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

Technical Documentation & Analysis: Precision Machining of Hard Materials

Reference Paper: Zhou et al., “A Study on the Material Removal Characteristics and Damage Mechanism of Lapping for Pressureless Sintered Silicon Carbide (SSiC) Microlens Cavity.” Micromachines 2023, 14, 1162.

Executive Summary

This analysis translates the findings regarding the precision machining of SSiC microlens molds into actionable material science requirements, highlighting 6CCVD’s superior CVD diamond solutions for high-wear optical applications.

Challenge: Pressureless Sintered Silicon Carbide (SSiC), while thermally stable, is extremely hard (Hs > 115) and brittle, making high-efficiency lapping for optical quality difficult due to micro-fracturing and subsurface damage.
Mechanism Identified: Material removal during lapping is governed by a combination of brittle mechanisms: ploughing, shearing, micro-cutting, and micro-fracturing, necessitating careful control of abrasive size.
Abrasive Optimization: The study confirms that diamond abrasive particle size is critical. Larger particles (W20, 15-17 µm) offer faster bulk removal via ploughing, while smaller particles (W7, 5-6 µm) achieve better surface finish via cutting mechanisms.
Process Solution: A “step lapping” approach (W7 followed by W20) was required to optimize both material removal rate and final surface quality, demonstrating the complexity of achieving optical finish on SSiC.
6CCVD Value Proposition: For high-wear applications like Precision Glass Molding (PGM) molds, 6CCVD’s Single Crystal Diamond (SCD) and Polycrystalline Diamond (PCD) offer intrinsic hardness and wear resistance far exceeding SSiC, simplifying the machining process and guaranteeing superior mold lifetime and optical fidelity.
Surface Quality Guarantee: 6CCVD provides as-grown or polished diamond wafers with guaranteed surface roughness (Ra < 1nm for SCD), eliminating the need for complex, multi-stage lapping processes to achieve the required optical finish.

Technical Specifications

The following hard data points were extracted from the experimental methodology:

Parameter	Value	Unit	Context
Workpiece Material	SSiC	N/A	Pressureless Sintered Silicon Carbide
SSiC Hardness	> 115	Hs (Shore)	High hardness, prone to brittle fracture
SSiC Elastic Modulus	410	GPa	High stiffness
Initial SSiC Roughness (Ra)	0.730	µm	Pre-lapping surface quality
Lapping Tool Material	WC	N/A	Cemented Tungsten Carbide (Spherical)
Lapping Tool Diameter	10	mm	Microlens cavity size constraint
Rotational Speed	720	degree/s	Lapping parameter
Feed Rate	0.5	µm/s	Material removal rate control
Expected Machining Depth	30	µm	Total material removed
Small Abrasive Size (W7)	5.0-6.0	µm	Used for defect removal/finish
Large Abrasive Size (W20)	15.0-17.0	µm	Used for bulk material removal
Abrasive Concentration	6	%	Diamond slurry composition
Minimum FEM Grid Size	5	nm	Simulation detail for surface analysis

Key Methodologies

The experimental study utilized a specialized lapping process combined with advanced characterization and simulation techniques:

Lapping Platform: Experiments were conducted on a self-developed, C-shaped lapping platform featuring a marble stage for rigidity and grating rulers for precise X, Y, and Z control.
Tooling: A spherical Cemented Tungsten Carbide (WC) tool head (10 mm diameter) was used. The rough surface morphology of the tool was intentionally designed to store abrasive slurry and facilitate chip extraction.
Abrasive Slurry: Diamond abrasive slurry was used, consisting of 6% abrasive, 20% thickening agent, 24% dispersant, 16% lubricant, 22% blending agent, and 12% lapping assisting agent.
Process Parameters: Lapping was performed at 720 degree/s rotational speed, 0.5 µm/s feed rate, and a starting contact pressure of 0.1 N. Total material removal depth was 30 µm.
Step Lapping: To optimize results, a two-step process was tested: initial machining with small W7 particles (5-6 µm) to remove surface defects, followed by W20 particles (15-17 µm) for faster bulk material removal.
Characterization: Surface morphology and elemental composition were analyzed using Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDX). Surface topography was measured using 3D laser confocal scanning microscopy.
Simulation: Finite Element Method (FEM) modeling using AdvantEdge FEM was employed to simulate the single abrasive micro-cutting process and validate the observed material removal mechanisms (ploughing, shearing, micro-cutting).

6CCVD Solutions & Capabilities

The research highlights the inherent difficulties in achieving high-quality optical surfaces on hard, brittle materials like SSiC, requiring complex, multi-step lapping and careful abrasive management. 6CCVD’s CVD diamond materials offer a direct, superior alternative for PGM mold applications, providing extreme hardness and thermal stability with simplified finishing requirements.

Applicable Materials

For replicating or extending this research into high-performance PGM molds, 6CCVD recommends the following materials:

Material Grade	Recommendation	Rationale for PGM Molds
Optical Grade SCD	Primary Recommendation	Highest hardness, thermal conductivity, and chemical inertness. Ideal for ultra-precision optics where Ra < 1nm is required. Eliminates brittle fracture issues inherent to SSiC.
Optical Grade PCD	Alternative for Large Molds	Available in large formats (up to 125 mm diameter). Excellent wear resistance for high-volume production. Can be polished to Ra < 5nm.
Boron-Doped Diamond (BDD)	For Integrated Heating	If the PGM mold requires integrated resistive heating elements, BDD provides the necessary electrical conductivity while maintaining diamond’s mechanical properties.

Customization Potential

6CCVD’s in-house manufacturing capabilities directly address the dimensional and finishing challenges encountered when machining hard materials for microlens arrays:

Requirement from Research	6CCVD Capability	Benefit to Customer
High-Precision Geometry	Custom laser cutting and shaping services.	We can supply diamond plates/wafers cut to precise dimensions (e.g., 15 mm x 15 mm, 0.5 mm thickness) or complex geometries required for mold inserts.
Large Format Molds	PCD wafers available up to 125 mm diameter.	Supports scaling up from the 10 mm tool diameter used in the study to industrial-scale microlens arrays.
Ultra-Smooth Finish	SCD polishing to Ra < 1nm; PCD polishing to Ra < 5nm.	Achieves optical quality finish before the customer’s final lapping/polishing steps, drastically reducing processing time and eliminating subsurface damage (cracks, pits) common in SSiC.
Tool Integration	Custom metalization (Au, Pt, Pd, Ti, W, Cu).	Allows for seamless integration of diamond molds into PGM heating and mounting systems, ensuring optimal thermal transfer and mechanical stability.
Thickness Control	SCD/PCD thickness control from 0.1 µm up to 500 µm (wafers) and substrates up to 10 mm.	Provides flexibility for thin-film diamond coatings or robust bulk mold inserts.

Engineering Support

The complexity of optimizing lapping parameters (abrasive size, pressure, speed, slurry concentration) on SSiC demonstrates the need for expert material consultation.

6CCVD’s in-house PhD team specializes in the mechanical, thermal, and optical properties of CVD diamond. We offer comprehensive engineering support to assist researchers and manufacturers in:

Material Selection: Determining the optimal SCD or PCD grade based on specific PGM temperature profiles and required mold lifetime.
Process Optimization: Consulting on post-processing techniques (such as final polishing or etching) tailored specifically for diamond, ensuring the highest possible surface quality for microlens array projects.
Failure Analysis: Utilizing our expertise to prevent issues like tool wear and non-uniform material removal observed in the SSiC study by leveraging the superior homogeneity and hardness of CVD diamond.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

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]