Nanocrystalline Diamond Coatings Research Articles

Optimization of diamond coated microdrills in aluminum alloy 7075 machining: A case study

Aluminum alloy 7075 is widely used for producing micro-scale heat sinks, micro-fluidic devices, micro-propellers and so on. This paper deals with optimizing microstructure and thickness of diamond coatings on microdrills used in 7075 aluminum alloy machining. Firstly, the friction tests between microcrystalline diamond (MCD), nanocrystalline diamond (NCD) films and aluminum alloy reveal that the stable coefficient of friction (COF) of MCD–aluminum alloy working pair is 0.240, much higher than that of NCD–aluminum alloy working pair (0.072). The decrease of COF is mainly attributed to the lower roughness of NCD films and the presence of more graphite or the non-diamond phases in NCD coatings. Afterwards, comparative cutting tests involving MCD, NCD, diamond-like coating (DLC) and TiAlN coated microdrills show that after drilling 200 holes, NCD coated microdrills exhibit the best cutting performance. Furthermore, NCD coated microdrills with coating thicknesses of 1μm, 2μm, 4.5μm and 7μm are fabricated and their cutting performance is studied in aluminum alloy machining. The cutting experiments show that the NCD coated microdrill with coating thickness of 4.5μm shows the best cutting performance, exhibiting not only lowest flank wear and no tool tipping or chipping on the main cutting edges but also the highest quality of drilled holes because of the outstanding adhesive strength and wear resistance of the NCD coating.

Impact of diamond nanoparticles on neural cells

Diamond nanoparticles (DNPs) are very attractive for biomedical applications, particularly for bioimaging. The aim of this study was to evaluate the impact of DNPs on neural cancer cells and thus to assess the possible application of DNPs for these cells imaging. For this purpose, the neuroblastoma SH-SY5Y cell line was chosen. Cells were cultured in medium with different concentrations (15, 50, 100 and 150 μg/ml) of DNPs. After 48 h of incubation, cell metabolic activity was evaluated by the XTT assay. For assessment of cellular metabolic activity, cells were also cultured on differently terminated nanocrystalline diamond (NCD) coatings in medium with 150 μg/ml of DNPs. Cell adhesion and morphology were evaluated by brightfield microscopy. Diamond nanoparticle internalization was determined by confocal microscopy. The obtained results showed that low concentrations (15, 50 and 100 μg/ml) of nanoparticles did not significantly affect the SH-SY5Y cell metabolic activity. However, a higher concentration (150 μg/ml) of DNPs statistically significantly reduced SH-SY5Y cell metabolic activity. After 48 h incubation with 150 μg/ml DNPs, cell metabolic activity was 23% lower than in medium without DNPs on standard tissue culture polystyrene.

Nanocrystalline diamond coatings on the interior of WC–Co dies for drawing carbon steel tubes: Enhancement of tube properties

Tungsten carbide (WC–Co) dies are commercially used for the tube drawing process. However they wear out progressively and are unable to meet the high demands required by the industry. In this study, the effect of nanocrystalline diamond (NCD) coatings on the interior of WC–Co drawing dies using a hot filament chemical vapour deposition technique is reported. A field trial was conducted on the production line for drawing AISI 1541 steel tubes to investigate the quality of the drawn tubes. The surface roughness of the tubes drawn through the NCD coated die was lower (Ra=381nm) when compared to the tubes drawn through a regular carbide die (Ra=527nm). The average residual stress of tubes drawn through the NCD coated drawing die was lowered by 25%. A pin-on-disc sliding wear test, carried out to estimate the coefficient of friction, showed that the coefficient of friction in the case of the NCD coated die was almost half that of the regular WC–Co dies. The excellent thermal conductivity and lower friction coefficient of NCD coatings also helped to decrease the working temperature of the tube drawing process, thereby resulting in a superior product.

