Oct 26 2004
.jpg)
sichkarenko.com / Shutterstock
The discovery of the float glass process followed by curtain wall glazing systems resulted in the integration of huge areas of glass products in the design of several large buildings.
However, the poor thermal performance of float glass stimulated the development of thin layer coating systems for glass, resulting in enhanced energy conservation. In Europe, almost 90% of all glass windows sold are coated. The performance and appearance of the glass are thus important in contemporary building technology. The following case study demonstrates the application of modern surface analysis methods for characterizing glass surface and coating integrity.
Case Study - Investigation of Multiple Defects on a Coated Optical Filter Glass
During long-term storage after production, circular defect features were identified on the surface of an optical filter product. Typically, the defects had a diameter of approximately 200 mm and the manufacturer identified a correlation between the rate of defect formation and storage conditions. Normally, ambient humidity increases the failure rate, while encapsulations or drier conditions decrease the rate.
The optical filter structure is formed on the basis of a glass substrate coated with a thin silicon layer followed by a multi-layer stack consisting of successive layers of zinc sulfide, yttrium oxide, and silver.
Initial analysis of the defects by optical microscopy demonstrated the following features:
- Topographic information denotes that the layer structure has become distorted, leaving fault lines that seem to spread out from the center of each defect
- All are circular defects with a particulate-like feature in the center of approximately1–5 mm in size
SIMS depth profile analysis of a non-defect area of the filter by DSIMS apparently reveals the alternating layer structure:
- The individual layer thicknesses are approximately 30 nm for zinc sulfide and approximately10 nm for silver and yttrium oxide layer.
- Chemical interaction between the neighboring silver and zinc sulfide layers resulted in the formation of silver sulfide.
- Carbon contamination exists at or close to the silver/zinc sulfide interfaces. This could be attributed to poor vacuum quality in a reactor vessel.
- OH is present in the zinc sulfide layer and mainly at the zinc sulfide/yttrium oxide interface. Again this may be an indication of the quality of vacuum during layer deposition, or moisture ingress after production.
Progressive SIMS image acquisitions via a defect area demonstrate the buckling and displacement of the layer structure and the potential cause of defect formation:
- A silicate-rich particulate is present at the middle of the defect area at the interface between the coating layer structure and the original glass substrate
- Localized physical stresses in the coating—induced by the particulate and expansion of any related moisture—led to the ultimate distortion of the coating, spreading outwards from the nucleation site
On the basis of this work, a more thorough cleaning and inspection process for the glass substrates was executed by the manufacturer, leading to subsequent yield enhancements.