Glass Coating Technology ComparisonA variety of techniques are available to deposit thin films onto 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-coatings and are applied using PVD processes. Pyrolytic coatings are applied using CVD methods and are often referred to as hard-coatings.
Both coating methods 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.
|A Stewart Engineers AcuraCoat® CVD Under-Coater for on-line pyrolitic CVD glass coating.|
Magnetron Sputtered TechnologySputtered coatings are applied off-line, 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, which is why the process is called "soft-coating."
Commercial sputtered coatings are produced by depositing between six and twelve layers of thin metallic and oxide coatings onto the surface of the glass in a vacuum chamber. Silver is the active layer for low-emissivity sputtered coatings. Additional layers include barriers, color modification, oxide layers, and sacrificial metal layers. Sputter manufacturers 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 required inventory per product
- By depositing more layers, sputtered coatings can offer better physical properties for some applications
- Capital costs are higher for sputtering equipment relative to production capacity
- Manufacturing costs are high due to required materials, energy, maintenance and depreciation (often two-to-three times the price of CVD coatings), resulting in low margins
- Yields decline as more layers are deposited (increased defects)
- Sputtered coatings are soft and are damaged easily during normal handling and fabrication
- Sputtered coatings are sensitive to moisture, requiring sealed bags with desiccant for storage and transit
- Shelf-life is limited with sealed packaging, and further reduced after packaging is opened
- 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.
- Sputtered coatings cannot typically be used for single-pane applications
- Toughened soft coat requires unusual skill and often results in increased toughening losses
Pyrolytic CVD TechnologyChemical Vapor Deposition (CVD) is used to produce aesthetic and functional coatings as an alternative to PVD. Specialized coaters produce CVD hard coatings by passing metal oxides over semi-molten glass (600 - 700C) during the glass manufacturing process. A chemical reaction occurs, joining the vapor with the glass surface permanently by means of a strong covalent bond. The result is a hard and robust coating that enhances the strength and stain resistance of the glass.
These hard coatings, which cannot be accidentally wiped off during normal handling, are more durable than soft and fragile sputtered coatings. Manufacturers and fabricators handle CVD hard coatings with the same procedures and equipment as standard float glass, resulting in higher yields, higher profit, excellent lead times, and improved customer service.
Pyrolitic CVD hard coatings offer other benefits as well:
- Lower capital investment
- Higher yields
- Lower manufacturing costs (CVD: ~$1/square-meter versus sputter: ~$4/square-meter)
- CVD is an on-line process, resulting in unmatched throughput
- CVD hard coatings are bonded to the glass with covalent bonds, resulting in increased shelf-life and scratch and stain resistance
- CVD hard coatings can be used in specialty applications such as monolithic, touchscreen, and anti-microbial. Conductive coating applications such as solar panels and appliance glass are more practical with CVD hard coatings
- Special handling equipment and procedures are not needed. Glass is handled and shipped with the same equipment as non-value-added float glass.
- CVD hard coatings are not susceptible to delamination due to humidity, resulting in a much-higher shelf-life
- CVD glass is toughened similarly to standard float glass products, resulting in improved throughput and cost effectiveness
- There is no visual differentiation between annealed or tempered CVD products. Projects requiring a mix of toughened and annealed glass benefit from a more pleasing uniformity of color.
- CVD for on-line use was not readily available on attractive commercial terms until very recently
- CVD systems must be installed on-line at a float glass manufacturing facility
- Some manufacturers mistakenly believe CVD cannot be installed due to space constraints
- Some manufacturers mistakenly believe CVD can only be installed during a cold repair
Which is best?Which type of coated glass is best depends on several factors for a glass manufacturer, including: where customers are located, the size and type of operation, inventory turns, and desired durability.
For most glass manufacturers, pyrolitic CVD technology is superior due to lower capital and operating costs and higher throughput.
For downstream customers, both sputter coated and pyrolytic glass are viewed as high-performance glass products. Architects and building owners are generally open to using both products and are primarily concerned with aesthetics that slightly favor CVD.
If your company is interested in determining the feasibility of a pyrolitic CVD hard coater, check out the Stewart Engineers AcuraCoat® CVD coater today.