Impact of differently modified nanocrystalline diamond on the growth of neuroblastoma cells

The aim of this study was to assess the impact of nanocrystalline diamond (NCD) thin coatings on neural cell adhesion and proliferation. NCD was fabricated on fused silica substrates by microwave plasma chemical vapor deposition (MPCVD) method. Different surface terminations were performed through exposure to reactive hydrogen and by UV induced oxidation during ozone treatment. Boron doped NCD coatings were also prepared and investigated. NCD surface wettability was determined by contact angle measurement. To assess biocompatibility of the NCD coatings, the neuroblastoma SH-SY5Y cell line was used. Cells were plated directly onto diamond surfaces and cultured in medium with or without fetal bovine serum (FBS), in order to evaluate the ability of cells to adhere and to proliferate. The obtained results showed that these cells adhered and proliferated better on NCD surfaces than on the bare fused silica. The cell proliferation on NCD in medium with and without FBS after 48h from plating was on average, respectively, 20 and 58% higher than that on fused silica, irrespective of NCD surface modification. Our results showed that the hydrogenated, oxygenated and boron-doped NCD coatings can be used for biomedical purposes, especially where good optical transparency is required.

Characterization of tribo-layer formed during sliding wear of SiC ball against nanocrystalline diamond coatings

Tribo-layer formation and frictional characteristics of the SiC ball were studied with the sliding test against nanocrystalline diamond coating under atmospheric test conditions. Unsteady friction coefficients in the range of 0.04 to 0.1 were observed during the tribo-test. Friction and wear characteristics were found to be influenced by the formation of cohesive tribo-layer (thickness~1.3μm) in the wear track of nanocrystalline diamond coating. Hardness of the tribo-layer was measured using nanoindentation technique and low hardness of ~1.2GPa was observed. The presence of silicon and oxygen in the tribo-layer was noticed by the energy dispersive spectroscopy mapping and the chemical states of the silicon were analyzed using X-ray photoelectron spectroscopy. Large amount of oxygen content in the tribo-layer indicated tribo-oxidation wear mechanism.

Nano Hardness and Elastic Modulus of Nanocrystalline Diamond Coating

Nanocrystalline diamond (NCD) coatings were prepared on Si3N4 substrate by hot filament chemical vapor deposition (HFCVD) method with Ar/H2/CH4 gas mixture. The nano hardness and elastic modulus of the NCD coatings were evaluated using a nano indenter. The results show that the diamond grains appear to be rounded with the average grain size of less than 100 nm. The average nano hardness is ∼53.5 GPa, the elastic modulus is ∼606.8 GPa, and the elastic recovery is as high as ∼80%, for the fine NCD coatings deposited with the substrate temperature of 700°C. The nano hardness and elastic modulus of the NCD coatings will increase with the substrate temperature increasing from 600°C to 700°C.

Adhesion characteristics of nano- and micro-crystalline diamond coatings: Raman stress mapping of the scratch tracks

Abstract Interfacial adhesion characteristics of nanocrystalline and microcrystalline diamond coatings deposited on tungsten carbide (WC–Co) substrates were studied and analysed using a scratch tester. Coating failure events and critical point loads were identified by acoustic emission, tangential force measurement and image analysis carried out on the scratch track. In this respect, enhanced scratch resistance properties were observed in microcrystalline diamond (MCD) coating in comparison to nanocrystalline diamond (NCD) coating. Significant difference in critical loads for adhesive failure was observed for MCD and NCD coatings. These loads were 42 N and 20 N for MCD and NCD coatings, respectively. The reason for these two distinctly different adhesive characteristics was attributed to the microstructure of the respective coatings. The surface morphologies at critical failure point and wedge spallation regions of the scratch tracks were completely different for NCD and MCD coatings. Critical point regions were analysed by Raman stress mapping to study the scratch induced residual stresses in the strained diamond flakes and deformed coating of the scratch track. In this respect, high tensile stresses were observed in the regions of critical failure. This behaviour is strongly dependent on magnitude of stress and nature of deformation during the scratch test of NCD and MCD coatings.

Superior hardness and Young's modulus of low temperature nanocrystalline diamond coatings

Nanocrystalline diamond (NCD) coatings with thickness of about 3 μm were grown on silicon substrates at four deposition temperatures ranging from 653 to 884 °C in CH4/H2/Ar microwave plasmas. The morphology, structure, chemical composition and mechanical and surface properties were studied by means of Atomic Force Microscopy (AFM), X-Ray Diffraction (XRD), Raman spectroscopy, nanoindentation and Water Contact Angle (WCA) techniques. The different deposition temperatures used enabled to modulate the chemical, structural and mechanical NCD properties, in particular the grain size and the shape. The characterization measurements revealed a relatively smooth surface morphology with a variable grain size, which affected the incorporated hydrogen amount and the sp2 carbon content, and, as a consequence, the mechanical properties. Specifically, the hydrogen content decreased by increasing the grain size, whereas the sp2 carbon content increased. The highest values of hardness (121 ± 25 GPa) and elastic modulus (1036 ± 163 GPa) were achieved in NCD film grown at the lowest value of deposition temperature, which favored the formation of elongated nanocrystallites characterized by improved hydrophobic surface properties.

