An atomic surface of an aluminum alloy induced by novel green chemical mechanical polishing using hybrid rare earth abrasives and mechanisms unraveled

Abstract

An aluminum (Al) alloy is a soft, plastic-like metal that is prone to embedding abrasives, scratches, corrosion pits, and deformation. Achieving an atomically smooth surface on an Al alloy presents a significant challenge. This study introduces a novel green chemical mechanical polishing (CMP) technique using hydrogen peroxide, tyrosine, sodium carbonate, and hybrid abrasives composed of silica, yttria, and ceria. The method produces a surface roughness (S_a) of 0.187 nm over a 50 × 50 μm² scanning area, with a material removal rate of 17.23 μm h⁻¹. Transmission electron microscopy (TEM) analysis shows a damaged layer thickness of 3.6 nm. To the best of our knowledge, this work reports the lowest S_a and damaged layer thickness for Al alloys to date. Molecular dynamics simulations are used to elucidate the mechanism of dynamic material removal during nanoscratching. As the cutting depth increases from 2.5 to 3 nm, the thickness of the damaged layer varies from 3.5 to 4.3 nm, aligning well with the TEM findings. The maximum von Mises stress recorded is 7.98 GPa, with the appearance of dislocation loops, vacancy defects, stacking faults, and an amorphous phase. The zeta potential measurements are −22.78, −9.46, −34.03, and −41.55 mV for silica in water, silica and yttria in water, silica at a pH of 10, and silica and yttria at a pH of 10, respectively. These correspond to polydispersity indices of 0.281, 0.412, 0.231, and 0.185. These data indicate that the hybrid abrasives of silica and yttria in a sodium carbonate solution exhibit the best stability and dispersion among the tested solutions. Characterization techniques, including X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and Raman spectroscopy, demonstrate the role of hydrogen peroxide in oxidizing the Al alloy surface to form aluminum hydroxide, alumina, and silica. Proposed complexing formulas suggest that tyrosine forms complexes with Al³⁺ ions, while ceria reacts with silica. The removal of oxides and complexes is facilitated by hybrid abrasives. The proposed novel green CMP method offers a new pathway for achieving atomic-level surface smoothness on Al alloys, potentially enhancing their application in high-performance devices.