Dense, Amorphous Boron and Boron Carbide Coatings through Vacuum Arc Deposition from Non-metal, Sintered Cathodes: Applications to Nuclear Fusion and other Extreme Environments

C.C. Klepper (Sp), J.M. Williams, R.C. Hazelton, E.J. Yadlowsky, J.J. Moschella, M.D. Keitz, E.P. Carlson, HY-Tech Research Corporation, Radford (USA) 
 
This paper will discuss the current state and potential benefits of the development of stable, near steady state, vacuum cathodic arc discharges on sintered cathodes made from powders of pure boron, as well as boron carbide and similar compounds. The development has overcome not only differences in arc discharge initiation and operation from that of metal cathodes, but also issues of mechanical integrity of the cathode compacts under the severe stresses induced by the arc discharge. Pure, dense, amorphous coatings of these materials can now be deposited at high deposition rates. Of particular current interest for extreme environments are amorphous boron coatings. The hardness of elemental boron is 33 GPa, a value greater than that of most other hard-coatings. Excellent corrosion resistance and refractory properties result from the highly passive nature of the native oxide. As an element, boron is highly reactive with many other elements. It forms hard and highly refractory compounds with many transition metals. Because of this reactivity (high negative heats of mixing) boron is strongly adherent to many engineering metals; substrates of several commercial alloys have been coated. These include Ti-6Al-4V for biomedical and aerospace applications, 52100 bearing steel for tribological applications, H13 steel for molten aluminum-alloy die-casting, and Co-Cr-Mo for a biomedical application. The high reactivity of the boron rich coatings together with the high energetics of the deposition process can result in deep transition layers. The inter-layer compounds and in-diffusion possible with transition metal substrates, many of which have attractive high temperature strength and corrosion properties in their own right, can result in engineering components for use in extreme environments. A summary of results of testing in extreme environments (including the die-casting application) will be provided and the potential for coating plasma-facing components for nuclear fusion will be discussed