Biomodification and biodeterioration of carbon coatings by fungal strains

Over the past years, surface protection by hard carbon coatings has been increasingly applied in many fields. This study aimed to characterize changes in the properties of carbon coatings brought about by the growth of fungi (the first report about these processes was published by Kaczorowska et al., Nanodiam, PWN, 2006, 99–116). Alterations in the structure of diamond-like carbon (DLC) and nanocrystalline diamond (NCD) coatings, examined by Raman spectroscopy, XRD, XPS, and FTIR, were found to be caused by the processes of oxidation (incorporation of oxygen atoms) and reduction (hydrogenation) occurring within graphite carbon and amorphous carbon domains (components of the surface layer of the tested coatings along with the diamond domain). Determination of the activity of selected enzymes synthesized by the studied strains of filamentous fungi revealed that the observed biomodification and biodeterioration processes involved, among others, laccase, Mn-dependent peroxidase, two catechol dioxygenases, and esterases. The biosynthesis of these enzymes by fungi was enhanced when graphite was added to their culture media. Because of the high risk of fungal infections, the quality of carbon coatings should be routinely controlled by standard microbiological tests employing suitable strains of filamentous fungi (e.g., Aspergillus niger) and yeasts synthesizing enzymes that attack carbon materials.

Tool Life Time Extension with Nano-crystalline Diamond Coatings for Drilling Carbon-fibre Reinforced Plastics (CFRP)

Carbon fibre reinforced plastics (CFRP) are delicate to machine as the material is highly abrasive and tends to show multiple workpiece damages like delamination, fibre pull out, uncut fibres and burnt matrix material. In result machining CFRP requires on the one hand tools which can withstand the massive wear, on the other hand geometrical highly adaptable tools to produce flawless workpieces. Diamond coated solid carbide tools can provide an excellent wear resistance in combination with a highly adaptable tool geometry. Nano-crystalline diamond coatings provide a good layer adhesion and can be applied to machining tools.This study focuses on required drilling tool characteristics for a large tool life time of diamond coated carbide tools. The workpiece material used is the M21/35%/370H5/AS4C-6K from Hexcel© which finds its main application in the aircraft industry. The machining tests are being conducted using a Mori Seiki NMV5000DCG at different feed rates and cutting velocities. Two different nano-crystalline diamond coatings are being evaluated regarding wear resistance depending on the drill geometry and tool diameter. The workpiece quality is measured using a microscope. It is shown that the tool geometry influences the wear resistance intensively. A change in wear resistance depending on the tool geometry is not unique for different diamond coatings. Finally, it is shown that cutting velocity and feed have only a small influence on tool life time.

Graded composite diamond coatings with top-layer nanocrystallinity and interfacial integrity: Cross-sectional Raman mapping

Cross-sectional structural characteristics of the CVD diamond coatings deposited on the tungsten carbide (WC-Co) substrates were analysed using Raman imaging technique. The grain size of the nanocrystalline diamond (NCD) coatings was observed to deviate from the nanocrystallinity with increasing thickness and exhibited the surface characteristics of microcrystalline diamond (MCD). However, thick diamond coatings with surface nanocrystallinity is the key requirement for load-bearing tribological applications. Tribological tests have clearly indicated the significance and need for the top-layer nanocrystallinity. Graded composite diamond coatings with an architecture of NCD/transition-layer/MCD/WC-Co are potentail candiadates to realize thick diamond coatings with top-layer nanocrystallinity. Residual stresses along the cross-section of the graded composite diamond coatings were analysed using Raman imaging technique, which confirmed the improved interfacial integrity of the graded composite diamond coatings

