Surface TreatmentsSurface Treatments

ME 355ME 355

Bill PedersenBill Pedersen

Major Surface Major Surface TreatmentsTreatments

Finishing and Polishing – covered previouslyFinishing and Polishing – covered previously CoatingsCoatings

Conversion Coatings (oxidation, anodizing)Conversion Coatings (oxidation, anodizing) Thermal Coatings (carburizing – flame spraying)Thermal Coatings (carburizing – flame spraying) Metal Coatings (electrochemical, electroless) Metal Coatings (electrochemical, electroless) DepositionDeposition

Physical Vapor DepositionPhysical Vapor Deposition Chemical Vapor DepositionChemical Vapor Deposition

OrganicOrganic

Conversion CoatingsConversion Coatings

OxidationOxidation Phosphate CoatingsPhosphate Coatings Chrome CoatingsChrome Coatings

Conversion Coatings - Conversion Coatings - OxidationOxidation OxidationOxidation

Not all oxides are Not all oxides are detrimental – many are detrimental – many are tightly adhering leading tightly adhering leading to passivation and to passivation and hardening of surfacehardening of surface AlAl22OO33

Chromium in Stainless Chromium in Stainless steel rapidly corrodes to steel rapidly corrodes to passivate the surfacepassivate the surface

Gun-bluingGun-bluing Heat steel to 700 deg F in Heat steel to 700 deg F in

steam or oilsteam or oil Blue coating offers some Blue coating offers some

corrosion resistance, but little corrosion resistance, but little wear benefitwear benefit

Chemical Baths – similar in Chemical Baths – similar in nature to gun-bluingnature to gun-bluing

Black Oxide – chemical Black Oxide – chemical applicationapplication Typically applied to steel, Typically applied to steel,

copper and stainless steelcopper and stainless steel Anodizing – electrochemical Anodizing – electrochemical

conversionconversion Usually done to AluminumUsually done to Aluminum 2-25 2-25 m thick typicallym thick typically Multiple colors possibleMultiple colors possible Improved Corrosion and Wear Improved Corrosion and Wear

ResistanceResistance

Conversion Coatings – Conversion Coatings – Phosphate CoatingPhosphate Coating

Immersion in a Zn-P bath with Phosphoric acid Immersion in a Zn-P bath with Phosphoric acid causes growth of a crystalline zinc phosphate causes growth of a crystalline zinc phosphate layerlayer Iron, Zinc or Manganese Phosphate layer formedIron, Zinc or Manganese Phosphate layer formed

Typically applied to C-steel, low alloy steel and Typically applied to C-steel, low alloy steel and cast ironscast irons Sometimes applied to Zinc, Cadmium, Aluminum Sometimes applied to Zinc, Cadmium, Aluminum

and Tinand Tin

Typically very thin ~ 2.5 Typically very thin ~ 2.5 mm

Conversion Coatings – Conversion Coatings – Chrome CoatingChrome Coating

Food cansFood cans Immersion in a chromic acid bath (pH ~ 1.8) Immersion in a chromic acid bath (pH ~ 1.8)

with other chemicals to coat surfacewith other chemicals to coat surface Known carcinogen chemicals used, so Known carcinogen chemicals used, so

alternatives are currently under researchalternatives are currently under research Molybdate chemicals currently best subsititute for Molybdate chemicals currently best subsititute for

aluminum coatingsaluminum coatings Very good to minimize atmospheric corrosionVery good to minimize atmospheric corrosion

Many household goods – screws, hinges (yellow Many household goods – screws, hinges (yellow brown appearance)brown appearance)

Typically very thin < 2.5 Typically very thin < 2.5 mm

Thermal TreatmentsThermal Treatments

Surface Heat TreatmentSurface Heat Treatment Diffusion CoatingDiffusion Coating Hot-Dip CoatingsHot-Dip Coatings Weld Overlay CoatingsWeld Overlay Coatings

Thermal Treatments – Thermal Treatments – Surface Heat TreatmentSurface Heat Treatment

