Nanocrystalline Diamond Coating Research Articles

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.

EFFECT OF EMBEDDED TUNGSTEN PARTICLES ON IMPACT RESISTANCE OF DIAMOND COATING

For diamond coating, natural fragile property easily leads to fracture, delamination and peeling, which seriously inhibits the applications in many industrial fields. In order to prolong the lifetime, improving the toughness under impact load is essential for diamond coating. In this work, a novel method was proposed by the conventional CVD diamond technique combining with the particles, with which the nanocrystalline diamond (NCD) coating with W particles (W-NCD) was fabricated to evaluate the impact behavior. The pure NCD coating was also produced for comparison. Repeating impact testing was performed to evaluate the impact resistance of the as-deposited NCD coatings. The results showed that the diamond coating can be fabricated on the substrate with W particles. The indentation scar revealed that the W-NCD coating had the stronger impact resistance than the NCD coating. Ratcheting effect was employed to discuss the impact properties of NCD coating for the first time. The coating integrity played a vital role in ratcheting displacement. Repeating impact can make the NCD and W-NCD coatings soft, and the W particles can accelerate the softening process. Hence, embedding particles can provide a potential and valid method to enhance the impact resistance of diamond coating that was very important for the fragile coating.

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.

Microstructure and biological evaluation of nanocrystalline diamond films deposited on titanium substrates using distributed antenna array microwave system

In this paper, the microstructure and biological properties through osteoblast cell viability and morphology study of Nano Crystalline Diamond (NCD) films deposited on Ti and TiAl6V4 substrates using a Distributed Antenna Array (DAA) microwave system are investigated. The morphology, purity and microstructure of such NCD coatings are comparable to those usually obtained on silicon wafers in the considered reactor, whereas the surface roughness is much higher due to the high roughness of Ti and TiAl6V4 bare substrates. The coated samples have a good stability in time and the various elements (C, Ti, Al, V) are uniformly distributed across the whole surface. The NCD-coated Ti samples show no sign of cytotoxicity with survival rate of 90% after 24 h. Besides, the NCD coating affects the spreading of the osteoblast cells since the cell coverage, which increases with time, is always higher for NCD/Ti systems compared to Ti substrates at least up to 24 h. As a matter of fact, the NCD coating of Ti and TiAl6V4 substrates changes the wettability from hydrophobic to hydrophilic regime, which can play a significant role in mediating cell interactions with material surfaces. NCD deposition on Ti and TiAl6V4 samples using the DAA reactor is therefore very promising for biomedical applications since it guaranties hard, biocompatible and hydrophilic as-grown coatings that can be achieved on large surfaces and complex-shaped substrates.

Load sensitive super-hardness of nanocrystalline diamond coatings

Synthetic diamond films have attracted great attention for their extreme properties and potential engineering applications as protective and wear-resistant coating for cutting tools. Nanocrystalline diamond (NCD) coatings were synthesized from CH4/H2/Ar (1/10/89%) microwave plasma at four deposition temperatures (TD) ranging from 653 to 884 °C. The hardness (H) and Young's modulus (E) of NCD coatings measured at three different loads (10, 25 and 47 mN) depended on the nanoindentation load-level. The NCD coating produced at the lowest TD showed values of H = 121 ± 25 GPa and E = 1036 ± 163 GPa at the highest load. This result was attributed to the formation of elongated nanocrystallites at low deposition temperature. Further, the NCD coating obtained at lower deposition temperature exhibited an anomalous indentation size effect (ISE), i.e. a reverse ISE (RISE), which was ascribed to the heterogeneity of grain sizes along the [220] and [111] directions. Finally, a positive and negative (inverse) Hall-Petch behavior was observed for grain sizes along the [111] and [220] directions, respectively.

