Enhanced oxidation and graphitization resistance of polycrystalline diamond sintered with Ti-coated diamond powders

To improve the oxidation and graphitization resistances of the polycrystalline diamond(PCD), Ti coating was deposited on the diamond powders via magnetic sputtering method, which achieved a uniform Ti C protection barrier in PCD during the sintering process. The phase compositions, microstructures and thermal stability of Ti-PCD were characterized by X-ray diffraction(XRD), Auger electron spectroscopy(AES),scanning electron microscopy(SEM) and thermal gravimetric-differential scanning calorimetry(TG-DSC).The results demonstrate that the oxidation and graphitization resistances of PCD are strengthened due to the existence of Ti C phase, which acts as an effective inhibitor. The as-received inhibitor delays the oxidation and graphitization of PCD, elevating their initial temperature by ～50°C and ～100°C, respectively. During the annealing treatment of Ti-PCD, the priory oxidation of Ti C, which produces Ti O2 as an oxygen barrier, postpones the diamond oxide. Moreover, the Ti C barrier also protects diamond grains from direct contact with cobalt, thus a lower cobalt-catalytic graphitization, and yields to an improved graphitization resistance of PCD. The enhanced oxidation and graphitization resistances of PCD are of significant importance for practical applications to elevated temperatures.     

High vacuum tribology of polycrystalline diamond coatings

Polycrystalline diamond coatings have been grown on unpolished side of Si(100) wafers by hot filament chemical vapour deposition process. The morphology of the grown coatings has been varied from cauliflower morphology to faceted morphology by manipulation of the growth temperature from 700°C to 900°C and methane gas concentration from 3% to 1·5%. It is found that the coefficient of friction of the coatings under high vacuum of 133·32×10^?7 Pa (10^?7 torr) with nanocrystalline grains can be manipulated to 0·35 to enhance tribological behaviour of bare Si substrates.     

Study of polycrystalline sintered compacts of diamond

Study of polycrystalline sintered compacts of diamond has been made using powder X-ray diffraction, scanning electron microscopy (SEM) and energy- dispersive X-ray analysis (EDX). Various crystalline phases formed at high temperature and high pressure, microstructure, particle-size distribution of diamond and binder and concentration of different elements in the sintered diamond compacts were determined.     

Junction like behavior in polycrystalline diamond films

We have successfully fabricated polycrystalline diamond rectifying junction devices on n-type (1 0 0) silicon substrates by Hot Filament Chemical Vapor Deposition (HFCVD) using methane/hydrogen process gas and trimethyl borate and trimethyl phosphite dissolved in acetone as p- and n-type dopants, respectively. Impedance spectroscopy and current-voltage analysis indicates that the conduction is vertical down the grains and facets and not due to surface effects. Electrical characteristics were analyzed with In and Ti/Au top metal contacts with Al as the substrate contact. Current-voltage characteristics as a function of temperature showed barrier potentials of 1.1 eV and 0.77 eV for the In and Ti/Au contacts, respectively. Barrier heights of 4.8 eV (In) and 4.4 eV (Ti/Au) were obtained from capacitance-voltage measurements.     

Wear of diamond particles for grinding ceramics

The relative wear of a single point diamond, defined in terms of the grinding force, was determined for a group of A1₂O₃ and Si₃N₄. It was found that the wear rate is directly related to the mechanical properties of the workpiece and the grinding environment.     

The grinding rate and specific cost of internal plunge-cut diamond grinding of hardmetals with periodic electrical discharge actions

The authors have studied the influence of diamond wheel grain size on the grinding rate and specific cost of internal plunge-cut elastic-mode grinding of VK15 (WC-15Co). Periodic electric-discharge actions on the wheel working surface, with an optimal cyclicity, provide an 396% improvement of the grinding rate and an 305% reduction of specific cost in comparison to grinding performed with no such actions.     

Electrochemical behaviour of molybdenum coated with flame CVD polycrystalline diamond film

Although natural diamond is a complete chemically-inert material for a wide range of aggressive environments, its comparative scarcity and problems for coating design have hampered its utility as a corrosion protective coating. The recent discovery and development of chemical vapour deposition methods for growing diamond crystals and polycrystalline diamond films has opened up a wide range of applications thanks to their excellent tribological, electronic and optical properties. Various applications are in progress for corrosion and combined wear and corrosion protection. This paper presents the first study of the corrosion behaviour of continuous polycrystalline diamond films using electrochemical impedance electroscopy. Diamond films have been deposited on molybdenum substrates by means of the acetylene flame combustion method (FCVD). Electrochemical behaviour has been studied in a 0.6 M NaCl solution, it being seen that despite the inert character and apparent continuity of the film, there are areas of the base material which are exposed to the electrolyte. This behaviour has been modelled by means of an equivalent circuit which allows for the corroboration of the proposed mechanism.     

