Nanocrystalline Diamond Coatings Research Articles

High-temperature tribological properties and wear mechanisms of multilayered cubic-BN/nanocrystalline diamond tool coatings

Wear failure of cutting inserts is the main form of failure that occurs when cutting difficult-to-machine materials at high speeds. To obtain cutting inserts with outstanding tribological properties, multilayered cubic boron nitride (cBN) and nanocrystalline diamond (NCD) coatings (cBN/NCD) were prepared on WC-6 wt% Co tool substrates. The tribological properties of the multilayered cBN/NCD coatings were studied at different temperatures, and the results showed that the coefficient of friction (COF) and wear rate of the coatings were closely interrelated to the temperatures, and the COF gradually decrease and the wear rate gradually increase as the temperature increased from room temperature (25 °C) to 600 °C. The COF and wear rate of the multilayer coatings at 600 °C were ∼0.05 and 8.43 × 10−6 mm3/(N⋅m), respectively. The wear resistance of the cBN/NCD coatings was higher than that of TiAlN coatings and PcBN tool materials by more than 3 and 4.5 times, respectively. The high-temperature wear failure modes of the multilayered cBN/NCD coatings were abrasive wear, diamond graphitization, and BN hydrolysis. Coating wear accelerated when pores were formed on the cBN surface of a multilayered cBN/NCD coating due to the low nucleation density of diamond. These results would guide the application of cBN/NCD-coated cutting tools in high-speed cutting processes.

Tribological properties of HFCVD diamond coatings over wide temperature range

Single and gradient multilayered diamond coatings, deposited on silicon nitride (Si3N4), were prepared by hot filament chemical vapour deposition. The tribological properties were evaluated in a wide temperature range (26 °C–300 °C), against Si3N4 ceramic balls, as the friction counterpart. The results showed that with the increase of temperature from 26 °C to 300 °C, the wear rate of NCD (nanocrystalline diamond) and GCD (gradient diamond) coatings rapidly increases. When the temperature reaches 300 °C, obvious cracks and detachment appear on the surface of the NCD coatings. Due to the special interlayer, the wear rate of the GCD coatings is lower than that of the NCD coatings. In the range of 26 °C to 100 °C, GCD exhibits excellent wear resistance and low friction coefficient due to its special gradient structure. When the temperature increases to 200 °C, NCD films with smaller grain sizes exhibit higher wear rates. From the wear morphology, it can be seen that cracks are generated in the NCD film, and as the temperature increases, the cracks continue to expand until the film spalling. GCD shows relatively good wear resistance at high temperatures. This study found that gradient multilayered diamond coatings exhibited superior wear resistance at high temperatures, compared with single-layer coatings tested in identical conditions.

Influence of Surface Roughness on Nanocrystalline Diamond Films Deposited by Distributed Antenna Array Microwave System on TA6V Substrates

In this study, the characteristics of nanocrystalline diamond films synthesized at low surface temperature on Ti-6Al-4V (TA6V) substrates using a distributed antenna array microwave reactor aiming at biomedical applications were investigated. The surface roughness of the TA6V substrates is varied by scratching with emery paper of 1200, 2400, 4000 polishing grit. Nanocrystalline diamond (NCD) coatings with morphology, purity, and microstructure comparable to those obtained on silicon substrates usually employed in the same reactor and growth conditions are successfully achieved whatever the polishing protocol. However, the latter has a significant effect on the roughness parameters and hardness of the NCD films. The use of the finest polishing grit thus permits us to enhance the hardness value, which can be related to the work-hardening phenomenon arising from the polishing process.

Various HFCVD diamond coatings synergistically tuned using CH4 gas flow and working pressure and key merit evaluation of their coated tools

Diamond-coated tools can meet the service requirements of high precision, high efficiency and long life with multi-element coupling, but the milling characteristics of graphite materials with high spindle speed, feed speed and high force may require tools with a high number of micro-edges and good adhesion strength. In the present study, microcrystalline diamond (MCD), nanocrystalline diamond (NCD) and gradient composite diamond (GCD) coatings were deposited on commercial end mills subjected to distinct CH4 gas flows and working pressure by hot filament chemical vapor deposition. The adhesion strength, erosion resistance and milling performance of the diamond coatings in milling graphite materials were evaluated. Besides, the effect of CH4 gas flows and working pressure on the growth model of diamond coatings and the mechanism of crack propagation when subjected to stress concentration were studied. The results show that the combination of high CH4 gas flows and low working pressure achieves the re-nucleation of diamond particles, the smaller grain, more graphite/amorphous carbon in the boundaries and larger residual stresses lead to continuous crack propagation, causing poor adhesion strength, erosion resistance and milling performance in NCD and GCD coatings. Conversely, the low CH4 gas flows to ensure the epitaxial growth required for MCD coating, the larger grain, higher diamond purity, columnar structure and smaller residual stresses allow the pinning effect between coating and substrate, altering and increasing the path of crack propagation, making it suitable for milling conditions with high speed and high force. This study provides significant insights into the links between diamond morphology, structure, grain size, boundaries, and phase composition and the mechanical properties of adhesion strength, erosion resistance, and milling properties by synergistically tuning or alternating the CH4 gas flows and working pressure.