Synthesis and characterisation of nanocrystalline, microcrystalline and functionally graded diamond coatings on reaction bonded SiC

Nanocrystalline diamond (NCD) films on different substrates have the potential for use in various tribological, thermal management and other applications. On the other hand, microcrystalline diamond (MCD) coatings exhibit higher hardness and better adhesion on many substrates. Hence, the realisation of a functionally graded MCD-NCD coating on a substrate have a high applicability, particularly for tribological and wear-related applications. In this study, reaction bonded silicon carbide was used as a substrate for the deposition and growth of NCD, MCD and functionally-graded NCD-MCD films using a large area hot filament chemical vapour deposition (HFCVD) equipment. The microstructure of the film was controlled by suitably adjusting the methane to hydrogen flow rate ratios during deposition. Characterization by high resolution scanning electron microscopy (HRSEM), atomic force microscopy (AFM), x-ray diffraction (XRD) and Raman spectroscopy confirmed the nature and microstructure of the coatings. Nanoindentation studies revealed the hardness values to lie in the range of 25 GPa for the NCD coating to 38 GPa for the MCD film, which drastically increased to 70 GPa when the thickness of the coating was reduced by changing the deposition time. The results of these studies on the NCD, MCD and functionally-graded NCD-MCD coatings are compared and contrasted.

Wear Rate of Nanocrystalline Diamond Coating under High Temperature Sliding Conditions

The present study deals with the tribological behavior of nanocrystalline diamond (NCD) coatings at high temperature sliding conditions. The NCD coatings were grown by plasma enhanced chemical vapor deposition (PECVD) method on the hard metal (WC-Co) substrates. The friction and wear tests were performed on ball-on-disc tribometer using a high-temperature chamber with rotary drive. The tests were carried out at room temperature, 300, 450 and 600 °C. The scanning electron microscopy (SEM), optical microscopy, mechanical profilometry and Raman spectrometry were used for investigation of the morphology and chemical composition of the wear scars and pristine surface. The depth and width of the wear scars measured after the high temperature sliding tests are larger in comparison with room temperature tests. It was observed that the coefficient of friction (COF) increased with increasing temperature. The wear rate of NCD coatings tested at 300-450° C was about 10 times higher than that at room temperature. The mechanisms involved for these variations are discussed.

Multilayer nano/micro crystalline diamond coating on boronizing cemented carbide

abstractBoronizing treatment was done to form a boride surface layer well bonded with cemented carbide substrate. Multilayer microcrystalline diamond (MCD) and nanocrystalline diamond (NCD) coatings were prepared on the boronizing cemented carbide substrate by hot filament chemical vapor deposition (HFCVD) method. The surface and the cross section micrograph, coating component, hardness and adhesion of the as-deposited diamond coatings were observed with a SEM, Raman spectroscopy, nano-indenter and a Rockwell hardness tester accordingly. The results show that the MCD coating consists of well-faceted grains and the hardness of the MCD coating is about 85GPa. The NCD coating has morphological features with grain sizes of several tens of nanometer. The hardness of the NCD coating is about 52GPa. The boron doping of cemented carbide substrate can improve the fracture strength of the diamond film to some extent, and the adhesion of multilayer NCD/MCD coating is better than that of monolayer MCD and monolayer NCD coating.

Preparation and optical properties of nanocrystalline diamond coatings for infrared planar waveguides

The high index of refraction planar optical waveguides operating in the near infrared spectrum were prepared from the intrinsic hydrogenated amorphous silicon (a-Si:H) layer coated by nanocrystalline diamond (NCD). The a-Si:H layers were grown by radio frequency plasma enhanced chemical vapor deposition on glass substrates from silane and hydrogen mixture at the substrate temperature about 250°C followed at 210°C by the NCD coating grown by the microwave plasma enhanced chemical vapor deposition from hydrogen rich plasma with adding methane and carbon dioxide at the substrate temperature. The optical absorptance spectra, Raman spectra and scanning electron microscopy reveal the diamond character of the NCD coatings as well as the stability of a-Si:H layer during NCD deposition. The critical issue of the NCD coated a-Si:H optical waveguide is the optical absorption in NCD coating. The Finite Element Method simulations show that the near infrared optical absorption coefficient of the NCD coating must be reduced below 10/cm to reduce the optical attenuation below 1dB/cm in the planar waveguide with the 50nm thick NCD coating.

Mechanical Properties and Uniformity of Nanocrystalline Diamond Coating Deposited Around a Sphere by MPCVD

Nanocrystalline diamond coatings were deposited by MPCVD on the spheres used for a ball bearing. The nanocrystalline coatings with a grain size of 50 nm were confirmed by the surface morphology and composition analysis. The hardness of the coating is 20–40 GPa tested by nanoindentation, which is higher than that of tungsten carbide and silicon nitride substrates. The coating around the sphere observed from the Micro CT images is uniform with a thickness of 12 μm.

