Key Takeaways & Executive Findings
- •• • Theoretical material removal rates deviate from experimental values by ≤13% across all tested conditions, with maximum errors of 10.6% at 7.5 kPa and 12.5% at 30 kPa, validating the model's predictive capability for industrial process control. • • Material removal rate increases monotonically with polishing pressure (7.5–30 kPa) and pad rotational speed (40–80 r/min), enabling deterministic throughput scaling without sacrificing surface integrity. • • Mechanical action reduces the activation energy and elevates the contact-zone temperature, accelerating the solid-phase reaction rate; this coupling mechanism governs the chemical-mechanical equilibrium critical for high-quality KDP processing. • • The solid-phase reaction layer thickness, derived from the reaction rate model and single-abrasive scratching theory, directly determines abrasive penetration depth and material removal rate, providing a quantitative link between reaction kinetics and mechanical removal.
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Abstract
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.
1. Introduction
KDP crystals are strategic optical materials for high-power laser systems and inertial confinement fusion devices, yet their extreme hygroscopicity, brittleness, and low hardness cause conventional machining approaches to fail: abrasive processes induce subsurface damage and microcracks, while wet chemical etching lacks spatial selectivity and dimensional control. Existing chemical mechanical polishing (CMP) techniques for KDP rely on aqueous slurries that exacerbate deliquescence and introduce corrosive attack, stalling the achievement of both high removal rates and sub-nanometer surface roughness.
This work addresses the bottleneck by implementing a fixed solid-phase chemical mechanical polishing method that eliminates liquid-phase corrosion. By integrating heterogeneous solid-phase reaction kinetics with the Arrhenius equation and single-abrasive scratching theory, the study establishes a material removal model that quantifies the effects of pre-exponential factor, activation energy, reaction mechanism function, and pad characteristics (asperity distribution, abrasive grain size, concentration). The model is validated across polishing pressures of 7.5–30 kPa and pad rotational speeds of 40–80 r/min, yielding theoretical-experimental discrepancies within 13% and demonstrating that mechanical action lowers activation energy and raises contact-zone temperature, thereby accelerating solid-phase reaction and enabling balanced chemical-mechanical removal for high-quality KDP crystal processing.
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SI Jialong, LU Xiao, LI Lan, YANG Liantong, CHEN Kai, YU Zhi, LI Jun (2026). A Material Removal Model for KDP Crystal in Chemical Mechanical Polishing Based on Solid-phase Chemical Reactions. Surface Technology (表面技术). https://doi.org/10.16490/j.cnki.issn.1001-3660.2026.09.004
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Frequently Asked Questions
What is the maximum observed error between the theoretical model and experimental material removal rates, and under which conditions does it occur?
The maximum error is 12.5% at a polishing pressure of 30 kPa, with a secondary maximum of 10.6% at 7.5 kPa. Across all tested pressures (7.5–30 kPa) and pad rotational speeds (40–80 r/min), the overall discrepancy remains within 13%, confirming the model's validity for process window prediction.
How does mechanical action quantitatively influence the solid-phase reaction rate in this CMP process?
Mechanical action, primarily polishing pressure and pad rotational speed, reduces the activation energy and elevates the contact-zone temperature between the KDP crystal and the fixed abrasive pad. This dual effect accelerates the solid-phase reaction rate, which in turn increases the reaction layer thickness and material removal rate. The model explicitly incorporates these parameters through the Arrhenius equation and true contact area calculations.
What is the fundamental coupling mechanism between chemical and mechanical actions that enables high-quality KDP processing?
The solid-phase chemical reaction generates a reaction layer on the KDP surface, which is subsequently removed by the mechanical action of abrasive grains. When the chemical reaction rate and mechanical removal rate reach equilibrium, the reaction layer thickness stabilizes, preventing subsurface damage and achieving high-quality processing. This balance is quantified by correlating abrasive penetration depth with reaction layer thickness in the model.
Can this solid-phase CMP model be scaled to industrial production, and what are the key operational parameters?
The model is validated at polishing pressures up to 30 kPa and pad rotational speeds up to 80 r/min, with material removal rate increasing monotonically with both parameters. This scalability indicates that industrial throughput can be enhanced by increasing pressure and speed, provided the chemical-mechanical equilibrium is maintained. The model's predictive accuracy within 13% supports its use for process optimization and control in manufacturing.
What are the limitations of the current model, and what further experimental validation is required?
The model assumes a fixed abrasive pad with uniform asperity distribution and does not account for pad wear or abrasive degradation over prolonged polishing. Additionally, the error at 30 kPa (12.5%) suggests that at higher pressures, secondary mechanical effects such as abrasive embedding or reaction layer fracture may become significant. Future work should incorporate time-dependent pad topography and validate the model over extended polishing durations and varying abrasive concentrations.
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