Cohesive Zone Models of Polyimide/Aluminum Interphases

The fracture energy required to delaminate PMDA/ODA polyimide films from aluminum substrates was determined using the circular blister test. Films were prepared by spin coating the polyamic acid of PMDA and ODA onto polished aluminum substrates, by vapor co-deposition of PMDA and ODA monomers onto polished aluminum substrates, or by spin-coating the polyamic acid onto polished aluminum substrates that were first coated with thin layers of γ-aminopropyltriethoxysilane (γ-APS). Elastic and elastoplastic analyses were used to extract the fracture energies from the blister test results. Elastoplastic analysis provided fracture energies that ranged from 579 J/m² for spin-coated films on polished substrates to 705 J/m² for vapor-deposited films on polished substrates and to 750 J/m² for spin-coated films on silanated substrates. These values were intermediate between those provided by the two different elastic analyses. Differences in fracture energy determined by the three different analysis methods were related to plastic deformation in the films and, in the case of the two elastic analyses, to differences in the approach used to extract the fracture energy from experimental results. Failure of specimens prepared by spin-coating PMDA/ODA films onto aluminum substrates occurred cohesively within the polymer, near the interface between well imidized polymer in the bulk of the films and poorly imidized polymer in a layer near the aluminum surface. For the case of specimens prepared by vapor codeposition of PMDA and ODA monomers, failure occurred within the vapor deposited films, close to the aluminum/film interface. Failure of spin-coated films on silanated substrates occurred mostly within the γ-APS but leaving 'islands' of polyimide and silane on the aluminum.