Surface Treatment Coating &
Cleaning
References:
• Materials & Processes in Manufacturing by De Garmo, Black & Kohser, Prentice Hall India
• Manufacturing Engineering & Technology by Kalpakjian
Surface Treatment
• Surface modification processes may be done for:
• Cleaning of surfaces and removal of defects like scratches, burrs etc.
• Modification of bulk properties like strength or fracture resistance by heat treatment
• Modification of surface properties like surface finish, wear or corrosion resistance, texture, color etc.
• Controlling friction and improving lubrication
• Impart decorative features like color and texture
Chemical Cleaning
• Help in removal of oil, dirt, scale, or other foreign material.
It prepares the surface for subsequent painting /plating.
• Problems of chemical cleaning:
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Safe disposal of spent or contaminated solutions: Due to the use of hazardous, toxic, or environmentally unfriendly materials.
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E.g. Chlorofluorocarbons (CFCs) and carbon tetrachloride cause ozone-depletion.
• Selection of the cleaning method will depend on:
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Cost of the equipment, power, cleaning materials, maintenance and labour, plus the cost of recycling and disposal of materials.
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Size, geometry, material and quantity of parts to be processed
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Desired surface finish.
Alkaline Cleaning
• A “soap and water” approach to clean a wide variety of soils (including oils, grease, wax, fine particles of metal, and dirt)
• Cleaners are usually complex solutions of alkaline salts, additives to enhance cleaning or surface modification, and surfactants or soaps that are selected to reduce surface tension and displace, emulsify, and disperse the insoluble soils.
• The actual cleaning occurs as a result of one or more of the following mechanisms:
• Saponification, the chemical reaction of fats and other organic compounds with the alkaline salts;
• Displacement, where soil particles are lifted from the surface;
• Dispersion or emulsification of insoluble liquids;
• Dissolution of metal oxides.
• Alkaline cleaners can be applied by immersion or spraying. They are often heated to accelerate cleaning action.
• Cleaning is followed by a water rinse to remove all residue of the cleaning solution,
• Corrosion inhibitor (or rust preventer) may be required, depending on subsequent use of product.
• Environmental issues relating to alkaline cleaning include
• Reducing or eliminating phosphate effluent
• Reducing toxicity and increasing biodegradability
Solvent Cleaning
• Oils, grease, fats, and other surface contaminants are removed by dissolving them in organic solvents derived from coal or petroleum, usually at room temperature.
• Common solvents:
• Petroleum distillates (such as kerosene, naphtha, and mineral spirits),
• Chlorinated hydrocarbons (such as methylene chloride and trichloroethylene),
• Liquids such as acetone, benzene, toluene, and the various alcohols.
• Small parts are cleaned by immersion or spraying while large parts may be cleaned using spraying or wiping.
• Advantages:
• The process is simple and capital equipment costs are low.
• It is suitable for cleaning large parts and heat-sensitive products
• Materials that might react with alkaline solutions and products with organic contaminants (e.g.
marking crayon) may be cleaned.
• Virtually all common industrial metals can be cleaned, and the size and shape of the workpiece are rarely a limitation.
• Limitations:
• Insoluble contaminants, such as metal oxides, sand, scale, and the inorganic fluxes used in welding, brazing, and soldering, cannot be removed by solvents.
• Resoiling can occur as the solvent becomes contaminated.
• Many of the common solvents have been restricted because of health, safety, and environmental concerns.
Vapor Degreasing
• Vapours of a chlorinated or fluorinated solvent are used to remove oil, grease, and wax from metal products.
• A non-flammable solvent, such as trichloroethylene, is heated to its boiling point, and the parts to be cleaned are suspended in its vapours. The vapor condenses on the work and washes the
soluble contaminants back into the liquid solvent.
• Vapour degreasing is more effective than cold solvent cleaning as contaminants do not reach the surface.
• Heating of the surface by condensing solvent results in its almost instantaneous drying.
• Now a days, sealed chamber machines use non-VOC, non-chlorinated solvents that are continuously recycled.