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.

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.

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.

Contact Stress Analysis of NCD Coating on Roll Ball of Ball Bearing

Chemical vapor deposition nanocrystalline diamond (NCD) film has numerous unsurpassed properties, among them, bulk modulus, hardness; abrasion resistance and thermal conductivity are notable ones like polycrystalline CVD diamond film. The super mechanical properties have made NCD film to be an effective way to improve the resistance performance of the ceramic bearing and extended its life. The contact stress of bearing ball with NCD coating in elastic contact was analyzed. Factors such as substrate material, thickness of NCD coating, load and interlayer material which affect the shear stress distribution of NCD coating on bearing ball were investigated. The results show that substrate with low elastic modulus and heavy load will enlarge the difference of equivalent stress value at NCD/substrate interface. Thick film will make the stress distribution at the interface uniform. Soft interlayer should be inhibited to avoid extra stress in contact status.

The failure mechanism of chromium as the interlayer to enhance the adhesion of nanocrystalline diamond coatings on cemented carbide

Chromium as an interlayer has been developed to enhance the adhesion of nanocrystalline diamond (NCD) coatings to cemented carbide cutting tool materials. This study showed that Cr did not play the expected role and that the NCD coating delaminated during the Rockwell C indentation test. A systematic study of the morphologies, composition, and phase components of the failed samples showed that although Cr prevented the outward diffusion of Co to the diamond interface, the chromium carbides formed during the NCD deposition contained microcracks, which provided a path for delamination in the presence of high tensile thermal stresses generated when cooling the coating to room temperature. This work provides additional criteria to choose an appropriate interlayer to enhance the adhesion of diamond coatings to cutting tool materials.

Cytotoxicity evaluation of nanocrystalline diamond coatings by fibroblast cell cultures

The cytotoxicity profile of nanocrystalline diamond (NCD) coatings on a Si3N4 ceramic was investigated. This material is envisaged to have biomedical dental applications such as burrs and surgical instruments. Two fibroblast cell culture systems were used to address the cytotoxicity of NCD-coated samples: L929 cells (a mouse permanent cell line) and human gingival fibroblasts. Cell behavior was evaluated in terms of cell adhesion, cell viability/proliferation (mitochondrial function, MTT assay) and the pattern of cell growth. Fibroblast cell behavior on standard polystyrene culture plates was used as control, as Si3N4 substrates have previously been shown to be biocompatible. NCD coatings provided a suitable surface for cell attachment, spreading and proliferation. Human gingival cells showed a homogeneous cytoplasm spreading, a flattened elongated morphology and a typical parallel alignment on confluent cultures. In comparison, L929 cells denoted a lower cytoplasm expansion, a heterogeneous spreading but a higher proliferation rate. For both cells, after few days, the NCD coating was completely covered with continuous cell layers. As compared to standard polystyrene culture plates, no deleterious or cytotoxic responses were observed with L929 and human fibroblast cell cultures, and in both a slight enhancement in cell proliferation was observed. In addition, the seeded NCD film allowed reproduction of the typical features of the two cell culture systems tested, further suggesting the lack of cytotoxicity of this coating.

Microstructural and biological properties of nanocrystalline diamond coatings

In this study, the microstructural, mechanical, adhesion, and hemocompatibility properties of nanocrystalline diamond coatings were examined. Microwave plasma chemical vapor deposition (MPCVD) was used to deposit nanocrystalline diamond coatings on silicon (100) substrates. The coating surface consisted of faceted nodules, which exhibited a relatively wide size distribution and an average size of 60 nm. High-resolution transmission electron microscopy demonstrated that these crystals were made up of 2–4 nm rectangular crystallites. Raman spectroscopy and electron diffraction revealed that the coating contained both crystalline and amorphous phases. The microscratch adhesion study demonstrated good adhesion between the coating and the underlying substrate. Scanning electron microscopy and energy dispersive X-ray analysis revealed no crystal, fibrin, protein, or platelet aggregation on the surface of the platelet rich plasma-exposed nanocrystalline diamond coating. This study suggests that nanocrystalline diamond is a promising coating for use in cardiovascular medical devices.