A.J.S. Fernandes

Diamond coating on steel with a titanium interlayer

Abstract Diamond coating on steel by microwave plasma chemical vapour deposition (MPCVD) is investigated. Direct growth of diamond film on steel substrate is possible, but its adhesion is poor because of the formation of a rather thick graphite layer at the diamond/steel interface. Calculation is performed in order to select a suitable interlayer. The results show that titanium is one of the promising candidates. Under optimized process conditions, adherent diamond film is coated on the steel with a titanium interlayer approx. 2 μm thick. Pull-off tests and micro-indentation tests are performed to evaluate the coating adhesion.

Microwave plasma chemical vapour deposition diamond nucleation on ferrous substrates with Ti and Cr interlayers

Abstract Diamond-coated steel is considered an important issue in synthetic diamond technology due to the great economical importance of enhancing the wear resistance and surface hardness of commercial Fe-based alloys. However, direct diamond coating by chemical vapour deposition (CVD) is rather problematic—adhesion and growth are seriously affected. The use of interlayers is a common approach to minimise these problems. This work reports an investigation on the establishment of good nucleation and growth conditions of diamond films by microwave plasma CVD (MPCVD) on ferrous substrates coated with Ti and Cr interlayers. Commercial grade ferrous substrates were pre-coated with commercial interlayers by sputtering (Ti, Cr) and electroplating (Cr) techniques. Steel substrates led to better results than iron cast substrates. The best films were obtained on Ti pre-coated steel substrate. The results on Cr interlayers pointed to the advantage of electroplating over the physical vapour deposition (PVD) sputtering. From the two selected parameter sets for diamond deposition, the one using lower power level conducted to the best results. Initial roughness and growth parameters were found to counteract on the uniformity of the diamond films. The morphology was studied by scanning electron microscopy (SEM), the roughness was estimated by profilometry, while diamond quality and stress state were evaluated by μ-Raman spectroscopy.

Influence of nucleation density on film quality, growth rate and morphology of thick CVD diamond films

Abstract The optimum growth parameters of our 5 kW microwave plasma CVD reactor were obtained using CH4/H2/O2 plasma and high quality transparent films can be produced reproducibly. Among the films prepared in this system, the film of best quality has very smooth crystalline facets free of second nucleation and the full width at half maximum (FWHM) of the diamond Raman peak is 2.2 cm−1, as narrow as that of IIa natural diamond. For this study, diamond films were grown on silicon substrates with low (104–105 cm−2) and high nucleation densities (>1010 cm−2), respectively. From the same growth run, a highly 〈110〉 textured 300 μm thick white diamond film with a growth rate of 2.4 μm/h was obtained from high nucleation densities (>1010 cm−2), and a white diamond film of 370 μm in thickness with a higher growth rate of 3 μm/h was obtained from low nucleation densities (5×104–105 cm−2) too. The effect of nucleation density on film quality, growth rate, texture and morphology was studied and the mechanism was discussed. Our results suggest that under suitable growth conditions, nucleation density has little effect on film quality and low nucleation density results in higher growth rate than high nucleation density due to less intense grain growth competition.

Structural and optical properties of europium doped zirconia single crystals fibers grown by laser floating zone

Yttria stabilized zirconia single crystal fibers doped with europium ions were developed envisaging optical applications. The laser floating zone technique was used in order to grow millimetric high quality single crystal fibers. The as-grown fibers are completely transparent and inclusion free, exhibiting a cubic structure. Under ultraviolet (UV) excitation, a broad emission band appears at 551 nm. The europium doped fibers are translucent with a tetragonal structure and exhibit an intense red emission at room temperature under UV excitation. The fingerprint transition lines between the D50 and F7J(0–4) multiplets of the Eu3+ ions are observed with the main emission line at ∼606 nm due to D50→F72 transition. Photoluminescence excitation and wavelength dependent the photoluminescence spectra confirm the existence of different Eu3+ optical centers.

Wear resistant CVD diamond tools for turning of sintered hardmetals

Abstract Sintered hardmetals are very hard materials that are usually machined using diamond grinding wheels and electro-discharge machining. Dry cutting with super-hard cutting tools like cubic boron nitride (c-BN), polycrystalline diamond (PCD) and chemical vapour deposition (CVD) diamond is an ecological alternative to reduce operation times and, therefore, to improve the productivity. In the present work, cylindrical forging dies of WC–27 wt.% Co hardmetal grade were turned at fixed operating parameters (cutting speed=15 m/min; depth-of-cut=0.2 mm; feed rate=0.03 mm/rev.) using CVD diamond tipped hardmetal inserts. Commercial PCD and c-BN inserts were tested for comparison. The cutting tool behaviour was studied in terms of both the tool wear and the finishing quality of the workpiece. The tool damage was evaluated using a special probe for edge roughness evaluation, together with scanning electron microscopy observations. The CVD diamond tools survived the task showing slight cratering, whereas flank wear was the main wear mode for the other superhard tools. Amongst all the tested tools, PCD presented the worst performance in terms of tool wear and workpiece surface quality. Furthermore, the operation time was reduced to one tenth with respect to conventional diamond wheel grinding.