Application of Nano-Crystalline Diamond in Tribology.

Nano-crystalline diamond has been extensively researched and applied in the fields of tribology, optics, quantum information and biomedicine. In virtue of its hardness, the highest in natural materials, diamond outperforms the other materials in terms of wear resistance. Compared to traditional single-crystalline and poly-crystalline diamonds, nano-crystalline diamond consists of disordered grains and thus possesses good toughness and self-sharpening. These merits render nano-crystalline diamonds to have great potential in tribology. Moreover, the re-nucleation of nano-crystalline diamond during preparation is beneficial to decreasing surface roughness due to its ultrafine grain size. Nano-crystalline diamond coatings can have a friction coefficient as low as single-crystal diamonds. This article briefly introduces the approaches to preparing nano-crystalline diamond materials and summarizes their applications in the field of tribology. Firstly, nano-crystalline diamond powders can be used as additives in both oil- and water-based lubricants to significantly enhance their anti-wear property. Nano-crystalline diamond coatings can also act as self-lubricating films when they are deposited on different substrates, exhibiting excellent performance in friction reduction and wear resistance. In addition, the research works related to the tribological applications of nano-crystalline diamond composites have also been reviewed in this paper.

The influence of droplet-based seeding of nanodiamond particles on the morphological, optical, and mechanical properties of diamond coatings on glass

This work investigates four different methods for the seeding of nanodiamond (ND) particles towards applying nanocrystalline diamond coatings on glass. The standard spin coating and dip coating are investigated together with two droplet-based techniques, ink-jet printing and ultrasonic spray coating (USSC). The application of these last two techniques for diamond coatings on glass is novel and is specifically designed for patterning and large area deposition, respectively. Comparing the seeding processes, their impact on the chemical vapor deposited nanocrystalline diamond (NCD) layers is unclear. Here, particularly the impact on the morphological, optical, and mechanical properties, are studied. In this work, borosilicate glass substrates were seeded, after which diamond layers were grown in a linear antenna microwave plasma-enhanced chemical vapor deposition reactor to thicknesses of 45, 100, and 500 nm. The coatings showed a roughness that is 2 to 3 times higher for the droplet-based seeding techniques compared to spin coating. It was found that the optical absorption coefficient for 100 nm thick samples prepared using USSC was 2400 cm−1 at the wavelength of 550 nm, compared to 1500 cm- 1 using spin and dip seeding techniques. The stress in the film was compressive as determined by curvature measurements. Sand-trickling tests on samples coated with 100 nm thick diamond films showed similar resistance against damaging for all seeding techniques without delamination. Therefore, the different techniques achieve mechanically competitive performance. On top USSC and ink-jet printing have the opportunity to fully cover substrates on large-scale flat and even 3D surfaces, or pattern fine structures, which is not possible for standard spin and dip coating. It can be concluded that diamond coatings on large glass plates for scratch resistance, and sensing due their somewhat higher roughness, are within reach using these innovative droplet-based seeding technologies.

Key Challenges in Diamond Coating of Titanium Implants: Current Status and Future Prospects.

Over past years, the fabrication of Ti-based permanent implants for fracture fixation, joint replacement and bone or tooth substitution, has become a routine task. However, it has been found that some degradation phenomena occurring on the Ti surface limits the life or the efficiency of the artificial constructs. The task of avoiding such adverse effects, to prevent microbial colonization and to accelerate osteointegration, is being faced by a variety of approaches in order to adapt Ti surfaces to the needs of osseous tissues. Among the large set of biocompatible materials proposed as an interface between Ti and the hosting tissue, diamond has been proven to offer bioactive and mechanical properties able to match the specific requirements of osteoblasts. Advances in material science and implant engineering are now enabling us to produce micro- or nano-crystalline diamond coatings on a variety of differently shaped Ti constructs. The aim of this paper is to provide an overview of the research currently ongoing in the field of diamond-coated orthopedic Ti implants and to examine the evolution of the concepts that are accelerating the full transition of such technology from the laboratory to clinical applications.

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.

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.

