Coatings
Selecting the best float glass coating technology Ron Hunt* and Chris Cording** discuss the advantages and disadvantages of the techniques available to deposit thin films on to flat glass. � An under coater.
A
variety of techniques is available to deposit thin films on to flat glass. The most widely used of these for producing high quality functional coatings can be subdivided into two classes: Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD). PVD processes include a number of approaches of which sputtering is one, and is also the one most widely used for glass. Sputtered coatings are generally referred to as soft coated and are applied using PVD processes. Pyrolytic coatings are applied using CVD methods and are often referred to as hard coated. Both have advantages and disadvantages. It is important to consider the performance and handling factors that best meet product and manufacturing needs when evaluating which system is right for a glass manufacturing facility.
www.glass-international.com
Magnetron sputtered technology Sputtered coatings are applied offline independently of the float glass manufacturing process. Thin films are formed by accelerating high energy ions from targets toward the glass surface at low temperatures. The ions bombard the glass surface forming uniform thin layers. The bond is weak, and this is why the process is called ‘soft coating’. Commercial sputtered coatings are produced by depositing 6 to 12 layers of thin metallic and oxide coatings on to the surface of the glass in a vacuum chamber.
Silver is the active layer for low emissivity sputtered coatings. Additional layers include barriers, colour modification oxide layers and sacrificial metal layers. Sputter manufactures can use these additional layers to offer more products than pyrolytic producers offer. Off-line batch sputtered soft coatings offer a few benefits such as: � Sputtering can be done by companies not involved in glass production; � Sputtering is an established process with a range of target materials; � Short delivery times are possible with less inventory per product; � By depositing more layers sputtered coatings can offer better properties for some applications. Many companies embrace sputter coating for the above advantages. Glass distributers have little choice but to install sputter systems to grow their businesses, which accounts for the popularity of sputter systems. However, glass manufacturers have other options and should also consider the disadvantages of sputter coating: 1. Capital costs are higher for sputtering equipment relative to production capacity. 2. Manufacturing costs are high due to materials, energy, maintenance and depreciation (often 2 to 3 times the price of CVD coatings), resulting in low margins.
3. Yields decline as more layers are deposited. 4. Sputtered coatings are soft and scratch easily during normal handling and fabrication. 5. Sputtered coatings are sensitive to moisture, requiring sealed bags with desiccant. 6. Shelf life is limited with sealed packaging and further reduced after packaging is opened. 7. Sputtered coatings have weak adhesion that can cause sealant failures in insulated glass units. Edge deletion of sputtered coatings is recommended, adding to manufacturing costs. Sputtered coatings that don’t require edge deletion can experience rapid seal failure due to certain detergents and brick washes. 8. Sputtered coatings cannot typically be used with single pane applications. 9. Toughened soft coat requires unusual skill and often results in increased toughening losses.
Pyrolytic CVD technology Chemical Vapor Deposition (CVD) is used to produce aesthetic and functional coatings. Specialised coaters produce CVD coatings by passing metal oxides over semi molten glass (600°C to 700°C) during the glass manufacturing process. A chemical reaction occurs joining the vapour on to the glass surface permanently with a Continued>>
46 Glass International December/January 2017
Stewart Engineers recovered.indd 1
13/12/2016 15:10:43