Nélia Ferreira Leite

Dispersion liquid properties for efficient seeding in CVD diamond nucleation enhancement

Abstract We investigated diamond seeding as an efficient method of enhancing the nucleation density in chemical vapor deposition (CVD) diamond growth. We prepared substrates by ultrasonification for a short time in a 0.25 m diamond powder suspension and studied the effects of several dispersion liquids on seeding. We identified five key liquid properties for efficient seeding: zero or low dipole moment, low density, high vapor pressure, low surface tension and low viscosity. n-Hexane and n-pentane are the most suitable liquids, and yield seeding at the limit of the powder grit. We also observed the phenomenon of chemical consolidation of diamond particles onto silicon substrates during the initial growth stages. This consolidation ensures film adherence comparable with that obtained by other methods of nucleation enhancement.

Improvements of ultra-high molecular weight polyethylene mechanical properties by nitrogen plasma immersion ion implantation

Nitrogen Plasma Immersion Ion Implantation (PIII) has been used to modify the surface chemical structure of Ultra High Molecular Weight Polyethylene (UHMWPE). Grinding and polishing processes based on abrasive papers and alumina pastes have been evaluated with regard to their results on the improvement of polymer surface roughness, which has shown to be of crucial importance for hardness characterization. Raman spectroscopy, XPS, and Nanoindentation tests were used to characterize the modified surfaces. Experimental results has shown that UHMWPE surface mechanical properties such as hardness and elastic modulus can be improved by induced chain cross-linking between the macromolecules on the polymer surface caused by nitrogen PIII. The new material formed on the surface is Diamond Like Carbon (DLC). As a significant improvement in hardness was obtained by DLC synthesis on the treated surface, it is expected a dramatic improvement of abrasion resistance and overall durability of prostheses made with PIII treated UHMWPE.

Columnar CVD diamond growth structure on irregular surface substrates

Abstract Columnar grain structure is always observed in CVD-diamond growth and is an important parameter to identify the morphology of thin and thick films. Structure defects, aspects of onset nucleation and film growth mechanisms can also be related to columnar growth. In this work we focused our attention on the columnar structure of CVD-diamond grown on irregular surfaces. We observed that there is a relationship among curvature radius of the substrate surface, the spread of the column volume and the growth rate of the diamond film. Growth rates on spherical surfaces of around 0.5 mm curvature radius have been observed to be up to three times bigger than the growth rates on planar surfaces. Also, the grain size distribution on planar and on the corner surfaces as a function of the growth rate has been studied. Characterization with scanning electron microscopy (SEM) and Raman scattering spectroscopy (RSS) has been performed.

Stress study of HFCVD boron-doped diamond films by X-ray diffraction measurements

Abstract Stress analysis on chemical vapor deposition (CVD) diamond films has demonstrated an apparent disagreement among various researchers in recent works even for similar deposition conditions. The type and the value of stress have shown a strong dependence on film thickness, which can be attributed to columnar growth and grain size and boundaries. X-Ray diffraction techniques appeared to be more suitable to study these effects and permit the evaluation of the average stress in larger sample areas when compared with micro-Raman spectroscopy, which feels a local strain inside the grains. In the case of boron-doped diamond films, boron incorporation on substitucional or interstitial sites can produce stresses according to the doping level. In order to investigate these effects, a series of diamond films were deposited on silicon (001) substrate in a hot filament (HF)-assisted CVD reactor at 800°C. The CH 4 flow is kept at 0.5 sccm for all experiments and the H 2 and B 2 O 3 /CH 3 OH/H 2 flows are controlled in order to obtain the desired B/C ratios. Stress behavior in HFCVD boron-doped diamond films has been investigated by X-ray diffraction measurements using the sin 2 ψ technique. Tensile and compressive stresses have been observed and the thermal and intrinsic components have been calculated. The diamond films were characterized by scanning electron microscopy and Raman spectroscopy.

