E. Cappelli

Diamond nucleation and growth on different cutting tool materials: influence of substrate pre-treatments

Abstract Diamond coated cutting tools seem to be one of the most promising economical and technological applications of the low-pressure diamond deposition methods. The main problem associated to this application is the nucleation and growth of a uniform diamond deposit with a suitable and reliable adhesion, withstanding thermal and mechanical stresses, originating by the severe cutting conditions. In this work we have studied the HF-CVD deposition conditions of diamond on different tool substrate materials: cer-mets (WC, Co); ceramics (SiAlON, Al2O3 + TiC) and whisker reinforced ceramics (Al2O3 + SiC and ZrO2). We have examined, by SEM-EDAX and XPS surface analysis, the influence of chemical, mechanical and physical substrate pretreatments on the substrate surface modification. Strong Co etching and reducing seems to be the best WC, Co substrate pre-treatment for a suitable diamond nucleation and adhesion; the SiAlON ceramics are very good substrates for high quality diamond growth. Raman spectroscopy have evidentiated an excellent diamond film quality, with a small energy shift (1337 cm−1), may be owing to some compressive deformation.

Wettability of HF-CVD diamond films

Wettability of HF-CVD diamond films grown in different conditions has been investigated. Wettability depends on surface tension: solids with high surface tension, as diamond, should be hydrophilic, while solids with low surface tension should be hydrophobic. In spite of these arguments, natural diamond exhibits a moderate hydrophobicity [J. Coll. Inter. Sci. 130 (1999) 35], depending on surface termination (hydrogen or oxygen terminated). In this work we find that CVD diamond films show wettability behaviours ranging from a small, up to clear hydrophobicity, probably according to surface carbon termination functionalities. Wettability does not seem to be influenced by characteristics as film structural morphology or growth orientation direction, which were analysed by SEM, while it seems dependent on surface reconstruction, as detected by Raman and XPS analysis. Moreover, in contrast with natural diamond [J. Coll. Inter. Sci. 130 (1999) 35] we found an enhancement to water wettability when CVD diamond films were treated in a hot filament activated hydrogen atmosphere. We argue that this effect may be due to the hydrogen etching of reconstructed surface layers with lower surface tension.

Relation between the HFCVD diamond growth rate, the line-width of Raman spectrum and the particle size

Abstract The microstructure of a thin diamond film determines its functionality and reliability. Thus, knowledge of the phenomena influencing the stabilisation of a particular microstructure is widely requested. Here the occurrence of a linear relation between the 1332 cm−1 Raman line-width and the growth rate of HFCVD diamond thin film is shown. A linear dependence of the Raman line-width on the reciprocal of the diamond particle size, measured by X-ray diffraction, already substantiated for MWCVD thin films, is shown to exist also for HFCVD films. Consequently the average particle size and the film growth rate are also shown to be related. The phonon confinement model has been reconsidered and a new phonon diffusion model is proposed to explain these results. According to this model the phonon energy, initially confined in a particle, is allowed to diffuse into the matrix with time. Finally, it is proposed that the relation of the film growth rate with the particle size is due to the concomitant effects of the matter diffusion process, necessary to reach and stabilize crystalline order, and the film growth that mainly depends on process deposition conditions.

WC–Co cutting tool surface modifications induced by pulsed laser treatment

Abstract Many innovative industrial materials like carbon fibers, metal matrix composites (MMC), hypereutectic Al/Si alloys are very hard and difficult to machine with traditional cutting tools. CVD diamond coated inserts seem to be the most promising system to overcome the problem. The WC, x Co hard metal, a widespread used and cheaper bulk material, could be a convenient substrate for diamond film coatings. The Co-rich binder phase, however, constitutes a severe obstacle for diamond deposition and adhesion, owing to its catalytic effect for amorphous carbon or soot formation. Several chemical and physical methods have been developed to etch Co from the surface; however, no definite and reliable procedure has been achieved. In our experiments, we treated the WC, x Co hard metal substrates with ArF ( λ =193 nm, hν =6.4 eV) and CO 2 ( λ =10.6 μm, hν =0.12 eV) pulsed lasers, at different fluences and incident angles, to modify both the chemical composition and the structure of the surfaces. The morphological and chemical effects have been studied by SEM/EDAX microscopy and XPS surface analysis. Diamond nucleation and adhesion are compared with results obtained on the plain tool substrates.

