lntrmationrl JaumJl Of REFRACTORY METALS &HARDMATERlAls zyxwvutsr ELSEV IER International Journal of Refractory Metals & Hard Materials 16 (1998) 201-205 Diamond coatings for tool shafts by induction plasma deposition Jiirg Oberste Berghausa*, Jean-Luc Meuniera, Franqois Gitzhofer” “Pl~rsmu khrwlo~ zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Rescwrch Center (CRTPJ. Departmmt of Chemical Engineering, McGill University, 3610 Universi~ Street, Montrial, Q&hrc, Canada H3A 282 “ Plusma 7Schnology Research Center (CRTP). D6partrmmt de G&tie Chimique, llniversiti de Sherbrooke, Sherhrooke, QuPbec, Canada JIK 2Rl Received 5 December 1997; accepted 8 June 1998 Abstract Induction plasma torches have inherent properties such as large plasma volume, generated without electrodes, with high plasma temperatures (10000 K) and axial injection capability. The potential benefits for diamond synthesis are large coverage as well as high growth rate. In this paper, induction plasma was used to synthesise diamond layers, without diamond preseeding, and the diamond growth (axially and radially) was investigated as a function of various thermal boundary layers thicknesses determined by emission spectroscopy. The small diameter substrate (4.76 mm) was used to probe the diamond quality as a function of the homogeneity of the generated plasma in order to get data on diamond growth all along the large plasma diameter. The results have shown that the gas velocity strongly influences the thermal boundary layer thickness as well as the radial growth of the diamonds. This was demonstrated by SEM as well as roughness measurements. The reduction of the overall diffusion time by the decrease in the boundary layer thickness can consequently improve the growth of the lateral faces of the diamond which are less directly seen by the plasma gases. 0 1998 Elsevier Science Ltd. All rights reserved. 1. Introduction Over the past few years, there has been an increasing number of research efforts directed towards producing diamond films under thermodynamically metastable conditions. The interest in diamond is due to its excel- lent physical and chemical properties. Diamond is the hardest known material and combines optical transpar- ency from ultraviolet to infrared, high electron and hole mobility, and very good thermally conducting and electrical insulating properties. It is also chemically inert. For all these reasons, the technical challenge of diamond synthesis in the form of films is very attractive in view of potential industrial applications. Crystalline diamond can be grown by an astonish- ingly wide variety of energetically assisted chemical vapor deposition processes (CVD). The activation methods range from thermally assisted chemical vapor deposition, for example hot filament processes [l-3], over low pressure microwave enhanced CVD [4-61 to highly energetic atmospheric pressure combustion flames [7], direct-current [8,9] and rf thermal plasma jets [9- 121. Using radio-frequency (rf) induction thermal plasmas at atmospheric pressure, very high deposition rates have been attained. These can be attri- buted to the high density of growth precursors avail- *Corresponding author. able at the elevated pressure and to the large volume of reactant gases that can be excited in a rf torch [13]. The homogeneity of the induction plasma is also prone to creating uniform deposits [g]. The rf plasma technology (rf-CVD) hence shows some advantages in view of attaining large surface coverage and high growth rates of diamond coatings suitable for industrial applications. Ultimately, the use of diamond films as practical industrial coatings will depend on whether they can be grown on non-diamond seeded substrates of desired geometry and material, such as drills, shafts, etc. In this work the diamond film deposition on a small molyb- denum shaft, 4.76 mm in diameter, is investigated as a way to check for the diamond spatial generation distri- bution in the plasma in order to further spray on cylin- drical rotating mandrels. The diamond formation in thermal plasma CVD strongly depends on the gas phase chemistry in the boundary layer region between the plasma and the substrate surface. Manipulation of process parameters directly affecting the boundary layer chemistry allows for enhancement both in deposi- tion rate and in diamond film quality [10,12,14]. In an attempt to manipulate the thickness of the boundary layer, while keeping most of the reactor conditions constant, the plate power of the power supply is adjusted to either 14.7 or 21.8 kW. The relationship between boundary layer thickness and diamond growth behavior is described. 0263-4368/98/$ - see front matter 0 1998 Elsevier Science Ltd. All rights reserved PII: SO263-4368(98)00029-8