• Advantages
• It is a rapid, flexible process
• Almost no visible effect on the surface being cleaned.
• It can be applied to all common industrial metals.
• Limitations
• Solvents may attack rubber, plastics, and organic dyes that might be present in product assemblies.
• Not suitable for removing insoluble soils.
Ultrasonic Cleaning
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Used for high-quality cleaning of small parts.
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Parts are suspended or placed in wire-mesh baskets that are then immersed in a liquid cleaning bath, often a water-based detergent.
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The bath contains an ultrasonic transducer that operates at a frequency of 10 to 40 kHz to cause cavitation in the liquid.
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The bubbles that form and implode provide the majority of the cleaning action, and if gross dirt, grease, and oil are removed prior to the
immersion, excellent results can usually be obtained in 60 to 200 seconds.
•
Because of the ability to use water-based solutions, ultrasonic cleaning
has replaced many of the environmentally unfriendly solvent processes.
Acid Pickling
• Process
• Cleaning of metal parts to remove oils and/or other contaminants.
• Parts are then dipped into dilute acid solutions to remove oxides and dirt.
• After the pickling bath, parts are rinsed to flush the acid residue from the surface and then dipped in an alkaline bath to prevent rusting
• Caution should be used to avoid over-pickling, since the acid attack can result in a roughened surface.
• Solutions used
• Most common solution is a 10% sulfuric acid bath at an elevated temperature between 150° and 185° F.
• Muriatic acid may be used, either cold or hot.
• At elevated temperatures dilute solutions may be used.
Surface Hardening: Why & How
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Components like gear, shaft or spindle need a hard/wear resistant surface with a soft / tough core.
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Section size of such components is often too large to be uniformly hardened even on severe quenching. Time lag between the
transformations at the surface and the core results in an unfavourable tensile residual stress at the surface.
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Residual tensile stress
• Reduce fatigue life of critical components like turbine shaft or landing gear of an aircraft.
• Lead to cracking and distortion
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Surface hardening develops a hard surface with compressive residual stress, to improve wear resistance, increase fatigue life and avoid susceptibility to distortion and cracking.
•
The most commonly used methods of surface hardening are as follows:
• Shot peening: general applicable to all metals
• Coating / hard facing
• Surface (local) heating & cooling: steel
• Surface diffusion & subsequent treatment
Peening
• It is a cold working process involving deformation of surface
• Results in development of beneficial, compressive residual stresses in the surface layer balanced by tensile residual stresses elsewhere.
• The layer of compressed surface material resists
the development and propagation of cracks and
increases resistance to fatigue failure, corrosion
fatigue, stress corrosion and cavitation erosion
Types of Peening
•
Shot peening:
• Cast steel, glass or ceramic shots, 0.125 to 5mm in diameter, are used to impinge the surface
• Result in overlapping indentations and plastic deformation up to 1.25 mm deep
• Used for shafts gears, springs etc.
• Suitable for all metals and alloys capable of plastic deformation
•
Water jet peening
• Uses water jet at a pressure of 400MPa
• Used for steels and aluminum alloys
•
Laser shot peening
• Uses laser shocks of high power (upto 1kW)
• Used for jet engine fan blades and materials like titanium and nickel alloys
• Depth of residual stresses > 1mm
•
Ultrasonic peening
• Uses a hand tool based on a piezoelectric transducer having a frequency of around 22 HZ
Roller Burnishing
• Surface is cold worked by a series of hard polished rollers
• Results in:
• considerable residual compressive stress on the surface of the workpiece thus increasing fatigue strength and wear resistance of the surface layer.
• Improved surface finish by removing scratches, tool marks etc.
• Used for flat cylindrical and conical surfaces
• Typical applications: seals, valves, spindles, fillets of shafts etc.
