briefly discusses various surface treatment processes

1. ahmadkhan_99@hotmail.com
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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. Finishing and Polishing
 Blast Finishing
 Shot Peening
 Tumbling
 Buffing
 Chemical Polishing

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. Conversion Coatings - Oxidation
 Oxidation
 Not all oxides are detrimental – many are tightly
adhering leading to passivation and hardening of
surface

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. 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. Metal Coatings
 Electroplating
 Electroless Plating
 Immersion Plating
 Metallizing of Plastics and Ceramics

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. 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. 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. 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. Thermal Treatments
 Surface Heat Treatment
 Diffusion Coating
 Hot-Dip Coatings
 Weld Overlay Coatings

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. 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. 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. Vapor Deposition
 Chemical Vapor Deposition (CVD)
 Physical Vapor Deposition (PVD)
 Thermal PVD
 Sputter Deposition
 Ion plating

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. Chemical Vapor Deposition
 Applications
 Diamond Coating, Carburizing, Nitriding, Aluminizing
and Siliconizing processes
 Semiconductor manufacturing

20. Organic Coatings - paint
 Enamels
 Lacquers
 Water-base paints
 Powder Coating