Comparative study on cracking behavior of sol-gel silica antireflective coating for high-powered laser system

Potassium dihydrogen phosphate (KDP) crystal is a kind of very crucial nonlinear optics for high-powered laser equipment as frequency conversion (doubler, tripler or quadrupler) components. In this paper, investigation on cracking behavior of sol-gel silica antireflective (AR) coatings has been performed using KDP crystal as substrate. Cracking induced by heating or moisturizing post-treatment is respectively studied, in which distinct cracking behaviors have been observed. It has been interestingly found that either temperature or humidity saltation can result in coating crack generation, but the corresponding residual stress demonstrates in completely reverse ways, giving rise to distinctive crack morphology on both crack edge and bottom. The origination and the derivation of such stress manner have been stated based on the intrinsic porous structure of sol-gel silica coating, and the critical cracking condition has been suggested dependent on the relationship between the accumulated residual stress against elastic recovery capability of such coating.