H. Sein

Application of diamond coatings onto small dental tools

Abstract Small dental tools such as burs and drills are commonly used in dental practice and laboratory. Conventional diamond burs used for grinding operations have a number of problems associated with heterogeneity of the crystallites, decreased cutting efficiency, need for repeated sterilisation and short life. These burs are manufactured by imbedding diamond particles into the burs using a suitable binder matrix material. The use of a diamond coating may offer an improvement in dental bur technology. Chemical vapour deposition (CVD) of diamond coatings onto the cemented tungsten carbide WC–Co substrate is problematic. Generally the adhesion of diamond coating to cemented carbide substrate is poor. It is obvious that the binder materials such as cobalt can suppress diamond growth and enhance graphitic deposits, which cause poor adhesion and low diamond nucleation density. The effects of key process parameters such as filament position, filament and substrate temperature and pre-treated substrate material on the coating properties have been investigated using a variety of analytical techniques. Characterisations of the substrates and polycrystalline diamond film morphology were analysed by scanning electron microscopy (SEM). The chemical composition was evaluated by energy dispersive spectroscopy (EDS). Raman spectroscopy was used to assess the carbon-phase purity and give an indication of the stress levels in the as-grown polycrystalline diamond films.

Diamond films grown on cemented WC–Co dental burs using an improved CVD method

Abstract A modified hot-filament chemical vapour deposition system was used to deposit adherent diamond films onto cutting edges of cemented tungsten carbide (WC–Co) dental burs. Generally, the as-grown films were found to be polycrystalline and displayed good coverage. Micro-Raman spectroscopy was used to characterise the stress distribution at three positions: (i) bur tip, (ii) middle of the bur and (iii) end of bur. The stress at the back, in the middle and at the tip of the bur was calculated to be −1.7, −2.3 and −3.4 GPa in compression, respectively. It was noted that the filament temperature, as measured by the two colour optical pyrometer, varied along the coils of the filament. The coiled filament was at higher temperatures around the central region as compared to the filament edges. The temperature of the cutting edges was higher at the tip of the bur compared to the middle and back of the bur. The cutting teeth were closer to the filament and therefore expected to be at a slightly higher temperature. Consequently, a thicker coating was deposited on the edges of the cutting teeth as compared to the flat bur base material.

Stress distribution in diamond films grown on cemented WC–Co dental burs using modified hot-filament CVD

Abstract A modified hot-filament chemical vapour deposition system has been used to deposit diamond films onto dental burs. The as-grown films were found to be polycrystalline and displayed good coverage. Micro-Raman spectra were taken at different positions of the coating to characterise stress distribution in the deposited film. The Raman diamond peak positions were used to calculate the stress along and across the diamond coating. It was found that the stress across the coating remained constant. However, the stresses along the coating displayed some interesting results. The stress at the base, in the middle and at the tip of the bur was calculated to be −1.7, −2.3 and −3.4 GPa in compression, respectively. The temperature of the bur during deposition was found to be different at different positions on the bur. At the base of the cutting area; at the middle of the bur; and at the tip of the bur the temperature was found to be 842, 908 and 952 °C, respectively. The stress in the films increased as the temperature of the bur increased. Scanning electron microscopy analysis was used to characterise the as-grown films for morphology and crystallinity.

The impact of inert gases on the structure, properties and growth of nanocrystalline diamond

For biomedical and electronic applications, it is highly desirable to deposit smooth diamond films with crystal sizes in the nanoscale range. We present experimental results of chemical vapour deposition diamond growth from CH4 with incremental substitution of H2 with He or Ar gases; the concentrations of the inert gases were varied between 0 and 98 vol%. Results show that initially the addition of either argon or helium increases the growth rate and significantly alters the film structure and crystallinity up to 60 vol%. With additions of argon or helium greater than 60 vol% in the gas phase the growth decreases and there is degradation of the crystal structure. In general, nanocrystalline diamond has been deposited at dilutions in excess of 90 vol% helium or argon.

Chemical vapour deposition diamond coating on tungsten carbide dental cutting tools

Diamond coatings on Co cemented tungsten carbide (WC-Co) hard metal tools are widely used for cutting non-ferrous metals. It is difficult to deposit diamond onto cutting tools, which generally have a complex geometry, using a single step growth process. This paper focuses on the deposition of polycrystalline diamond films onto dental tools, which possess 3D complex or cylindrical shape, employing a novel single step chemical vapour deposition (CVD) growth process. The diamond deposition is carried out in a hot filament chemical vapour deposition (HFCVD) reactor with a modified filament arrangement. The filament is mounted vertically with the drill held concentrically in between the filament coils, as opposed to the commonly used horizontal arrangement. This is a simple and inexpensive filament arrangement. In addition, the problems associated with adhesion of diamond films on WC-Co substrates are amplified in dental tools due to the very sharp edges and unpredictable cutting forces. The presence of Co, used as a binder in hard metals, generally causes poor adhesion. The amount of metallic Co on the surface can be reduced using a two step pre-treatment employing Murakami etching followed by an acid treatment. Diamond films are examined in terms of their growth rate, morphology, adhesion and cutting efficiency. We found that in the diamond coated dental tool the wear rate was reduced by a factor of three as compared to the uncoated tool.

