Ultraviolet irradiation hardening of hafnia films prepared by sol-gel processes

Abstract

Sol-gel-derived hafnia films containing formic acid as an organic ligand were hardened by ultraviolet (UV) irradiation. Transparent hafnia sols were prepared by treating hafnium hydroxide with a formic acid aqueous solution. The gel films were hardened to scratch hardness of over 9H by UV irradiation using a high-pressure mercury lamp for 2.5-10 min. A formato ligand was found to be bridged to Hf ions in a bond involving two metal ions by analyzing the FTIR spectra of the hafnia powders before and after UV irradiation. The hardening process of the hafnia gel films was investigated using the thermal programmed desorption (TPD) technique. The amount of CO and CO2 generated from the formato ligands decreased after UV irradiation, as did the amount of H 2O generated from Hf-OH in the gel, indicating that the formato ligands and OH in the gel were eliminated by UV irradiation, resulting in pencil hardness of over 9H.