Sn Whisker Growth During Mechanical Loading and Unloading: Highlighting the Critical Role of Stress in Whisker Growth

Whisker formation in metallic coatings has been studied extensively over the past few decades. A wide consensus exists for stress as a driving force behind whisker growth. However, many of the details are not understood, and other theories have been proposed that challenge stress-based explanations. In this paper, we quantify the kinetics of stress-driven whisker growth by demonstrating that the whisker growth can be turned on and off by application and removal of mechanical load. We observed that (i) whiskers formed during the loading stage grow linearly with time, (ii) they stop growing soon after the load was removed, and (iii) they resume growth when the load was reapplied. The experimental results are explained using finite element analysis (FEA) of stress evolution due to applied load and average growth rate predictions. The results have implications for applications in which the coatings experience stresses, for example, due to fasteners or thermal cycling.

Automated summary

Whisker formation in metallic coatings has been extensively studied, with stress generally accepted as the driving force behind whisker growth. However, this paper presents experiments that demonstrate the ability to control whisker growth by applying and removing mechanical load. The observed linear growth during loading, cessation of growth upon load removal, and resumption of growth with load reapplication are explained using finite element analysis and growth rate predictions. These results have important implications for stress-related applications of coatings, such as fasteners or thermal cycling.