Chemical vapour deposition of microdrill cutting edges for micro- and nanotechnology applications

Conventional cemented tungsten carbide-cobalt (WC-Co) microdrills generally have a low cutting efficiency and short lifetime mainly because they operate at very high cutting speeds. Since it is relatively expensive to make microtools it is highly desirable to improve their lifetime and in-service performance. Microtools used to make microelectronic and mechanical systems (M.E.M.S) devices with sharp cutting edges, such as milling or drilling tools need protective coating in order to extend life and improve performance. One method of achieving this objective is to use a suitable surface engineering technology to deposit a hard wear resistant coating, such as diamond. Diamond has excellent mechanical properties, such as ultra-high hardness and a low friction coefficient. One of the most promising surface treatment technologies for depositing diamond onto complex shaped components is chemical vapour deposition (CVD). However, CVD of diamond coatings onto the cemented WC-Co tool has proved to be problematic. Binder materials such as cobalt can suppress diamond nucleation resulting in poor adhesion between the coating and substrate. In this paper the effects of pre-treated substrate material on the coating structure are reported. The morphology and the crystallinity of the as-grown films was characterised by using scanning electron microscopy (SEM). Raman spectroscopy was used to assess the carbon-phase purity and give an indication of the stress levels in the as-grown polycrystalline diamond films. The diamond coated tools have potential applications in micro- and nanomachining of micro- and nano-sized components used in M.E.MS.

Diamond-coated cutting tools for biomedical applications

Diamond coatings are attractive for cutting processes due to their high-hardness, low-friction coefficient; excellent wear resistance, and chemical inertness. The application of diamond coatings on cemented, tungsten carbide (WC-Co) burs has been the subject of much attention in recent years as a method to improve cutting performance and tool life. WC-Co burs containing 6% Co and 94% WC substrate, with an average grain size of 1–3 µm, were used in this study. To improve the adhesion between diamond and WC substrates, it is necessary to etch away the surface Co and prepare the surface for subsequent diamond growth. Hot filament chemical vapor deposition (HFCVD), with a modified vertical filament arrangement, has been used for the deposition of diamond films. Diamond film quality and purity has been characterized using scanning electron microscopy (SEM) and micro-Raman spectroscopy. The performance of diamond-coated WC-Co burs, uncoated WC-Co burs, and diamond-embedded (sintered) burs have been compared by drilling a series of holes into various materials such as human teeth, borosilicate glass, and acrylic teeth. Flank wear has been used to assess the wear rates of the burs when machining biomedical materials such as those just described.

Time-modulated chemical vapor deposition of diamond films

This article investigates the role of substrate temperature in the deposition of diamond films using a newly developed time-modulated chemical vapor deposition (TMCVD) process. TMCVD was used to deposit polycrystalline diamond coatings onto silicon substrates using hot-filament chemical vapor deposition system. In this investigation, the effect of (a) substrate temperature and (b) methane (CH4) content in the reactor on diamond film deposition was studied. The distinctive feature of the TMCVD process is that it time-modulates CH4 flow into the reactor during the complete growth process. It was noted that the substrate temperature fluctuated during the CH4 modulations, and this significantly affected some key properties of the deposited films. Two sets of samples have been prepared, in each of which there was one sample that was prepared while the substrate temperature fluctuated and the other sample, which was deposited while maintaining the substrate temperature, was fixed. To keep the substrate temperature constant, the filament power was varied accordingly. In this article, the findings are discussed in terms of the CH4 content in the reactor and the substrate temperature. It was found that secondary nucleation occurred during the high timed CH4 modulations. The as-deposited films were characterized for morphology, diamond-C phase purity, hardness, and surface roughness using scanning electron microscopy, Raman spectroscopy, Vickers hardness testing, and surface profilometry, respectively.

Pulsed biased growth of nanocrystalline diamond by hot filament chemical vapour deposition

Abstract For many industrial applications such as biomedical instruments, optical devices and microelectromechanical systems, the control of the film structure, crystallinity and morphology is of critical importance. The crystallite size, orientation and surface roughness have a profound effect on the mechanical, optical and electrical properties of the films and therefore the final product performance. In order to reduce the crystallite size and surface roughness, inert gases were added to the methane and hydrogen mixture during chemical vapour deposition of nanocrystalline diamond films. In addition, the results on the influence of pulsed biasing on the morphology of these films are reported. Bias voltages in the range –300-0 V were investigated. Increasing the bias voltage significantly alters the crystallite size and morphology of the deposited films. Raman spectroscopy, SEM and atomic force microscopy were used to characterise the nanocrystalline diamond films. SE/501

Novel diamond-coated tools for dental drilling applications

The application of diamond coatings on cemented tungsten carbide (WC-Co) tools has been the subject of much attention in recent years in order to improve cutting performance and tool life in orthodontic applications. WC-Co tools containing 6% Co metal and 94% WC substrate with an average grain size of 1 – 3 μm were used in this study. In order to improve the adhesion between diamond and WC substrates it is necessary to etch cobalt from the surface and prepare it for subsequent diamond growth. Alternatively, a titanium nitride (TiN) interlayer can be used prior to diamond deposition. Hot filament chemical vapour deposition (HFCVD) with a modified vertical filament arrangement has been employed for the deposition of diamond films to TiN and etched WC substrates. Diamond film quality and purity has been characterized using scanning electron microscopy (SEM) and micro Raman spectroscopy. The performances of diamond-coated WC-Co tools, uncoated WC-Co tools, and diamond embedded (sintered) tools have been compared by drilling a series of holes into various materials such as human tooth, borosilicate glass, and acrylic tooth materials. Flank wear has been used to assess the wear rates of the tools when machining biomedical materials such as those described above. It is shown that using an interlayer such as TiN prior to diamond deposition provides the best surface preparation for producing dental tools.