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Arc ion plating had been employed on a cobalt-base superalloy K40S to deposit a NiCrAlYSi coating to improve its oxidation resistance at 1323-1423 K in air. The K40S superalloy had poor oxidation resistance because a non-protective and easy spalling surface oxides scale mixed of Cr2O3 and CoCr2O4 was formed on its surface. After coated with NiCrAlYSi coating, a dense and protective alpha-Al2O3 scale was formed on the coating and excellently improved its oxidation resistance. Inter-diffusion obviously occurred between the coating and the substrate K40S superalloy in oxidation process, which resulted from Co atoms in K40S outwards diffused. A richen Cr and W carbides inter-diffusion layer was formed, which could acted as a diffusion barrier that barred Al atoms in coating inwards diffusion. Though the NiCrAlYSi changed into NiCoCrAlYSi during oxidation process, it still possessed a good oxidation resistance and. had a considerable long-term life. (C) 2008 Elsevier Ltd. All rights reserved.