Microcrystalline Diamond Coatings Research Articles

Superior tribological and sealing performance of micro-crystalline diamond coated silicon carbide seals under dry friction and high load

Silicon carbide seals suffer severe wear under the conditions of dry friction and high load, posing a great challenge in achieving a stable performance and a long lifetime. In this work, micro-crystalline diamond coatings were deposited on silicon carbide sealing rings to prevent the failure caused by wear. The deposited micro-crystalline diamond coatings exhibit high-quality crystallinity and strong adhesive strength with the silicon carbide substrate. In sealing tests, the micro-crystalline diamond coated silicon carbide sealing pair demonstrates superior tribological and sealing properties under dry friction and high load of 1800 N, featuring a low coefficient of friction of 0.16, a low wear rate of 4.00 × 10−7 mm3·N−1·m−1 and a high friction limit of 5.24 MPa·m·s−1. Characterizations of morphology, chemical composition and microstructure confirm that the smoothening effect on the contact face and the formation of graphite during the sealing test synergistically contribute to the outstanding tribological and sealing performance.

Tribological Performance of Microcrystalline Diamond (MCD) and Nanocrystalline Diamond (NCD) Coating in Dry and Seawater Environment

In the present study, the tribological properties of diverse crystalline diamond coating with micro (MCD) and nanometer (NCD) sizes, fabricated by the microwave plasma chemical vapor deposition (MPCVD) method, are systematically investigated in dry and seawater environments, respectively. Owing to the SiO2 lubricating film with extraordinary hydrophilicity performance by a tribochemical reaction, the average friction coefficient (COF) and wear rate of NCD coating under seawater decreased by 37.8% and 26.5%, respectively, comparing with in dry conditions. Furthermore, graphite would be generated with the increment of surface roughness. Graphite transformed from the diamond under high contact pressure. Thus, with the synergism between SiO2 lubricating film with extraordinary hydrophilicity performance and graphite, the corresponding COF and wear rate of MCD would be further decreased by up to 64.1% and 39.5%. Meanwhile, various characterizations on morphology, spectra, and tribological performance of the deposited diamond coating were conducted to explore the in-depth mechanism of the enhanced tribological performance of our NCD and MCD coatings in the extreme under seawater working conditions. We envision this work would provide significant insights into the wear behavior of diamond coatings in seawater and broaden their applications in protective coatings for marine science.

Micro and Nano-Crystalline Diamond Coatings of Co-cemented Tungsten Carbide Tools with Their Characterization

In this study, a bias-enhanced hot filament CVD system with methane and hydrogen as reactant gasses was used to deposit microcrystalline diamond (MCD) and nanocrystalline diamond (NCD) films on Co-cemented tungsten carbide substrates. They were then characterized using FE-SEM, Optical microscopy, XRD, and Raman Spectroscopy. Nanocrystalline diamond coating displays wide carbon peaks near 1350 cm−1 and 1580 cm−1 in Raman Spectroscopy, which correspond to D band and G band of carbon, and a sharp band increase near 1140 cm−1 because of the presence of trans-polyacetylene, confirming the sample as a nanocrystalline diamond. Similarly, the microcrystalline diamond phase was observed at 1350 cm−1 as broadened carbon peaks. The average grain size of the homogenous diamond film was roughly less than 90 nm, several tens of nanometres. The average indentation depths were 45 nm and 63 nm of MCD and NCD coatings and their average hardness values were found 80GPa and 50GPa, respectively. The morphology of the samples was observed using AFM, the average root mean square roughness of the MCD sample in the scanned area was found to be 354.11 nm and of the NCD sample was found to be 142.71 nm this indicates that the nanocrystalline diamond-coated WC was much smoother and had a smaller grain size than microcrystalline diamond coated ones. The values of the coefficient of friction vary from 0.38 to 0.42 for MCD coated sample. In contrast, for the NCD samples, the variations in the coefficient of friction were 0.30–0.36. A low coefficient of friction was observed in NCD as compared to MCD due to the presence of the graphite carbon phase.

Temperature stabilization of WC-Co cutting inserts with feedback to IR pyrometer upon growth of multilayer diamond coatings by microwave plasma chemical vapor deposition

We produced microcrystalline diamond coatings with nanocrystalline top diamond layer on WC-Co cutting inserts in a microwave plasma (2.45 GHz) chemical vapor deposition reactor. The coatings have a high uniformity of the substrate surface owing to elimination of the plasma edge effect by application of a moveable quartz ring above the substrate holder. Computer modeling showed how the positioning of the ring changes the electromagnetic field around the substrates. A new approach was used to stabilize the temperature of the cutting inserts during the coating process, which is based on in situ control of the quartz ring position by proportional-integral-differential (PID) regulation. The coatings have been characterized with microRaman spectroscopy in a mapping mode in cross section of the samples. Depth profiles of diamond Raman peak position and width are demonstrated and the stress in the films is estimated, the minimum stress being determined for the cutting insert apex.