Secondary electron emission from diamond: Physical modeling and application to scanning electron microscopy

Secondary electron emission from diamond films is studied as a function of the primary electron beam energy and bulk materialproperties. A formulation of a simple model of the secondary electron emission coefficient, as a function of the primary electron beam energy, has been found to be helpful in defining physical criteria able to guide the optimization of the diamond film electron emission performance. The secondary electron mean escape depth deduced from the model is indeed related to the density of defects in the material and represents the main factor in determining the low energy secondary electron yield. These results are supported by Raman spectroscopy measurements, indicating a lower graphitic content and a higher crystalline quality of the diamond phase in films showing better secondary electron and photoemission yields. We demonstrate that a diamond film, acting as a stable and proportional electron multiplier, can be used as a converter of backscattered electrons into secondary electrons in scanning electron microscopy. It will be shown that the use of a diamond film converter is suitable to improve the signal to noise ratio of images providing an enhanced compositional contrast.

Ceramic surface modifications induced by pulsed laser treatment

Abstract Technical polycrystalline sintered Al2O3 (90%) substrates have been irradiated, in a vacuum chamber, at grazing incident angles (∼30°), with pulsed ArF (λ=193 nm, hν=6.4 eV) excimer laser, at different fluences and numbers of pulses, to modify the structure and morphology of the surface. Vacuum, inert gas and oxygen atmospheres, at different substrate temperatures, ∼25°C and ∼700°C, have been used to study surface chemistry and morphology modifications induced by laser energy. Surface chemistry has been analysed by XPS spectroscopy. Morphological modifications have been studied by SEM/EDS microscopy. Changes in surface roughness have been quantified by a standard profilometer.

CVD diamond dosimetric response evaluated by X-ray absorbers method

Abstract The DC electronic response of a small area polycrystalline diamond film irradiated by a low energy X-ray beam has been analysed. The relationship between the induced photocurrent and the X-ray flux has been investigated making use of an X-ray absorber method. Aluminium foils have been used to attenuate the direct beam and the metal–diamond–metal device response tested at several electric field values. The X-ray photocurrent shows a linear response of the device that has been observed for a large range of X-ray flux values.

Effect of nanostructure and back contact material on the field emission properties of carbon films

The emission properties of carbon films grown by pulsed laser ablation are investigated in relation to their nanostructure, which changes from amorphous to nanostructured carbon, according to the substrate temperature. In addition to an increasing number and size of six-member carbon rings, Raman scattering measurements reveal light polarisation sensitivity, reflecting a temperature-induced orientation of graphene domains. Such characteristics largely affect the electron emission properties, resulting in a close relation among threshold field values, graphene domain size and probably their average orientation. These results are interpreted suggesting that hot electron generation and transport through graphene domains is one of the main mechanisms enhancing the electron emission probability. A lowering of the threshold field strength is also observed when carbon films are deposited on titanium substrate in respect to the silicon one.

Grain boundary transport in x-ray irradiated polycrystalline diamond

The transport properties of a “thin” polycrystalline diamond film are analyzed after the sample exposure to 8.06-keV x-ray radiation. Structure and morphology of the as-grown film have been evaluated by Raman, x-ray diffraction, and scanning electron microscopy techniques. The transport properties have been investigated by measuring dark current–voltage characteristics in the temperature range of 60 to 360 K. Ohmic transport has been evidenced on the as-grown film up to 1.16×105 V/cm. After irradiation, nonlinear contributions to the dark current have been evidenced and related to field-assisted thermal ionization of traps. Below 200 K, hopping mechanisms have been observed. Correlations have been found among x-ray irradiation, density of traps involved in the transport processes, and the nonhomogeneous nature of the sample. A simple model of the grain boundary structure is proposed to explain the observations.

Metal-semiconductor-metal photodiodes based on CVD diamond films

Abstract We present a study on diamond-based UV photodetectors with lateral metal-semiconductor-metal configuration having the gap between Al electrodes of about 5 μm, similar to the average grain size in diamond films. Using a simple model for the photocurrent collection, mobility-lifetime values of diamond layers are derived and compared with available transport data. The spectral response of these devices is also measured from 1 to about 7 eV, showing the presence of internal photoemission processes in the near infrared region, the existence of localized transitions in the visible and the occurrence of interband transitions above 5 eV, responsible for a large UV photoresponse.