Adhesion and Progressive Debonding of Polymer/Metal Interfaces: Effects of Temperature and Environment
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the above time dependent failure mechanisms. Williams and Marshall [1] determined the existence of a stress corrosion process in bulk polymeric materials. They proposed that there exist three regions in a crack growth rate, da/dN, versus applied stress intensity factor, K1, plot, designated regions I, 11,and III. They suggested that relaxation processes, hydrodynamic flow properties of the environment, and failure properties of the polymer in air controlled these three regions, respectively. To date, however, such time dependent delamination data for interfaces with polymers is almost non-existent, particularly under cyclic loading condition where mechanical fatigue effect might occur. The absence of such data along with the material and processing determinants of subcritical crack-growth behavior currently represents a serious limitation to life prediction strategies and to the design of complex multi-layered structures and devices with superior structural integrity and long term reliability. An understanding of the effects of environment on the debond growth-rate is of considerable importance from the viewpoint of elucidating the complex mechanical and environmental interactions which occur during debond propagation. The measured rate of fatigue crack growth in an aggressive environment, (da/dN)e, is considered to be the sum of three components [21:
dN-e =- dN
kdN scc
.
dN)f
where (da/dN), is the fatigue crack-growth rate in a reference environment (pure mechanical fatigue), (da/dN)scc represents the contribution from the environment-induced stress corrosion process, and (da/dN)cf is an environmentally assisted enhancement of the rate from a synergistic interaction of fatigue 263 Mat. Res. Soc. Symp. Proc. Vol. 563 © 1999 Materials Research Society
and environmental attack. In the present study, we do not explicitly consider each contribution to the measured fatigue debond growth rate, (da/dN),, but rather assume that the overall growth rate is proportional to the chemical reaction rate of the environmental species (H 20) with the strained crack tip bonds. By modifying Williams and Marshall's model [1] for describing stress corrosion cracking in bulk polymers, the fatigue debond growth rate in an aggressive environment, (da/dN),, may be written as:
(2)
da= Co(AG)me -AH RT =C(AGr
dN
where Co represents a constant that is related to the chemical activity of the reacting species, m is a constant, AH is the activation energy of the process, R is the gas constant, and T is the absolute temperature. EXPERIMENTAL Double cantilever beam (DCB) specimens were made by electroplating a thin nickel layer onto two copper substrates and then bonding together with an epoxy adhesive (Phenol-Novolac) to form a Phenol-Novolac/Ni/Cu sandwiched structure. A silica filler is added to adjust the coefficient of thermal expansion of the polymer to that of the substrate. Fracture mechanics samples containing the interfaces of interest were prepared and details have been described elsewhere [3, 4]. All tests were performed insid
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