Dip coating of alumina films by the sol-gel method
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Chang-Whan Won and Byong-Sun Chun ERC for Rapidly Solidified Advanced Materials and the Department of Metallurgical Engineering, Chungnam National University, Daejeon, Korea, 305-764
H. Y. Sohn Department of Metallurgical Engineering, University of Utah, Salt Lake City, Utah 84112-1183 (Received 12 February 1993; accepted 10 August 1993)
Thin alumina films were prepared on stainless steel and glass surfaces by dip coating from an aluminum alkoxide solution. Multiple coatings from a low-viscosity solution were necessary for high quality and sufficient thickness of the films. To determine the bonding mechanism and strength, wavelength dispersive x-ray was used. Aluminum was found to diffuse into the substrate in both cases. The possible bonding mechanism is a combination of diffusion and the formation of chemical compound between the alumina and the substrates. The surface properties of stainless steel, such as microhardness, electric resistance, and corrosion resistance, were substantially improved when coated with alumina.
I. INTRODUCTION It is already known that the dip coating of glasses, metals, and plastics by the sol-gel method is very useful for modifying the surface properties of substrates with large surface areas and provides the substrates with new functional properties.1"5 Alumina is one of the important technical ceramics because of its useful properties. Many studies have been made to prepare coatings of this ceramic by the sol-gel method.6"8 Several papers discussed the preparation of alumina films as membranes9"11 by the sol-gel method, but few reports have been published on the coating on metals to improve the surface properties. In the present work, the coating of alumina on stainless steel sheets by the sol-gel method was studied. Conditions necessary for producing high-quality films and the bonding mechanism, as well as the strength of the coated alumina films, were investigated.
Hydrolysis
Alkoxide/H20 = 1 mol / 100 mol
Catalyst (HCI)
Peptization
Mol ratio of HCI/Alkoxide = 0.07
Sol
Dipping
with metal or glass substrate
Drying
at 9 5 t for 10min
Heat treatment
Thin film
at 500X; for 30 min
on metal or substrate
FIG. 1. Flow chart of experimental procedure.
II. EXPERIMENTAL PROCEDURE A. Preparation of coating films The preparation of coating films was carried out by the dipping method with alumina hydroxide sol which was produced by the hydrolysis and polycondensation of an aluminum alkoxide, aluminum secondary butoxide [A1(OC4H9)3], using the procedure described by Yoldas,8 as shown in Fig. 1. The viscosity of the coating solution was controlled by the gelation time. The alumina (A12O3) films were J. Mater. Res., Vol. 8, No. 12, Dec 1993 http://journals.cambridge.org
Al - Alkoxide (AI(OC4H9)3
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made by dipping samples of slide glass and type 304 stainless steel, which had been cleaned by the vapor degreasing process with trichloroethane, into the solution and removing the substrates from the solution at a constant withdrawal speed of 15 cm/min. All sli
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