Multi-layer structure for chemical vapor deposition diamond on electroplated diamond tools

In this work we established a process to overcome the deposition difficulty on electroplated diamond tools by a multi-layer structure. The process consists of the following steps: (1) diamond powder aggregation with nickel (this step is the conventional method for the production of electroplated tools); (2) electrochemical deposition of a chromium layer, but leaving the diamond grains partially uncovered; (3) nitridation of the chromium layer; and (4) deposition of the chemical vapor deposition (CVD) diamond layer. This method uses the advantages and overcome the disadvantages of each step. Electroplating with nickel is conventionally used due to its relatively good wettability to diamond. The direct aggregation of the diamond powder with a chromium layer results in looser grains and is not usable. The nickel layer is inadequate for diamond deposition; even after treatment in hydrogen atmosphere, diamond does not grow on it. The chromium nitride layer is well known to be very suitable for diamond growth, however, the thermal stress between these layers is very high limiting film thickness and its applicability. With the multi-layer structure obtained the CVD diamond film is deeply anchored by the diamond grains into the metal matrix and helps considerably to decrease the stress. The process has been developed on flat surfaces and tested on small conventional diamond burrs. The diamond films have been characterized by scanning electron microscopy (SEM), energy dispersive X-rays (EDS) and Raman spectroscopy (RS).

Cylindrical CVD diamond as a high-performance small abrading device

Columnar grain structure studies have been carried out on a small-curvature radius cylinder substrate for the development of small abrading devices. An enhanced hot filament-assisted technique was used. The substrate holder has a movable mechanism magnetically coupled to a d.c. motor placed outside the reactor chamber. Free-standing diamond films as thick as 1 mm were obtained on molybdenum wire substrates. The substrates varied from 500 up to 1200 μm diameter and were as long as 50 mm. A relationship of the curvature radius of the substrate surface with the growth rate and the spread of the column volume has been observed. A preferential diamond (111) surface morphology has been obtained, which is strongly dependent on diamond growth parameters, including the substrate position and its angular velocity. Grain size up to around 0.15 mm was obtained. In this work we also report an interesting application involving the characteristics of the (111) diamond surface as a small abrading device. Some results of cylindrical diamond burrs and diamond drills for non-ferrous wear are shown.

Very adherent CVD diamond film on modified molybdenum surface

In this work, studies have been carried out on surface modification on a pure molybdenum surface to enhance CVD diamond adherence. Pure molybdenum exhibits a phase transformation temperature close to that of diamond growth, but it is possible to obtain a surface modification by ion sub-implantation at an appropriate lower temperature without phase transformation. In this study, it was possible to create an appropriate interface by using appropriate ion compounds and impact energy on the substrate surface at a very narrow window of working temperature. This interface efficiently avoided carbon diffusion during diamond growth and also improved the chemical bond density between atoms from the interface and carbon from the onset of diamond nucleation. Diamond films from a few micrometers up to millimeters have been obtained with very good adherence. Characterization by XPS and small angle X-ray diffraction of this interface prior to diamond growth and after diamond nucleation reveals different compound contributions to the chemical bonds. Indentation and SEM analyses were also carried out.

Diamond growth with CF4 addition inhot-filament chemical vapour deposition

Tetrafluoromethane (CF4) was added to standard CH4/H2 mixtures for diamond growth in hot-filament-assisted chemical vapour deposition. CF4 concentrations in the range of 0.3%–3% were studied. Mass spectrometry of the exhaust gas showed that only a small fraction (< 15%) of CF4 was thermally dissociated for filament temperatures over 1800 °C. The observed stable products of its dissociation were mainly C2H2, CH4 and HF. This CF4 addition considerably enhanced the nucleation and growth characteristics on silicon and molybdenum. Diamond growth was observed with substrate temperature as low as 390 °C. A comparative study for the growth dependence on substrate temperature with and without CF4 addition in the gas mixture is presented. The growth rate was measured by post-growth weighing with a micro balance. An activation energy of 11 kcal mol-1 for growth with CF4 addition was obtained. Raman spectra and atomic force microscopy were used to characterize the diamond films.

CVD diamond burrs — Development and applications

Abstract Some studies about application devices that have been developed in our laboratory are presented. Some characterizations show the quality of free-standing CVD diamond film. An improvement of CVD diamond burrs will be presented, including laser cutting of diamond film, processes for soldering the film to the stainless steel tips, and statistical performance of the burrs. Basic studies were carried out in order to improve the film quality. These results gave us support for the scaling-up of a hot-filament system for the manufacturing of CVD diamond burrs.

Low temperature chemical vapour deposition of diamond on tungsten carbides using CF4 gas doping for machine tool applications

Abstract We report here the improvement of the adhesion of CVD diamond films on WC-Co (6%) tools and the enhancement in the cutting tool life-time by the introduction of CF4 in the CH 4 H 2 gas mixture. By the use of this halogen precursor it was possible to reduce the substrate temperature during the deposition without degradation in the diamond film quality. The low-temperature deposition improves the smoothness of the coating and minimizes the thermal stress induced by the CVD process.