• Burnishing of internal cylinders is called ball burnishing or ballizing
Explosive Hardening
• Explosive sheet is directly detonated on the workpiece surface
• Surface is subjected to transient pressure resulting in large increase in surface hardness
• Contact pressure up to 35GPa lasting 2 to 3 μs
• Application: Railroad surfaces
Thermal Treatments
• They may be used to improve friction and wear resistance and increase surface hardness
• Hard Facing
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Thick layer, edge or point is deposited on the surface using welding techniques
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Improves wear resistance
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E.g. hard coatings of tungsten carbide, chromium or molybdenum
•
Applications: valve seats, dies for hot metal working
• Case Hardening
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Induces residual stresses on surface
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Improve fatigue life by delaying crack initiation
Thermal Spraying/ Metallizing
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Involve the projection of small softened particles onto a cleaned and prepared surface where they adhere to form a continuous coating.
•
Combined thermal and kinetic energy causes the particles to flatten or
’splat’ onto the surface, and onto each other, to produce a cohesive coating of successive layers.
•
Sources of heat
• Combustion spraying: Source of heat is an oxyfuel flame/mixture
• Electric Spraying: Source of heat is electric arc
•
The process can be used to apply coatings to a wide range of materials and components
•
It is used to
• Extend life of new components due to
• Improved resistance to wear, erosion, corrosion, abrasion
• Better electrical conductivity or insulation, lubricity, high or low friction, sacrificial wear, chemical resistance
• Repair and re-engineer worn or damaged components.
•
Applications: automotive components, rocket motor nozzles and tank
cars are sprayed with Zinc or Aluminum up to 0.25 mm thick
Electropolishing
• Electropolishing is the reverse of electroplating.
• Here, the workpiece acts as anode and the material is removed from the workpiece by electrochemical
dissolution.
• The process is suitable for polishing irregular surface since there is no mechanical contact between workpiece and polishing medium.
• The electrolyte electrochemically etches projections on the workpiece surface at a faster rate than the rest, thus
producing a smooth surface.
• This process is also used for deburring operation.
Cladding
• If the material is deposited on a substrate as a liquid or
organic gas (or from a liquid or gas medium), the process is called coating. If the added material is a solid during
deposition, the process is known as cladding.
• In cladding a thin layer of corrosion resistant material is deposited on a substrate by pressure
• Cladding pressure may be achieved by:
•
Rollers
•
Dies
•
Explosives
• Typical Application: Al-clad is a corrosion resistant aluminum
alloy clad over pure aluminum
Mechanical Plating
• Used for hardened steel parts of automobiles
• The process
• Small products are cleaned then placed in a tumbling barrel, along with a water slurry of the metal powder to be plated, glass or ceramic
tumbling media, and chemical promoters or accelerators.
• Media particles peen the metal powder onto the surface, producing uniform-thickness deposits. Plating thickness < 0.25μm
• Any metal that can be made into fine powder can be deposited, but the best results are obtained for soft materials, such as
cadmium, tin, and zinc.
• Coatings may be layered or involve mixtures with bulk chemistries that would be chemically impossible due to solubility limits.
• Since coatings are deposited at room temperature mechanical
Ceramic Coating
• Impart high temperature and electrical resistance
• Powders of hard metals or ceramics are used as spraying materials
• Typical applications: nozzles for rockets and wear
resistant parts
Vapor Deposition
• Workpiece is surrounded by chemically reactive gasses containing the chemical compounds of materials to be deposited
• Deposited layer is a few μm thick
• Deposited material: metals, alloys, carbides, nitrides, borides, ceramics or oxides
• Substrate: metal, plastic glass or paper
• Applications: coating of cutting tools, dies, punches and
wear surfaces
Types of Vapor Depositions
• Physical vapor deposition (PVD) :
• Method of depositing a thin film by the condensation of a vaporized form of the material onto various surfaces
• Used to deposit titanium, titanium nitrate, tantalum,
tantalum nitrate, aluminum and a very thin film of copper called seed layer.
• Carried out in high vacuum in a temperature range of 473- 773K
• Typical thickness of deposited layer is a few to 1000s of nanometers
• Chemical vapor deposition
• CVD processes deposit material through chemical reactions and generally require significantly higher temperatures.
