Surface treatment an overview


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briefly discusses various surface treatment processes

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Surface treatment an overview

  1. 1. 1
  2. 2. Major Surface Treatments Finishing and Polishing Coatings  Conversion Coatings (oxidation, anodizing)  Metal Coatings (electrochemical, electroless)  Thermal Coatings (carburizing – flame spraying)  Deposition  Physical Vapor Deposition  Chemical Vapor Deposition  Organic Coatings
  3. 3. Finishing and Polishing Blast Finishing Shot Peening Tumbling Buffing Chemical Polishing
  4. 4. Conversion Coatings A conversion coating is a coating produced by chemical treatment of a metallic surface that gives a superficial layer containing a compound of the metal  Oxidation  Phosphate Coatings  Chromate Coating  Various Coloring Processes
  5. 5. Conversion Coatings - Oxidation Oxidation  Not all oxides are detrimental – many are tightly adhering leading to passivation and hardening of surface
  6. 6. Conversion Coatings - Oxidation Black Oxide – chemical application  Typically applied to steel, copper and stainless steel Anodizing – electrochemical conversion  Usually done to Aluminum  2-25 mm thick typically  Multiple colors possible  Improved Corrosion and Wear Resistance
  7. 7. Conversion Coatings – Phosphate Coating Immersion in a Zn-P bath with Phosphoric acid causes growth of a crystalline zinc phosphate layer  Iron, Zinc or Manganese Phosphate layer  Primarily this is used as a base for subsequent painting and it also serves as corrosion protection coating Typically applied to C-steel, low alloy steel and cast irons Sometimes applied to Zinc, Cadmium, Aluminum and Tin Typically very thin ~ 2.5 micron
  8. 8. Metal Coatings Electroplating Electroless Plating Immersion Plating Metallizing of Plastics and Ceramics
  9. 9. Metal Coatings - Electroplating  Used to increase wear and corrosion resistance  Electrochemical process used to create a thin coating bonding to substrate  Process is slow so coating thickness can be closely controlled (10-500 mm)
  10. 10. Metal Coatings - Electroplating  Applications  Tin and Zinc are deposited on steel for further working  Zinc and Cadmium are deposited on parts for corrosion resistance  Copper is deposited for electrical contacts  Nickel for corrosion resistance  Chromium can be used to impart wear resistance  Precious metals for decoration or electronic devices
  11. 11. Metal Coatings – Electroless Coatings Part is submerged into an aqueous bath filled with metal salts, reducing agents and catalysts  Catalysts reduce metal to ions to form the coating Excellent for complex geometries as deposition is uniform across surface regardless of geometry (except very sharp corners (0.4 mm radii))
  12. 12. Metal Coatings – Immersion Plating Deposition of a metallic coating on a substrate by chemical replacement from a solution of a salt of the coating metal It differs from electroless plating in not requiring a chemical reducing agent; the substrate metal itself act as a reducing agent,
  13. 13. Thermal Treatments Surface Heat Treatment Diffusion Coating Hot-Dip Coatings Weld Overlay Coatings
  14. 14. Thermal Treatments – Surface Heat Treatment  Basic concept is to heat the surface to austenitic range, then quench it to form surface martensite - workpiece is steel  Heating Methods  Flame Treatment  Induction Heating  Copper coil wraps around part to heat by induction  Electron Beam or Laser Beam Hardening  Typically heat small area and allow the bulk solid heat capacity to quench the small heated area
  15. 15. Thermal Treatments – Diffusion Coating  With low carbon steel, the surface can be enriched by diffusion of C or N into surface  Carburizing  Nitriding  Metal Diffusion  Chromizing – Chromium diffuses into surface to form corrosion resistant layer.  Take care with carbon steels as surface will decarburize  Aluminizing – Used to increase the high temperature corrosion resistance of steels and superalloys
  16. 16. Thermal Treatments – Hot-Dip Coatings  These coatings are used for corrosion protection  Galvanizing  Parts are dipped into a molten zinc bath  Galv-annealing  Galvanized parts are then heat treated to ~500 ºC to form Fe-Zn inter-metallic  Used for metals that need spot welded to protect copper electrode from alloying with zinc and reducing its life
  17. 17. Vapor Deposition Chemical Vapor Deposition (CVD) Physical Vapor Deposition (PVD)  Thermal PVD  Sputter Deposition  Ion plating
  18. 18. Chemical Vapor Deposition  Deposition of a compound (or element) produced by a vapor-phase reduction between a reactive element and gas  Process typically done at elevated temps (~900ºC)  Coating will crack upon cooling if large difference in thermal coefficients of expansion
  19. 19. Chemical Vapor Deposition  Applications  Diamond Coating, Carburizing, Nitriding, Aluminizing and Siliconizing processes  Semiconductor manufacturing
  20. 20. Organic Coatings - paint  Enamels  Lacquers  Water-base paints  Powder Coating