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Diamond has got a unique wear resistance, therefore diamond coatings are often used to improve the performance of cutting tools and wear parts. In order to obtain diamond coatings over adequate substrates, it is necessary to combine the suitable amounts of the right gas precursors and heat them for reactions to occur.
There are several methods for the excitation of the gas phase: microwave plasma, flame jet or even laser. However, the present application note concerns the hot-filament vapour deposition technique (HFCVD). Typically in this method a dilute mixture of carbon containing gas such as methane in hydrogen is thermally activated at sub atmospheric pressures by a hot filament. The gas mixture and flow must be cautiously controlled; hence the use of high performance thermal mass flow controllers is imperative.
It is of utmost importance that the mass flow controllers used can guarantee both the proper total amounts of gases and the repeatability of the process; otherwise the uniformity and overall quality of the attained thin films will be compromised.
The instruments must be extremely reliable and possess analogue or digital communication, because careful control and monitoring are essential due to safety issues related to the ignitable and explosive nature of the gases involved in the process.
One of the most versatile methods for the production of diamond films is the hot-filament vapour deposition technique (HFCVD) where the gas mixture is heated by being passed along thin W or Ta wires (∅100 to 300 μm) that are heated up to 2400ºC.
Usually only two gases are needed: H2 and CH4, the methane being diluted at 1 to 2 vol% in the hydrogen. The total pressure inside these cold-wall HFCVD reactors can vary typically between 20 mbar and 200 mbar, the total flow depending on the size and geometry of the reactor chamber.
A recent type of diamond coatings is termed nanocrystalline diamond (NCD), in opposition to the microcrystalline diamond films (MCD). NCD is characterized by a nanometric crystallite size (1 nm to 50 nm) and an extremely smooth surface that retains most of the hardness of MCD and has got improved wear and friction behaviour relatively to MCD. These coatings generally need the addition of a third, inert gas, that contributes to the formation of NCD by enhancing re-nucleation processes during growth an by changing the thermal load of the gases inside the chamber, also affecting the substrate heating. This system is more complex than the MCD one and further care is needed in the control and monitoring of the feed gases.
A further modification of such reactors consists in doping the diamond coatings with Boron (MCD and NCD) during growth in order to make them electrically conductive. For this liquid precursor containing Boron (B) species is generally used and a gas is bubbled through it, carrying the Boron containing vapour to the hot filaments and to the diamond coatings. The doping level is adjusted by selecting the right concentration of Boron in the precursor and by adjusting the gas flow through the precursor. The task becomes increasingly difficult when doing NCD since three gases are already at play. The role of the thermal mass flow controllers (such as EL-FLOW Select, LOW-dP-FLOW or IN-FLOW series) is insurmountable in this application or any other involving CVD processes for diamond growth from gas phases.