Basic concept is to heat the surface to Basic concept is to heat the surface to austenitic range, then quench it to form surface austenitic range, then quench it to form surface martensite - workpiece is steelmartensite - workpiece is steel

Heating MethodsHeating Methods Flame TreatmentFlame Treatment Induction HeatingInduction Heating

Copper coil wraps around part to heat by inductionCopper coil wraps around part to heat by induction Electron Beam or Laser Beam HardeningElectron Beam or Laser Beam Hardening

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

Thermal Treatments – Thermal Treatments – Diffusion CoatingDiffusion Coating

With low carbon steel, the surface can be enriched by With low carbon steel, the surface can be enriched by diffusion of C or N into surfacediffusion of C or N into surface

CarburizingCarburizing Heat steel to austenitic range (850-950 Heat steel to austenitic range (850-950 ºC)ºC) in a carbon rich in a carbon rich

environment, then quench and temperenvironment, then quench and temper NitridingNitriding

Nitrogen diffusion into steels occurs around 500-560 Nitrogen diffusion into steels occurs around 500-560 ºC to ºC to form a thin hard surfaceform a thin hard surface

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

Metal DiffusionMetal Diffusion Chromizing – Chromium diffuses into surface to form corrosion Chromizing – Chromium diffuses into surface to form corrosion

resistant layer. resistant layer. Take care with carbon steels as surface will decarburizeTake care with carbon steels as surface will decarburize

Aluminizing – Used to increase the high temperature corrosion Aluminizing – Used to increase the high temperature corrosion resistance of steels and superalloysresistance of steels and superalloys

Thermal Treatments – Thermal Treatments – Hot-Dip CoatingsHot-Dip Coatings

These coatings are used for corrosion protectionThese coatings are used for corrosion protection GalvanizingGalvanizing

Parts are dipped into a molten zinc bathParts are dipped into a molten zinc bath Galv-annealingGalv-annealing

Galvanized parts are then heat treated to ~500 Galvanized parts are then heat treated to ~500 ºC to form Fe-Zn inter-ºC to form Fe-Zn inter-metallicmetallic

Used for metals that need spot welded to protect copper electrode from Used for metals that need spot welded to protect copper electrode from alloying with zinc and reducing its lifealloying with zinc and reducing its life

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

(can greatly reduce spangles easily observed on galvanized parts)(can greatly reduce spangles easily observed on galvanized parts) Aluminum CoatingsAluminum Coatings

Alloyed with SiAlloyed with Si Coatings used on steel for high temperature applications that need a Coatings used on steel for high temperature applications that need a

lustrous appearancelustrous appearance Example – Automobile exhaustExample – Automobile exhaust

Thermal Treatments –Thermal Treatments –Weld Overlay coatingsWeld Overlay coatings

Typically used to improve wear resistance by creating Typically used to improve wear resistance by creating a hard surface over a tough bulk bodya hard surface over a tough bulk body

Hard FacingHard Facing Weld buildup of parts – alloy composition controls final Weld buildup of parts – alloy composition controls final

propertiesproperties Examples – cutting tools, rock drills, cutting bladesExamples – cutting tools, rock drills, cutting blades Cladding of material for corrosion resistanceCladding of material for corrosion resistance

Thermal sprayingThermal spraying Molten particle deposition – a stream of molten metal particles Molten particle deposition – a stream of molten metal particles

are deposited on the substrate surfaceare deposited on the substrate surface Major difference from hard facing is that the surface of the Major difference from hard facing is that the surface of the

substrate is not subjected to welding. Instead it just substrate is not subjected to welding. Instead it just undergoes a bonding process with the molten particles.undergoes a bonding process with the molten particles.

