Lecture 25 - Surface Treatments for Web

8/13/2019 Lecture 25 - Surface Treatments for Web

1/26

Surface TreatmentsME 355

Bill Pedersen


2/26

Major Surface Treatments Finishing and Polishingcovered previously

Coatings

Conversion Coatings (oxidation, anodizing)

Thermal Coatings (carburizingflame spraying)

Metal Coatings (electrochemical, electroless)

Deposition

Physical Vapor Deposition

Chemical Vapor Deposition

Organic


3/26

Conversion Coatings Oxidation

Phosphate Coatings

Chrome Coatings


4/26

Conversion CoatingsOxidation Oxidation

Not all oxides are

detrimentalmany are

tightly adhering leadingto passivation and

hardening of surface

Al2O3

Chromium in Stainlesssteel rapidly corrodes to

passivate the surface

Gun-bluing Heat steel to 700 deg F in

steam or oil

Blue coating offers somecorrosion resistance, but littlewear benefit

Chemical Bathssimilar innature to gun-bluing

Black Oxidechemicalapplication Typically applied to steel,

copper and stainless steel

Anodizingelectrochemicalconversion Usually done to Aluminum

2-25 mm thick typically

Multiple colors possible

Improved Corrosion and WearResistance


5/26

Conversion CoatingsPhosphate Coating Immersion in a Zn-P bath with Phosphoric acid

causes growth of a crystalline zinc phosphate

layer Iron, Zinc or Manganese Phosphate layer formed

Typically applied to C-steel, low alloy steel and

cast irons

Sometimes applied to Zinc, Cadmium, Aluminumand Tin

Typically very thin ~ 2.5 mm


6/26

Conversion CoatingsChrome Coating Food cans

Immersion in a chromic acid bath (pH ~ 1.8)with other chemicals to coat surface

Known carcinogen chemicals used, soalternatives are currently under research Molybdate chemicals currently best subsititute for

aluminum coatings

Very good to minimize atmospheric corrosion Many household goodsscrews, hinges (yellow

brown appearance)

Typically very thin < 2.5 mm


7/26

Thermal Treatments Surface Heat Treatment

Diffusion Coating

Hot-Dip Coatings

Weld Overlay Coatings


8/26

Thermal TreatmentsSurface Heat Treatment Basic concept is to heat the surface to

austenitic range, then quench it to form surfacemartensite - 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


9/26

Thermal TreatmentsDiffusion Coating With low carbon steel, the surface can be enriched by

diffusion of C or N into surface

Carburizing Heat steel to austenitic range (850-950 C) in a carbon rich

environment, then quench and temper

Nitriding Nitrogen diffusion into steels occurs around 500-560 C to form

a thin hard surface

Good for Cr, V, W, and Mo steels. Will embrittle surface of

Aluminum. Metal Diffusion

ChromizingChromium diffuses into surface to form corrosionresistant layer. Take care with carbon steels as surface will decarburize

AluminizingUsed to increase the high temperature corrosionresistance of steels and superalloys


10/26

Thermal TreatmentsHot 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 C 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

Zn-Al Coatings Gives a different corrosion protect and a more lustrous appearance

(can greatly reduce spangles easily observed on galvanized parts)

Aluminum Coatings Alloyed with Si

Coatings used on steel for high temperature applications that need alustrous appearance ExampleAutomobile exhaust


11/26

Thermal TreatmentsWeld Overlay coatings Typically used to improve wear resistance by creating

a hard surface over a tough bulk body

Hard Facing Weld buildup of partsalloy composition controls final

properties Examplescutting tools, rock drills, cutting blades

Cladding of material for corrosion resistance

Thermal spraying Molten particle depositiona 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 undergoesa bonding process with the molten particles.


12/26

Metal Coatings Electroplating

Electroless Coatings

Metallizing of Plastics and Ceramics


13/26

Metal CoatingsElectroplating Used to increase wear and corrosion resistance

Electrochemical process used to create a thin coatingbonding to substrate

Process is slow so coating thickness can be closelycontrolled (10-500 mm)

Applications Tin and Zinc are deposited on steel for further working

Zinc and Cadmium are deposited on parts for corrosionresistance (Cadmium is toxic and can not be used for food

applications) Copper is deposited for electrical contacts

Nickel for corrosion resistance

Chromium can be used to impart wear resistance to dies andreduce adhesion to workpieces such as aluminum or zinc

