Diamond Coatings Research Articles

Micro-grit blasting to enhance adhesion of diamond coating on Ti6Al4V

Diamond coatings are of industrial importance due to their tribological properties like wear resistance and low friction. These properties can be exploited only when the coating has good adhesion to the substrate. Substrates such as titanium alloys exhibits poor adhesion due to differences in the coefficient of thermal expansion (CTE) between the diamond coating and the substrate. This study investigates the enhancement of diamond coating adhesion to substrate by micro-grit blasting. The effect of micro-grit blasting pressure and angle of blasting are systematically studied. The micro-grit blasted surface is characterised by the roughness parameters such as skewness and kurtosis. Diamond coating is carried out on the grit-blasted substrates using the hot filament chemical vapour deposition (HFCVD) method and then characterised using electron microscopy, Raman spectroscopy, X-ray diffraction, and scratch testing. The skewness was seen to be negative, and did not vary much with grit-blasting conditions. A high value of residual stress has been observed in all the samples. Pressure was inversely correlated with kurtosis and is one of the dominant factors influencing kurtosis variation. Surfaces with lower kurtosis value were found to have higher adhesion strength. The effect of angle on adhesion depends on the relative value of skewness at the given angle. Adhesion strength is proportional to the skewness in the case of angle variation. The crystal size of the diamond grown for both parameters (angle & pressure) initially increases and then decreases depending on the surface free energy. Higher the surface free energy lower will be the average crystal size.

Numerical study of thermal shock on infrared windows and their composites with diamond coatings under harsh conditions

Infrared windows such as sapphire, aluminum oxynitride (AlON) and zinc sulfide (ZnS) have excellent optical properties, but restricted for application due to low thermal conductivity (<30 W/m °C). Diamond could provide a solution for overcoming the problems due to ultrahigh thermal conductivity (~2000 W/m °C). In this paper, a finite element simulation software (ANSYS) was used to simulate the thermal shock behavior of the infrared materials and their composite with diamond coatings (0–0.04 mm in thickness). The performance in an extremely hot environment was simulated by analyzing the thermal distribution, temperature uniformity, and central stagnation temperature. A constant heat flow of 0.5 MW/m2 was applied to investigate the influence of coating thickness on thermal transfer. The results showed that the central stagnation temperatures of the single sapphire and the corresponding composite windows were the lowest. Temperature difference of the windows was mainly determined by the intrinsic thermal parameters, which became narrow with the increase of diamond thickness. Thermal stress and deformation of diamond films and the substrates were calculated and the results indicated that the diamond film can significantly improve the structural stability of these windows. For single diamond windows (1–5 mm in thickness), ultralow central stagnation temperatures of ~86–226 °C were obtained even at ultrahigh heat flux of 2.0 MW/m2. Diamond film could greatly help to decreasing central temperature and improving the thermal uniformity. Related research could provide theoretical reference for the design and application of the “heat free” infrared window.

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.

Low-volume-loss surface polishing with a krypton fluoride excimer laser for polycrystalline diamond films

Surface polishing of hard diamond coatings is a key technique for applying such coatings in industry. In this study, we demonstrated surface modification of polycrystalline diamond films with minimal volume loss of the films by vertical irradiation of KrF excimer laser beams. Optimized laser scanning selectively removed surface asperities and reduced the surface roughness from ∼0.1 to ∼0.05 μm. Raman spectroscopic measurements revealed that laser polishing involves the phase transformation of diamond to amorphous carbon phases and thermal oxidation or evaporation of the amorphous phases. The residual amorphous carbon was almost completely removed by subsequent laser irradiation at appropriate fluences. We consider that the surface texture inherent to polycrystalline diamond films plays an important role in the concentration of laser beams on the nearby film surface. From reciprocating slide-type friction tests, we found that laser polishing decreased the initial friction coefficient from 0.5 to 0.1–0.3 against alumina balls and reduced the abrasion of the mating materials.

Cutting Performance and Wear Mechanism of Multilayer Diamond‐Coated Tools

Monolayer and multilayer diamond films are deposited on WC‐Co cemented carbide by hot‐filament chemical vapor deposition. The growth characteristics of diamond coatings are analyzed. Cutting performance characteristics such as tool life and the stability of machining process in the machining of presintered ZrO2 are compared based on the variation of cutting speed and resultant cutting force, and workpiece surface roughness. For the monolayer diamond coatings, as the concentration of CH4 increases from 1% to 5%, the diamond crystal is transformed from micron columnar crystal to nanocluster crystal. The multilayer diamond coatings combine the advantages of micron‐ and nanocrystalline structures. The multilayer diamond‐coated tool exhibits longer service life and better machining quality. Because of the appearance of the brittle–plastic conversion mechanism, the surface integrity of ZrO2 processed by multilayer diamond‐coated tool is relatively high. As for the uncoated tool, the workpiece is mainly machined by brittle spalling. The interfacial stratified fracture system between the interlayers is proposed to be the toughening mechanism of the multilayer structure.

