Optics and photonics play an increasingly significant role in technology and daily life. Precision glass molding (PGM) enables the production of complex-shaped optical components that are required. As glass is heated and pressed into the desired shape between two ultraprecision molding tools, protecting their surface quality in the high-stress, high-temperature molding environment is of paramount importance. Even with protective thin film coatings on the molding surfaces of the tools, the biggest current deficit of the PGM process is the low and/or inconsistent life of the molding tools.
The coatings used in PGM are currently developed and selected by trial-and-error. The reason for this is the lack of knowledge about the fundamental mechanisms that lead to the failure of the coated tools. The objective of this work is to provide a better understanding of these mechanisms and how they are influenced by material properties and process parameters, so that the appropriate measures can be taken to delay or prevent failure.
First, the processes and materials involved are analyzed and possible interaction mechanisms are identified. Based on this, simple methods to individually test the most probable failure mechanisms under application-specific conditions are devised and applied.
Next, a new methodology for quickly assessing the interactions between different glasses and coatings is developed, based on the principles of accelerated life testing. The results are analyzed qualitatively and quantitatively and the effect of various input parameters (glass, coating, temperature etc.) as well as the mechanisms leading to coating failure are identified. In addition, a descriptive/predictive model for the outcome of these stress tests is developed.
These results should be validated by molding tests. However, performing these tests in a production glass molding machine is very time-consuming and cost-intensive. Therefore, a completely new, automated testing rig that duplicates the failure-relevant molding process steps but is much more time-efficient is developed. The molding tests performed there confirm the already-observed failure mechanisms and provide additional insights regarding the effect of mechanical loads.
Finally, the observed and potential failure mechanisms are organized in an easy-to-reference root cause analysis diagram describing to their contribution to the various failure modes, providing an explanatory and predictive model.
The Failure Mechanisms of Coated Precision Glass Molding Tools
Molding tools in precision glass molding fail easily, even with protective thin film coatings applied. In this work, various efficient methods for assessing glass-coating interactions are developed, including a new, automated testing rig. Analysis of the testing results provides a better understanding of these mechanisms and how they are influenced by material properties and process parameters, so that the appropriate measures can be taken to prolong the life of the molding tools.