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High-quality nanodiamond coatings have immense potential in cutting tools, electronic devices, and biomedicine. Introducing Ar is a crucial method of enhancing the quality of these films. This study examines the influence of Ar on the spatial distribution of CH 4 /H 2 microwave plasma parameters, group density, and the subsequent deposition of nanodiamond coatings. The electron number density, discharge volume, gas temperature and molecular mean free path increase with the increase of Ar volume fraction. In addition, the dissociation rate of H 2 increases with the increase of Ar volume fraction, which leads to more H. For the Ar-rich deposition atmosphere, the hydrocracking rate of CH 4 reduces and the hydrocracking rate of C 2 H increases, resulting in a decline in the number density of CH 3 and an increase in the amount of C 2 at the core of the discharge region. C 2 H 2 is the primary neutral hydrocarbon in the discharge, while CH 3 is the leading neutral single‑carbon hydrocarbon. The total flux of atomic hydrogen and C 2 increases with Ar, which is a key factor in improving the quality of nanocrystalline diamond crystals and enhancing renucleation. The optical emission spectroscopy of CH 4 /H 2 /Ar plasma analysis indicates an accelerated generation rate of H and C 2 with an increase in Ar volume fraction. The deposition experiments demonstrate that an increase in Ar volume fraction results in a reduction in the size of diamond grains, an augmentation in the coating coverage on the substrate surface, and an acceleration in the coating deposition rate. Meanwhile, the quality of diamond crystal is almost unaffected. • Self-consistent multiphysics modeling of microwave plasma of CH 4 /H 2 /Ar gas mixture. • Kinetics of chemical reaction in microwave plasma of CH 4 /H 2 /Ar-rich gas mixture. • Nanocrystalline diamond films with higher surface coverage, faster growth rate and finer grains were prepared by adding Ar. • The average grain size of nanocrystalline diamond coating is a linear function of the total flux of C 2 species. • The growth rate was positively correlated with substrate temperature, rather than total species flux. [ABSTRACT FROM AUTHOR]

• A self-consistent multi-physics coupling model including microwave electromagnetic field, plasma field and fluid heat transfer field is established. • CH 4 -H 2 plasma characteristics from 18 to 100 W∙cm−3. • Kinetics of CH 4 -H 2 plasma chemical reaction at high average power density. • High-rate growth of high-quality Φ 60 mm diamond films at high average power density of 100 W∙cm−3. Investigating the mechanism behind the high-efficiency deposition of high-quality diamond has always been a prominent topic in related research. This paper investigates the CH 4 -H 2 plasma characteristics and related chemical reaction kinetics in the average power density range of 18–100 W·cm−3, which is a typical range for large-area diamond film deposition. The number densities of H, CH 3 and C 2 H 2 increase linearly with the average power density. C 2 H 2 is the main hydrocarbon in the discharge, and CH 3 is the main single-carbon hydrocarbon. The number density and radial uniformity of CH 3 and C 2 H 2 increase with the average power density, which is necessary for the high rate and uniform deposition of diamond films. The plasma's optical emission spectra indicate an acceleration in the C 2 production rate at high power density. The diamond film deposition experiment demonstrates the efficacy of high average power density in achieving high-quality diamond deposition at a high growth rate. [ABSTRACT FROM AUTHOR]

Deposition of a consistent diamond coating on intricate surfaces has historically posed a difficulty. This article presents the advancement in research on depositing a uniform diamond coating on three-dimensional (3D) silicon spheres, utilizing a faraday cage within a microwave plasma chemical vapor deposition (MPCVD) reactor. A self-consistent three-dimensional multi-physics field model has been established for simulating the microwave electric field and plasma inside a reactor with or without a faraday cage. Results indicate the cage effectively reduces the discharge at the top of the silicon sphere and improves uniformity of the distribution of the microwave electric field and plasma electron density on the sphere's surface. MPCVD trials have demonstrated that a powerful discharge takes place at the apex of the silicon sphere resulting in an excessive thermal gradient if the cage is not added. The apex of the silicon sphere is covered with a blend of diamond and graphite phases. Due to the excessive internal stress of the coating, approximately 50 % of the samples experience coating detachment. After the addition of the cage, the strong discharge is transferred to the top of the cage, effectively suppressing the microwave electric field and plasma inside the cage. Improved quality, reduced internal stress, and enhanced uniformity of the polycrystalline diamond coating are achievable on the silicon spheres within the cage. Moreover, this technique offers a novel approach to coat diamond on complex 3D geometries. [Display omitted] • Three-dimensional multi-physics modeling verifies that faraday cage suppresses microwave electric field and plasma. • The Faraday cage improves the uniformity of the distribution of microwave electric fields and plasma in the internal space. • Uniform, high crystal quality, low stress diamond coating. [ABSTRACT FROM AUTHOR]

