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Metallizing is the general name for the technique of coating
metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...
on the surface of objects. Metallic coatings may be decorative, protective or functional. Techniques for metallization started as early as
mirror A mirror or looking glass is an object that reflects an image. Light that bounces off a mirror will show an image of whatever is in front of it, when focused through the lens of the eye or a camera. Mirrors reverse the direction of the im ...
making. In 1835, Justus von Liebig discovered the process of coating a glass surface with metallic silver, making the glass mirror one of the earliest items being metallized. Plating other non-metallic objects grew rapidly with introduction of ABS plastic. Because a non-metallic object tends to be a poor electrical conductor, the object's surface must be made conductive before plating can be performed. The plastic part is first etched chemically by a suitable process, such as dipping in a hot
chromic acid The term chromic acid is usually used for a mixture made by adding concentrated sulfuric acid to a dichromate, which may contain a variety of compounds, including solid chromium trioxide. This kind of chromic acid may be used as a cleaning mixtu ...
- sulfuric acid mixture. The etched surface is sensitised and activated by first dipping in tin(II) chloride solution, then palladium chloride solution. The processed surface is then coated with electroless copper or nickel before further plating. This process gives useful (about 1 to 6 
kgf KGF or ''kgf'' may refer to: *Keratinocyte growth factor *King George's Fields, UK, recreation grounds *Kolar Gold Fields *The IATA code for Sary-Arka Airport, Karaganda, Kazakhstan * ''K.G.F'' (film series), Indian Kannada-language film series * ...
/cm or 10 to 60  N/cm or 5 to 35  lbf/in) adhesion force, but is much weaker than actual metal-to-metal adhesion strength. Vacuum metallizing involves heating the coating metal to its boiling point in a vacuum chamber, then letting condensation deposit the metal on the substrate's surface. Resistance heating, electron beam, or plasma heating is used to vaporize the coating metal. Vacuum metallizing was used to deposit aluminum on the large glass mirrors of reflecting telescopes, such as with the
Hale telescope The Hale Telescope is a , 3.3 reflecting telescope at the Palomar Observatory in San Diego County, California, US, named after astronomer George Ellery Hale. With funding from the Rockefeller Foundation in 1928, he orchestrated the planning, de ...
.
Thermal spray Thermal spraying techniques are coating processes in which melted (or heated) materials are sprayed onto a surface. The "feedstock" (coating precursor) is heated by electrical (plasma or arc) or chemical means (combustion flame). Thermal sprayi ...
processes are often referred to as metallizing. Metals applied in such a manner provide corrosion protection to steel for decades longer than paint alone. Zinc and aluminum are the most commonly used materials for metallizing steel structures. Cold sprayable metal technology is a metallizing process that seamlessly applies cold sprayable or puttyable metal to almost any surface. The composite metal consists of two (waterbased binder) or three different ingredients: metal powder, binder and hardener. The mixture of the ingredients is cast or sprayed on the substrate at room temperature. The desired effect and the necessary final treatment define the thickness of the layer, which normally varies between 80 and 150  µm.


See also

* List of telescope parts and construction *
Thin film deposition A thin film is a layer of material ranging from fractions of a nanometer (monolayer) to several micrometers in thickness. The controlled synthesis of materials as thin films (a process referred to as deposition) is a fundamental step in many ap ...
* Electroplating *
Sputtering In physics, sputtering is a phenomenon in which microscopic particles of a solid material are ejected from its surface, after the material is itself bombarded by energetic particles of a plasma or gas. It occurs naturally in outer space, and ca ...
* Chemical vapor deposition


References

* * Industrial processes Coatings {{metal-stub