Characterization and growth mechanisms of adhesion-induced microcavities during debonding of softened glass

The glass/mold interaction is crucial for controlling the surface quality of high-precision glass products and elongating the lifespan of precious molds in hot forming techniques. Here we employ the probe tack test to separate a typical glass molding interface composed of N-BK7 glass and tungsten carbide molds at different temperatures from 655 to 690°C. The macroscale debonding behavior translates from interfacial fracture to cohesive bulk deformation as temperature increases. The glass surfaces after debonding are covered by numerous randomly distributed cavities in micrometer. With temperature increasing, the maximum depth of microcavities greatly increases from less than 0.5 to over 10 μm; the area fraction overall increases and reaches 15% at maximum. These microcavities could result from the development of localized deformation at the gas-trapping spots, due to the separation of the adhesive glass/mold interface. A large-sized cavity evolves from the cyclic growth and coalescence of small cavities. For the interfacial fracture cases, cavities mainly propagate as cracks along the interface, and thus develop into shallow disc-like shapes. However, for the cohesive cases, cavities prefer to grow in the bulk. The growth bifurcation could be governed by the competition between strain energy release rate and viscoelastic complex modulus.