Structure and antiwear behavior of micro-arc oxidized coatings on aluminum alloy

The coating deposition mechanism of aluminum alloys by means of micro-arc oxidation has been investigated, which may be adapted to the further application of wear resistant protection of Al alloys. The phase composition of the micro-arc oxidized coatings has been examined using X-ray diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy. The antiwear behavior of the coatings was evaluated with a scratch tester, reciprocating tester and Timken tester. The results show that the micro-arc oxidized coatings on Al alloys show two distinct regions, i.e. a porous overlayer region consisting predominantly of gamma-Al2O3 which was produced at the higher cooling rate, and a dense internal region consisting predominantly of alpha-Al2O3 which was generated at the lower cooling rate. They are well bonded on the Al substrate in 40-110 mum coatings. The polished coating mainly composed of alpha-Al2O3, registers a lower wear of 3.00-5.00 x 10(-6) mm(3)/N m in reciprocating sliding against ceramic counterpart at a speed of 0.33 m/s and a contact pressure of 2 MPa. The antiwear life of the polished coating reached 2500 m at a speed of 1.25 m/s and load of 300 N and the friction coefficient was more than 0.45 against the steel ring in a Timken tester. Thus, the micro-arc oxidation coating mainly composed of alpha-Al2O3, could be a promising candidate as a protective coating of Al alloy-based components in terms of wear-resistance, and further improved lubrication effects as a self-lubrication coating or using oil, grease and solid lubrication films. (C) 2002 Elsevier Science B.V. All rights reserved.