Cutting performance of time-modulated chemical vapour deposited diamond coated tool inserts during machining graphite

Time Modulated Chemical Vapour Deposition (TMCVD) process regime has been used to deposit diamond coatings onto commercially available tungsten carbide tool inserts. The TMCVD process was developed in our laboratories so that diamond films with fine grains could be deposited. It accomplishes this by promoting secondary nucleation during larger methane flow modulations. The average surface roughness of the diamond coatings were correlated with the cutting performance of the coatings when dry machining of graphite. Inserts coated were characterised by Scanning Electron Microscopy (SEM) and Raman spectroscopy and were tested for turning performance using graphite as the workpiece material. The cutting forces were measured by the DynoWareT™ data acquisition system. Polycrystalline Diamond (PCD) inserts were also used for comparison. Repeated turning tests showed that the surface roughness of the coatings is a limiting factor when achieving better chip flow during machining.

Scribing wheel (SW) is an important tool for separating glass panels in thin-film transistor liquid crystal display industry. In this study, unlike the traditional SW completely made of polycrystalline diamond (PCD) or cemented tungsten carbide (c-WC), an alternative partially taking advantage of chemical vapor deposition diamond (CVDD) was newly developed. The fabrication of such unique sandwich-like CVDD-SW combined hot filament chemical vapor deposition (HFCVD), welding, and other machining processes. Both hard CVDD scribing edge and tough c-WC supporting layers contributed to SW structure. CVDD was prepared by adjusting the concentration of methane fed into HFCVD chamber. Morphological observation confirmed the reproducibility of microcrystal diamond (MCD), submicrocrystal diamond (SMCD), and nanocrystal diamond (NCD) diamond. Besides grain size, the existence of columnar structure, the nondiamond carbon content, the residual stress, and I(220)/I(111) ratio of CVDD films were characterized by scanning electron microscopy, Raman spectroscopy, and X-ray diffraction. Based on results, SMCD was predicted as the optimized CVDD for making a scribing edge. After three CVDD films were respectively integrated into SW, this prediction was supported by preliminary scribing test. Selecting Corning-1737 as the cutting object, among three CVDD-SWs and one self-made PCD-SW, only the scribing edge of SMCD-SW kept almost undamaged. The outperformance of our design was thus confirmed.

Tribological and mechanical properties of HFCVD diamond-coated WC-Co substrates with different Cr interlayers

Diamond is generally accepted as a material with individual properties such as superlative hardness, low coefficient of friction and very high thermal conductivity. Synthetic diamond can be achieved in the form of thin and thick films using experimental chemical vapor deposition (CVD) methods. Cemented tungsten carbide (WC–Co) is the commonly used tool material with high hardness (~18 GPa) and high elastic modulus (~550 GPa) and, also compatible to the growth of synthetic diamond films. Presently, synthetic diamond coatings have been widely used on carbide cutting tools from long time to prevent the abrasive wear occurred on conventional tools. Based upon the size of the grains, synthetic diamond coatings are basically classified into nanocrystalline diamond (NCD) and microcrystalline diamond (MCD). In the work reported in the present chapter, smooth and adhesive thin NCD and MCD coatings were deposited on chemically treated tungsten carbide substrates using pre-determined process parameters in the hot filament chemical vapor deposition (HFCVD) method. Tungsten carbide with 6% Co is the mostly accepted grade of base material used for the successful growth of synthetic diamond films on its surface and, also to minimize the thermal residual stresses existing during the deposition and cooling down process between the interfaces of coating and substrate. Mostly, these thermal residual stresses are produced due to difference in thermal expansion coefficients between the coating and substrate. During deposition process, the process parameters such as methane concentration (%CH4/H2) and chamber pressure were controlled automatically using pre-programmed recipe for the growth of NCD and MCD films. The structural characteristics and quality of the synthetic diamond films were confirmed using X-ray diffraction and Raman spectroscopy techniques, respectively. The surface morphology was studied using a high resolution scanning electron microscope (HRSEM) and atomic force microscope (AFM). Moreover, the hardness measurement of coatings were done using a Berkovich nanoindenter. After that, a comparative evaluation between these two types of coatings was done.

