Mechanical and tribological investigations of sol–gel derived SiO2 optical coatings

Sol–gel silica coatings were deposited on soda lime glass substrates by dip-coating. The silica films were amorphous, crack free, and smooth with an average roughness of less than 3 nm and thickness of 0.8–1.2 μm. Mechanical and tribological properties were investigated using nanoindentation, Taber abrasion and a polishing wear test. The hardness increased with increased thermal exposure from 2200 MPa for samples fired at 480 °C for 2 min to 3800 MPa for those fired at 560 °C for 14 min. In Taber abrasion, failure was due to penetration of the coatings by the abrasive particles of the Taber grinders, which caused scratches and subsequent delamination. For polishing wear experiments, hardness was inversely proportional to polishing wear rate. This suggests that hardness is a good indicator for the resistance of sol–gel silica coatings to polishing.