Characterization of chemically bonded composite sol–gel based alumina coatings on steel substrates

Phosphate-bonded sol–gel composite alumina coatings were prepared on stainless steel substrates at processing temperatures of 300, 400 and 500°C. Mechanical property and electrochemical characterisation has been carried out and relationships sought between both processing temperature and coating microstructure. Coating corrosion and electrochemical behaviour is largely controlled by the degree of cracking and porosity in the coatings, which is minimum in those processed at the lowest temperature. Little correlation was found between coating processing or microstructure and coating mechanical properties (micro and scratch hardness, elastic modulus) or residual stresses but the interface toughness, as measured by a high load indentation method, increased with process temperature on coatings deposited on sand-blasted substrate surfaces.