1. Root Cause of the Machining Challenge
Single-crystal silicon is a core material in modern semiconductor and infrared optical applications. Its inherent characteristics of high hardness and high brittleness make it exceptionally sensitive to tool conditions during Single Point Diamond Turning (SPDT). Rapid and severe tool wear represents the fundamental contradiction throughout the SPDT machining of single-crystal silicon.For a long time, the research focus in the industry has been concentrated on process optimization for planar silicon wafers, resulting in a relatively mature set of process parameters. However, with the growing engineering demand for single-crystal silicon components with complex surface shapes such as concave and convex geometries, the existing theoretical framework proves insufficient in explaining the influence mechanisms of workpiece surface form. This knowledge gap has left the consistently inferior machining quality of concave workpieces—compared to planar and convex ones—without a systematic explanation.
2. Cutting Edge Motion Analysis: Identifying the Core Issue
Through a refined analysis of cutting edge motion trajectories during the turning of workpieces with different surface shapes, researchers identified the root cause of the problem.
For planar and convex workpieces, the contact and disengagement of the cutting edge with the workpiece surface follow a certain periodic pattern, and wear fluctuates within a relatively reasonable range. In contrast, during the turning of concave workpieces, the cutting edge region responsible for generating the final workpiece surface—referred to as the “surface-generating cutting edge”—remains in a state of severe wear throughout the entire machining cycle, with no effective opportunity for natural recovery or load redistribution.This finding reveals the fundamental mechanism of quality degradation in concave surface machining at the kinematic level: the workpiece surface shape influences the motion state of the cutting edge, which in turn governs the degree of wear and ultimately determines the quality of the machined surface and subsurface.
3. Molecular Dynamics Simulation: Microscale Verification of the Wear-Quality Relationship
To further validate the above mechanism at the microscopic scale, Molecular Dynamics (MD) simulation was introduced to analyze cutting edges under different wear conditions.
The simulation results clearly demonstrate a strong and direct correlation between cutting edge wear state and machining quality. As wear severity increases, surface roughness rises correspondingly, and the depth of subsurface damage deepens significantly. This microscale verification aligns closely with the conclusions drawn from macroscopic cutting edge motion analysis, forming a complete mechanistic chain from macroscopic kinematics to atomic-scale dynamics.The deepening of subsurface damage deserves particular attention. In optical component and semiconductor wafer applications, subsurface damage directly affects optical transmittance, mechanical strength, and the reliability of subsequent processes. It is a critical yet often overlooked indicator of ultra-precision machining quality.
4. Four-Axis Linkage Turning: A Solution for Concave Workpieces
Based on the mechanistic understanding established above, the research team proposed a four-axis linkage turning method. By introducing an additional coordinated axis control on top of conventional three-axis SPDT, the method actively optimizes and intervenes in the cutting edge motion trajectory.The core logic of the four-axis linkage approach is to adjust the relative motion between the tool and the workpiece during machining, preventing the surface-generating cutting edge from being confined to a single zone of severe wear throughout the process. This achieves dynamic equalization of cutting edge wear, thereby breaking the vicious cycle of quality degradation in concave surface machining.
5. Quantitative Validation: Significant Quality Improvement
The application of the four-axis linkage turning method on concave single-crystal silicon workpieces has been fully validated through quantitative measurement.
In terms of surface roughness, the concave workpiece surface roughness was significantly reduced from Ra 5 nm to Ra 2 nm after adopting the four-axis method—an improvement of 60%—approaching the quality levels achievable on planar and convex workpieces.In terms of subsurface damage depth, the damage depth was reduced from 515 nm to 202 nm, representing a reduction of more than 60%. The substantial reduction in subsurface damage translates directly into improved reliability of the workpiece during subsequent thinning, polishing, and end-use stages.
6. Engineering Implications
The core contribution of this research lies in establishing a systematic correlative mechanism among workpiece surface shape, cutting edge wear, and machining quality, filling a critical knowledge gap in SPDT processing of complex-surface single-crystal silicon. The four-axis linkage strategy provides a practically viable optimization path for engineering applications, and offers direct guidance for the high-quality manufacturing of high-value components such as single-crystal silicon infrared optical elements and non-planar semiconductor devices.