Coating thickness effect of metallic glass thin film on the fatigue-properties improvement of 7075 aluminum alloy

Abstract

The outstanding strength-to-weight performance of 7075 aluminum alloy (AA 7075) has prompted its wide-scale application to light-weight sport equipment, automobile bodies, and airframe design. However, the fatigue performance of AA 7075 cannot match that of steel. In this study, we used direct current sputtering to coat AA 7075 substrates with (Zr53Cu30Ni9Al8)99.5Si0.5 (Zr-based) metallic glass thin film (MGTF) coupled to a 50-nm titanium thin-film buffer layer. The Zr-based MGTF coating was applied at thicknesses of 200, 300, 400, and 500 nm. The fatigue characteristics of as-polished and coated AA 7075 samples were then tested in accordance with ASTM-C1161-02c specifications. The 200-nm-thick Zr-based MGTF coating was shown to extend the fatigue life of the AA 7075 by 26 times at a stress level of 250 MPa. The application of 200-nm-thick Zr-based MGTF coating also extended the fatigue limit from 150 MPa (as-polished sample) to 235 MPa, which represents an improvement of 56.7%.Transmission electron microscopy analysis revealed that the Zr-based MGTF coating restricted the formation of offsets and cracks beneath the surface of specimen. This can be attributed to the high strength, good flexibility, and strong adhesion of the Zr-based MGTF coating, which provided sufficient compressive stress to suppress slip band protrusions. The increases in fatigue life and fatigue limit were proportional to the thickness of the MGTF coating, due to changes in surface roughness and adhesion capability. A coating thickness of 200 nm was found to provide the best resistance to fatigue, due to its superior flexibility. These results demonstrate the outstanding potential of Zr-based MGTF coatings to improve the fatigue resistance of AA 7075.