Studies on Protective Coatings for Molding Tools Applied in a Precision Glass Molding Process for a High Abbe Number Glass S-FPM3

At a Glance

Metadata	Details
Publication Date	2023-08-16
Journal	Coatings
Authors	Chong Chen, Marcel Friedrichs, Cheng Jiang, Liang Wang, Ming-Yang Dang
Institutions	Fraunhofer Institute for Production Technology IPT, Changchun University of Science and Technology
Citations	2
Analysis	Full AI Review Included

Technical Documentation & Analysis: High-Performance Diamond Coatings for Precision Glass Molding

This document analyzes the research paper “Studies on Protective Coatings for Molding Tools Applied in a Precision Glass Molding Process for a High Abbe Number Glass S-FPM3” to highlight the critical role of diamond-based materials in high-stress thermal-mechanical applications and to position 6CCVD’s capabilities as the ideal solution provider.

Executive Summary

Diamond Superiority Confirmed: The research validates that Diamond-Like Carbon (DLC, specifically ta-C) coatings offer vastly superior durability for Precision Glass Molding (PGM) tools compared to precious metal (PtIr) and ceramic hard coatings (CrAlN).
Exceptional Lifetime: The DLC coating survived 500 molding cycles of high Abbe number glass (S-FPM3) at 540 °C with only isolated micro defects and glass adhesion streaks, demonstrating robust performance.
Failure Mechanism Identified: Coating degradation (delamination and macroscopic glass adhesion) in the PtIr and CrAlN samples (which failed within 1-5 cycles) was attributed to high shear stress (up to 55 MPa) concentrated at the mold’s peripheral contact edge.
Friction Coefficient is Key: Finite Element Method (FEM) analysis proved that the low friction coefficient (f=0.1) inherent to DLC coatings is critical for mitigating shear stress and preventing premature failure, contrasting sharply with the high-friction coatings (f=0.6).
6CCVD Relevance: As experts in MPCVD diamond, 6CCVD provides the ultra-high purity Single Crystal Diamond (SCD) and Polycrystalline Diamond (PCD) substrates necessary for developing or utilizing advanced, low-friction diamond coatings and tools for next-generation PGM applications.

Technical Specifications

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

Parameter	Value	Unit	Context
Glass Material	S-FPM3	N/A	High Abbe number glass (used in endoscopes).
Glass Transition Temperature (T_g)	496	°C	Critical temperature for S-FPM3.
Molding Temperature (T_molding)	540	°C	Peak temperature during the PGM process.
Maximum Molding Force	2.0	kN	Applied force during the pressing stage.
DLC Coating Thickness	100	nm	Thin film thickness of the high-performing ta-C layer.
PtIr / CrAlN Coating Thickness	600	nm	Thickness of the rapidly failing coatings.
Required Surface Roughness (Ra)	< 5	nm	Achieved surface quality for all coated molds.
DLC Friction Coefficient (f)	0.1	N/A	Low friction value used in FEM simulation.
PtIr / CrAlN Friction Coefficient (f)	0.6	N/A	High friction value used in FEM simulation.
Maximum Shear Stress (f=0.6)	55	MPa	Calculated stress at the mold surface during pressing (high friction case).
Demonstrated DLC Lifetime	500	Cycles	Achieved lifetime before significant degradation.

Key Methodologies

The experiment combined a rigorous thermal-mechanical PGM process with advanced analytical techniques to determine coating degradation mechanisms.

Molding Process (PGM Cycle):
- Evacuation: Chamber pressure reduced to below 3 Pa.
- Heating: Infrared lamps used to heat the mold and glass preform at a rate of 3 K/s.
- Soaking: Temperature held constant at 540 °C for 120 s to ensure homogeneity.
- Molding: Lower mold moved upward, applying a constant force of 2.0 kN for 70 s.
- Gradual Cooling: Nitrogen flow introduced (0.2 K/sec rate) down to 480 °C (below T_g).
- Rapid Cooling: System fast-cooled to 200 °C for lens removal.
Coating Deposition:
- DLC (ta-C): Applied using Filtered Cathodic Vacuum Arc (FCVA) technology.
- CrAlN & PtIr: Applied using custom Direct Current Magnetron Sputtering (DCMS).
- Note: A 20 nm thick Cr adhesion layer was required for the PtIr coating on the WC substrate.
Specimen Characterization & Analysis:
- Metallography: Light Microscopy (LM), White Light Interferometry (WLI) for Ra measurement, Scanning Electron Microscopy (SEM) (up to 25,000× magnification), and Energy Dispersive X-ray Spectroscopy (EDX).
- Simulation: Finite Element Method (FEM) using ABAQUS commercial software to model the thermo-mechanical coupled process and stress distribution, incorporating the Coulomb friction law.

