Preparation of compact Al2O3 film on metal for oxidation resistance by polyvinylpyrrolidone

Oxidation resistance of metal at high temperature can be improved by an environmentally friendly solution deposition approach. Stable precursor solution with high oxide concentration, favorable viscosity and low surface tension was prepared using aluminum sec-butoxide (ASB) and polyvinylpyrrolidone (PVP) as starting raw materials. Alumina sol-gel films were deposited onto metal by spin-coating followed by heat treatment. When PVP was added according to an amount of 50 mg/mL into a sol with an ASB/H2O molar ratio of 1:35, the as-obtained sol exhibited favorable gelation time and viscosity. The surface tension of the alumina sol with PVP was examined to be lower by 32% than the sol (ASB:H2O = 1:100) without PVP. TG-DTA analyses show the densification of the alumina gel film with PVP was progressed within a wide temperature range from 200 to 650 degrees C. Crack-free Al2O3 film with a thickness up to 1.5 mu m was successfully produced on metallic substrate by three spin-coating cycles. SEM and XRD analyses revealed the gel film transformed into compact alpha-Al2O3 material after calcined at 1,000 degrees C for 0.5 h. The weight gained by the samples during firing at 1,000 degrees C indicated that the Al2O3 coating film could reduce the rate of oxidation by degrees 81%. The hardness of the Al2O3 film coated metal was higher by 260% than the uncoated metal that was calcined at 1,000 degrees C for 0.5 h. It was confirmed by adhesion test that both the alumina/PVP hybrid film and the as-produced alpha-Al2O3 coating film had strong adhesion.