Effects of solvents on the tribological behaviour of sol–gel Al2O3 films

Thin films of Al2O3 and modified Al2O3 were prepared on a glass substrate by dip coating process from specially formulated aqueous/ethanol sols. Thermogravimetric analysis (TGA) and differential scanning calorimetric analysis (DSC) of precursor gels were performed to explore the thermal events occurring during the annealing process of the sol–gel Al2O3 films. The morphologies of the unworn and worn surfaces of the films were analyzed by atomic force microscope (AFM) and scanning electron microscopy (SEM). The chemical composition of the film was characterized by X-ray photoelectron spectroscopy (XPS). The tribological properties of the films sliding against Si3N4 ball were evaluated on a one-way reciprocating friction and wear tester. AFM and XPS results confirm that the solvents can influence the structure and composition of the film significantly. The film from ethanol solution process shows longer wear life than the one from distilled water solution, especially under lower load. The added polyethylene glycol (PEG) modifier in sols generates totally different effects on the tribological behaviour of the films with respect to solvents. The addition of PEG to ethanol solution deteriorates the wear resistance of the corresponding film. Differently, the wear resistance of the modified film from aqueous solution is significantly improved. Actually, the modified film from water solution shows the best wear resistance among all the films. The wear mechanisms of the films are discussed based on SEM observation of the worn surface morphologies. Results show that brittle fracture and severe abrasion dominates the wear of glass substrate. The wear of the film from aqueous solution is characteristic of micro-fractures, the corresponding modified film shows better plasticity and fewer micro-cracks, whereas, the wear of the film from ethanol solution was characterized by abrasive and slight plastic deformation, and the corresponding modified one shows evidence of abrasive and brittle fracture.