A Study on the Material Removal Characteristics and Damage Mechanism of Lapping for Pressureless Sintered Silicon Carbide (SSiC) Microlens Cavity

At a Glance

Metadata	Details
Publication Date	2023-05-31
Journal	Micromachines
Authors	Tianfeng Zhou, Zhongyi Li, Weijia Guo, Peng Liu, Bin Zhao
Institutions	Beijing Institute of Technology, Chongqing University of Technology
Citations	4
Analysis	Full AI Review Included

Executive Summary

This study experimentally and theoretically investigated the material removal characteristics and damage mechanisms during the lapping of Pressureless Sintered Silicon Carbide (SSiC) microlens cavities for Precision Glass Molding (PGM) applications.

Material Removal Mechanism (MRM): The lapping process on SSiC, a hard and brittle material, is dominated by brittle removal, involving a combination of ploughing, shearing, micro-cutting, and micro-fracturing. This mechanism was validated using Finite Element Method (FEM) simulations.
Abrasive Size Impact: Larger W20 diamond particles (15.0-17.0 µm) resulted in a higher material removal rate and deeper lapping trails, primarily utilizing a ploughing mechanism. Smaller W7 particles (5.0-6.0 µm) resulted in lower removal rates and shallower scratches, favoring a cutting mechanism.
Surface Damage: Lapped surfaces consistently exhibited damage features characteristic of brittle fracture, including micropores, discrete breakouts, fracture pits, and brittle cracks.
Tool Wear Effect: Wear on the Cemented Tungsten Carbide (WC) lapping tool head significantly impacted surface quality, leading to non-uniform pressure distribution, uneven material removal, and increased surface roughness/waviness.
Chemical Anomaly: EDX analysis revealed localized carbon enrichment (up to 70.54% atomic percentage) in dark areas of the lapped surface, hypothesized to be caused by friction-induced carbon diffusion or trapping in micro-cracks.
Optimization: A step lapping approach (W7 followed by W20) was demonstrated to improve uniformity, remove initial defects, and achieve a faster bulk removal rate.

Technical Specifications

Parameter	Value	Unit	Context
Workpiece Material	Pressureless Sintered SiC (SSiC)	N/A	PGM mold material
Workpiece Dimensions	15 x 15 x 0.5	mm	Sample size
SSiC Hardness	> 115	Hs	Material property
SSiC Elastic Modulus	410	GPa	Material property
Initial Surface Roughness (Ra)	~0.730	µm	SSiC workpiece
Lapping Tool Material	Cemented Tungsten Carbide (WC)	N/A	Spherical head
Lapping Tool Diameter	10	mm	Tool head size
Rotational Speed	720	degree/s	Lapping process parameter
Feed Rate (Drilling)	0.5	µm/s	Lapping process parameter
Expected Machining Depth	30	µm	Material removal target
Abrasive Size (W7)	5.0-6.0	µm	Diamond slurry (Small)
Abrasive Size (W20)	15.0-17.0	µm	Diamond slurry (Large)
Initial Surface Composition (C)	40.45	Atomic%	Pristine SSiC (EDX)
Max Local Carbon Content	70.54	Atomic%	Darker lapped area (EDX)
Slurry Composition (Abrasive)	6	%	Diamond content
Slurry Composition (Thickening Agent)	20	%	Slurry component

Key Methodologies

The lapping experiments were conducted on a custom-built C-shaped platform designed for precision control and force measurement.