Interaction of Methane Concentration and Deposition Temperature in Atmospheric Laser Based CVD Diamond Deposition on Hard Metal

For laser-based plasma chemical vapour deposition (CVD) of diamond on hard metal at atmospheric pressure, without a vacuum chamber, the interaction between the deposition temperature and the methane concentration has to be understood to adjust the coating thickness, deposition duration, and medium diamond crystal size. The hypothesis of this study is that a wider range of methane concentrations could be used to deposit microcrystalline diamond coatings due to the increasing etching and deposition rates with rising deposition temperatures. The deposition of the CVD diamond coatings was carried out on K10 hard metal substrates. The process temperature and the methane concentration were varied from 650 to 1100 °C and from 0.15% to 5.0%, respectively. The coatings were analysed by scanning electron and 3D laser-scanning confocal microscopy, energy dispersive X-ray and micro-Raman spectroscopy, as well as cryofracture-based microscopy analysis. The results showed that microcrystalline diamond coatings could be deposited in a wider range of methane concentrations when increasing the process temperature. The coating thickness saturates depending on the process temperature even though the methane concentration constantly increases. The coating thickness increases with an increasing deposition temperature until the cobalt diffusion hinders the deposition at the process temperature of 1100 °C.

Diamond/β-SiC film as adhesion-enhanced interlayer for top diamond coatings on cemented tungsten carbide substrate

In present work, diamond/β-SiC composite interlayers were deposited on cemented tungsten carbide (WC-6%Co) substrates by microwave plasma enhanced chemical vapor deposition (MPCVD) using H2, CH4 and tetramethylsilane (TMS) gas mixtures. The microstructure, chemical bonding, element distribution and crystalline quality of the composite interlayers were systematically characterized by means of field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), X-ray photoelectron spectrometer (XPS), electron probe microanalysis (EPMA), Raman spectroscopy and transmission electron microscropy (TEM). The influences of varying TMS flow rates on the diamond/β-SiC composite interlayers were investigated. Through changing the TMS flow rates in the reaction gas, the volume fraction of β-SiC in the composite interlayers were tunable in the range of 12.0%–68.1%. XPS and EPMA analysis reveal that the composite interlayers are composed of C, Si element with little cobalt distribution. The better crystallinity of the diamond in the composite is characterized based on the Raman spectroscopy, which are helpful to deposit top diamond coatings with high quality. Then, the adhesion of top diamond coatings were estimated using Rockwell C indentation analysis, revealing that the adhesion of top diamond coatings on the WC-6%Co substrates can be improved by the interlayers with the diamond/β-SiC composite structures. Comprehensive TEM interfacial analysis exhibits that the cobalt diffusion is weak from WC-6%Co substrate to the composite interlayer. The homogeneous microcrystalline diamond coatings with the most excellent adhesion can be fabricated on the substrates with the composite interlayer with the β-SiC/diamond ratio of about 45%. The composite structures are appropriate for the application in high-efficiency mechanical tool as a buffer layer for the deposition of the diamond coating.

Application of Raman Spectroscopy for Analyzing Diamond Coatings on a Hard Alloy

Single- and multilayer coatings of micro- and nanocrystalline diamond were deposited on tungsten carbide substrates (WC + 6 wt.% Co alloy) in a microwave plasma of methane/hydrogen and methane/hydrogen/nitrogen mixtures. Barrier tungsten layers were used to improve adhesion of the diamond coatings to the substrates. Raman spectroscopy was used to study the properties of the nano- and microcrystalline diamond coatings depending on the deposition parameters. Data on structural transformations of the coatings under different tribological contact conditions were obtained, and secondary nucleation processes were investigated on different diamond faces during the deposition from the gas phase.

Comparative Analysis of Wear Rates of Microcrystalline Diamond and Diamond-Like Carbon Coatings Deposited on WС-Co Substrates

The study investigates the wear of microcrystalline diamond (MCD) and diamond-like carbon (DLC) coatings. The MCD and DLC coatings were grown by plasma enhanced chemical vapor deposition (PECVD) method on WC-Co substrates. The sliding wear tests were performed on the ball-on-plate type of tribometer in reciprocating mode. The ball-cratering wear tests were carried out using Calo tester. The mechanical profilometer, optical and scanning electron microscopes (SEM) were used for investigation of the surface morphology of the wear scars. The wear of DLC coating is more intense in comparison to the MCD coating. In contrast to the MCD coating, no evidence of the DLC coating deflection was found.

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.

