Surface Treatments ME 355 Bill Pedersen

Major Surface Treatments  Finishing and Polishing – covered previously  Coatings  Conversion Coatings (oxidation, anodizing)  Thermal Coatings (carburizing – flame spraying)  Metal Coatings (electrochemical, electroless)  Deposition  Physical Vapor Deposition  Chemical Vapor Deposition  Organic

Conversion Coatings  Oxidation  Phosphate Coatings  Chrome Coatings

Conversion Coatings - Oxidation  Oxidation  Not all oxides are detrimental – many are tightly adhering leading to passivation and hardening of surface  Al 2 O 3  Chromium in Stainless steel rapidly corrodes to passivate the surface  Gun-bluing  Heat steel to 700 deg F in steam or oil  Blue coating offers some corrosion resistance, but little wear benefit  Chemical Baths – similar in nature to gun-bluing  Black Oxide – chemical application  Typically applied to steel, copper and stainless steel  Anodizing – electrochemical conversion  Usually done to Aluminum  2-25  m 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 formed  Typically applied to C-steel, low alloy steel and cast irons  Sometimes applied to Zinc, Cadmium, Aluminum and Tin  Typically very thin ~ 2.5  m

Conversion Coatings – Chrome Coating  Food cans  Immersion in a chromic acid bath (pH ~ 1.8) with other chemicals to coat surface  Known carcinogen chemicals used, so alternatives are currently under research  Molybdate chemicals currently best subsititute for aluminum coatings  Very good to minimize atmospheric corrosion  Many household goods – screws, hinges (yellow brown appearance)  Typically very thin < 2.5  m

Thermal Treatments  Surface 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 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

Thermal Treatments – Diffusion Coating  With low carbon steel, the surface can be enriched by diffusion of C or N into surface  Carburizing  Heat steel to austenitic range ( ºC) in a carbon rich environment, then quench and temper  Nitriding  Nitrogen diffusion into steels occurs around ºC to form a thin hard surface  Good for Cr, V, W, and Mo steels. Will embrittle surface of Aluminum.  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

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  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 a lustrous appearance  Example – Automobile exhaust

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.

Metal Coatings  Electroplating  Electroless Coatings  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 (  m)  Applications  Tin and Zinc are deposited on steel for further working  Zinc and Cadmium are deposited on parts for corrosion resistance (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 and reduce adhesion to workpieces such as aluminum or zinc  Precious 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 - Electroless Nickel Plating  Has the appearance of stainless steel  Autocatalytic immersion process  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

Electroless Nickel vs. Chrome Plating ELECTROLESS NICKELHARD CHROME METAL DISTRIBUTIONVERY GOODPOOR CORROSION RESISTANCE 1,000 HOURS ASTM B HOURS ASTM B117 HARDNESS: AS DEPOSITED HEAT TREAT Rc 70 Rc Rc Rc MELTING POINT1800 o F2900 o F WEAR RESISTANCEGOODVERY GOOD CO-EFFICIENT OF FRICTION: DYNAMIC STATIC DUCTILITY1-2%Very Low Almost 0 EFFLUENT COSTRELATIVELY LOWHIGH DEPOSITION RATE (PER HOUR PER HOUR) EFFECTIVE OF HYDROGEN EMBRITTLEMENT ON PLATED COMPONENTS FAIR/NOT SERIOUSUSUALLY SERIOUS

Metal Coatings – Metallizing of Plastics and Ceramics  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), electrical conduction (electronic touchpads), and EMF shielding

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

Physical Vapor Deposition – Thermal 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  m to as thick as 1 mm.

Physical Vapor Deposition – Sputter 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 from the surface (sputtering), which are then deposited on the substrate anode  If the chamber is full of oxygen instead of Ar, then the sputtered atoms will oxidize immediately and an oxide will deposit (called reactive sputtering)

Physical Vapor Deposition – Ion Plating  Combination of thermal PVD and sputtering  Higher rate of evaporation and deposition  TiN coating is made this way (Ar-N 2 atmosphere)  The gold looking coating on many cutting tools to decrease the friction, increase the hardness and wear resistance

Chemical Vapor Deposition  Deposition of a compound (or element) produced by a vapor-phase reduction between a reactive element and gas  Produces by-products that must be removed from the process as well  Process typically done at elevated temps (~900ºC)  Coating will crack upon cooling if large difference in thermal coefficients of expansion  Plasma CVD done at ºC (reaction is activated by plasma)  Typical for tool coatings  Applications  Diamond Coating, Carburizing, Nitriding, Chromizing, Aluminizing and Siliconizing processes  Semiconductor manufacturing

Organic Coatings - paint  Enamels  Form film primarily by solvent evaporation  30 % Volatile Organic Content (VOC)  Lacquers – solvent evaporation  Water-base paints – water evaporation, therefore much better  Powder Coating – superior – more detail to follow

Powder Coating  Fully formulated paint ground into a fine powder  Powder is sprayed onto part, retained by static electricity  Heat cured onto part  Can virtually eliminate VOCs

Teflon and dry lubricant coatings  Sprayed, dipped or tumbled to coat, followed by heating to bond  Key characteristics:  Low friction coefficient (0.02 – 0.08)  Can sustain load of 250 ksi

Summary  Don’t overlook coatings importance to overall product performance  Coating technology is very specialized – get a couple of expert opinions before committing