Optimization of Crystalline Diamond Coating Structure Architecture for Improving Adhesion and Cutting Performance in Milling with Cemented Carbide Inserts

The adhesion, structure architecture, and residual stresses of crystalline diamond coatings (CDCs) on cemented carbide inserts are the factors that significantly affect tool life. The influence of these factors on cutting performance cannot be investigated separately since interactions among them exist. The paper elucidates such dependencies to optimize the CDC architecture and improve cutting performance. In this context, diamond coatings possessing different architectures were deposited on cemented carbide tools. The fatigue endurance and the milling performance of the coated tools were investigated using impact and milling tests, respectively. The residual stresses in the film structures were determined through impact tests and appropriate (Finite Element Analysis) FEA evaluation of the corresponding results. According to the obtained results, the application of a bottom micro-structured CDC prior to the deposition of an upper nanolayered one with inferior thickness improves the coated tools’ cutting performance. An optimum coating architecture is associated with a thickness ratio between the micro-structured bonding to the upper nanolayered CDCs of 2/1. Hereupon, the augmentation of coated tool life via the application of an optimum diamond coating architecture compensates for the high tool cost and improves milling productivity. The latter is further enhanced as the number of tool replacements decreases.

Evolution of the Growth of a Micro-Nano Crystalline Diamond Film on an Axial Carbide Tool Model in Microwave Plasma

Conformal multilayer micro-nanocrystalline diamond coatings were grown on substrates of a hard alloy with 9% Co with a high aspect number in microwave plasma from gas mixtures CH4/H2 and CH4/H2/N2. The substrates were of a cylindrical axial tool model with a size ratio: d = 12 mm to l = 75 mm. An original tool holder made of molybdenum, in the form of a sector of the excessive ring with the axis of the hole parallel to the central conductive platform, protects part of the substrate from heating due to the edge effect of plasma. The uniformity of heating of the growth part, which is located inside the excessive ring, is calculated using mathematical modeling and is provided by rotation at a speed of at least 12 rpm, which ensures the uniformity of the coating. The average grain size of the nanocrystalline film measured along the cylinder forming was 41 nm.

MODELING OF THE SUBSTRATE COOLING SYSTEM IN THE INSTALLATION FOR THE DEPOSITION OF COATINGS BY THE GAS PLASMA METHOD

The efficiency of diamond coating synthesis depends on both the parameters of the plasma flow and the uniform temperature distribution on the surface of the substrate on which the coating is synthesized. Mathematical modeling of the substrate cooling system in the installation for the deposition of coatings by the gas plasma method was carried out in order to find optimal parameters at which high density and radial uniformity of energy and chemically active particle flows are simultaneously achieved on the substrate in the process of synthesis of diamond coatings. The task was solved by direct search methods using the FlowSimulation module of the SolidWorks package.

Erosive wear characteristics and damage model of polycrystalline diamond compact for enhancing mud pulser rotor

The rotor of the mud pulser will suffer severe erosive wear due to long-term scouring by solid particles in drilling mud. An important way to solve this problem is to improve the erosion resistance of the rotor. Polycrystalline diamond compact (PDC) has excellent mechanical properties. To explore the erosive wear characteristics of the PDC for enhancing mud pulser rotor, the influence of the impact angle (30°, 45°, 60°, 75°, and 90°) and diamond grain size (4 μm, 10 μm, 20 μm, and 30 μm) on the erosion rate of the PDC was investigated. The results show that the PDC erosion rate is positively correlated with the impact angle and diamond grain size. When the impact angle is 90° and the diamond grain size is 4 μm, the PDC erosion rate is the smallest, 0.81%, and when the diamond grain size is 30 μm, the PDC erosion rate is the largest, 2.12%. The erosive wear characteristics of the PDC are cobalt removal and diamond grain cutting at a low impact angle. At high-impact angles, there is a micro-fracture of the edge of the diamond grains and an intersection of transverse and radial cracks. The fine diamond in PDC is removed, and the coarse diamond in PDC seems to have a trans-granular fracture. An innovative cutting-crack damage model considering the diamond grain sizes and impact angles is proposed based on the erosion damage mechanism, which can better predict the evolving law of the PDC erosion rate. The results provide data support and theoretical guidance for the application of PDCs to enhance erosion resistance rotors.

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.

