Surface treatment an overview

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SURFACE TREATMENTAN OVERVIEW

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Major Surface TreatmentsFinishing 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

Finishing and PolishingBlast Finishing

Shot Peening

Tumbling

Buffing

Chemical Polishing

Conversion CoatingsA 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

Conversion Coatings - Oxidation

OxidationNot all oxides are detrimental – many are tightly

adhering leading to passivation and hardening of surface

Conversion Coatings - OxidationBlack 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

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

Metal CoatingsElectroplating

Electroless Plating

Immersion Plating

Metallizing of Plastics and Ceramics

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)

Metal Coatings - Electroplating

ApplicationsTin and Zinc are deposited on steel for further

workingZinc and Cadmium are deposited on parts for

corrosion resistanceCopper is deposited for electrical contactsNickel for corrosion resistanceChromium can be used to impart wear resistancePrecious metals for decoration or electronic devices

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))

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,

Thermal TreatmentsSurface Heat Treatment

Diffusion Coating

Hot-Dip Coatings

Weld Overlay Coatings

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 MethodsFlame TreatmentInduction Heating

Copper coil wraps around part to heat by inductionElectron Beam or Laser Beam Hardening

Typically heat small area and allow the bulk solid heat capacity to quench the small heated area

Thermal Treatments – Diffusion CoatingWith low carbon steel, the surface can be enriched by

diffusion of C or N into surface Carburizing Nitriding

Metal DiffusionChromizing – Chromium diffuses into surface to

form corrosion resistant layer. Take care with carbon steels as surface will

decarburizeAluminizing – Used to increase the high

temperature corrosion resistance of steels and superalloys

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

Vapor DepositionChemical Vapor Deposition (CVD)Physical Vapor Deposition (PVD)

Thermal PVDSputter DepositionIon plating

Chemical Vapor DepositionDeposition 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

Chemical Vapor DepositionApplications

Diamond Coating, Carburizing, Nitriding, Aluminizing

and Siliconizing processes

Semiconductor manufacturing

Organic Coatings - paint

Enamels

Lacquers

Water-base paints

Powder Coating