A Material Removal Model for KDP Crystal in Chemical Mechanical Polishing Based on Solid-phase Chemical Reactions
Potassium dihydrogen phosphate (KH2PO4, KDP) crystals are indispensable optical components in high-power laser systems and inertial confinement fusion devices, yet their hygroscopicity, brittleness, and low hardness render them among the most difficult materials to machine. This study establishes a solid-phase chemical reaction rate model grounded in heterogeneous solid-phase reaction kinetics and the Arrhenius equation, quantifying the influence of mechanical action and temperature on reaction kinetics. The true contact area between the KDP crystal and the fixed abrasive pad is computed, and the reaction layer thickness is derived by coupling the reaction rate model with single-abrasive scratching theory. A material removal model is subsequently formulated, linking abrasive penetration depth to reaction layer thickness. Experiments were conducted at polishing pressures of 7.5, 15, 22.5, and 30 kPa and pad rotational speeds of 40, 50, 60, 70, and 80 r/min. Theoretical material removal rates were compared with empirical data, yielding discrepancies within 13% (maximum errors of 10.6% at 7.5 kPa and 12.5% at 30 kPa). Material removal rate increases monotonically with polishing pressure and pad rotational speed. The model elucidates the coupling mechanism between chemical and mechanical actions: solid-phase reactions generate a reaction layer on the workpiece surface, which is removed by abrasive mechanical action; equilibrium between chemical and mechanical actions enables high-quality KDP crystal processing.