Deposition of Superhard Cubic Boron Nitride and other Materials from the BCN Ternary System by Supersonic Plasma Jet Chemical Vapor Deposition
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| Introduction: The synthesis of hard materials has long been a goal of industry, as such coatings have many potential applications in the fields of metal coating and electronics. For example, if machine tools could be coated with superhard coatings, up to 40% of the costs associated with the use and disposal of environmentally hazardous coatings could be eliminated [1]. Two materials with extremely attractive properties for such applications are diamond and the cubic phase of boron nitride (cBN). Diamond and cubic boron nitride (cBN), with hardness of approximately 70 and 50 GPa , are respectively the hardest and second hardest known materials. Cubic boron nitride does not dissolve in most metals and is stable to oxidation in air to temperatures greater than 1000oC, thus making cBN preferable to many other materials, including diamond, for metal coating applications. Despite the great advantages of cBN as a tool coating, proliferation of functional cBN coatings has been limited due to the inability to deposit cBN films greater than ~1 micron thick due to great amounts of film stress in cBN films. One possible solution to this limitation is to deposit materials from the BCN ternary system, possibly with high cubic contents. While various researchers have deposited materials from the BCN ternary system several microns in thickness with hardnesses up to 64 GPa [2], the published hardness measurements of these novel materials are relatively rare. Also, published diagnostics of deposition systems used for BCN deposition, which would lead to an enhanced understanding of the BCN deposition process, are also quite rare. Therefore, our current research involves depositing films from the BCN ternary system using a supersonic thermal plasma jet deposition system, and employing plasma diagnostics to enhance our understanding of the deposition process. Experimental Approach: Our deposition system is shown schematically in Figure 1. Mixtures of argon and hydrogen or nitrogen flow through the torch. A bias is applied to the cathode and an arc strikes between the cathode and grounded anode nozzle. The gases are heated by the arc, form a thermal plasma, and accelerated towards the substrate at supersonic speeds. Reactants such as boron trifluoride, methane, hydrogen, and mixtures thereof are injected in the expansion portion of the nozzle, dissociated in the plasma, and react to form a film on the substrate. To enhance the deposition process a secondary discharge is superimposed over the plasma jet by applying a positive or negative bias to the substrate with respect to the grounded anode (see Figure 2). To enhance our understanding of the film deposition process in this system we are using plasma diagnostics to measure the effects of various deposition conditions on the plasma parameters operative in the deposition of BCN materials in the described deposition system. Optical emission spectroscopy is carried out with focusing lenses and a spectrometer scanned axially along the plasma jet, while Langmuir probe measurements are carried out by inserting a tungsten probe through the back of the substrate (see Figure 1). The probe may be axially translated to take measurements at or within a few millimeters in front of the substrate. Probe measurements are taken under conditions for which a film is not deposited to examine the plasma environment under different deposition conditions. |
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References: 1.S. Veprek. Journal of Vacuum Science and Technology A, v. 17, n. 5 (1999) pp. 2401-2420. 2.D. Hegemann, R.Riedel, C.Oehr. Thin Solid Films v.339 (1999), pp. 154-159.
