Study of the influence of hardening on sol-gel-type anti-reflective coatings characterized by ultrasound

The present article focuses on the non-destructive characterization of sol-gel-type anti-reflective thin films, produced by dip-coating, with surface acoustic waves, an ultrasound technique. In particular, the changes that occur as a result of a post-treatment called ammonia hardening, carried out to improve the mechanical characteristics of these coatings, have been highlighted. These layers are employed to coat the main optical components working in transmission for the ignition facilities. Despite the robust nature of these coatings, damage occasionally occurs, primarily on the rear face of the silica optics. The observed damages manifest as a combination of molten, spalling, and fractured morphologies. These occasional damages highlight the importance of controlling the mechanical characteristics (Young's modulus and Poisson's ratio) of these layers so that their thickness. The article also details characterization techniques that exploit surface acoustic wave dispersion to determine the mechanical and dimensional properties of thin films. Differences in properties are observed between faces exposed and unexposed to ammonia vapors during hardening, suggesting an influence of exposure angle in the post-treatment process. The findings of this study highlight the impact of ammonia hardening on the thickness and Young's modulus of thin films, showing that the treatment strengthens the bonds between nanoparticles, reducing thickness and increasing mechanical stiffness. It has been observed that, currently, post-treatment induces a slight asymmetry of properties (mechanical and optical) between the front and rear faces, which nevertheless remains negligible for the present applications of these treatments. • Two silica samples coated with sol gel layers were characterized by ultrasonic method • Sol Gel layers undergoing ammonia curing were characterized by ultrasonic method • Ultrasonic characterizations showed that the cured film strengthens mechanically • A difference in mechanical properties could be observed between the two cured faces • The angle of the ammonia flow could explain this difference • Only the ultrasonic method exploiting surface waves was able to highlight this difference To our knowledge, this is the first characterization of the mechanical properties of sol-gel silica layers, obtained by dipping and curing with ammonia vapors, using a non-destructive technique involving SAWs.