A new elegant technique for polishing CVD diamond films

Abstract It is well known that the columnar growth nature of CVD diamond results in a very rough growth surface and the surface roughness steeply increases with film thickness, especially for thick CVD diamond films. In this paper, we report the successful implementation of a new elegant technique for polishing thick polycrystalline CVD diamond films at high polishing rate of up to 10 μm/h. This technique involves polishing the as-grown polycrystalline diamond films with another thick as-grown polycrystalline diamond film, which acts as a polishing abrasive. Two types of diamond films were prepared using microwave plasma CVD and then polished for 2 h using the new polishing technique. A stylus profilometer, scanning electron microscopy and Fourier transform infrared spectroscopy were used to measure the surface roughness, characterize morphology and optical transmission of the samples before and after polishing, respectively. By polishing, thickness of 20–30 μm was removed from the top surface, and the mean surface roughness Ra of the films reduced significantly, e.g. for one film Ra reduced initially from 5.2 to 1.35 μm and the other from 3.2 to 0.55 μm. The principal advantages of this new polishing technique are simplicity, flexibility and time saving. This simple method can serve as ‘rough chipping’ to quickly remove the rough top surface and then combine with conventional polishing methods for precision machining to further reduce the surface roughness to a specific desired degree.

MPCVD diamond tool cutting-edge coverage: dependence on the side wedge angle

Abstract Microwave plasma CVD usually produces uniform diamond coatings and high-quality diamond films. However, abnormal deposits appear near the sample edges — the so-called ‘edge effect’. Wedge-shaped silicon nitride inserts with 30°-, 60°-, 75°- and 90°-edge angles were vertically and horizontally exposed to MPCVD diamond coating to systematically study this effect. Finite element method (FEM) analysis was used to simulate the temperature distribution on such geometries. Diamond morphology and quality were assessed by SEM and micro-Raman techniques. The edge effect, a consequence of plasma concentration and thermal phenomena in this experimental set-up (activation by electromagnetic gas discharge), is more accentuated on samples that are vertically wedge-oriented towards the plasma. A grain-size gradient is established along the exposed surface, steeply increasing at the hot edge. An extreme effect occurs in the sharpest wedge samples, avoiding diamond growth at the edge.

Tailored Si3N4 Ceramic Substrates for CVD Diamond Coating

Abstract A review is presented of chemical vapour deposition (CVD) diamond coating of silicon nitride (Si3N4) materials. Microcrystalline and nanocrystalline diamond films were grown using microwave plasma (MPCVD) and hot filament (HFCVD) reactors, respectively. Scanning electron and atomic force microscopy, μ-Raman spectroscopy, low incident angle and classical X-ray diffraction, acoustic emission assisted Brale indentation and thermal conductivity measurements were employed for the full characterisation of the diamond/ Si3N4 system. Using these techniques, the nucleation and growth stages as a function of substrate composition and surface pretreatment were characterised, as well as the diamond quality, the existence of residual stresses and the adhesion between the diamond film and the substrate. Based on this study, a tailored material was developed and tested in the machining of hardmetal workpieces with encouraging results.

Thermal conductivity enhancement in cutting tools by chemical vapor deposition diamond coating

Abstract Two different types of chemical vapor deposition (CVD) diamond-coated tools have been investigated: (i) 300–1000 μm films brazed to WC-6 wt.% Co substrates; and (ii) 70-μm-thick films directly deposited on Si3N4–SiC composites. Thermal diffusivity perpendicular to the tool surface was measured up to 1000 °C by the laser flash method. Multiple-layer heat conduction models were used to appraise the effective thermal diffusivity of the film/substrate system. The thermal conductivity of the brazed samples increased up to 65% with respect to the hard metal substrate, evidencing a strong dependence on the diamond thickness. In the directly coated Si3N4 based tool substrates, the thermal diffusivity and the mean thermal conductivity increased up to ∼10%.

Residual stress minimum in nanocrystalline diamond films

Nanocrystalline diamond films have been deposited on silicon nitride substrates by hot filament chemical vapor deposition. Gas mixtures of CH4–H2–Ar were used with variation of the Ar∕H2 ratio in order to study the influence of the Ar content on the formation of nondiamond phases at the grain boundaries and thus in the film residual stress assessed by x-ray diffraction techniques. By varying this ratio it is possible to optimize conditions, decreasing the film’s residual stress to a minimum of 0.09GPa.