Close-to-Atomic\nPrecision Surface Achieved through\na Neutral and Eco-friendly Slurry in Chemical Mechanical Polishing\nof TC4 Alloy

Atomic\nand close-to-atomic scale manufacturing represents a leading-edge\nmethod for improving the operational performance of titanium alloy\ncomponents within the aerospace, biomedical, and new energy fields.\nRegrettably, despite chemical mechanical polishing (CMP) being presently\none of the most efficient techniques for achieving atomic layer polishing,\nthe absence of a neutral and environmentally sustainable slurry capable\nof producing ultrasmooth and low-damage surfaces in titanium alloy\nCMP is notable. In this study, a neutral and eco-friendly slurry composed\nof H<sub>2</sub>O<sub>2</sub> and alkali metal ions is proposed, which\nis different from the traditional TC4 alloy polishing where H<sup>+</sup>, OH<sup>–</sup>, or hazardous chemicals require deep\ninvolvement. The results demonstrate that the decreasing dynamic ionic\nradius of alkali metal ions significantly promotes the interfacial\nmechanochemical reactions. Therefore, an ultrasmooth surface with\nclose-to-atomic precision (<i>S</i><sub>a</sub> ≤\n1 nm) can be achieved by adding only 150 mmol/L K<sub>2</sub>SO<sub>4</sub> and 10 wt % H<sub>2</sub>O<sub>2</sub>. The presence of H<sub>2</sub>O<sub>2</sub> predominantly affects the electrochemical behavior,\ncontributing to enhanced chemical corrosion. K<sub>2</sub>SO<sub>4</sub> causes the effective thickness of the electrical double layer to\ndecrease to 0.56 nm. The thinner electrical double layer can amplify\nthe action of chemical corrosion and mechanical wear. They synergistically\ncontribute to the equilibration of mechanochemical reactions at the\nsliding interface. The novel slurry is highly desirable for both the\nenvironment and people as well as for Ti alloy CMP industries.