Metal CoatingsMetal Coatings

ElectroplatingElectroplating Electroless CoatingsElectroless Coatings Metallizing of Plastics and CeramicsMetallizing of Plastics and Ceramics

Metal Coatings - Metal Coatings - ElectroplatingElectroplating

Used to increase wear and corrosion resistanceUsed to increase wear and corrosion resistance Electrochemical process used to create a thin coating Electrochemical process used to create a thin coating

bonding to substratebonding to substrate Process is slow so coating thickness can be closely Process is slow so coating thickness can be closely

controlled (10-500 controlled (10-500 m)m) ApplicationsApplications

Tin and Zinc are deposited on steel for further workingTin and Zinc are deposited on steel for further working Zinc and Cadmium are deposited on parts for corrosion Zinc and Cadmium are deposited on parts for corrosion

resistance (Cadmium is toxic and can not be used for food resistance (Cadmium is toxic and can not be used for food applications)applications)

Copper is deposited for electrical contactsCopper is deposited for electrical contacts Nickel for corrosion resistanceNickel for corrosion resistance Chromium can be used to impart wear resistance to dies and Chromium can be used to impart wear resistance to dies and

reduce adhesion to workpieces such as aluminum or zincreduce adhesion to workpieces such as aluminum or zinc Precious metals for decoration or electronic devicesPrecious metals for decoration or electronic devices

Metal Coatings – Metal Coatings – Electroless CoatingsElectroless Coatings

Part is submerged into an aqueous bath Part is submerged into an aqueous bath filled with metal salts, reducing agents filled with metal salts, reducing agents and catalystsand catalysts Catalysts reduce metal to ions to form the Catalysts reduce metal to ions to form the

coatingcoating Excellent for complex geometries as Excellent for complex geometries as

deposition is uniform across surface deposition is uniform across surface regardless of geometry (except very regardless of geometry (except very sharp corners (0.4 mm radii))sharp corners (0.4 mm radii))

Metal Coatings -Metal Coatings -Electroless Nickel PlatingElectroless Nickel Plating

Has the appearance of Has the appearance of stainless steelstainless steel

Autocatalytic immersion Autocatalytic immersion processprocess

Key characteristics:Key characteristics: Heat treatable coating (to Heat treatable coating (to

68 Rc) very hard68 Rc) very hard Non-porousNon-porous Corrosion resistantCorrosion resistant .001” thick typical.001” thick typical Withstand load to 45 ksiWithstand load to 45 ksi

Can be applied to:Can be applied to: steel and stainless steel, steel and stainless steel,

iron, aluminum, titanium, iron, aluminum, titanium, magnesium, copper, magnesium, copper, brass, bronze, and nickel brass, bronze, and nickel

Electroless Nickel vs. Electroless Nickel vs. Chrome PlatingChrome Plating  ELECTROLESS NICKEL HARD CHROME

METAL DISTRIBUTION VERY GOOD POOR

CORROSION RESISTANCE1,000 HOURS

ASTM B117400 HOURS

ASTM B117

HARDNESS:   AS DEPOSITED

   HEAT TREAT

  48-52 Rc70 Rc

  64-69 Rc48-52 Rc

MELTING POINT 1800oF 2900oF

WEAR RESISTANCE GOOD VERY GOOD

CO-EFFICIENT OF FRICTION:   DYNAMIC   STATIC

 0.190.20

 0.160.17

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

Metal Coatings –Metal Coatings –Metallizing of Plastics and Metallizing of Plastics and CeramicsCeramics

Poor adhesion is the major challenge (As Poor adhesion is the major challenge (As in all coating processes, however it is in all coating processes, however it is more challenging in this case.)more challenging in this case.)

ApplicationsApplications Decorative (plumbing fixtures, automotive Decorative (plumbing fixtures, automotive

parts), reflectivity (headlights), electrical parts), reflectivity (headlights), electrical conduction (electronic touchpads), and EMF conduction (electronic touchpads), and EMF shieldingshielding

Vapor DepositionVapor Deposition

Physical Vapor Deposition (PVD)Physical Vapor Deposition (PVD) Thermal PVDThermal PVD Sputter DepositionSputter Deposition Ion platingIon plating

Chemical Vapor Deposition (CVD)Chemical Vapor Deposition (CVD)

Physical Vapor Physical Vapor Deposition – Thermal PVDDeposition – Thermal PVD

Thermal PVD – also called Vacuum Thermal PVD – also called Vacuum DepositionDeposition Coating material (typically metal) is evaporated by Coating material (typically metal) is evaporated by

melting in a vacuummelting in a vacuum Substrate is usually heated for better bondingSubstrate is usually heated for better bonding Deposition rate is increased though the use of a DC Deposition rate is increased though the use of a DC

current (substrate is the anode so it attracts the current (substrate is the anode so it attracts the coating material)coating material)

Thin ~0.5 Thin ~0.5 m to as thick as 1 mm.m to as thick as 1 mm.

