Fatigue crack growth along interface between metal and ceramics submicron-thick films in inert environment

The mechanical crack growth along the interface between submicron films (Cu/SiN) under fatigue is experimentally investigated in an inert environment. A modified four-point bend specimen which has only one interface crack to facilitate the control of crack growth is proposed for the tests. The results reveal that the clear interface crack between Cu and SiN grows under the cyclic load even in the low humidity of the inert environment (3.5 (plus-minus) 1% relative humidity (RH)). The crack growth rate, da/dN, is governed by the stress intensity factor range, (delta)Ki, and the following three stages are observed in the da/dN - (delta)Ki curve: threshold, stable growth and critical growth. The region of subcritical crack growth is quite narrow due to the high yield stress of the films and the constraint by the substrate. A similar sigmoidal relationship is obtained in the air (50 (plus-minus) 10% RH), though the growth is greatly accelerated by the humidity.