Hardness and Modulus Properties in Ion-beam-modified Amorphous Carbon: Temperature and Dose Rate Dependences
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Hardness and modulus properties in ion-beam-modified amorphous carbon: Temperature and dose rate dependences Deok-Hyung Lee, Hyukjae Lee, and Byungwoo Parka) School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245 (Received 18 September 1996; accepted 12 February 1997)
Ion implantation into amorphous carbon has been initiated to investigate the possibility of superhard carbon-nitride formation. Studies of implantation-temperature effects by 100 keV N+ or 80 keV C+ ions at 50 mA show a narrow temperature window at approximately 2100 ±C for the optimum surface hardness and elastic modulus (measured by nanoindentation), both values much higher than those for the unimplanted amorphous carbon. No distinguishable properties are found between nitrogen and self (carbon) implantations. At a dose rate of 5 mA, however, the optimum hardness and modulus are found at a lower implantation temperature, with a broader temperature window. The enhanced strengths are well correlated with the asymmetric diffuse peak at around 1500 cm–1 in Raman spectroscopy, and the increased ratio of sp3 - over sp2 -bonded carbon sites observed by electron energy loss spectroscopy.
I. INTRODUCTION
II. EXPERIMENTAL PROCEDURE
Metastable carbon-nitride phase, which was first predicted to have a bulk modulus comparable to that of diamond by a scaling relationship and pseudopotential total-energy calculations,1,2 has been the focus of considerable research activities for over a decade. Numerous attempts have been made to synthesize this hypothetical b –C3 N4 phase, employing nonequilibrium processes. Promising indications of covalent C–N bonding and relatively good mechanical properties have been claimed,3–9 even though a full understanding of stability, crystallinity, and stoichiometry of the compounds is lacking. Other possible C–N structures have been predicted by molecular dynamics,10 having comparable stability with that of b –C3 N4 . However, due to the complex nature of the processes imposing kinetic limitations for metastable phases during synthesis, it is difficult to predict the nitride microstructures and physical properties. Ion implantation is one feasible way to synthesize metastable carbon-nitrogen compounds with energetic collision cascades. Still, there are few reports showing improved structural properties in the attempted synthesis of carbon nitride by ion implantation. In this article, ionbeam modification of amorphous carbon is presented for the enhanced surface hardness and elastic modulus, by studying the effects of chemical nature for the implanting species, implantation temperatures, and dose rates.
The experiments were performed by transforming amorphous carbon (from Kobe Steel, having a density of 1.8 gycm3 and a thickness of 0.6 mm)11 with nitrogen or carbon implantation. To remove any adsorbed contamination on the substrates, those were preannealed at 200 ±C for 20 min in the implanter vacuum chamber (,10–7 Torr) before ion impl
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