Precious metals for decoration or electronic devices


14/26

Metal CoatingsElectroless 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 surfaceregardless of geometry (except verysharp corners (0.4 mm radii))


15/26

Metal CoatingsElectroless Nickel Plating Has the appearance of

stainless steel

Autocatalytic immersionprocess

Key characteristics: Heat treatable coating (to

68 Rc) very hard

Non-porous

Corrosion resistant

.001 thick typical Withstand load to 45 ksi

Can be applied to: steel and stainless steel,

iron, aluminum, titanium,

magnesium, copper,brass, bronze, and nickel


16/26

Electroless Nickel vs.Chrome PlatingELECTROLESS NICKEL HARD CHROME

METAL DISTRIBUTION VERY GOOD POOR

CORROSION RESISTANCE1,000 HOURS

ASTM B117

400 HOURS

ASTM B117

HARDNESS:

AS DEPOSITED

HEAT TREAT48-52 Rc70 Rc

64-69 Rc48-52 Rc

MELTING POINT 1800oF 2900oF

WEAR RESISTANCE GOOD VERY GOOD

CO-EFFICIENT OF FRICTION:

DYNAMIC

STATIC

0.19

0.20

0.16

0.17

DUCTILITY 1-2% Very Low Almost 0

EFFLUENT COST RELATIVELY LOW HIGH

DEPOSITION RATE

(PER HOUR PER HOUR).0002 - .0003 .001 - .002

EFFECTIVE OF HYDROGEN

EMBRITTLEMENT ON PLATED

COMPONENTS

FAIR/NOT SERIOUS USUALLY SERIOUS


17/26

Metal CoatingsMetallizing of Plastics andCeramics

Poor adhesion is the major challenge (As

in all coating processes, however it is

more challenging in this case.) Applications

Decorative (plumbing fixtures, automotive

parts), reflectivity (headlights), electricalconduction (electronic touchpads), and EMF

shielding


18/26

Vapor Deposition Physical Vapor Deposition (PVD)

Thermal PVD

Sputter Deposition Ion plating

Chemical Vapor Deposition (CVD)


19/26

Physical Vapor DepositionThermal PVD Thermal PVD also called Vacuum

Deposition

Coating material (typically metal) is evaporated by

melting in a vacuum

Substrate is usually heated for better bonding

Deposition rate is increased though the use of a DC

current (substrate is the anode so it attracts the

coating material)

Thin ~0.5 mm to as thick as 1 mm.


20/26

Physical Vapor DepositionSputter Deposition Vacuum chamber is usually backfilled with Ar gas

Chamber has high DC voltage (2,000-6,000 V)

The Ar becomes a plasma and is used to target the

deposition material. The impact dislodges atoms fromthe surface (sputtering), which are then deposited onthe substrate anode

If the chamber is full of oxygen instead of Ar, then thesputtered atoms will oxidize immediately and an oxidewill deposit (called reactive sputtering)


21/26

Physical Vapor DepositionIon Plating Combination of thermal PVD and

sputtering

Higher rate of evaporation and deposition TiN coating is made this way (Ar-N2

atmosphere)

The gold looking coating on many cuttingtools to decrease the friction, increase the

hardness and wear resistance


22/26

Chemical VaporDeposition Deposition of a compound (or element) produced by a

vapor-phase reduction between a reactive element andgas Produces by-products that must be removed from the process

as well Process typically done at elevated temps (~900C)

Coating will crack upon cooling if large difference in thermalcoefficients of expansion

Plasma CVD done at 300-700C (reaction is activated byplasma) Typical for tool coatings

Applications Diamond Coating, Carburizing, Nitriding, Chromizing,

Aluminizing and Siliconizing processes

Semiconductor manufacturing


23/26

Organic Coatings paint Enamels

Form film primarily by solvent evaporation

30 % Volatile Organic Content (VOC) Lacquerssolvent evaporation

Water-base paintswater evaporation,therefore much better

Powder Coatingsuperiormore detailto follow


24/26

PowderCoating Fully formulated

paint ground into afine powder

Powder is sprayedonto part, retained bystatic electricity

Heat cured onto part

Can virtuallyeliminate VOCs


25/26

Teflon and dry lubricantcoatings Sprayed, dipped or

tumbled to coat,

followed by heating

to bond

Key characteristics:

Low friction coefficient

(0.020.08) Can sustain load of

250 ksi


26/26

Documents

Lecture 25 - Surface Treatments for Web