Polishing of polycrystalline diamond by hot nickel surface

A microwave plasma technique has been employed to deposit polycrystalline diamond film over a molybdenum substrate button using a gas mixture of hydrogen and methane at a substrate temperature of 851°C. A CVD diamond coated molybdenum substrate button was mounted with a load against hot nickel plate and rotated for 3.45 h in a hydrogen ambient. Hot tungsten filament was used as a heat source to maintain the temperature of the nickel block and CVD diamond coated molybdenum button at 848°C. This experiment has reproducibly shown the successful polishing of polycrystalline CVD diamond by hot nickel. A Tencor profilometer and scanning electron microscope have been used to evaluate the surface smoothness and morphology before and after polishing the polycrystalline diamond thin films.     

Neutron detection and dosimetry using polycrystalline CVD diamond detectors with high collection efficiency

Polycrystalline chemical vapour deposited (CVD) diamond film is an interesting material for neutron detection and dosimetry. However, the use of CVD diamond detectors is still limited by the low-level signal pulse produced because of the high energy required to produce an electron-hole pair in diamond (13.2 eV) and by the reduced charge collection efficiency owing to several types of traps for electrons and holes in CVD films. A new type of CVD diamond detector with high gain (HG) contacts was produced as part of the collaboration between the ENEA Fusion Division and the Faculty of Engineering of Rome 'Tor Vergata' University. In this paper the performance of the HG CVD diamond detector is presented and possible applications of CVD diamond detectors to neutron dosimetry are also discussed.     

Infrared properties of chemical-vapor deposition polycrystalline diamond windows

Low-resolution transmittance and reflectance spectra of high-quality chemical-vapor deposition (CVD) diamond windows were measured in the infrared in the 2.5-500-mum wavelength range (20-4000 cm(-1)). High-resolution measurements on a window with nearly parallel surfaces show well defined interference fringes at low frequencies. By standard procedures the optical constants n and k of CVD diamond were determined, for the first time to the author's knowledge, in the far-infrared region. It is shown that a window with a large wedge angle, close to 1 degrees , does not produce appreciable interference fringes. Modeling of these results confirms that interference fringes can be avoided by use of properly wedged CVD diamond windows. This result is of considerable relevance to the use of CVD diamond windows in spectroscopic applications for which fringe suppression is a major requirement.     

Nanoscale ductile grinding of glass by diamond fibres

Chemical vapour-deposited diamond fibres have been used to grind soda glass. The surface topography was studied using scanning electron microscopy and atomic force microscopy techniques. The diamond surface facets and edges led to grinding without surface cracking and to surface roughness, Ra, values in the range 3–50 nm. The grinding mechanism involved the formation by ductile flow of glass ribbons adjacent to the grinding grooves. This grinding mechanism was similar to that reported for single-point diamond machining. The potential for ductile grinding with diamond fibres is discussed.     

Reversible wettability on polycrystalline diamond films between superhydrophobicity and superhydrophilicity

Reversible wettability on the polycrystalline diamond film was investigated through the alternation of hydrogen and oxygen plasma treatment. This can be ascribed to the reversible switching of the surface terminal chemical properties between two different states. The water contact angle could be varied between 5.9 degrees and 88.9 degrees when the diamond film was prepared on a flat substrate. When diamond films with multi-scaled roughness were fabricated, the wettability conversion could be amplified, leading to switching between superhydrophobicity and superhydrophilicity.     

Electron emission from nitrogen-doped polycrystalline diamond/Si heterostructures

The field electron emission from polycrystalline diamond/silicon and nitrogen-doped polycrystalline diamond/silicon structures obtained by HF CVD deposition method has been investigated. Electron emission currents from the samples were measured in a chamber at the pressure equal to 2·10^?6 Pa in sphere-to-plane diode configuration with the 5 ??m distance between electrodes. As expected, the results confirm the relation between the structure of diamond films and their emission properties. The type of silicon substrate also influences the value of emission currents and the diamond/n-Si heterostructures exhibit better electron emission than diamond/p-Si ones. The nitrogen doping significantly enhances the electron emission from the heterostructures and their emission parameters. The values of the threshold field between 2 V/??m and 3 V/??m were registered, the values of emission current close to 1 mA/cm² at 5 V/??m for the nitrogen-doped films were obtained. The shape of current-voltage characteristics for nitrogen-doped polycrystalline films may be interpreted in terms of stochastic distribution of diameters of conducting channels which form the emission centers.     

