Counterion-Driven Mechanochemical Reactions at TC4 Alloy/SiO₂ Interfaces: Electrical Double Layer and Dynamic Ionic Radius

Ti-6Al-4V (TC4) alloy with ultra-smooth surfaces has found extensive application in biomedical fields. Chemical mechanical polishing is a crucial method for achieving ultra-smooth surfaces, but its efficiency in polishing TC4 alloy surfaces is low. This study proposes a new approach to enhance the polishing efficiency by tuning counterions, which significantly influence both chemical corrosion and microscopic interaction forces. The mechanism involves Li⁺/Na⁺/K⁺ regulating the action intensity at the tribological interface by altering the thickness of the electrical double layer and dynamic ionic radius. On the one hand, reducing the thickness of the electrical double layer from 1.41 to 0.46 nm can enhance the intensity of chemical reactions, and the smaller the dynamic ionic radius of the counterion, the more pronounced the chemical corrosion caused by H₂O₂ becomes. Combining the two, the reaction products of H₂O₂ (HO₂⁻ and OOH⁻) can more readily react with Ti to form fragile reaction products with the help of K⁺. On the other hand, as the electrostatic repulsion force weakens, the SiO₂ particles exert a stronger mechanical force, allowing for quicker removal of the fragile reaction products. Thus, in the presence of 10 wt%H₂O₂ and 200 mM K₂SO₄, a polishing efficiency of 1197 nm/min is achieved, with the S_a of 2.7 nm over a scanning area of 195.8 × 195.8 μm², and without polishing damage layer on the substrate. The findings provide mechanistic insight for further exploring the limits of polishing performance in CMP of titanium alloys.