Fabrication and Ultraviolet Irradiation Hardening of Sol-Gel-Derived Hafnia Films

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This paper describes the ultraviolet (UV) irradiation hardening of sol-gel-derived hafnia films containing oxalic acid as an organic ligand. Colloidal and viscous hafnia sols were prepared by treating hafnium hydroxide with an oxalic acid aqueous solution. The transparent gel films were hardened to scratch hardness of over 9H by UV irradiation using a high-pressure mercury lamp for 2.5-10 min. The films showed a maximum water droplet contact angle of 89°, indicating that the hardened films displayed hydrophobicity similar to that of fired hafnia films obtained at 550°C. The bonding structure of the oxalato ligands was analyzed on the basis of their FTIR spectra. It was found that an oxalato ligand bridges to Hf ions in a bond involving two metal ions. The hardening process of the hafnia gel films was investigated using the thermal programmed desorption (TPD) technique. Species of C, O, CO and CO₂ evolved from the oxalato ligands in a temperature range of 200-400°C. The amount of CO and CO₂ decreased markedly after UV irradiation, indicating that the oxalato ligands were eliminated by UV irradiation, resulting in pencil hardness of over 9H.