A two-dimensional computational model for the hot-filament chemical vapour deposition (HFCVD) process to produce diamond films is presented. The model solves the overall continuity, momentum, energy and species continuity equations inside the reaction chamber of a HFCVD reactor. The gas-phase homogeneous reactions are represented by a simplified reaction mechanism. The model incorporates the catalytic production of H radical at the filament surface. Expressions representing the recombination of the H radical at surfaces and the growth rate of diamond film on the substrate were coupled to the gas-phase transport equations. The computational model was solved numerically by means of a commercial software. The model predictions showed good agreement with the experimental data reported in the literature in terms of both gas temperature and CH3 concentration profiles along the filament-to-substrate centre distance and with experimental data of the growth rate of diamond films obtained in a laboratory HFCVD reactor. Numerical simulations considering one, three and five filaments were conducted. The results showed that as the number of filaments increases, the concentration of CH3 in the reaction chamber increases as well as the rate of growth of the diamond film being produced. The shape and dimensions of the reaction chamber, filaments and substrate were found to significantly affect the model predictions. Therefore, the use of computational fluid dynamics techniques in the analysis of HFCVD reactors must be exercised with caution.

The dc plasma jet chemical vapour deposition (CVD) process is one of the most promising processes for the synthesis of polycrystalline diamond films. The obtainable linear growth rates in the range of 100 μm/h are much higher than the growth rates for other CVD processes like microwave or hot filament CVD (1 to 10 μm/h). Wear resistant diamond coatings on cutting tools are very interesting for industrial applications. The poor adhesion of the diamond films is the main problem with these systems. Therefore a mechanical or chemical pretreatment of the substrate surface is used to improve the adhesion of the coating. The influence of mechanical pretreatment by grinding and polishing with diamond powder as well as chemical etching with different solutions or combinations of these pretreatments were examined on WC-Co hard metals. The diamond films were synthesized on these pretreated cutting tools by dc plasma jet CVD. Scanning electron microscopy, EDS analysis, X-ray diffraction, and Raman spectroscopy have been used to characterize the coatings. Adhesion and wear resistance of the diamond films have been investigated using turning tests on metal matrix composites without lubricant. The improvement in adhesion of the diamond coatings is found to be highest after a combined mechanical and chemical pretreatment.

It is known that in the condition of chemical vapor deposition (CVD) diamond process, molybdenum is capable of forming carbide known as the "glue" which promotes growth of the CVD diamond, and aids its adhesion by (partial) relief of stresses at the interface. Furthermore, the WC grains are reaction bonded to the Mo 2 C phase. Therefore, molybdenum is a good candidate material for the intermediate layer between WC–Co substrates and diamond coatings. A molybdenum intermediate layer of 1–3 μm thickness was magnetron sputter-deposited on WC/Co alloy prior to the deposition of diamond coatings. Diamond films were deposited by hot filament chemical vapor deposition (HFCVD). The chemical quality, morphology, and crystal structure of the molybdenum intermediate layer and the diamond coatings were characterized by means of SEM, EDX, XRD and Raman spectroscopy. It was found that the continuous Mo intermediate layer emerged in spherical shapes and had grain sizes of 0.5–1.5 μm after 30 min sputter deposition. The diamond grain growth rate was slightly slower as compared with that of uncoated Mo layer on the WC–Co substrate. The morphologies of the diamond films on the WC–Co substrate varied with the amount of Mo and Co on the substrate. The Mo intermediate layer was effective to act as a buffer layer for both Co diffusion and diamond growth.

Diamond-coated tools can greatly improve the productivity of machining highly abrasive materials such as high silicon–aluminium alloys used in the automotive industry. Cemented-carbide diamond-coated tool inserts have not become an off-the-shelf product owing to several difficulties including insufficient adhesion of diamond to the substrate and questionable reproducibilty in their machining performance in the manufacturing. In order to overcome these difficulties, a better understanding of the effects of the chemical vapour deposition (CVD) conditions such as methane concentration, reactor pressure and substrate temperature is important. In this work, cemented tungsten carbide tool inserts with 6 wt% Co (WC–Co) were coated with diamond films deposited at five different methane concentrations (1–9 vol%). Here we present preliminary results of the effect of methane concentration variation on the following physical properties of the diamond coating: surface morphology; crystal structure; chemical quality; surface roughness; residual stress. The results indicate that the best physical properties of diamond-coated tool inserts using hot-filament CVD are achieved with diamond coatings deposited at methane concentrations ranging from 1 to 3%.

The diamond coating of tungsten carbide (WC)–cobalt (Co) hard metals has been the focus of various research activities mainly due to growing interest in diamond-coated tools for industrial machining of different materials. In this work, multilayered diamond coatings have been grown on pretreated tungsten carbide–cobalt substrates by hot-filament chemical vapor deposition. Two types of multilayer coatings have been prepared: (a) three-layer (alternating nanocrystalline diamond and microcrystalline diamond films) and (b) five-layer (alternating ultrananocrystalline and microcrystalline diamond films). The morphology, roughness, structure, thickness, adhesion of these coatings have been investigated using atomic force microscopy, field emission gun scanning electron microscopy, energy-dispersive X-ray spectroscopy, Raman spectroscopy, nanoindentantion and Rockwell C indentation tests at 600 N load. Raman spectra of the two types of samples exhibited sharp characteristic bands of nanocrystalline diamond and a centered peak at 1340 cm−1. The displacement of this peak in relation to the natural diamond peak (1332 cm−1) can be attributed to graphitic inclusions in diamond grain boundaries and the thermal coefficient mismatch of the multilayer coating–substrate system. Roughness surface measurements showed very smooth surfaces for both samples. Although the substrate has a high binder content, the results show that well-adhered chemical vapor deposition diamond multilayer coatings were produced.