Structure of diamond polycrystalline films deposited on silicon substrates

The article presents results of structural studies of polycrystalline diamond thin films deposited by hot filament CVD on silicon substrates. The films were characterized using Scanning Electron Microscopy (SEM), Raman Spectroscopy (RS), Electron Backscattered Diffraction (EBSD), Energy Dispersive Spectroscopy (EDS) and Secondary Ion Mass Spectroscopy (SIMS). Both the EBSD patterns and Raman spectra confirm that the grains visible in the electron micrographs are diamond micro-crystallites. The residual stress in the films is found to be in the range between −4.29 GPa and −0.56 GPa depending on the sample thickness. No evidence of lonsdalite and graphite has been registered in the polycrystalline material of the investigated samples. Evidence of the existence of silicon carbide at the diamond/silicon interface is presented. It is also suggested that an amorphous carbonaceous film covers the silicon surface in the regions of holes in the thin diamond layers.     

Study on grinding of pre-sintered zirconia using diamond wheel

Zirconia-based ceramics are the most preferred materials used in implants due to their excellent mechanical properties compared with other alternatives. These materials available in the pre-sintered form are appropriate to grind due to their soft nature. However, it is challenging to achieve a required surface finish in grinding these ceramic materials owing to chipping, which limits its usage in industries. In this work, the pre-sintered yttria stabilized tetragonal zirconia (Y-TZP) component was ground using a resin-bonded diamond-grinding wheel under different cooling environments. The components ground under the minimum quantity lubrication conditions exhibited a reduced grinding force with better surface finish compared to wet (flood coolant) and dry conditions. The resultant specific energy was reduced with the increase in maximum chip thickness for different cooling conditions. The critical depth of cut estimated from the pre-sintered zirconia was witnessed to be higher, which indicated that the initiation of ductile to brittle transition occurred at a deeper depth of cut. The material removal mechanism observed in the pre-sintered zirconia was dominated by brittle fracture. This was evident from the obtained chips and ground surface morphology.     

Surface integrity and removal mechanism in grinding sapphire wafers with novel vitrified bond diamond plates

In order to improve machining efficiency of sapphire wafer machining using the conventional loose abrasive process, fixed-abrasive diamond plates are investigated in this study for sapphire wafer grinding. Four vitrified bond diamond plates of different grain sizes (40?µm, 20?µm, 7?µm, and 2.5?µm) are developed and evaluated for grinding performance including surface roughness, surface topography, surface and subsurface damage, and material removal rate (MRR) of sapphire wafers. The material removal mechanisms, wafer surface finish, and quality of the diamond plates are also compared and discussed. The experiment results demonstrate that the surface material is removed in brittle mode when sapphire wafers are ground by the diamond plates with a grain size of 40?µm and 20?µm, and in ductile mode when that are ground by the diamond plates of grain sizes of 7?µm and 2.5?µm. The highest MRR value of 145.7?µm/min is acquired with the diamond plate with an abrasive size of 40?µm and the lowest surface roughness values of 3.5?nm in Ra is achieved with the 2.5?µm size.     

Comparative study on dry etching of polycrystalline diamond thin films

The reactive ion etching (RIE) technique was used to etch polycrystalline diamond thin films. In this study we investigate the influence of process parameters (total pressure, rf power, gas composition) of standard capacitively coupled plasma RIE system on the etching rate of diamond films. The surface morphology of etched diamond films was characterized by Scanning Electron Microscopy and the chemical composition of the etched film part was investigated by Raman Spectroscopy.We found that the gas composition had a crucial effect on the diamond film morphology. The use of CF₄ gas resulted in flatter surfaces and lateral-like etching, while the use of pure O₂ gas resulted in needle-like structures. Addition of argon to the reactant precursors increased the ion bombardment, which in turn increased the formation of non-diamond phases. Next, increasing the rf power from 100 to 500 W increased the etching rate from 5.4 to 8.6 μm/h. In contrast to this observation, the rise of process pressure from 80 to 150 mTorr lowered the etching rate from 5.6 down to 3.6 μm/h.     

Machining and morphological evaluation of diamond coated tungsten carbide drills

This paper reports the results of drilling tests on various workpiece materials using diamond coated tungsten carbide drills. The performance of the coated drills were compared with uncoated control drills. Hot filament chemical vapor deposition was used for coating the pre-treated drills and film coating morphology and stress characteristics were studied prior to drilling. Forces and torques were measured during drilling and the results indicate catastrophic failure and short tool-life of the coated drills for all workpiece materials. Drill failure reasons are attributed to crystal clustering and point loading of the cutting edges. Further research issues are also identified.