Optical diamond materials are essential for infrared optical windows, microwave windows, and laser windows due to their exceptional properties. However, dark features inside the diamond are significant factors limiting optical properties, particularly at high deposition rates. This work focuses on adjusting crystallographic parameters to prepare (110) oriented optical-grade polycrystalline diamond films, aiming to mitigate dark features. Diamond films deposited via microwave plasma chemical vapor deposition (MPCVD) exhibit discernible quality stratification. Characterization analysis of diamond film samples with varying levels of black defects reveals a correlation: films with higher dark feature content tend to exhibit a preference for the (111) orientation, accompanied by a significant presence of penetration twins and interstitial voids. Conversely, transparent diamond films demonstrate a prominent (110) orientation, featuring numerous contact twins that alleviate competition from penetration twins and original grains during growth. Electron backscatter diffraction (EBSD) and scanning electron microscope (SEM) results reveal that transparent diamond films cultivated with a pronounced preference for the (110) orientation manifest consistent and seamless contact twins, leading to fewer dark features and improved optical transmittance. In contrast, (111) oriented diamond film exhibits numerous penetration twins, and holes form between them. Based on the analysis, 5-inch optical polycrystalline diamond films with (110) orientation were achieved successfully by suppressing dark features. [ABSTRACT FROM AUTHOR]

A piece of diamond film (Ø 125 mm) was polished based on prefabricated stress. The diamond film was glued to a metal workpiece at high temperatures. The difference in the thermal expansion coefficients between the diamond film and the workpiece was exploited to prefabricate stress in the middle of the two materials. This prefabricated stress had a compressive effect on the diamond film, offset the stress generated in the grinding and polishing process, and prevented the diamond film from cracking. The feasibility of the prestressing method was verified by a combination of experiments and finite element simulations. The prestressing method was used to bond the diamond film to a 10 mm-thick brass fixture at 20, 50, 70, and 90 °C. It was found that when the temperature of the bonded diamond film reached 90 °C, the increase in the prefabricated stress was −410.71 MPa. The finite element simulation results were found to be consistent with the experimental data. The diamond film was cooled down to room temperature after bonding at high temperatures to simulate the prefabricated stress generation process. The diamond film with the prefabricated stress was again heated to the bonding temperature to simulate the stress change caused by the frictional heat generated during the polishing process. The stress of the bonded sample increased when it was cooled down to room temperature. The stress on the diamond surface became smaller during the polishing process, and the stress almost disappeared with the rise in bonding temperature, verifying the viability of the prestressing method. [Display omitted] • Prestressing method to polish a large-sized diamond film was proposed. • Diamond film was bonded to the metal fixture at high temperature and cooled. • The prefabricate stress was measured and simulated. • A Ø125 mm diamond film was successfully polished using this idea. [ABSTRACT FROM AUTHOR]

In this study, the effect of defects on fracture toughness in different directions was investigated. Two diamond films with a diameter of 122 mm and as-deposited thicknesses of 2.2 and 1.5 mm, respectively, were deposited by DC arc plasma jet chemical vapor deposition. To accurately obtain the fracture toughness, the films were ground and polished to 1.6 and 0.8 mm, respectively. The results indicate that many defects, including pores, are introduced into the films during the growth process, particularly on the side close to the growth surface in thicker film. The size of pores reaches micrometres, which affects the fracture toughness under loading in different directions. Due to the minimum grain size, both films exhibit a maximum toughness of 8.2 and 10.0 MPa·m1/2, respectively, with growth-surface cracks. For the thinner sample, the maximum value is followed by that of the edge-surface cracks. As the grain size of the edge surface falls between that of the growth surface and that of the nucleation surface, the results indicate that fracture toughness is affected by grain size. However, the number of pores near the growth side increases when the thickness exceeds 0.8 mm. Pores reduce the toughness, resulting in the minimum fracture toughness (6 MPa·m1/2) of the thicker diamond film with edge-surface laser cracks. The sample with an edge-surface sharp pre-crack has a similar fracture toughness of 5.6 MPa·m1/2. This study provides guidance for selecting the applied load direction. • Two thick diamond films with thicknesses 2.18 and 1.48 mm were deposited. • Fracture toughnesses of the diamond films in three directions were investigated. • Numerous pores were formed in the diamond films during orientation growth. • Due to the effect of pores, the toughness values among three directions differ. [ABSTRACT FROM AUTHOR]