Adsorption, wetting, and friction properties of phosphonic acid self-assembled monolayers on the surface of Zr-based bulk metallic glasses

Self-assembled monolayers (SAMs) technology is considered a potential surface modification technique for addressing challenges in micro/nanostructure fabrication. Bulk metallic glasses (BMGs) are ideal materials for micro/nanostructure moulds. In this study, the surface of Zr-based bulk metallic glasses (Zr-based BMGs) was modified using phosphonic acid SAMs. The findings indicate that phosphonic acid SAMs can effectively reduce the surface free energy of Zr-based BMGs, successfully converting its surface from hydrophilic to hydrophobic and significantly reducing its friction coefficient by up to 90.0 %. Moreover, compared to traditional crystalline Zr-3 alloy, SAMs technology provides significantly better modification effects on amorphous Zr-based BMG surfaces. Experimental results show that among the three different SAMs, the packing density is highest for the mixed hexylphosphonic acid–dodecylphosphonic acid (HPA–DDPA) SAMs, followed by pure dodecylphosphonic acid (DDPA) SAMs, with pure hexylphosphonic acid (HPA) SAMs showing the lowest packing density. The water contact angle for HPA–DDPA SAMs reaches 101.4°, representing a 68.7 % increase compared to the bare plate. Additionally, phosphonic acid SAMs can significantly reduce the friction coefficient of Zr-based BMG surfaces, effectively suppressing oxidative wear and abrasive wear , thereby improving surface wear resistance. The packing density of phosphonic acid SAMs is the primary factor influencing surface wettability and frictional performance, with both wettability and frictional performance increasing monotonically as packing density increases. This is attributed to shear forces and van der Waals forces at the interface. Under specific loads, HPA–DDPA SAMs exhibit excellent lubrication and wear resistance, attributed to the elastic response and compliance of the SAMs. In conclusion, this paper successfully prepares various phosphonic acid SAMs on Zr-based BMGs surfaces and provides a detailed elucidation of the enhancement mechanisms in hydrophobicity and wear resistance through the study of SAMs molecular packing density.