Growth, structure, and mechanical properties of transition metal carbide superlattices
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. Birch Department of Physics, Thin Film Physics Division, Linko¨ping University, SE-58183 Linko¨ping, Sweden
M. Ode´n Department of Mechanical Engineering, Division of Engineering Materials, Linko¨ping University, SE-58183 Linko¨ping, Sweden
J-O. Malm National Center of HREM, Department of Inorganic Chemistry 2, Lund University, P.O. Box 124, SE-22100 Lund, Sweden
L. Hultman Department of Physics, Thin Film Physics Division, Linko¨ping University, SE-58183 Linko¨ping, Sweden
U. Jansson Department of Inorganic Chemistry, The Ångstro¨m Laboratory, Uppsala University, P.O. Box 538, SE-75121 Uppsala, Sweden (Received 4 April 2000; accepted 14 February 2001)
Superlattices of TiC/VC have been deposited on MgO(001) substrates by simultaneous direct current metal magnetron sputtering and C60 evaporation in the temperature range 200–800 °C. Thin superlattices (approximately 1000 Å) deposited at 400 °C exhibited an epitaxial growth with abrupt interfaces while films deposited at 200 °C showed a partial loss of epitaxy. At 800 °C roughening by surface diffusion started to degrade the superlattices and introduced a columnar microstructure. A loss of epitaxy was observed for thicker (>7000 Å) superlattice films deposited at 400 °C. The results suggest that this observation is due to difficulties in depositing epitaxial VC.
I. INTRODUCTION
Ceramic superlattice films have been reported to exhibit a hardness increase of 2–3 times relative to the observed values for the homogeneous materials.1–3 The main contribution to the hardness enhancement in these films has been attributed to barriers for moving dislocations across the layer interfaces generated by the difference in shear modulus between the constituting layers of the superlattice.4 To date, however, there are no results for the strengthening of transition metal carbide superlattice films. The reason for this situation may be the lack of a suitable deposition technique for growth of epitaxial carbide films and superlattices at reasonably low temperatures, where the interdiffusion between the individual carbide layers is minimized and where abrupt interfaces can be formed. Recently, however, there has been a breakthrough in the deposition of epitaxial carbide films by the use of C60 as a carbon source.5–8 The processes are based on the simultaneous evaporation of J. Mater. Res., Vol. 16, No. 5, May 2001
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C60 in combination with either metal evaporation or metal dc (direct current) magnetron sputtering. Epitaxial films of TiC, VC, and NbC as well as TiC/VC and TiC/NbC superlattices have been deposited at temperatures below 500 °C. In fact, epitaxial TiC films on MgO(001) have been deposited at a temperature as low as 100 °C using metal sputtering.8 The mechanism behind the low epitaxial temperatures is yet unknown, but it is clear that it must be attributed to some unique property of the C60 molecule. The new C60-based processes make it possible to study the mechanical properties of epitaxial metal carbide films and
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