Effect of the interfacial microstructure on hardness of multi-layer diamond coatings

Multi-layer diamond coatings have been widely employed in cutting tools due to their superior mechanical performance, which mainly stems from their complex multi-layer structures and versatile interfaces. However, it is still lack of investigation on how these interfaces may be tuned to enhance the mechanical properties. Herein, the multi-layer diamond coatings with different modulation periods (ʌ = 1.70, 0.87, 0.29 μm), containing microcrystalline diamond (MCD), sub-microcrystalline diamond (SMCD) and nanocrystalline diamond (NCD) as sub-layers, were synthetized by hot filament chemical vapor deposition (HFCVD). The interfacial microstructures were characterized by scanning transmission electron microscopy (STEM). The hardness (H) and Young's modulus (E) of the coatings were evaluated by nanoindentation tests with two different modes. The planar defects including stacking faults (SFs) and twinning boundaries (TBs) are observed at both MCD/SMCD and NCD/MCD interfaces but absent at SMCD/NCD interfaces. Owing to more interfaces, SFs and TBs as barriers inhibiting the dislocation motion effectively, the H and E of the coatings are enhanced with the decrease of ʌ. Hence, the coating with ʌ of 0.29 μm, possessing more SFs, TBs and more sharp interfaces, obtains the highest hardness (78.39 ± 1.10 GPa). In summary, the mechanical properties of the multi-layer diamond coatings can be greatly improved by the elaborately designed interfacial microstructure, which is of great significance to promote the far-ranging application of diamond coatings.

Ultrananocrystaline diamond (UNCD™) coatings for new generations high-tech/ medical devices/prostheses

Ultrananocrystaline diamond (UNCD™) coatings for new generations high-tech/ medical devices/prostheses This article provides a summary of materials science, materials integration strategies, materials properties, and design and development of new generations of industrial products, high-tech and external/implantable medical devices/prostheses, and treatment of medical conditions, based on the unique biocompatible Ultrananocrystalline Diamond (UNCD™) coating co-developed by Auciello and colleagues, and key oxide films and nanoparticles, to improve the quality of life of people worldwide. The target audience for this article includes undergraduate/graduate students, postdocs, scientists and engineers in Academia and Industry in different fields of science and technology, including materials scientists, mechanical engineers, bioengineers, applied physicists/chemists, medical device designers/manufacturers, and medical doctors/surgeons, who, by knowing more about biomaterials and devices they are implanting in people, may make better decisions when selecting a device based on the appropriate material for implantation.

Frequently weak bleeding from bone marrow that was exposed by drilling of the cortical bone occurs. This kind of bleeding can be easily stopped by further low speed drilling with a fine diamond burr.

Frequently weak bleeding from bone marrow that was exposed by drilling of the cortical bone occurs. This kind of bleeding can be easily stopped by further low speed drilling with a fine diamond burr.

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.

Optimization and evaluation of a three-step pretreatment process for PCB microdrills

The traditional Murakami two-step and three-step acid pretreatment processes were introduced to reduce the negative impact of Co on the cemented carbide tools during deposition of the diamond coatings. However, treatment methods requiring a longer etching time and a higher etching solution concentration lead to the formation of pores and cavities on the surface of the substrate, which would damage the substrate and thus significantly decrease the fracture load of the tools, especially microdrills with a small edge diameter. In this study, the pretreatment process of microdrills is optimized based on the traditional three-step process. The surface morphologies and Co content of the microdrills after pretreatment are investigated via scanning electron microscopy and energy-dispersive spectroscopy. The effect of the pretreatment on the fracture toughness of the microdrills and the adhesion of the diamond coatings is evaluated. The results show that the surface microstructure and fracture load of the microdrills are inevitably damaged during the acid and Murakami etching processes. Furthermore, the adhesion of the diamond coating depends on the final step of the acid etching process. The optimized three-step process consists of first etching with an acid solution for 10 s, then etching with a Murakami solution for 4 min, and finally etching with the acid solution for 30 s. This process improves the pretreatment efficiency and coating adhesion without significantly reducing the fracture load of the microdrills.

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.

Introducing and assessing the efficacy of a novel method to reduce enamel damage after orthodontic bracket removal using two herbal-based resin colouring agents: An in vitro study

The differentiation of resin remnants from enamel is a critical factor to minimize enamel damage after bracket debonding. This study was conducted to produce, and ascertain the efficacy of two colouring agents in minimizing enamel loss, adhesive and bonding remnants, and surface roughness after debonding. Two dyes containing annatto (orange colour) and curcumin (yellow colour) were produced. Seventy-two maxillary premolars were divided into three groups. After bracket bonding and debonding, the adhesive remnant was removed with a fine diamond bur. In groups 1 and 2, the orange and yellow dyes were utilized during the removal process, respectively. In group 3 (control) adhesive was removed with no colouring agent. The buccolingual dimension of the teeth was measured at the occlusal, middle, and apical areas, before bonding and after clean-up. The adhesive remnant index (ARI) and bonding remnant index (BRI) scores were recorded and the surface roughness parameters were measured. Data were analysed by ANOVA, Tukey, and Fisher's exact tests (α=0.05). Enamel loss was significantly lower in the groups cleaned by the use of colouring agents than that of the control group (P<0.05). No bonding agent was observed when the dyes were used, whereas 65% of teeth in the control group showed the remaining bonding material (P<0.001). There was no significant difference in ARI scores or surface roughness alterations among the study groups (P>0.05). The two dyes produced in this study were effective in enhancing the visibility of residual resin materials and minimizing enamel loss during the clean-up process.

