The effects of aqueous solution chemistry on the fracture of nanoporous thin-films

Nanoporous thin-film glasses are being considered for a number of emerging technologies ranging from biosensors to microelectronic devices. The integration of these materials into such devices, however, has been limited by their extremely fragile nature and susceptibility to stress corrosion cracking in reactive environments. Despite the intense interest in these materials, there is currently a paucity of experimental data and little understanding regarding the effect of aqueous solutions on reducing adhesion and accelerating crack growth. The objective of this research is to characterize the effect salient aqueous solution chemistries have on crack growth in hybrid organic-inorganic thin-film glasses with controlled volume fractions of nanometer scale porosity.; Crack growth rates were characterized as a function of the mechanical driving energy over the range of 10-4 to 10-10 m/s using load relaxation fracture mechanics techniques. Results from crack growth studies were rationalized in terms of a stress enhanced chemical reaction between strained Si-O crack tip bonds and reactive environmental species. In general, growth rates were inhibited by acidic solutions and enhanced by basic solutions. This was true of both buffered and non-buffered aqueous solutions and was attributed to the differences in hydroxide ion concentration between the environments. However, anomalously high growth rates were observed in weakly acidic hydrogen peroxide solutions and crack velocities were accelerated by over three orders of magnitude beyond those expected for equivalent pH solutions. Kinetic mechanisms responsible for the anomalous behavior observed in hydrogen peroxide solutions are proposed.; While the chemistry of the solutions has a significant effect on the chemical reaction kinetics, the porosity of the films was also demonstrated to dramatically affect the rate of crack growth. By systematically increasing porosity from ∼10 to ∼40 vol.% the films' cohesive fracture energy was decreased by 50% resulting in over four orders of magnitude increase in the crack growth rate. This behavior was rationalized in terms of the affect increasing porosity has on increasing the local crack tip driving energy. Finally, design strategies for increasing the fracture resistance of nanoporous films are demonstrated and involve incorporating ductile layers into the thin-film structure which dissipate energy by plastic deformation.