The chemical vapor deposition (CVD) diamond and diamond-like carbon (DLC) films are deposited on the cobalt cemented tungsten carbide (WC-Co) cutting tools respectively using the hot filament chemical vapor deposition (HFCVD) technique and the vacuum arc discharge with a graphite cathode. The scanning electron microscope (SEM), optical interferometer profiler and Raman spectroscopy were adopted to characterize the as-deposited diamond and DLC films. The cutting performance of as-fabricated CVD diamond and DLC coated milling tools is evaluated in dry milling SiC particulate reinforced Al-metal matrix composite material (Al/SiC-MMCs), comparing with the uncoated WC-Co milling tool. The milling results demonstrate that the uncoated WC-Co milling tool suffers severest wear in its circumferential cutting edge, while the wear of DLC coated milling tool is slightly lower. Comparatively, the CVD diamond coated milling tool exhibits much stronger wear resistance. The wear on its circumferential cutting edge is less than 0.07 mm at the end of milling test, only a half of that of DLC coated milling tool. This result is attributed to the extremely high hardness and strong adhesive strength of CVD diamond film covered on the WC-Co milling tool.

Diamond chemical vapour deposition on seeded cemented tungsten carbide substrates

Aiming at the key problems of the material removal rate and surface integrity of existing tools in the lapping of sapphire hard and brittle crystals, an efficient lapping tool has been developed to explore a new process for HFVCD (hot filament chemical vapor deposition) diamond tools to efficiently lap sapphire wafers. With the premise of ensuring the surface roughness of the wafer is Ra ≤ 0.5 μm, the material removal rate is increased to more than 1 μm/h. To explore a high-efficiency lapping process for sapphire wafers using HFCVD diamond tools. The influence of key preparation parameters on the surface characteristics of CVD (chemical vapor deposition) diamond films was systematically investigated. Three types of CVD diamond coating tools with distinct surface morphologies were fabricated. These tools were subsequently employed to conduct lapping experiments on sapphire wafers in order to evaluate their processing performance. The experimental results demonstrate that the gas pressure, methane concentration, and substrate temperature collectively influenced the surface morphology of the diamond coatings. The fabricated coatings exhibited well-defined grain boundaries and displayed pyramidal, prismatic and spherical features, corresponding to high-quality microcrystalline and nanocrystalline diamond layers. In the lapping experiments, the prismatic CVD diamond coating tool exhibited the highest material removal rate, reaching approximately 1.7 μm/min once stabilized. The spherical diamond coating tool produced the lowest surface roughness on the lapped sapphire wafers, with a value of about 0.35 μm. Surface morphology-controllable diamond tools were used for the lapping processing of the sapphire wafers. This achieved a good surface quality and high removal rate and provided new ideas for the precision machining of brittle hard materials in the plane or even in the curved surface.

Analyzing the performance of diamond-coated micro end mills

Diamond coatings on cemented tungsten carbide tools by low-pressure microwave CVD

High nucleation densities and short incubation times of polycrystalline diamond films can be promoted by the dc-glow discharge–surface interaction process as in situ surface pretreatment method. As a result of the pretreatment, a carbon nanophase film whose properties are strongly affected by the dc-glow discharge deposition parameters is formed. At optimal conditions this nanophase has a predominant diamond character which promotes diamond growth in the subsequent chemical vapor deposition (CVD) process. In this work, the evolution of the diamond film during a standard hot filament (HF) CVD process, following the dc-glow discharge pretreatment, was studied by nonreactive ion implantation of the precursor film and C13 isotopic labeling during different deposition stages in addition to spectroscopic and high resolution scanning electron microscopic (HR-SEM) techniques. Our results indicate that only part of the diamond growth centers formed by the dc-glow discharge process are placed on the surface but rather they are distributed homogeneously within the precursor film. Using 13CH4- isotopic labeling it was shown that the nanodiamond films formed by the dc-glow discharge process are stable under standard HF CVD conditions. Only a fraction of the nanodiamond particles deposited by the dc-glow discharge processes serve as nucleation centers for the growth of diamond. A complex interlayer composed of a nanodiamond composite is formed between the silicon substrate and the microcrystalline diamond film.