Abstract:
Objectives Single-crystal gallium nitride (GaN) is a pivotal semiconductor material widely utilized in high-power, high-frequency electronic devices, and optoelectronic applications. However, its inherent hardness and brittleness pose significant challenges in achieving damage-free surfaces during ultra-precision machining. Understanding the fundamental deformation mechanisms induced by cutting, particularly the critical role of tool geometry, is essential for advancing GaN machining technology. This study aims to comprehensively elucidate the influence mechanism of diamond tool angles, specifically the rake angle and flank angle, on the cutting-induced deformation behavior and subsurface damage formation in single-crystal GaN at the nanoscale. The primary objective is to establish clear relationships between tool angles, material removal mechanisms, defect generation (dislocations, phase transformation, amorphization), and final surface integrity, thereby providing foundational knowledge for optimizing ultra-precision machining processes. Methods To achieve these objectives, a rigorous multi-scale investigation is conducted, combining molecular dynamics simulation with experimental verification. Large-scale MD simulations are meticulously performed to model the nanoscale cutting process of single-crystal GaN using a diamond tool. The simulations employed highly validates interatomic potentials capable of capturing the complex bonding and deformation behavior of GaN. The model incorporates realistic crystal orientations and environmental conditions. The influence of tool angles is systematically explored by simulating cutting processes with a wide range of rake angles (−18°, −12°, −6°, 6°, 12°, 18°) and flank angles (−18°, −12°, −6°, 6°, 12°, 18°). Post-simulation analysis utilizes sophisticated algorithms to dissect the deformation mechanisms: employed to identify, characterize, and quantify the evolution of dislocations, including their types (e.g., perfect dislocations, partial dislocations), Burgers vectors, and densities within the workpiece. Used to distinguish between the pristine wurtzite GaN structure, transformed phases (e.g., possible local zinc-blende or other metastable structures under high stress), and amorphous regions generated during cutting. Local atomic stress (Von Mises or equivalent stress) and strain distributions are calculated and visualized to correlate mechanical loading with observed deformation and damage. Atomic kinetic energy is tracked to map the temperature evolution within the cutting zone and subsurface layers. To corroborate the simulation findings, controlled nanocutting experiments are conducted on single-crystal GaN substrates. Crucially, two distinct diamond abrasive grains with differing morphologies are employed as cutting tools: rake angle of −70° and a flank angle of 10°; rake angle of −43° and a flank angle of 20°.This direct comparison allows for the experimental assessment of the impact of varying rake and flank angles on surface morphology, chip formation behavior, and subsurface damage extent, using techniques such as transmission electron microscopy (TEM) and optical microscopy for cross-sectional analysis. Results The integrated simulation and experimental approach yields profound insights into the role of tool angles: Increasing the positive rake angle or reducing the magnitude of a negative rake angle is found to significantly enhance the shear-dominated material removal mechanism. This promotes more efficient and continuous chip formation while effectively suppressing undesirable lateral atomic flow and material pile-up at the groove sides, leading to improved groove definition. Conversely, increasing the magnitude of the negative rake angle dramatically exacerbates subsurface damage. The highly compressed wedge beneath the tool tip induces severe plastic deformation deeper into the substrate. Comprehensive analysis using DXA, and stress-strain fields reveals the fundamental mechanisms triggered by large negative rake and flank angles: These tool geometries induce substantially higher compressive and shear stresses within the primary deformation zone directly ahead of the tool and the subsurface region. Consequently, localized temperatures rise significantly due to intense plastic work and friction. The extreme mechanical and thermal loading promotes prolific nucleation of dislocations. These dislocations readily propagate and interact, forming complex networks. The high von Mises stress and shear stress beneath the tool facilitate solid-state phase transformations from the stable wurtzite structure to other phases. Furthermore, the intense deformation and temperature lead to extensive amorphization (loss of long-range crystalline order) within the subsurface layer. Employing tools with positive rake angles and adequate positive flank angles demonstrably alleviates subsurface damage. The cutting mechanics shift towards efficient shearing at the primary shear zone, minimizing the crushing effect below the tool. This promotes cleaner material removal, reduces dislocation density and amorphization depth, and consequently facilitates the generation of high-quality surfaces with minimal subsurface damage. Nanocutting experiments using the two specific diamond grains provids clear validation. more negative rake consistently produced scratches with significantly greater pile-up, more pronounced lateral cracks, and deeper subsurface damage zones compared to less negative rake and larger flank angle, as evidenced by TEM characterization. This directly supports the simulation predictions regarding the detrimental effects of highly negative rake angles. Conclusions This comprehensive study, synergizing high-fidelity molecular dynamics simulations with targeted experimental validation using distinct tool geometries, has significantly deepened the understanding of the nanoscale deformation and damage mechanisms in single-crystal GaN during diamond cutting. It unequivocally establishes that: Tool rake angle is a paramount factor governing the dominant material removal mode, chip formation efficiency, and the severity of subsurface damage. Large negative rake angles, while sometimes necessary for tool edge strength, induce extreme stress and temperature conditions that promote massive dislocation activity, phase transformation, and amorphization, leading to deep subsurface damage. Positive rake angles and sufficient positive flank angles promote shear-dominated cutting, suppress deleterious lateral flow and deep damage, and are highly conducive to achieving superior surface integrity with minimal subsurface defects. The mechanistic insights gained, particularly the detailed characterization of defect evolution (dislocations, phase changes, amorphous layers) linked directly to specific tool angles, provide crucial theoretical guidance and a robust scientific foundation for the rational design and optimization of ultra-precision machining (e.g., diamond turning, grinding, polishing) processes for single-crystal GaN. This knowledge is vital for enhancing the performance and reliability of next-generation GaN-based devices.
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