Influence of the substrate bias on the size and thermal stability of grains in magnetron-sputtered nanocrystalline Ag fi
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N. Schell and R.M.S. Martins Institute of Ion Beam Physics and Materials Research, Forschungszentrum Rossendorf, D-01314 Dresden, Germany (Received 11 November 2004; accepted 2 February 2005)
The nanostructural evolution during heat treatments of direct-current magnetronsputtered Ag films, deposited at room temperature at different substrate bias voltages, was experimentally studied. A growth chamber equipped with a magnetron and Kapton windows for in-situ x-ray diffraction was mounted on a six-circle goniometer at a synchrotron beam line. Bragg–Brentano x-ray diffraction was used to monitor the (111) Bragg peak during thermal annealing of the Ag films. In addition, to investigate the 〈111〉 fiber texture, one-dimensional pole figures were measured ex situ. The thermal stability of the nanostructure was sensitively dependent on the substrate bias voltage. Increasing the bias voltage resulted in significantly lower rates of grain growth, which we ascribe mainly to the formation of Ar bubbles. Furthermore, the grain size in the as-deposited films decreased with increasing bias voltage while the width of the one-dimensional pole figures increased.
I. INTRODUCTION
Nanocrystalline (nc) materials can be considered a new class of materials with a great potential for industrial applications. They are characterized by grain sizes between a few nanometers and 100 nm, which result in extremely large grain-boundary areas.1 This area gives the nc materials a variety of improved properties compared with those of the conventional, polycrystalline counterparts.2 Control of the grain size is of vital importance for the optimization of the mechanical properties of nc materials. As described by the empirical Hall–Petsch equation,3,4 due to the restricted dislocation motion, a decrease in grain size increases the hardness. However, at sufficiently small grain sizes, dislocations are absent, and other deformation mechanisms like Coble creep and/or grain sliding may take over, in some cases even resulting in a decreasing hardness with smaller grain size.1 In conventional polycrystalline materials, the toughness decreases with increasing hardness. However, as the grain boundaries are effective barriers against crack
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0143 J. Mater. Res., Vol. 20, No. 4, Apr 2005
propagation, a decrease in the grain size of the nc materials with a simultaneous increase in grain-boundary area results in improved hardness and toughness.5 The large, stored grain-boundary energies result in significant driving forces for grain growth, and thus in some nc materials grain growth takes place even at room temperature. However, in most nc materials, the onset of grain growth was observed at temperatures significantly above room temperature due to specific structures and compositions of the grain boundaries.6 Although the onset temperature for grain growth is dependent on the method of synthesis (variations in grain boundary structure and composition), it has been fou
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