Background Yttrium aluminum garnet (YAG) crystals, used as gain media in high-power laser devices, require stringent surface quality. However, their high hardness and low fracture toughness make it difficult for conventional polishing methods to simultaneously achieve a high material removal rate (MRR) and ultra-low surface roughness (Sa), which limits their application in advanced laser systems.

Purpose This study aims to investigate the process optimization and underlying mechanisms of acidic chemical mechanical polishing (CMP) for YAG crystals.

Methods An orthogonal experimental design was employed to systematically evaluate the main effects and interactions of pressure, rotational speed, and slurry concentration on polishing responses.

Results The results revealed that MRR is predominantly controlled by pressure, while Sa is primarily determined by slurry concentration. Based on this finding, a "decoupling matching criterion for removal-quality dual-response parameters" was proposed, leading to the identification of an optimal synergistic process window (A3B2C2: 110 kPa, 4 r/min, volume fraction 10%). This optimized process achieved an MRR of 30 nm/min, an Sa as low as 0.14 nm, a PV value of 0.04λ, and a defect-free surface. The MRR represents an 11% improvement over existing SiO₂ abrasive processes, and the Sa value is below the application threshold for Nd:YAG (≤0.2 nm). Mechanistic studies indicated that in an acidic CMP environment, H⁺ preferentially attacks Al–O–Al bonds, inducing surface reconstruction and lattice distortion. The broadening of XRD peaks and the positive shift of the Al 2p binding energy in XPS (from 73.46 eV to 75.23 eV) confirmed surface amorphization and the formation of a hydrated Al–OH layer.

Conclusions These findings establish a four-step removal mechanism: proton-mediated action, selective bond cleavage, interfacial bonding, and softened layer stripping. Furthermore, a theoretical framework termed “Chemical–Mechanical Temporal Matching” (CM-TM) is proposed for the first time, elevating CMP surface quality control from parameter optimization to the synergistic regulation of reaction kinetics and mechanical removal. This study provides a systematic process guideline and mechanistic explanation for atomic-scale, damage-free polishing of YAG and similar hard and brittle optical materials, offering both theoretical value and engineering guidance.