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	RWTH Aachen University, Fraunhofer Institute for Production Technology IPT
Citations	2
Analysis	Full AI Review Included

Executive Summary

This study evaluates the performance and degradation mechanisms of three protective coatings—Diamond-Like Carbon (DLC), Platinum-Iridium (PtIr), and Chromium Aluminum Nitride (CrAlN)—applied to tungsten carbide (WC100) molds used in Precision Glass Molding (PGM) of high Abbe number S-FPM3 glass.

Superior Durability: The DLC (tetrahedral amorphous carbon, ta-C) coating exhibited the highest resistance, surviving 500 molding cycles with minimal degradation (isolated micro glass adhesions).
Premature Failure: CrAlN failed after only 1 cycle, and PtIr failed after 5 cycles, both due to macroscopic glass adhesion and coating delamination.
Failure Mechanism Identified: Degradation consistently initiated at the outer edge of the mold contact area, not the center, across all three coating types.
FEM Validation: Finite Element Method (FEM) simulation confirmed that the high friction coefficient (f = 0.6 assumed for PtIr/CrAlN) generated significantly higher shear stress (up to 55 MPa) at the mold edge compared to the low friction DLC (f = 0.1, 10 MPa).
Material Selection Insight: The early failure of PtIr was linked to the diffusion and subsequent oxidation of the underlying Cr adhesion layer at high temperatures (540 °C).
Methodology: A novel combined approach utilizing metallographic analysis (SEM, EDX) and thermo-mechanical FEM simulation was successfully employed to explain coating degradation location and mechanism.

Technical Specifications

Parameter	Value	Unit	Context
Glass Material	S-FPM3	N/A	High Abbe number glass (74.70)
Glass Transition Temp (T_g)	496	°C	S-FPM3 glass
Molding Temperature (T_max)	540	°C	Set point for PGM process
Heating Rate	3	K/s	Controlled by PID controller
Molding Force (Max)	2.0	kN	Applied during pressing stage
Holding Time (Soaking)	120	s	Time held at 540 °C
Gradual Cooling Rate	0.2	K/sec	Down to 480 °C (below T_g)
WC100 Granularity	<0.08	µm	Mold substrate material
WC100 Hardness	2700	HV	Mold substrate material
DLC Coating Thickness	~100	nm	Deposited via FCVA
PtIr/CrAlN Coating Thickness	~600	nm	Deposited via DCMS
PtIr Adhesion Layer	~20	nm	Cr layer used for PtIr coating
DLC Friction Coefficient (FEM)	0.1	N/A	Assumed for simulation
PtIr/CrAlN Friction Coefficient (FEM)	0.6	N/A	Assumed for simulation
Max Shear Stress (f=0.6)	55	MPa	Calculated at mold edge during gradual cooling
Max Shear Stress (f=0.1)	10	MPa	Calculated at mold edge during gradual cooling
Initial Surface Roughness (Ra)	<5	nm	Required for all coated molds

Key Methodologies

Mold and Preform Preparation:
- Molding tools were fabricated from binderless WC100 using ultra-precision grinding (Ra < 5 nm). Simplified geometry: spherical concave lower mold (R8) and plane upper mold.
- Coatings applied: DLC (ta-C type) via Filtered Cathodic Vacuum Arc (FCVA); PtIr and CrAlN via custom Direct Current Magnetron Sputtering (DCMS). PtIr utilized a 20 nm Cr adhesion layer.
- Glass preforms were polished S-FPM3 spheres (Ø4 mm).
Precision Glass Molding (PGM) Experiment:
- Tests conducted in an industrial Toshiba GMP-315 machine.
- Process sequence: Evacuation (<3 Pa), Heating (3 K/s to 540 °C), Soaking (120 s), Molding (2.0 kN force applied for 70 s).
- Controlled Cooling: Gradual cooling (0.2 K/sec) under reduced force (1.6 kN) down to 480 °C (below T_g), followed by rapid cooling to 200 °C for lens removal.
Specimen Characterization (Metallography):
- Surface quality monitored using Light Microscopy and White Light Interferometry (WLI).
- Microscopic defects and morphology analyzed using Scanning Electron Microscopy (SEM) up to 25,000x magnification.
- Chemical composition of adhesions and degradation products determined via Energy Dispersive X-ray Spectroscopy (EDX).
Finite Element Method (FEM) Simulation:
- Commercial software ABAQUS was used for thermo-mechanical coupled simulation of the PGM process.
- Model utilized a mirrored 2D rotational symmetric geometry to reduce computational effort.
- The Coulomb friction law was applied, comparing low friction (f=0.1, representing DLC) and high friction (f=0.6, representing PtIr/CrAlN) to analyze Mises stress and shear stress distribution.

Commercial Applications

The findings regarding durable protective coatings for PGM tools are critical for high-volume, cost-effective manufacturing of precision optics used in the following sectors:

Medical Imaging: Manufacturing of high-precision micro-lenses for advanced endoscopic systems and minimally invasive surgical probes.
Consumer Electronics: Production of miniaturized, high-resolution imaging optics for mobile phones and other portable devices.
Laser Systems: Fabrication of aspheric glass components requiring high form accuracy and low dispersion (using high Abbe number glass).
Optical Manufacturing: Improving the efficiency and reducing the cost of replicative manufacturing processes for high-quality optical glass components by extending mold tool lifetime.

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]