Amorphic Diamond Films Produced By Laser Ablation

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AMORPHIC DIAMOND FILMS PRODUCED BY LASER ABLATION C. B. COLLINS, F. DAVANLOO, T. J. LEE, J. H. YOU AND H. PARK Center for Quantum Electronics, University of Texas at Dallas, P. 0. Box 830688, Richardson, Texas 75083-0688

ABSTRACT Four types of diamond films have been shown to be stable and free from hydrogen: crystalline diamond, amorphic diamond, i-C and defected graphite. Each is significantly harder than other forms of carbon, but only now are quantitative values of hardness being reported. In this work nanoindentation techniques were used on these highly elastic films and linear ranges of plastic response were identified. Values of hardness were extracted from the on-load data for amorphic diamond and crystalline diamond that could not be distinguished from the hardness of naturally occurring diamond. This seems to explain why thin layers of amorphic diamond have been so effective in protecting substrates from the erosive impacts of particles and droplets reported in previous studies. INTRODUCTION The search for a thin film coating of carbon with the properties of diamond has attracted an intense level of interest over the past five years. Conventional approaches have been based upon techniques of chemical vapor deposition (CVD) and its variants. To date only two products have been realized from those efforts, polycrystalline diamond films with columnar patterns of growth and "diamondlike carbon" coatings which are noncrystalline hydrocarbons usually containing more hydrogen than carbon. Both are critically flawed by the impractical temperatures and aggressive chemical environments needed for growth. To survive the conditions for depositing either of these coatings, a material has to be so robust that it really needs no protection. Recently, we reported the discovery of three additional structures of pure carbon having many of the properties of diamond.1. 2 These new materials have been called defected graphite, i-C, and amorphous ceramic diamond - an appellation shortened to amorphic diamond for convenience. They were condensed without catalysts from laser plasmas prepared at successively higher energies and all three required the precursive ions in the plasma to carry more energy to the point of condensation than has been available in the traditional approaches. Both defected graphite and i-C have now been synthesized by other means as well, but the best of the new materials, amorphic diamond seems to be the unique product of laser processing. 3"1' It is prepared by the laser ablation of pure carbon feedstock under UHV conditions for peak intensities at the focus in excess of 10"l watts cm- 2 . Notwithstanding the requirement for the highest of the input energies to make amorphic diamond, the average power transferred to the item to be coated is manageable, so process temperatures do not exceed 35°C at the substrate. Almost any material can be coated that is compatible with a vacuum environment and layers of amorphic diamond have been grown at rates of 0.5 pm/hr over 100 cm 2 areas on a wide variety of subst