Fatigue strength of diamond coatings' interface assessed by inclined impact test

The inclined impact test is an efficient method for characterizing the fatigue strength of interfaces of nanocrystalline diamond coatings (NCD). During this test, an oscillating oblique load induces repetitive shear stresses into the region between film and its substrate. Inclined impact tests were conducted on NCD coated specimens with a thickness of ca. 5μm at forces up to 850N and 1.5million loading cycles. The related imprints were evaluated by confocal microscopy measurements and EDX micro-analyses. Dependent on the applied load, after a certain number of impacts, damages in the film interface region develop resulting in coating detachment. In this way, residual stresses of the film are released leading to its lifting (bulge formation). The bulges are destroyed by further repetitive impacts and the coating is totally removed. The geometry of the developed NCD coating's bulges can be effectively described by appropriate Finite Element Method (FEM) supported calculations. Based on the attained impact test results, Woehler-like diagrams were developed for monitoring the fatigue failure or endurance of NCD coatings at various impact conditions. Employing such diagrams, the fatigue strength of NCD coatings can be assessed. According to the attained Woehler-like diagrams, a threshold load of 730N exists for the film delamination by fatigue in the investigated case.

Nanodiamond films with dendrite structure formed by needle crystallites

Nanocrystalline diamond coatings, consisting of tightly packed globular structures, were obtained using a direct current plasma assisted chemical vapor deposition method. Composition of the globules from diamond crystallites of different size and from graphitic material was revealed using a combination of thermogravimetry analysis, Raman spectroscopy and scanning electron microscopy. The smallest diamond and graphitic fractions were selectively removed from the nanodiamond films by thermal oxidation in air at a temperature of 620°C. The rest of the film material consisted of diamond needle-like crystallites, forming the “skeletons” of the globules. Mechanisms including the formation of the dendrite structure of the nanodiamond films with the “skeletons” formed by the needle crystallites are discussed.

Growth and characterization of integrated nano- and microcrystalline dual layer composite diamond coatings on WC–Co substrates

In this work, integrated composite diamond (ICD) coatings have been achieved with top layer nanocrystallinity, low friction coefficient and enhanced integrity. ICD coatings were deposited on chemically treated tungsten carbide (WC–Co) substrates using hot filament chemical vapour deposition technique. Nanocrystalline diamond (NCD) layer was deposited over microcrystalline diamond (MCD) layer with a coating architecture of NCD/transition layer/MCD/WC–Co. Graded transition layer thickness of ~1μm was realized by controlling the process parameters such as methane concentration and chamber pressure in order to integrate the MCD and NCD layers. Integrity of the coatings was examined by the cross-sectional studies. Structural and microstructural characteristics of ICD coatings were compared with those of MCD coatings. The measured average nanohardness of ICD coating was ~96GPa. A low and stable friction coefficient of ~0.06 was observed for ICD coatings against silicon nitride (Si3N4). ICD coatings were anticipated to exploit the advantages of both NCD and MCD coatings and these coatings can be promising candidates for various mechanical applications.

An investigation on the cutting performance of nano-crystalline diamond coatings on a micro-end mill

In conventional milling, appropriate coatings can be used on tools to enhance their cutting performance and efficiency of the production process. In an effort to improve the tooling performance in micro-scale milling, nano-crystalline diamond films were applied on a micro-end mill by a hot filament chemical vapour deposition process. The cutting performance of the nano-crystalline diamond-coated tool, including cutting forces, tool integrity, surface roughness and burr formation, were evaluated in slot milling of aluminium 6061-T6 against that of an uncoated tool in dry cutting conditions. Reduced cutting forces, free chip adhesion, lower tool wear, improved surface roughness, as well as smaller burrs, were observed using the nano-crystalline diamond-coated tool. The investigation illustrates the possibility of applying nano-crystalline diamond coatings on micro-milling tools for dry cutting and better tooling performance.

Composition profiles and adhesion evaluation of conductive diamond coatings on dielectric ceramics

Sintered silicon nitride (Si3N4) ceramic substrates were investigated as dielectric substrates for the growth of metal-like boron-doped nanocrystalline diamond (NCD) and microcrystalline diamond coatings via the Hot Filament Chemical Vapor Deposition (HFCVD) technique. The structural, electrical and chemical properties of both the ceramic substrates and the diamond coatings may potentiate their applicability in particular in harsh environments and highly demanding situations. Boron doping was achieved via a boron oxide solution in ethanol dragged into the reaction chamber with argon. The coatings were characterized by scanning electron microscopy, UV μ-Raman scattering, X-ray diffraction, time-of-flight secondary ion mass spectroscopy, Brale indentation for adhesion evaluation and two-point contact probe for resistivity measurements. The HFCVD technique led to a maximal growth rate of about 1μm/h. Several metal-like boron doped diamond coatings were obtained. It was found that at lower substrate temperature, lower system pressure and higher methane concentration, the resistivity of the conducting NCD coatings is about 3 orders of magnitude higher when compared with samples obtained with higher substrate temperature, higher system pressure and lower methane concentration. Nevertheless, for every metal-like boron-doped coating the use of the Si3N4 ceramic substrate guaranteed a superior adhesion level.

