1、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 Va
2、por 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 Al2O3 Chromium in Stainless steel rapidly corrodes to passivate t
3、he 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 conver
4、sion 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 formed
5、 Typically applied to C-steel, low alloy steel and cast irons Sometimes applied to Zinc, Cadmium, Aluminum and Tin Typically very thin 2.5 mm,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, s
6、o 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 mm,Thermal Treatments,Surface Heat Treatment Diffusion Co
7、ating 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 i
8、nduction 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 austen
9、itic 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 Chromizing Chromium diffuses into surface to
10、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
11、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 a
12、ppearance (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 re
13、sistance 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 me
14、tal 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 a
15、nd 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) Applications Tin and Zinc are deposited on steel for furthe
16、r 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
17、 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 depo
18、sition 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 res
19、istant .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,Metal Coatings Metallizing of Plastics and Ceramics,Poor adhesion is the major challenge (As
20、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 Depos
21、ition 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
22、 current (substrate is the anode so it attracts the coating material) Thin 0.5 mm 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 depositio
23、n 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
24、 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 cutting tools to decrease the friction, increase the hardness and wear resistance,Chemical Vapor Deposition,Deposition of a co
25、mpound (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 (900C) Coating will crack upon cooling if large difference in thermal coefficients of expansion Plasm
26、a CVD done at 300-700C (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 % Vol
27、atile 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 ont
28、o 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,Dont overlook coatings importance to overall product performance Coating technology is very specialized get a couple of expert opinions before committing,
