1. Chromate Coating
(old)
Chromate Coating is formed on base metals such as zinc, cadmium,magnesium, or aluminium by the combined action of sodium chromate and dilute sulphuric acid. The dissolution of the base metal in the dilute acid increases the pH of the solution adjacent to the metal surface facilating the reduction of chromium to chromium ions. At some critical pH basic chromium chromate is precipated on the base metal surface forming the chromate coating. The Chromate Coating is relativelly more corrosion resistant compared to phosphate coating and is amenable for painting but has low abrasion resistance.
(Engineering Chemistry, Sivasankar, p.486)
(new/better)
Chromate coatings are used on non-ferrous materials for added corrosion resistance and a base for paint. The coating media are: chromate dichromate, and chromic acid. The coatings are provided by spraying or dippimg in a chromate bath, which is an acidic solution containing hexavalent chromium compounds.
( Kaushish, Manufacturing Processes, p.447)
2. Rack Plating
(old)
Rack Plating is used for parts where the apperance of surfaces is critical e.g. decorative strips,faucets, or slower heads. Here, workpieces are mounted on racks or jigs and dipped into the processing solutions. Electrical current is fed through the complete racks when they are immersed in the processing bath. In order to guarantee that only the workpieces and not the rack itself is coated, racks are resistant to chemicals and coated with non-conductive coatings. The only uncoated parts of racks are constants where the workpieces are mounted, in order to ensure proper flow of electrical power to the substrate workpieces.
(Modern Surface Technology, F.-W. Bach,A. Laarmann,T. Wenz, p.111)
(new/better)
Rack plating involves the use of a well-designed jig or rack for suspending the workpieces to be plated. If well designed and maintained, this will give very high productivity during the plating operation.
( A. C. Tan, Tin and Solder Plating in the Semiconductor Industry: A Technical Guide, p.119)
3. Physical Vapor Deposition(PVD)
(old)
Physical vapor deposition (PVD) is a group of coating processes in which a material is converted into its vapor phase in a vacuum chamber and condensed onto a substrate surface as a very thin film. PVD can be used to apply a wide variety of coating materials: metals, alloys, ceramics and other inorganic compounds, and even certain polymers. Possible substrates include metals, glass, and plastics. Thus, PVD represents a versatile coating technology, applicable to an almost unlimited combination of coating substances and substrate materials.
Applications of PVD include thin decorative coatings on plastic and metal parts such as trophies, toys, pens and pencils, watchcases, and interior trim in automobiles. The coatings are thin films of aluminum (around 150 nm) coated with clear lacquer to give a high gloss silver or chrome appearance. Another use of PVD is to apply antireflection coatings of magnesium fluoride (MgF2) onto optical lenses. PVD is applied in the fabrication of electronic devices, principally for depositing metal to form electricalconnections in integrated circuits.Finally, PVD is widely used to coat titanium nitride (TiN) onto cutting tools and plastic injection molds for wear resistance.
All physical vapor deposition processes consist of the following steps: (1) synthesis of the coating vapor, (2) vapor transport to the substrate, and (3) condensation of vapors onto the substrate surface. These steps are generally carried out inside a vacuum chamber, so evacuation of the chamber must precede the actual PVD process.
Synthesis of the coating vapor can be accomplished by any of several methods, such as electric resistance heating or ion bombardment to vaporize an existing solid (or liquid), These and other variations result in several PVD processes. They are grouped into three principal types: (1) vacuum evaporation, (2) sputtering, and (3) ion plating.
(Mikell P. Groover,Fundamentals of Modern Manufacturing,4th Edition,pg.675)
(new/better)
Physical vapor deposition is the general name given to coating processes where the transport of material to the substrate is effected by a phsical driving mechanism. Such mechanism include evaporation, sputtering, ion-plating, and ion-assisted sputtering. Often chemical reactions do take place to form coatings of oxides, nitrides and carbides, but the transport process does not generally rely on chemical reactions. The coated samples can usually be maintained at a temperature between ambient and 100 C during the deposition and the working pressure is below 10^-2 Pa. PVD thus differs quite markedly from Chemical Vapor Deposition which occurs at atmospheric pressure, or just below, and at temperatures normally in excess of 500 C.
(C. Suryanarayana Non-Equilibrium Processing of Materials, p. 225)
4. Electroless Plating
(old)
There is no old definition
(new/better)
Electroless plating is a surface treatment process that does not require ellectric current, as does electrolytic plating, to precipitate the metal onto the plastic part. Electroless plating deposits a dissolved metal (such as copper or nickel) onto the surface of a plastic part through the use of a chemical solution.
(Edward A. Muccio Plastic Part Technology, p.212)
5. Acicular ferrite
(old)
There is no old definition
(new/better)
Far from being organized, this microstructure is better describes as chaotic. The plates of acicular ferrite nucleate heterogeneously on small non-metalic inclusions and radiate in many different directions from these ‘point’ nucleation sites.
(Harshad Kumar Dharamshi Hansraj Bhadeshia,Robert William Kerr Honeycombe, Steels: Microstructure And Properties, p.155(
No comments:
Post a Comment