CVD nanocrystalline diamond coatings on Ti alloy: A synchrotron-assisted interfacial investigation

Diamond coating on Ti-6Al-4V alloy was carried out using microwave plasma enhanced CVD with a super high CH4 concentration, and at a moderate deposition temperature close to 500 °C. The nucleation, growth, adhesion behaviors of the diamond coating and the interfacial structures were investigated using Raman, XRD, SEM/TEM, synchrotron radiation and indentation test. Nanocrystalline diamond coatings have been produced and the nucleation density, nucleation rate and adhesion strength of diamond coatings on Ti alloy substrate are significantly enhanced. An intermediate layer of TiC is formed between the diamond coating and the alloy substrate, while diamond coating debonding occurs both at the diamond-TiC interface and TiC-substrate interface. The simultaneous hydrogenation and carburization also cause complex micro-structural and microhardness changes on the alloy substrates. The low deposition temperature and extremely high methane concentration demonstrate beneficial to enhance coating adhesion strength and reduce substrate damage.

Carbon Nanostructures for Orthopedic Medical Applications

Carbon nanostructures (including carbon nanofibers, nanostructured diamond, fullerene materials and so forth) possess extraordinary physiochemical, mechanical and electrical properties attractive to bioengineers and medical researchers. In the past decade, numerous developments towards the fabrication and biological studies of carbon nanostructures have provided opportunities to improve orthopedic applications. Therefore, the aim of this article is to provide an up-to-date review on carbon nanostructure advances in orthopedic research. Orthopedic medical device applications of carbon nanotubes/carbon nanofibers and nanostructured diamond (including particulate nanodiamond and nanocrystalline diamond coatings) are emphasized here along with other carbon nanostructures that have promising potential. In addition, widely used fabrication techniques for producing carbon nanostructures in both the laboratory and in industry are briefly introduced. In conclusion, carbon nanostructures have demonstrated tremendous promise for orthopedic medical device applications to date, and although some safety, reliability and durability issues related to the manufacturing and implantation of carbon nanomaterials remain, their future is bright.

Synthesis and Characterization of Multilayered Diamond Coatings for Biomedical Implants

With incredible hardness and excellent wear-resistance, nanocrystalline diamond (NCD) coatings are gaining interest in the biomedical community as articulating surfaces of structural implant devices. The focus of this study was to deposit multilayered diamond coatings of alternating NCD and microcrystalline diamond (MCD) layers on Ti-6Al-4V alloy surfaces using microwave plasma chemical vapor deposition (MPCVD) and validate the multilayer coating’s effect on toughness and adhesion. Multilayer samples were designed with varying NCD to MCD thickness ratios and layer numbers. The surface morphology and structural characteristics of the coatings were studied with X-ray diffraction (XRD), Raman spectroscopy, and atomic force microscopy (AFM). Coating adhesion was assessed by Rockwell indentation and progressive load scratch adhesion tests. Multilayered coatings shown to exhibit the greatest adhesion, comparable to single-layered NCD coatings, were the multilayer samples having the lowest average grain sizes and the highest titanium carbide to diamond ratios.

Effect of CH 4/H 2 plasma ratio on ultra-low friction of nano-crystalline diamond coating deposited by MPECVD technique

Nano-crystalline diamond coatings were deposited on the silicon substrate using Microwave Plasma Enhanced Chemical Vapor Deposition (MPECVD). Experiments were performed by varying the H 2 content in CH 4/H 2 plasma during synthesis. Raman spectral analysis revealed that with decrease in hydrogen content in the CH 4 plasma, the I D/ I G ratio decreases with the formation of smaller crystallites. Such a film possesses a large grain boundary fraction containing hydrogenated amorphous carbon (a-C:H). During tribological test, sufficient amount of hydrogen present in the grain boundary passivates the dangling σ-bond causing ultra-low friction and extremely low wear evident by improvement in microstructure.