The investigation of creep of electroplated Sn and Ni-Sn coating on copper at room temperature by nanoindentation

Sn and Sn-based alloy coatings (such as Ni-Sn) are important electrode materials in lithium ion batteries. The mechanical performance of such coatings is essential because they undergo severe volume change induced stress during charge-discharge cycling. As Ni-Sn and Sn anode materials will operate for long periods under stress during charge-discharge cycling at or near room temperature time-dependent relaxation mechanisms such as creep may take place. In this study, the nanoindentation creep of these materials at room temperature (RT) has been investigated. It was found that the creep very easily reaches exhaustion for Ni-Sn and the copper substrate even at high load holds and thus both exhibit a high stress exponent. For low melting temperature material such as Sn the behaviour is different: the stress exponent obtained at RT is around 3 to 8 which is consistent with conventional creep tests. The creep behaviour of an as-deposited polycrystalline Sn thin film with a rough surface strongly depends on its microstructure which makes the nanoindentation creep analysis much more complex than in single crystal materials or polycrystalline bulk materials with large grain size. The microstructural influence on creep mechanisms in Sn films is highlighted in this paper. (C) 2008 Elsevier B.V. All rights reserved.