Fatigue crack propagation properties of submicron-thick freestanding copper films in vacuum environment

Abstract

Fatigue crack propagation experiments were conducted in approximately 500 nm thick freestanding copper (Cu) films in both air and vacuum environments to clarify the effects of vacuum environment on fatigue crack propagation properties. First, we newly developed an experimental setup for fatigue crack propagation experiments of the freestanding Cu films inside a vacuum chamber of a field-emission scanning electron microscope (FESEM). Fatigue crack propagation experiments were conducted in ambient air and vacuum environment of the FESEM chamber (˜10-4 Pa) under load-control conditions with constant maximum stress and at a stress ratio R of 0.1. In situ FESEM observations of fatigue crack propagation confirmed that preceding intrusions/extrusions were formed ahead of the fatigue crack tip, and the fatigue crack then propagated preferentially through these intrusions/extrusions in the lower stress intensity factor range (ΔK). In the higher ΔK, the fatigue crack propagated in tensile fracture mode. These mechanisms of fatigue crack propagation were similar to those in air. The relationships between fatigue crack propagation rate (da/dN) and stress intensity factor range (ΔK) in both environments were roughly within a narrow band in the region of ΔK ≳ 4—5 MPam1/2. On the other hand, da/dN in vacuum became smaller than that in air in the region of ΔK ≲ 4—5 MPam1/2. FESEM observations confirmed that the fracture surfaces morphologies depended on the environments in ΔK ≲ 4—5 MPam1/2: flat fracture surface were mainly observed in air, whereas, in vacuum environment, blunt fracture surface with fine roughness were mainly observed. This suggests that reversible cyclic slip deformation and rewelding occurred in vacuum environments, resulting in smaller da/dN in vacuum than air.