Effect of process variables on the structure, residual stress, and hardness of sputtered nanocrystalline nickel films
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R.A. Hoffman and A. Madan Advanced Coating Technology Group, Northwestern University, Evanston, Illinois 60208
J.R. Weertman Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208 (Received 8 June 2000; accepted 17 January 2001)
Nanocrystalline nickel films of about 0.1 m thickness grown by sputtering with and without substrate bias possessed average grain sizes of 9–25 nm. Variation in substrate bias at room and liquid nitrogen temperature of deposition strongly affected grain structure and size distribution. Qualitative studies of film surfaces showed variation in roughness and porosity level with substrate bias and film thickness (maximum of 8 m). The films had tensile residual stress, which varied with deposition conditions. The hardness values were much higher than those of coarse-grained nickel but decreased with an increase in the film thickness because of grain growth.
I. INTRODUCTION
Nanocrystalline metals have generated a great deal of interest in recent years because they demonstrate impressive mechanical behavior characterized by very high strengths.1–3 Strengthening is induced by the restricted deformation of nanoscale grains, which inhibit dislocation activity, and also by the presence of a large volume fraction of grain boundaries. Studies2– 4 on bulk nickel processed by inert gas condensations5 (IGC) have shown very high hardness and yield strengths, which are as much as 25% of the theoretical shear strength of the metal and half that of dislocation-free single-crystal whiskers. The compaction of nanocrystalline powders produced by methods like IGC has often led to grain growth6 (in the case of warm compaction) and flaws such as closed porosities, heterogeneities in grain size distribution,7 etc. These consequences of the compaction process could adversely affect the mechanical properties. Hence, it is necessary to experiment with alternative methods of synthesis. Other methods of synthesis include electrodeposition,8–10 ball milling,ll and physical vapor deposition processes such as magnetron sputteringl2–16 and electron beam evaporation.17–20 Nanocrystalline Ni specimens processed by electrodeposition8,9 have shown very high hardness and strength values. However, samples prepared by electrodeposition can have impurities from the plating process.10 While ball milling results in contamination from different sources, magnetron sputtering leads to high purity powders. Both pure metallic and alloy 1010
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J. Mater. Res., Vol. 16, No. 4, Apr 2001 Downloaded: 30 May 2014
powders have been generated by magnetron sputtering of suitable targets.12 It has been found that the size of the powder particles produced by magnetron sputtering depends on the Ar pressure; a pressure of around 150 mtorr yielded powders with nanocrystalline particles.l3 The goal of these experiments was to generate nanocrystalline powders and not film or bulk samples. Nanocrystalline metallic films have been processed using both magnetron sputtering14 –16 a
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