low friction. The worn surfaces and the transfer films were analyzed using different analytical techniques.

Low temperature synthesis of transparent conductive boron doped diamond films for optoelectronic applications: Role of hydrogen on the electrical properties

Transparent conductive electrodes are principal components in various optoelectronic devices and technologies. As such, diamond coatings in the form of electrically conductive thin films are envisioned to provide advantageous chemical and mechanical characteristics/stability in a variety of modern technologies including optoelectronics, biosensing, electrochemical and micromechanical systems. However, deposition of electrically conductive polycrystalline diamond coatings for such applications is currently a challenging task, since temperatures above 600 °C are usually required to ensure good diamond layer quality, which in turn limits the selection of substrates, to materials capable of withstanding exposure to high temperatures. In the present work, we investigate routes toward enhancement of electrical characteristics of nanocrystalline boron-doped diamond (BDD) films fabricated at low temperatures via chemical vapour deposition. We found that post-growth processing of BDD layers enhances their electrical properties, which otherwise are dependent on the employed deposition temperature regime. Finally, we show that integration of an electrically conductive Ti grid opens a route for fabrication of highly transparent and conductive composite nanocrystalline BDD electrodes over large areas at temperatures as low as 250 °C.

Picosecond-laser polishing of CVD-diamond coatings without graphite formation

Abstract Laser polishing of polycrystalline diamond coatings was up to now investigated by the use of nanosecond-lasers or femtosecond–lasers. We investigate polishing diamond coatings by the use of a picosecond-laser (ps-laser) to combine a high removal rate with an interaction time in the range of the free time between lattice collisions to prohibit heating up of the sample and thereby the formation of graphite. Polycrystalline CVD-diamond coatings are deposited by a laser based plasma CVD process. 10 µm thick diamond coatings with an initial surface roughness Sa of 1.85 µm are deposited on tungsten carbide substrates (6% Co). The ps-laser TruMicro 5050 with a wavelength of 515 nm was used under an inclination angle of 80°. The coating is analyzed by laser scanning microscopy, scanning electron microscopy and Raman spectrometry. High ablation rates are possible, but quickly lead to a formation of indentations in case of working in the focus of the laser beam. Therefore the laser was used 4.5 mm defocused which enables polishing a track width of 600 µm. The surface roughness could be reduced by 59% from 1.85 µm to 0.76 µm. By rotating the sample three times by 90° the roughness could be further reduced to 0.51 µm, which means a relative reduction of 72%. The results of Raman spectroscopy showed that diamond coatings can be polished by a ps-laser without the formation of graphite.

Photoluminescence of silicon vacancy centres as conceptual indicator for the condition of diamond protection coatings

Due to the outstanding properties polycrystalline diamond coatings are used for wear protection on tools and work pieces. Thereby adhesive and abrasive wear as well as spalling of the coating can lead to damage and downtimes of the working machines. By depositing silicon doped multilayer diamond coatings, an indication of the condition of the coatings could be achieved. In this study the behaviour and transmission of the zero-phonon line of silicon vacancy centres is investigated in doped multilayer diamond coatings. The in-situ silicon doped diamond coatings were synthesized in use of an atmospheric laser-based plasma chemical vapour deposition. Photoluminescence measurements were performed with an excitation area of 18 mm2 and a wavelength of 248 nm. While the photoluminescence of the doped layers conceptually proves suitability as an indicator for the condition of the coating, the undoped diamond layers in the coatings show a high transmissivity to the zero-phonon line for the used parameters.

Polishing of Black and White CVD Grown Polycrystalline Diamond Coatings

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

Polycrystalline Diamond Coating of Additively Manufactured Titanium for Biomedical Applications.

Additive manufacturing using selective laser melted titanium (SLM-Ti) is used to create bespoke items across many diverse fields such as medicine, defense, and aerospace. Despite great progress in orthopedic implant applications, such as for "just in time" implants, significant challenges remain with regards to material osseointegration and the susceptibility to bacterial colonization on the implant. Here, we show that polycrystalline diamond coatings on these titanium samples can enhance biological scaffold interaction improving medical implant applicability. The highly conformable coating exhibited excellent bonding to the substrate. Relative to uncoated SLM-Ti, the diamond coated samples showed enhanced mammalian cell growth, enriched apatite deposition, and reduced microbial S. aureus activity. These results open new opportunities for novel coatings on SLM-Ti devices in general and especially show promise for improved biomedical implants.

Influence of Cobalt on the Adhesion Strength of Polycrystalline Diamond Coatings on WC–Co Hard Alloys

The influence of cobalt on the phase composition and adhesion strength of polycrystalline diamond coatings has been studied using scanning electron microscopy, Raman spectroscopy, and X-ray microanalysis. The coatings have been deposited on WC–Co hard alloy substrates in glow discharge plasma. It has been found that the catalytic amorphization of carbon only takes place during the direct synthesis of the diamond coating, when the cobalt vapor pressure over the substrate is high and the cobalt-related degradation of the synthesized diamond is absent.

Deposition and Patterning of Polycrystalline Diamond Films Using Traditional Photolithography and Reactive Ion Etching

Given the exceptional characteristics of diamond films, they have become increasingly popular in the fields of medicine, microelectronics, and detector electronics. However, despite all the advantages, there are many technological problems that complicate their widespread application and impose limitations on diamond use in technological processes. In this study, we proposed a new technique for obtaining a complex topology of polycrystalline diamond coatings by selective seeding of the substrate by nucleation centers and subsequent surface treatment with reactive ion etching to reduce the number of parasitic particles. As a result, diamond films were obtained with a high particle concentration in the film region and high repeatability of the pattern. Moreover, parasitic particles influenced neutralization in areas where film coverage was not needed. The effect of the diamond nanoparticle concentration in a photoresist and the effect of reactive ion etching on the formation of a continuous film and the removal of parasitic nucleation centers were examined. The relative simplicity, low power consumption, and high efficiency of this method make it attractive for both industrial and scientific applications.

Plateholder design for deposition of uniform diamond coatings on WC-Co substrates by microwave plasma CVD for efficient turning application

Polycrystalline diamond coatings have been grown on cemented carbide WC-6% Co substrates with different aspect ratios by microwave plasma CVD in CH4/H2 gas mixtures. Special plateholder with holes for group growth has been used to protect the edges of the substrates from non-uniform heating due to the plasma edge effect. The difference in heights Δh of the substrates and plateholder, and its influence on the diamond film mean grain size, growth rate, phase composition and stress was investigated. Diamond growth rate of 0.3–1μm/h and compressive stress of 2.2–2.5GPa, respectively were determined in the optimal Δh region. The substrate temperature range of 740–760°C, within which uniform diamond films are produced with good adhesion, is determined. The diamond-coated samples produced at optimized process conditions exhibited a reduction of cutting force and wear resistance by a factor of two, and increase of cutting path length up to 8150m or by 4.3 times upon turning А390 Al-Si alloy as compared to performance of uncoated tools. Prime noveltyPolycrystalline diamond coatings with uniform structure and thickness distribution are grown by microwave plasma CVD (MPCVD) on WC-6% Co substrates using an optimized pocket-type substrate holder. Edge effect during MPCVD on high substrates (height/diameter>0.1) was suppressed. Influence of distance between the substrate holder and substrate surface on deposition temperature and other parameters was investigated. Cutting force and wear rate of the coated samples upon cutting of Si-Al abrasive alloy were twice less than that of uncoated WC-6% Co.