Assessment of stress relaxation experiments on diamond coatings analyzed by digital image correlation and micro-Raman spectroscopy

The analysis and control of residual stresses are important for understanding the fracture and delamination behavior of coatings and thin films. In this work, the stress relaxation in microcrystalline diamond coatings after focused ion beam (FIB) milling was assessed by micro-Raman spectroscopy and digital image correlation. Using the FIB, the so called H-Bar geometry was milled in the diamond coating. The cutting introduced a strain relief in the coating perpendicular to the longer sides of the bar. The relaxation strains were recorded from high resolution scanning electron microscopy images of the H-Bar before and after FIB cutting. Using finite element modeling of the milled geometry, the residual stress level in the coating is evaluated from the relaxation strains. Furthermore, μ-Raman spectroscopy was used to assess the biaxial residual stress level in the coating before and after FIB cutting. The μ-Raman signal inside the bar showed strong incidence for stress relaxation and confirmed to some extent the applicability of the FIB–DIC method.

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.

Wear resistance of nano- and micro-crystalline diamond coatings onto WC–Co with Cr/CrN interlayers

Cr/CrN bi-layers have been used recently to promote the growth of high quality Hot Filament Chemical Vapour Deposition (HFCVD) diamond coatings onto Co-cemented tungsten carbide (WC–6 wt.%Co) substrates. In the present investigation, the influence of the crystalline size of the diamond coatings on their wear endurance is looked into. Nano- (NDC) and micro-crystalline Diamond Coatings (MDC) were deposited by HFCVD onto untreated and Fluidized Bed (FB) treated Cr/CrN interlayers. NDCs, characterized by a cauliflower-like morphology, showed improved wear resistance. However, the superimposition of NDCs onto Cr/CrN interlayers micro-corrugated by FB treatment was found to be the most promising choice, leading to the formation of highly adherent and wear resistant coatings.

Chemical erosion of different carbon composites under ITER-relevant plasma conditions

We have studied the chemical erosion of different carbon composites in Pilot-PSI at ITER-relevant hydrogen plasma fluxes (∼1024 m-2 s-1) and low electron temperatures (Te∼1 eV). Optical emission spectroscopy on the CH A–X band was used to characterize the chemical sputtering. Fine grain graphite (R 6650, SGL Carbon Group), ITER-reference carbon fiber composite material (SNECMA NB31 and NB41; Dunlop 3D), nano- and micro-crystalline diamond coatings on molybdenum and SiC (Silit® SKD Reaction-Bonded, Saint-Gobain Ceramics) were compared. The chemical sputtering was similar for the different composites under comparable plasma conditions, except for SiC, which produced a ten times lower rate. The CH emission was constant at electron temperatures Te>1 eV and ion fluxes ranging between 1023 and 1024 m- 2 s-1, but decreased at lower temperatures. This decrease is possibly due to changes in the excitation of CH and not due to a change in the chemical erosion rate.

Nanodiamond-based tribosystems

The tribological behaviour of homologous contacts between nanocrystalline diamond (NCD) chemically vapour deposited (CVD) coated parts is reviewed. The major part of the published works is related to silicon nitride ceramic coated materials. The NCD-on-NCD tribo-tests were performed under unlubricated and lubricated sliding conditions in water and in simulated physiological fluids (Hank's balanced salt solution, HBSS, and dilute fetal bovine serum, FBS). The friction and wear behaviour was assessed using a reciprocating motion setup under moderated to high applied normal loads (10–90 N), and up to 6 km of sliding distance. The inherent very low surface roughness of the NCD films was responsible for a very low steady-state friction response (0.01–0.05) in unlubricated and water-lubricated tests. Moreover, the running-in period of accommodation observed for conventional microcrystalline diamond coatings is greatly suppressed by the much lower starting surface roughness found in the NCD coatings. In distilled water, the NCD coatings displayed a remarkable tribological performance characterised by a high resistance to wear damage (10 − 9 mm 3 N − 1 m − 1 ), characteristic of very mild wear regimes, and high tribomechanical threshold loads (85 N) before film failure. Biotribological experiments conducted in HBSS resulted in similar friction and wear coefficients, while tribotesting in FBS revealed higher friction values (0.10) due to a protein attaching effect.

Influence of grain size on the wear behavior of CVD diamond coatings in micro-EDM

In order to reduce wear and therewith enhance the microelectrical discharge machining (µ-EDM) results, a coated electrode was used. The paper presents the results of investigations into the influence of grain size of the boron-doped CVD diamond coating on the wear behavior in microsinking EDM. Experimental investigations show that nanocrystalline coatings exhibited smaller discharge craters compared to those for microcrystalline diamond coatings, and microcrystalline coating showed melted material around the discharge crater. The finding will be subjected to further investigations.

