Potassium dihydrogen phosphate (KH2PO4, KDP) crystal is a key optical material employed in high-power laser systems and inertial confinement fusion devices. However, its hygroscopicity, brittleness, and low hardness make KDP one of the most challenging materials to machine. Therefore, investigating the complex coupling mechanisms between chemical and mechanical actions during chemical mechanical polishing, and elucidating the role of solid-phase chemistry in material removal, are of great significance for achieving high-quality processing of KDP crystals.
In this work, a fixed solid-phase chemical mechanical polishing method was utilized. Drawing on the kinetics of heterogeneous solid-phase reactions and the Arrhenius equation, the effects of mechanical action and temperature on the reaction rate were examined, and the true contact area between the KDP crystal and the reactants in the fixed abrasive pad was computed. A solid-phase chemical reaction rate model was thus formulated. By integrating this model with single-abrasive scratching theory, the thickness of the solid-phase reaction layer was derived. A material removal model grounded in solid-phase chemical reactions was subsequently developed by correlating the abrasive penetration depth with the reaction layer thickness. This model elucidated the effects of critical reaction parameters such as the pre-exponential factor, activation energy, and reaction mechanism function on the material removal rate, along with the effect of polishing pad characteristics, including the distribution of surface asperities, abrasive grain size, and abrasive concentration. To validate the proposed material removal model, experiments were performed with polishing pressures set at 7.5, 15, 22.5, and 30 kPa, and fixed abrasive pad rotational speeds at 40, 50, 60, 70, and 80 r/min, respectively. The experimental parameters were input into the model to compute the theoretical material removal rates, which were then compared with empirical data.
The discrepancy between theoretical and experimental values was within 14%, affirming the model's validity. In the experiments where the material removal rate was investigated as a function of pressure and rotational speed, relatively larger errors were observed under specific conditions. Notably, the errors reached 10.6% and 12.5% at pressures of 7.5 kPa and 30 kPa, respectively, and 12.2% at a rotational speed of 40 r/min. The material removal rate was observed to increase with both polishing pressure and fixed abrasive pad rotational speed.
In fixed solid-phase chemical mechanical polishing, both polishing pressure and fixed abrasive pad rotational speed serve as key factors affecting the reaction activation energy and temperature. Specifically, an increase in pressure reduces the chemical activation energy, while a higher pad rotational speed elevates the interfacial temperature at the contact zone between the KDP crystal and the fixed abrasive pad. These effects collectively enhance the solid-phase reaction rate. As a result, polishing pressure and fixed abrasive pad rotational speed are also identified as dominant factors governing the material removal rate.
Key words
KDP crystal /
chemical mechanical polishing /
chemical reaction kinetics /
solid-phase reaction rate model /
reaction layer thickness /
material removal model
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Funding
National Natural Science Foundation of China (General Program) (52375439); Postgraduate Research & Practice Innovation Program of NUAA（xcxjh20250515)
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