Planification of amorphous carbon coatings via periodically transitional sp²/sp³ bonds to enhance cavitation erosion resistance

In the marine sector, hydraulic machinery suffers severe damage from cavitation erosion. Although amorphous carbon (a-C) coatings are an effective measure to enhance the cavitation erosion resistance (CER), their widespread application is hindered by their metastable nature characterized by internal stress. In this study, a planification strategy was employed via different sp²/sp³ distributions (including uniform ones with different sp²/sp³ levels and periodically transitional ones with different periods), aiming to construct a pure a-C coating that enhances CER on 316L stainless steel by modulating the bias during magnetron sputtering. The effects of the sp²/sp³ distributions on the microstructure, chemical bonding, adhesion strength, fracture toughness, corrosion resistance and CER was investigated and compared with the coatings featuring uniform sp²/sp³ distributions. The results show that the periodic-mode coatings exhibit much higher CER due to their lower internal stress, excellent adhesion strength, enhanced fracture toughness and high corrosion resistance. After the cavitation erosion test for 4 h, the periodic-mode coatings only show striped structures without any coating delamination, along with less graphitization tendency during cavitation erosion. Hence, the design approach of periodic sp²/sp³ carbon bonds provides a promising new direction for marine equipment with improved CER and potential applications in various other environments.