Improvement of corrosion fatigue strength of magnesium alloy by multilayer diamond-like carbon coatings

Abstract

The fatigue behaviour of a wrought magnesium alloy AZ80A coated with multilayer diamond-like carbon (DLC) films with the total thicknesses of 3 μm and 12 μm was studied in laboratory air and distilled water. The effects of the multilayer DLC films on fatigue and corrosion fatigue behaviour were discussed. In laboratory air, the fatigue strengths of the multilayer DLC-coated specimens significantly increased compared with those of the single layer DLC-coated ones with equivalent film thicknesses. Fatigue cracks initiated at the substrate under the defects of the film. Higher fatigue strengths of the multilayer DLC-coated specimens could be attributed to the smaller defect size in the film. However, the single layer films and the multilayer 3 μm DLC film could not improve corrosion fatigue strength in distilled water. The defects were found in the films, through which the corrosive water could reach the substrate. The degradation of fatigue strength under the corrosive environment was not recognized in the multilayer 12 μm DLC-coated specimens, indicating that the thick multilayer DLC coating was effective as a protection film. Through-film thickness defects were successfully removed by the thick multilayer fabrication process.