Additive-dependent lateral connectivity of modified surface sites and shear accommodation in sapphire polishing

Sapphire (α-Al₂O₃) polishing under magneto-chemical rheological polishing (MCRP) relies on the coupling between chemically induced surface modification and shear. Polishing behavior depends not only on chemical reactivity, but also on the lateral connectivity of modified surface sites, and the ability of the corresponding relaxed surface states to accommodate shear during sliding. Triethanolamine (TEA), β-alanine, and potassium hydroxide (KOH) were investigated through polishing tests, electrochemical measurements, post-polishing XPS, DFT-based electronic descriptors, classical adsorption modeling, and sliding molecular dynamics simulations. TEA achieved the highest material removal rate (5.46 μm/h) together with the best surface quality Ra = 0.08 nm, PV < 1 nm, followed by β-alanine and KOH. Electrochemical and DFT results indicated that the three additives differed in functional-group reactivity and in the distribution of potential interaction sites. Connectivity analysis showed that TEA produced the highest lateral connectivity of modified surface sites, whereas KOH and β-alanine produced more fragmented modified-site distributions. Sliding molecular dynamics simulations showed that the TEA-associated relaxed surface state exhibited stronger near-surface shear accommodation and reduced stress transmission into the subsurface lattice. These findings support a tribo-chemical interpretation of sapphire polishing behavior in MCRP, in which additive-dependent differences in the lateral connectivity of modified surface sites and the shear response of the resulting relaxed surface states together influence polishing performance.