Enhancing mechanical stability of sol-gel silica antireflection coatings via ammonia treatment at low temperature

Silica antireflection coatings from sol-gel processes are of great importance in optical areas because of their capability to reduce light reflection and hence to increase transmission. However, it retained a challenge to enhance the mechanical durability of the sol-gel antireflection coatings. One option is to introduce strong chemical bonding between neighboring silica nanoparticles in the coatings. Here, we demonstrate that ammonia treatment is an effective approach to enhance the mechanical durability of the silica antireflection coatings. The coatings after ammonia treatment exhibited excellent mechanical properties, with tensile hardness of 4.4 GPa, pencil hardness of 4H, and good abrasion resistance, which is noticeably better than the coatings without ammonia treatment. The ammonia treatment did not impair the optical property of the coatings. The coatings after ammonia treatment remained their good transmission, with peak transmittance as high as 99.8%. Mechanism analysis suggests that the enhancement of the mechanical durability is likely due to the condensation of silanol groups between the neighboring silica nanoparticles under ammonia catalysis. The ammonia treatment provides a promising alternative to current approaches for improving the mechanical durability of the sol-gel silica coatings.