Kinetics of channeling cracks in polymeric coatings

Techniques to measure the relationship between crack velocity and energy-release rate for channeling cracks in thin films and coatings are presented in this paper. The approach uses linear-elastic fracture mechanics to relate the crack spacing, material properties and geometry to the energy-release rate of a channeling crack, by means of detailed finite-element calculations. The velocities of individual cracks are monitored and related to their distance to nearest neighbors as part of these calculations. Other parameters that were identified as being important in determining the energy-release rate were the depth below the surface to which the crack penetrated, and the residual strain in the cracked layer. A technique to determine the crack depth was developed which involved focused-ion beam milling of the specimen, followed by electron microscopy. The residual strain in the crack layer was determined by measuring the crack-mouth opening using atomic-force microscopy, and comparing this to numerical analyses. These approaches were used to analyze a multi-layer system consisting of a polymeric, colloidal-silica nano-composite layer and a primer layer coating a polycarbonate substrate. The crack growth showed a threshold value of energy-release rate corresponding to about 6.6 J/m2, with a minimum detectable crack velocity of about 3 nm/s. When the energy-release rate was increased to about 25 J/m2, the resultant crack velocity was 10-2 m/s. The channeling cracks in this system exhibited the characteristics of a thermally-activated rupture process, but showed no evidence of fatigue or stress-corrosion cracking under the conditions studied.