Physical Vapor Deposition – Physical Vapor Deposition – Sputter DepositionSputter Deposition

Vacuum chamber is usually backfilled with Ar gasVacuum chamber is usually backfilled with Ar gas Chamber has high DC voltage (2,000-6,000 V)Chamber has high DC voltage (2,000-6,000 V) The Ar becomes a plasma and is used to target the The Ar becomes a plasma and is used to target the

deposition material. The impact dislodges atoms from deposition material. The impact dislodges atoms from the surface (sputtering), which are then deposited on the surface (sputtering), which are then deposited on the substrate anodethe substrate anode

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

Physical Vapor Physical Vapor Deposition – Ion PlatingDeposition – Ion Plating

Combination of thermal PVD and Combination of thermal PVD and sputteringsputtering

Higher rate of evaporation and depositionHigher rate of evaporation and deposition TiN coating is made this way (Ar-NTiN coating is made this way (Ar-N22

atmosphere)atmosphere) The gold looking coating on many cutting The gold looking coating on many cutting

tools to decrease the friction, increase the tools to decrease the friction, increase the hardness and wear resistancehardness and wear resistance

Chemical Vapor Chemical Vapor DepositionDeposition

Deposition of a compound (or element) produced by a Deposition of a compound (or element) produced by a vapor-phase reduction between a reactive element and vapor-phase reduction between a reactive element and gasgas Produces by-products that must be removed from the process Produces by-products that must be removed from the process

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

Coating will crack upon cooling if large difference in thermal Coating will crack upon cooling if large difference in thermal coefficients of expansioncoefficients of expansion

Plasma CVD done at 300-700ºC (reaction is activated by Plasma CVD done at 300-700ºC (reaction is activated by plasma)plasma) Typical for tool coatingsTypical for tool coatings

ApplicationsApplications Diamond Coating, Carburizing, Nitriding, Chromizing, Diamond Coating, Carburizing, Nitriding, Chromizing,

Aluminizing and Siliconizing processesAluminizing and Siliconizing processes Semiconductor manufacturingSemiconductor manufacturing

Organic Coatings - paintOrganic Coatings - paint

EnamelsEnamels Form film primarily by solvent evaporationForm film primarily by solvent evaporation 30 % Volatile Organic Content (VOC)30 % Volatile Organic Content (VOC)

Lacquers – solvent evaporationLacquers – solvent evaporation Water-base paints – water evaporation, Water-base paints – water evaporation,

therefore much bettertherefore much better Powder Coating – superior – more detail Powder Coating – superior – more detail

to followto follow

Powder Powder CoatingCoating Fully formulated paint Fully formulated paint

ground into a fine ground into a fine powderpowder

Powder is sprayed Powder is sprayed onto part, retained by onto part, retained by static electricitystatic electricity

Heat cured onto partHeat cured onto part Can virtually Can virtually

eliminate VOCseliminate VOCs

Teflon and dry lubricant Teflon and dry lubricant coatingscoatings

Sprayed, dipped or Sprayed, dipped or tumbled to coat, tumbled to coat, followed by heating followed by heating to bondto bond

Key characteristics:Key characteristics: Low friction coefficientLow friction coefficient

(0.02 – 0.08)(0.02 – 0.08) Can sustain load of Can sustain load of

250 ksi250 ksi

SummarySummary

Don’t overlook coatings importance to Don’t overlook coatings importance to overall product performanceoverall product performance

Coating technology is very specialized – Coating technology is very specialized – get a couple of expert opinions before get a couple of expert opinions before committingcommitting