1. SURFACE TREATMENT AN OVERVIEW

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
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 ( 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. Thermal Treatments – Weld Overlay coatings
Typically used to improve wear resistance by creating a hard surface over a tough bulk body
Hard Facing
Weld buildup of parts – alloy composition controls final properties
Examples – cutting tools, rock drills, cutting blades
Cladding of material for corrosion resistance
Thermal spraying
Molten particle deposition – a stream of molten metal particles are deposited on the substrate surface
Major difference from hard facing is that the surface of the substrate is not subjected to welding. Instead it just undergoes a bonding process with the molten particles.

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

19. 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

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

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