Characterization and tribological investigation of SiO ₂ and La ₂ O ₃ sol-gel films

Thin films of SiO ₂ and La ₂ O ₃ were prepared on a glass substrate by a dip-coating process from specially formulated sols. The tribological properties of the resulting thin films sliding against a Si ₃ N ₄ ball were evaluated on a one-way reciprocating friction and wear tester. The morphologies of the unworn and worn surfaces of the films were examined by an atomic force microscope (AFM) and a scanning electron microscope (SEM). La ₂ O ₃ shows the best tribological performance. The coefficient of friction is about 0.1 and the wear life is over 5000 sliding passes both under higher (3 N) and lower load (1 N). The SiO ₂ film derived from a specially formulated aqueous solution shows much better performance in resisting wear and reducing friction than the one derived from an ethanol solution. The wear mechanisms of the films are discussed based on SEM observation of the worn surface morphologies. SEM observation of the morphologies of worn surfaces indicates that the worn surface of La ₂ O ₃ is too slight to be observed by SEM. The wear of SiO ₂ derived from TEOS solution is the characteristic of delaminating, which is responsible for the abrupt failure of the film. The wear of SiO ₂ derived from aqueous solution is the characteristic of fracture. Brittle fracture and severe abrasion dominate the wear of glass substrate.