Tool and Workpiece Setup:
- SSiC workpieces (15 x 15 x 0.5 mm) were fixed onto a movable stage.
- A spherical WC lapping tool (10 mm diameter) was fixed to the spindle. The tool head featured a rough surface design to retain abrasive slurry and facilitate chip extraction.
Lapping Parameters:
- Rotational speed was fixed at 720 degree/s.
- Feed rate (drilling) was set at 0.5 µm/s, targeting a 30 µm material removal depth.
- Contact pressure was monitored and gradually increased from a starting point of 0.1 N to maintain a consistent material removal rate.
- Bottom delay duration (100, 200, or 300 s) was applied after reaching the target depth.
Abrasive Slurry Application:
- Diamond abrasive slurry was sprayed regularly. Two particle sizes were tested: W7 (5.0-6.0 µm) and W20 (15.0-17.0 µm).
- The slurry composition included 6% abrasive, 20% thickening agent, 24% dispersant, 16% lubricant, 22% blending agent, and 12% lapping assisting agent.
Step Lapping Optimization:
- An optimized process utilized small W7 particles first to remove initial surface defects and scratches, followed by large W20 particles to achieve faster bulk material removal.
Characterization and Modeling:
- Surface morphology and damage were analyzed using Scanning Electron Microscopy (SEM).
- Surface topography and height variations (ploughing trails, pits) were measured using 3D laser confocal scanning microscopy.
- Elemental composition was verified using Energy Dispersive X-ray Spectroscopy (EDX).
- The material removal mechanism was simulated using AdvantEdge FEM, employing the Drucker-Prager criterion to model SSiC behavior under stress.

Commercial Applications

The findings directly support the optimization of manufacturing processes for high-performance optical components and molds made from hard ceramics.

Precision Glass Molding (PGM): Directly enables high-efficiency, high-precision fabrication of SSiC molds, which are essential for the mass production of complex optical elements like microlens arrays.
Optical Systems: Improves the quality and yield of microlens arrays (MLAs) used in consumer electronics, automotive lighting, medical imaging, and advanced display technologies (e.g., AR/VR optics).
Hard Ceramic Machining: Provides critical data on the brittle-dominant material removal mechanisms (ploughing vs. cutting) in SSiC, which is transferable to the precision machining of other ultra-hard materials (e.g., Reaction-Bonded SiC, Si₃N₄).
High-Reliability Components: SSiC’s superior wear resistance and thermal stability make the resulting molds suitable for demanding industrial applications requiring long mold life and high-temperature operation.
Tooling and Abrasives Industry: The research informs the selection and optimization of diamond abrasive particle size and slurry composition for achieving specific surface finish requirements on ceramic substrates.

View Original Abstract

Microlens arrays have been widely employed to control the reflection, refraction, and diffraction characteristics of light due to its distinctive surface properties. Precision glass molding (PGM) is the primary method for the mass production of microlens arrays, of which pressureless sintered silicon carbide (SSiC) is a typical mold material due to its excellent wear resistance, high thermal conductivity, high-temperature resistance, and low thermal expansion. However, the high hardness of SSiC makes it hard to be machined, especially for optical mold material that requires good surface quality. The lapping efficiency of SSiC molds is quite low. and the underlying mechanism remains insufficiently explored. In this study, an experimental study has been performed on SSiC. A spherical lapping tool and diamond abrasive slurry have been utilized and various parameters have been carried out to achieve fast material removal. The material removal characteristics and damage mechanism have been illustrated in detail. The findings reveal that the material removal mechanism involves a combination of ploughing, shearing, micro-cutting, and micro-fracturing, which aligns well with the results obtained from finite element method (FEM) simulations. This study serves as preliminary reference for the optimization of the precision machining of SSiC PGM molds with high efficiency and good surface quality.

Tech Support

Original Source

DOI: https://doi.org/10.3390/mi14061162

References

2019 - Preparation of dense and high-purity SiC ceramics by pressureless solid-state-sintering [Crossref]
2017 - Machining processes of silicon carbide: A review
2018 - Corrosion resistance of silicon-infiltrated silicon carbide (SiSiC) [Crossref]
2020 - Material removal characteristics of ultra-precision grinding silicon carbide ceramics [Crossref]
2017 - Investigation of silicon carbide ceramic polishing by simulation and experiment [Crossref]
2010 - Removal behaviors of different SiC ceramics during polishing [Crossref]
2014 - Study on removal mechanism and removal characters for SiC and fused silica by fixed abrasive diamond pellets [Crossref]
2017 - Experimental investigation on the surface and subsurface damages characteristics and formation mechanisms in ultra-precision grinding of SiC [Crossref]
2019 - 3D fabrication of spherical microlens arrays on concave and convex silica surfaces [Crossref]
2020 - Manufacturing of a microlens array mold by a two-step method combining microindentation and precision polishing [Crossref]