Adhesion and Wear Behaviour of NCD Coatings on Si<SUB>3</SUB>N<SUB>4</SUB> by Micro-Abrasion Tests

Nanocrystalline diamond (NCD) coatings offer an excellent alternative for tribological applications, preserving most of the intrinsic mechanical properties of polycrystalline CVD diamond and adding to it an extreme surface smoothness. Silicon nitride (Si3N4) ceramics are reported to guarantee high adhesion levels to CVD microcrystalline diamond coatings, but the NCD adhesion to Si3N4 is not yet well established. Micro-abrasion tests are appropriate for evaluating the abrasive wear resistance of a given surface, but they also provide information on thin film/substrate interfacial resistance, i.e., film adhesion. In this study, a comparison is made between the behaviour of NCD films deposited by hot-filament chemical vapour deposition (HFCVD) and microwave plasma assisted chemical vapour deposition (MPCVD) techniques. Silicon nitride (Si3N4) ceramic discs were selected as substrates. The NCD depositions by HFCVD and MPCVD were carried out using H2-CH4 and H2-CH4-N2 gas mixtures, respectively. An adequate set of growth parameters was chosen for each CVD technique, resulting in NCD films having a final thickness of 5 microm. A micro-abrasion tribometer was used, with 3 microm diamond grit as the abrasive slurry element. Experiments were carried out at a constant rotational speed (80 r.p.m.) and by varying the applied load in the range of 0.25-0.75 N. The wear rate for MPCVD NCD (3.7 +/- 0.8 x 10(-5) mm3 N(-1) m(-1)) is compatible with those reported for microcrystalline CVD diamond. The HFCVD films displayed poorer adhesion to the Si3N4 ceramic substrates than the MPCVD ones. However, the HFCVD films show better wear resistance as a result of their higher crystallinity according to the UV Raman data, despite evidencing premature adhesion failure.

In-Situ Friction Monitoring of Self-Mated CVD Diamond Coatings Using Acoustic Emission

In-situ measurements of acoustic emission (AE) in self-mated tribological pairs of CVD diamond coated silicon nitride (Si3N4) were made with the purpose of investigating the relationship between AE signal and friction events. A good correlation is found between the energy dissipation/emission processes, therefore enabling the possibility of monitoring the different friction regimes occurring during the sliding contact of microcrystalline diamond (MCD) coatings. Deposition of MCD on flat and ball-shaped Si3N4 samples was accomplished using microwave plasma assisted chemical vapour deposition (MPCVD) with H2/CH4 gas mixtures. The friction behaviour of self-mated MCD coatings was assessed using a reciprocating ball-on-flat geometry. The tests were run in ambient atmosphere without lubrication, the frequency (1Hz) and stroke (6mm) were kept constant while the applied normal load varied in the range 10-80N. The microstructure, surface topography and roughness of the MCD coatings were characterised by SEM and AFM techniques. The diamond quality was assessed from micro-Raman spectroscopy. The friction evolution was characterised by a short running-in period where the main feature is a sharp peak reaching values as high as approximately 0.6 followed by a steady-state regime with very low values in the range 0.03-0.04.

Deposition of Adherent Diamond Coatings on WC-Co Substrates

ABSTRACTHigh quality well-adhered microcrystalline diamond coatings have been produced by the microwave plasma enhanced chemical vapor deposition (MPECVD) technique on cemented carbide substrates. A multi-interlayer system Cr/CrN/Cr was deposited on the WC-Co substrate before diamond deposition to work as a diffusion barrier. The coated substrate was peened with friable diamond powder to roughen the surface resulting in high nucleation density. Adherent diamond film has been successfully deposited on the substrate at temperature around 700°C with 1.0 % CH4 in Hydrogen plasma. The surface morphology and film structure has been studied by Scanning Electron Microscopy (SEM) and X-Ray diffraction technique. The adhesion of the diamond film has been evaluated by Rockwell indentation tests.

Adhesion Improvement of CVD Diamond Coatings on WC-Co Substrates for Machining Applications

ABSTRACTIn order to improve the performance of WC-Co cutting tools, high quality microcrystalline diamond coatings were produced using microwave plasma enhanced chemical vapor deposition (MPECVD) method. The adhesion of the diamond film deposited on the substrate has been considered to play an important role in the performance of the cutting tools in machining applications. A thin layer of Cr was coated on the WC-Co substrate before the diamond deposition; 75 μm diamond powders were sandblasted on the surface at 40 Psi to increase the nucleation density. Diamond film has been successfully deposited on the substrate at temperature around 750°C with 1.5 % CH4 in Hydrogen plasma. Scanning electron microscopy (SEM) has been used to study the surface morphology and Raman spectroscopy has been performed to characterize the quality of the diamond films and measure the residual stress. The adhesion of the diamond film has been evaluated by Rockwell indentation test. The results indicated that film grown on the Cr interlayer with diamond powder sandblasting has much better adhesion strength.