6CCVD Solutions & Capabilities

The research unequivocally demonstrates that low-friction, diamond-based materials are essential for achieving long tool lifetimes in high-temperature PGM. 6CCVD is uniquely positioned to supply the foundational materials and engineering services required to replicate and advance this critical research.

Applicable Materials

The success of the DLC coating (ta-C) highlights the need for materials with extreme hardness and low friction. 6CCVD provides the highest quality CVD diamond materials, which are ideal for PGM tooling, either as substrates for advanced coatings or as the tool material itself.

6CCVD Material	Application Relevance	Key Capability Match
Optical Grade SCD	Ideal substrate for ultra-high quality DLC deposition or direct use in PGM tools requiring Ra < 1 nm. Offers maximum thermal conductivity and hardness.	Polishing: Ra < 1 nm (SCD). Thickness: 0.1 µm - 500 µm.
High-Purity PCD	Cost-effective alternative for larger PGM tools (up to 125mm diameter) requiring excellent mechanical stability and low friction.	Custom Dimensions: Plates/wafers up to 125mm. Polishing: Ra < 5 nm (Inch-size PCD).
Boron-Doped Diamond (BDD)	Relevant for PGM tools requiring conductive properties for active heating/sensing or electrochemical applications related to mold cleaning/maintenance.	Materials: Boron-Doped (BDD).

Customization Potential

The paper utilized simplified spherical tools with specific dimensions (e.g., Ø9.8 mm, R=8 mm) and required precise adhesion layers (Cr). 6CCVD offers comprehensive customization services to meet exact research and production specifications:

Custom Dimensions: 6CCVD can supply SCD and PCD plates/wafers in custom sizes up to 125 mm diameter, suitable for scaling up PGM tool production beyond the test dimensions used in the study.
Precision Polishing: We guarantee surface roughness down to Ra < 1 nm for SCD and Ra < 5 nm for PCD, ensuring the ultra-smooth surfaces required to minimize friction and shear stress, as identified by the FEM analysis.
Advanced Metalization: The research required a Cr adhesion layer for the PtIr coating. 6CCVD offers in-house metalization services, including Ti, W, Cu, Au, Pt, and Pd, allowing researchers to rapidly prototype and test complex multi-layer coating systems on diamond substrates.

Engineering Support

6CCVD’s in-house PhD team specializes in the material science and tribology of CVD diamond. We can assist researchers and engineers in:

Material Selection: Guiding the choice between SCD and PCD based on required thermal properties, optical clarity, and mechanical load for similar Precision Glass Molding (PGM) projects.
Tribological Optimization: Consulting on surface preparation and polishing techniques to achieve the lowest possible friction coefficient, directly addressing the critical shear stress failure mechanism identified in this paper.
Global Logistics: Ensuring reliable, DDU or DDP global shipping of sensitive, high-value diamond materials directly to research facilities worldwide.

Call to Action: For custom specifications or material consultation regarding high-performance diamond substrates for PGM tooling, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

Precision glass molding (PGM) is an efficient process used for manufacturing high-precision micro lenses with aspheric surfaces, which are key components in high-resolution systems, such as endoscopes. In PGM, production costs are significantly influenced by the lifetimes of elaborately manufactured molding tools. Protective coatings are applied to the molding tools to withstand severe cyclic thermochemical and thermomechanical loads in the PGM process and, in this way, extend the life of the molding tools. This research focuses on a new method which combines metallographic analysis and finite element method (FEM) simulation to study the interaction of three protective coatings—diamond-like carbon (DLC), PtIr and CrAlN—each in contact with the high Abbe number glass material S-FPM3 in a precision glass molding process. Molding tools are analyzed metallographically using light microscopy, white light interferometry, scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX). The results show that the DLC coating improved process durability more than the PtIr and CrAlN coatings, in which the phenomenon of coating delamination and glass adhesion can be observed. To identify potential explanations for the metrological results, FEM is applied to inspect the stress state and stress distribution in the molding tools during the molding process.

Tech Support

Original Source

DOI: https://doi.org/10.3390/coatings13081438

References

2021 - Wide field of view lens design with uniform image illumination in capsule endoscope system [Crossref]
2011 - Optical design and imaging performance testing of a 9.6-mm diameter femtosecond laser microsurgery probe [Crossref]
2014 - A study on the optical parts for a semiconductor laser module [Crossref]
2009 - A study for stray light distribution of mobile phone camera consisting of two aspheric lenses [Crossref]
2017 - Precision glass molding: Toward an optimal fabrication of optical lenses [Crossref]
1983 - Precision molded-glass optics [Crossref]
2005 - Compression molding of aspherical glass lenses—A combined experimental and numerical analysis [Crossref]
2015 - Corrective finishing of a micro-aspheric mold made of tungsten carbide to 50 nm accuracy [Crossref]