Performance evaluation of DLC and NCD coatings in micro-milling of Al7075-T6 alloy

The main problem in machining of aluminum alloys is the adhesion of chips to the cutting tool. In micro milling, chip adhesion causes a decrease in surface quality and contributes to micro burr formation. There are several ways to minimize chip adhesion, one of which is to coat the tool with the appropriate material. Carbon-based coatings are coating materials that can minimize chip adhesion. In this study, the performances of Diamond-Like Carbon (DLC) and Nano-Crystalline Diamond (NCD) coating materials in the micro-milling process of Al7075-T6 alloy were investigated in all aspects. The tests are performed under dry cutting conditions using various cutting parameters. Experimental results show that DLC and NCD coated tools produce approximately 20 % and 40 % lower forces, respectively. For fz ≤ 0.5 μm/tooth, no decisive influence of the coating material on the cutting forces is observed. For fz ≤ 0.5 μm/tooth, surface roughness and burr width increase significantly in uncoated and NCD coated tools due to size effect. A strong size effect was observed at feed values less than approximately 36 % of the tool edge radius in terms of surface roughness and <18 % of the tool edge radius in terms of burr width.

Diamond-Coated Silicon ATR Elements for Process Analytics

Infrared attenuated total reflection (ATR) spectroscopy is a common laboratory technique for the analysis of highly absorbing liquids or solid samples. However, ATR spectroscopy is rarely found in industrial processes, where inline measurement, continuous operation, and minimal maintenance are important issues. Most materials for mid-infrared (MIR) spectroscopy and specifically for ATR elements do not have either high enough infrared transmission or sufficient mechanical and chemical stability to be exposed to process fluids, abrasive components, and aggressive cleaning agents. Sapphire is the usual choice for infrared wavelengths below 5 µm, and beyond that, only diamond is an established material. The use of diamond coatings on other ATR materials such as silicon will increase the stability of the sensor and will enable the use of larger ATR elements with increased sensitivity at lower cost for wavelengths above 5 µm. Theoretical and experimental investigations of the dependence of ATR absorbances on the incidence angle and thickness of nanocrystalline diamond (NCD) coatings on silicon were performed. By optimizing the coating thickness, a substantial amplification of the ATR absorbance can be achieved compared to an uncoated silicon element. Using a compact FTIR instrument, ATR spectra of water, acetonitrile, and propylene carbonate were measured with planar ATR elements made of coated and uncoated silicon. Compared to sapphire, the long wavelength extreme of the spectral range is extended to approximately 8 μm. With effectively nine ATR reflections, the sensitivity is expected to exceed the performance of typical diamond tip probes.

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.

Study on the effect of Ar-containing work gas on the microstructure and tribological behavior of nanocrystalline diamond coatings

Using a combined pretreatment of sand-blasting and two-step etching, nanocrystalline diamond coatings with primarily diamond crystallites and good adhesion were deposited onto carbide cement by tuning the Ar/H2 ratio in the hot filament chemical vapor deposition (HFCVD) work gas. The microstructure, bonding structure, surface morphology and roughness, and wear scar were characterized using a scanning electron microscope, an atomic force microscope, a white light interferometer, and a Raman spectrometer. The dry sliding behavior of the coating against Si3N4 was tested using a pin-on-disc instrument. It was found that the addition of Ar to the HFCVD work gas can enhance grain refinement and tune the morphology of the nanocrystalline coatings. The roughness was reduced from 87.7 nm for the coating prepared without Ar to 6.21 nm for the coating prepared with 90% Ar. With the addition of Ar, the friction coefficient and wear rate decrease monotonically. The wear rate of the coating prepared with 90% Ar was less than 5% that of the coating prepared without Ar, and the wear mechanism transforms from spallation to be limited by the stress-induced sp2 tribo-layer.

Improvement of the Interfacial Fatigue Strength and Milling Behavior of Diamond Coated Tools via Appropriate Annealing

This article deals with the potential to reduce the amount of the residual stresses in the diamond films on cemented carbide inserts for improving their effective interfacial fatigue strength and thus their wear resistance. In this context, nano-crystalline diamond coatings (NCD) were deposited on cemented carbide inserts. A portion of these coated tools were annealed in vacuum for decreasing the amount of residual stresses in the film structure. The annealing temperature was appropriately selected for keeping the substrate strength properties invariable after the coating annealing. Inclined impact tests at ambient temperature on the untreated and heat-treated diamond coated tools were conducted for evaluating their effective interfacial fatigue strength. Depending upon the impact load, after a certain number of impacts, damages in the film-substrate interface develop, resulting in coating detachment and lifting. Via appropriate FEM (Finite Element Method)-evaluation of the impact imprints, the residual stresses in the diamond film structure were determined. Milling experiments were conducted for evaluating the cutting performance of the coated tools using aluminum foam as workpiece material. A correlation between the interfacial fatigue strength of diamond coatings and their residual stresses affected by annealings contributed to the explanation of the attained cutting results.