Scratch adhesion evaluation of diamond like carbon coatings with alternate hard and soft multilayers

A series of four diamond like carbon coatings comprising alternating soft and hard layers was prepared by closed field unbalanced magnetron sputtering. A constant bias voltage of −40 V was used for soft layer deposition, while the hard layers were deposited at different bias voltages for different coatings in series. A uniform hard-to-soft layer thickness ratio of approximately 1:1.33 was maintained for all four coatings. Cross-sectional transmission electron microscopy of the coatings confirmed that these diamond like carbon coatings indeed exhibited distinct soft and hard layers. Moreover, increasing bias voltage during hard layer deposition also increased the residual stress in these coatings. The coatings were also subjected to scratch testing to determine the effect of increasing bias voltage on the scratch toughness. Scratch cracking mechanism changed from chevron tensile cracking at low bias voltages to buckle spallation at higher bias voltages. All the coatings exhibited substrate exposure at high normal loads beyond the upper critical load. The scratch crack propagation resistance parameter was reduced by ∼1.5 times when the bias voltage was increased to −140 V. It was demonstrated that coatings deposited at a hard layer bias voltage of −100 V demonstrated the best combination of mechanical properties, including the plastic deformation resistance (H3/E2 ratio) and elastic strain to failure (H/E ratio).

Hot-filament CVD diamond coatings for optical applications

The demand for highly scratch resistant transparent coatings is motivated by the industrial sector of consumer goods for example for the protection of displays, touch-screens, watches, and all kinds of optical instruments. In a wide variety of applications, scratch-free and highly wear-resistant surfaces are the enabling factor for improvement of lifetime and thus sustainability of the products. Diamond with its unique properties such as the highest known hardness, low thermal expansion coefficient and chemical resistance (to acids and bases) [1,2] has outstanding potential for optical applications [3]. The use of thin ultrahard diamond layers with adapted optical properties is expected to enhance the resistance against abrasive wear to a new level. At Fraunhofer IST large-area Hot-Filament Chemical Vapor Deposition (HFCVD) processes have been developed [4,5] and now utilized for depositing highly uniform and transparent diamond coatings on optical components with sizes of up to 200 mm × 200 mm. The use of single-layer diamond coatings is investigated as well as the integration of diamond films into multilayer film stacks combining anti-reflective properties with mechanical stability (strength). Four-layer anti-reflective coatings (Si3N4/SiO2/Diamond/SiO2) entirely deposited with hot-filament/hot-wire processes and their optical and mechanical performance will be presented.

Effect of doping level on residual stress, coating-substrate adhesion and wear resistance of boron-doped diamond coated tools

Boron doping can change the basic properties of diamond coatings prepared by the hot filament chemical vapor deposition (HFCVD) method, including residual stress, coating-substrate adhesion and wear resistance. However, the specific effect of boron doping level on the properties of diamond coatings and the mechanism needs further investigation. In the present study, the tungsten cemented carbide (WC-Co) material is selected as the substrate to deposit boron-doped diamond coatings with different doping levels ranging from 0 to 15,000 ppm. To elaborate the influences of doping level on the boron-doped diamond (BDD) coatings, microstructure, surface roughness, grain orientation, nanohardness, Raman spectral, Kernel Average Misorientation (KAM) and Rockwell indentation of diamond coatings are characterized, then friction tests and milling experiments are conducted on all specimens. The results indicate that the appropriate doping level of 8000 ppm can change the grain orientation to (110) orientation, guarantee diamond quality and hardness, reduce the residual stress, minimize the coefficients of friction (COF) and improve the tool life. Raman and KAM analysis demonstrate that the specimen generates suitable tensile stress in the diamond lattice and offsets the compressive stress of the coating to obtain the lowest residual stress and the highest coating-substrate adhesion strength. Friction test results illustrate that the specimen has the best wear resistance because the grain with (110) orientation has high hardness and low COF. In addition, milling experiments illustrate that the specimen has the best machining life due to the combination of good coating-substrate adhesion and wear resistance, with a tool life of 355 min, which is 29.6 % longer than the machining life of the undoped diamond coated tool.