ADHESION OF SILICON-OXIDE LAYERS ON POLY(ETHYLENE-TEREPHTHALATE) .2. EFFECT OF COATING THICKNESS ON ADHESIVE AND COHESIVE STRENGTHS

Fragmentation tests in the uniaxial mode were performed on poly(ethyle ne terephthalate) (PET) films coated with a silicon oxide layer of thi ckness ranging from 30 to 156 nm. The coating's fragmentation process was investigated to reveal the crack onset strain and the crack densit y at fragmentation saturation. Adhesive strength was modeled from the Kelly-Tyson approach, including a Weibull distribution of the coating strength. The prediction was found to be independent of coating thickn ess, and equal to the substrate shear stress at saturation. The cohesi ve strength of the coating was characterized from the crack onset stra in. The measured decrease in crack onset strain with coating thickness increase was modeled by means of Weibull and fracture mechanics theor ies, the latter providing the best predictions. (C) 1997 John Wiley & Sons, me.