Al2O3 coatings were in situ formed by means of microarc oxidation technique to solve the problem of difficult preparation of alumina coatings on the inner surface of the complicated bimetallic parts. The effect of current density and oxidation time on the Al2O3 coatings was investigated in this work. The phase compositions, microstructure and cross-section morphologies of Al2O3 coatings were characterised respectively by XRD and SEM. Then the bonding strength between the substrate and Al2O3 coatings was tested by WS-2005 automatic scratch tester. The results showed that the critical load of as prepared Al2O3 coatings reached 38 N, which was higher than that of the Al2O3 coatings prepared by other methods such as sol–gel method, vapour deposition and chemical deposition. Moreover, the maximum content of α-Al2O3 in the compact alumina coatings achieved about 67·2 wt-%.
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