Physical vapor deposition
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Physical vapor deposition (PVD) is a variety of vacuum deposition methods used to deposit thin films by the condensation of a vaporized form of the desired film material onto various workpiece surfaces (e.g., onto semiconductor wafers).
The coating method involves purely physical processes such as high temperature vacuum evaporation with subsequent condensation, or plasma sputter bombardment rather than involving a chemical reaction at the surface to be coated as in chemical vapor deposition.
The term physical vapor deposition originally appeared in the 1966 book Vapor Deposition by C. F. Powell, J. H. Oxley and J. M. Blocher Jr., (but Michael Faraday was using PVD to deposit coatings as far back as 1838). Physical vapor deposition coating is a product that is currently being used to enhance a number of products, including automotive parts like wheels and pistons, surgical tools, drill bits, and guns. The current version of physical vapor deposition was completed in 2010 by NASA scientists at the NASA Glenn Research Center in Cleveland, Ohio. This physical vapor deposition coating is made up of thin layers of metal that are bonded together through a rig that NASA finished developing in 2010. In order to make the coating, developers put the essential ingredients into the rig, which drops the surrounding atmospheric pressure to one torr (1/760 of our everyday atmosphere). From there, the coating is heated with a plasma torch that reaches 17,540.33 degrees Fahrenheit. In the automotive world, it is the newest alternative to the chrome plating that has been used for trucks and cars for years. This is because it has been proven to increase durability and weigh less than chrome coating, which is an advantage because a vehicle's acceleration and fuel efficiency will increase. Physical vapor deposition coating is gaining in popularity for many reasons, including that it enhances a product’s durability. In fact, studies have shown that it can enhance the lifespan of an unprotected product tenfold. Variants of PVD include, in alphabetical order:
- Cathodic Arc Deposition: In which a high power electric arc discharged at the target (source) material blasts away some into highly ionized vapor to be deposited onto the workpiece.
- Electron beam physical vapor deposition: In which the material to be deposited is heated to a high vapor pressure by electron bombardment in "high" vacuum and is transported by diffusion to be deposited by condensation on the (cooler) workpiece.
- Evaporative deposition: In which the material to be deposited is heated to a high vapor pressure by electrically resistive heating in "low" vacuum.
- Pulsed laser deposition: In which a high power laser ablates material from the target into a vapor.
- Sputter deposition: In which a glow plasma discharge (usually localized around the "target" by a magnet) bombards the material sputtering some away as a vapor for subsequent deposition.
PVD is used in the manufacture of items, including semiconductor devices, aluminized PET film for balloons and snack bags, and coated cutting tools for metalworking. Besides PVD tools for fabrication, special smaller tools (mainly for scientific purposes) have been developed. They mainly serve the purpose of extreme thin films like atomic layers and are used mostly for small substrates. A good example are mini e-beam evaporators which can deposit monolayers of virtually all materials with melting points up to 3500 °C.
The source material is unavoidably also deposited on most other surfaces interior to the vacuum chamber, including the workholders.
Some of the techniques used to measure the physical properties of PVD coatings are:
- Calo tester: coating thickness test
- Nanoindentation: hardness test for thin-film coatings
- Pin on disc tester: wear and friction coefficient test
- Scratch tester: coating adhesion test
• PVD coatings are sometimes harder and more corrosion resistant than coatings applied by the electroplating process. Most coatings have high temperature and good impact strength, excellent abrasion resistance and are so durable that protective topcoats are almost never necessary.
• Ability to utilize virtually any type of inorganic and some organic coating materials on an equally diverse group of substrates and surfaces using a wide variety of finishes.
• More environmentally friendly than traditional coating processes such as electroplating and painting.
• More than one technique can be used to deposit a given film.
• Specific technologies can impose constraints; for example, line-of-sight transfer makes coating annular shapes practically impossible.
• Some PVD technologies typically operate at very high temperatures and vacuums, requiring special attention by operating personnel.
• Requires a cooling water system to dissipate large heat loads.
Application: (Reference: Achal Singh SRM-CEL) As mentioned previously, PVD coatings are generally used to improve hardness, wear resistance and oxidation resistance. Thus, such coatings use in a wide range of applications such as:
• Dies and moulds for all manner of material processing
• Cutting tools
• Fire arms
See thin-film deposition for a more general discussion of this class of manufacturing technique.
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