The invention of the float glass process followed by curtain wall glazing systems led to the incorporation of large areas of glass products in the design of many large buildings. The poor thermal performance of float glass, however, prompted the development of thin layer coating systems for glass which improve energy conservation. In Europe, approximately 90% of all glass windows sold are coated. The appearance and performance of glass is therefore paramount in modern building technology. The case study below illustrates the application of modern surface analysis techniques to characterising glass surface and coating integrity.
Case Study - Investigation of Multiple Defects on a Coated Optical Filter Glass

Circular defect features were observed on the surface of an optical filter product during long term storage after production. The defects were typically ~200mm in diameter and the manufacturer observed a correlation between the rate of defect formation and storage conditions. Typically, the failure rate increased with ambient humidity whereas drier conditions or encapsulation reduced the rate.

The optical filter structure is based on a glass substrate coated with a thin layer of silicon followed by a multi-layer stack containing successive layers of yttrium oxide, zinc sulphide and silver.

Initial investigation of the defects by optical microscopy (see figure 1) showed the following features:

· All defects are circular with a particulate-like feature in the centre ~1 - 5mm in size.

· Topographic information indicates that the layer structure has buckled leaving fault lines that appear to radiate from the centre of each defect.