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.

Effect of the crystallinity of NCD and MLD diamond coatings characterized by same level of residual stresses after annealing on their fatigue strength and wear behavior in milling

Nano-composite (NCD) and multi-layered (MLD) diamond coatings were deposited on cemented carbide tools using hot filament chemical vapour deposition (HFCVD) techniques. Appropriate annealings were conducted on the examined diamond coatings in order to be characterized by the same level of residual stresses. The crystalline structure of the employed diamond coatings was investigated by conducting Raman spectra. Inclined impact tests at ambient and elevated temperatures were carried out for assessing their temperature-dependent fatigue strength. Moreover, the wear behaviour of diamond coated inserts was investigating in milling aluminum foam. Raman spectra were also conducted on the treated diamond coatings for capturing potential crystalline changes developed due to the exercised thermal and dynamic mechanical loads during cutting. According to the attained results, the coexistence of sp2– and sp3-bonded phases in the cases of MLD diamond coatings results in an accelerated wear development, despite their structure capability to decelerate the crack propagation. As a result, nano-crystalline diamond coatings characterized only by sp3-bonded phase exhibit an improved wear behaviour. The cutting performance of the NCD coated inserts is further improved due to the enhanced tribological properties of the NCD coatings.

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.

Design and fabrication of HfC, SiC/HfC and HfC-SiC/HfC interlayers for improving the adhesion between diamond coatings and cemented carbides

Three types of interlayers named Hafnium carbide (HfC) single-interlayer, SiC/HfC double-interlayer, and HfC-SiC/HfC double-interlayer were designed and fabricated on cemented carbide (WC–Co) to seek out the optimal coating structure for well-adhered diamond coatings. The phase composition, microstructure, adhesion and effectiveness in diamond coating adhesion of these interlayers were investigated and compared. The results showed that the HfC single-interlayer was a HfC-dominant nanostructured layer composed of HfC, Si, and SiC. The SiC/HfC and the HfC-SiC/HfC double-interlayers had an inner layer with a structure similar to the HfC single-interlayer. However, the former had a SiC-dominant outer layer, while the latter had a gradient HfC-SiC mixed outer layer. The double-interlayers possessed better adhesion and higher hardness than those of the HfC single-interlayer. This could be attributed to the introduction of SiC in the outer layer, which endowed the substrates with significantly enhanced microhardness and decreased TEC, closer to those of diamond coatings. After the subsequent CVD process, dense nanocrystalline diamond coatings with a thickness of ∼5.0 μm were deposited onto the double-interlayered substrates. The coatings possessed good adhesion. Between them, the diamond coating on the HfC-SiC/HfC double-interlayered substrate had better adhesion (HF1∼HF2) and lower residual stress (−1.7 GPa) than those of the coating on the SiC/HfC double-interlayered substrate. These experimental results provide a feasible way for fabrication of well-adhered diamond coating on WC-Co tools.

Tribological Behavior of Diamond Coatings against Carbon Fiber Reinforced Plastics under Dry Conditions

The frictional resistance and machining quality when cutting carbon fiber reinforced plastics (CFRP) laminates are associated with tribological behavior of tool materials. In the present study, the tribological properties of three types of monolayer microcrystalline diamond (MCD) coatings, nanocrystalline diamond (NCD) coatings and dual-layer MCD/NCD coatings sliding against CFRP are investigated under dry lubricated conditions using the rotational friction tester. The coefficients of friction (COF), wear rate and worn surfaces of the contacted surfaces are analyzed for the MCD-CFRP, NCD-CFRP and MCD/NCD-CFRP contacting pairs. The results show that compared with the monolayer MCD and NCD, the bilayer of MCD/NCD coating displays the lowest COF with the value of ~0.13, it is 42% and 55% of the values for MCD and NCD coatings. Due to the rough surfaces of MCD, the wear debris of CFRP on MCD samples exhibits the plowing effect. While for the NCD and MCD/NCD samples, the wear fragments display the planar shapes. The wear rate of CFRP against MCD is more than twice that of CFRP against NCD, due to the excellent loading capacity. While the wear rate of CFRP against MCD/NCD is about twice than that of CFRP-NCD pairs. The bilayer of MCD/NCD combines the excellent advantages of high hardness of MCD and the smooth surface of NCD. It shows the broad application potential for the bilayer coatings.