• Used to deposit titanium nitride on cutting tools.
Surface Texturing
• For functional or aesthetic reasons for e.g.
• Textures rolled onto the sheets used for refrigerator panels serve to conceal dirt, smudges, and fingerprints.
• Embossed or coined protrusions an enhance the grip of metal stair treads and walkways.
• Corrugations provide enhanced strength and rigidity.
• Some textures can be used to modify optical or acoustical characteristics of a material.
• May be done by
• Etching
• Electric arcs
• Laser pulses
Plating
• It imparts
• Wear resistance
• Corrosion resistance
• High electrical conductivity
• Better finish
• Repair worn out parts
• Common processes
• Electroplating
• Electroless plating
• Electroforming
Electroplating
• Preparation of surface:
• Pinholes, scratches, and other surface defects must be removed if a smooth, lustrous finish is desired.
• Combinations of degreasing, cleaning, and pickling are used to ensure a chemically clean surface, one to which the plating material can adhere.
• Workpiece (cathode) and anode are suspended in a water based electrolytic solution. An external source of energy discharges metal ions form the anode and are deposited at the cathode
• The plated metal tends to be preferentially attracted to corners and
protrusions. This makes it particularly difficult to apply a uniform plating to irregular shapes, especially ones containing recesses, corners, and edges.
• Design features can be incorporated to promote plating uniformity, and improved results can often be obtained through the use of multiple spaced anodes or anodes whose shape resembles that of the workpiece.
• Electroplating thicknesses range from a few atomic layers to 0.05mm
Electroplating: Applications
• The most common platings are zinc, chromium, nickel, copper, tin, gold, platinum, and silver.
• Imparts corrosion or wear resistance, improves appearance (through color or luster), or increases the overall dimensions.
• Some practical applications
• Copper plating on aluminium wires
• Gold, silver or platinum plating for electronics and jewellery
• For plating plastics they are first coated with processes like electroless nickel plating
• The electrogalvanized zinc platings are thinner than the hot-dip coatings and can be
produced without subjecting the base metal to the elevated temperatures of molten zinc.
• Nickel plating provides good corrosion resistance.
• Chromium plate is used to impart lustre. An initial layer of copper produces a levelling effect thereby reducing the thickness of the subsequent nickel layer (typically 0.0006 in).
• Hard chromium plating, with Rockwell hardness between 66 and 70, can be used to build up worn parts to larger dimensions and to coat products for reduced friction and good wear and corrosion resistance. They are applied directly to the base material and are
usually much thicker than the decorative treatments, typically ranging from .003 to .010 in.
thick.
• Even thicker layers are used in applications such as diesel cylinder liners. Since hard
chrome plate does not have a leveling effect, defects or roughness in the base surface will be amplified. If smooth surfaces are desired, subsequent grinding and polishing may be necessary.
Electroless Plating
• Uses a chemical reaction
• May be used for non conducting materials
• More expensive than electroplating
• Cavities, recesses and inner surfaces may be coated successfully.
• Gives a uniform coating thickness
• Most common plating material is Nickel. Copper may also be used.
• Nickel Coating
•
Nickel chloride is reduced using a reducing agent to nickel metal,
which is then deposited on the substrate
Electroforming
• Metal is deposited on a mandrel, which is later removed. So the coating becomes the product
• Mandrel may be metallic (Zn or Al) or non metallic.
Low melting alloys, wax or plastics may also be used.
• Wall thickness as small as 0.025 mm
• May be used for complex shapes also.
• Suitable for low volumes of production or intricate parts made of nickel, copper, gold or silver
• Applications: aerospace, electronics, electro-optics
Porcelain Enamels
• Porcelain enamels are glassy inorganic coatings of metal oxides characterised by smooth and hard surfaces.
• Coatings may be applied by dipping, spraying or
electrodeposition with thicknesses in the range of 0.05 to 0.6mm
• They impart resistance to corrosion and abrasion, provide decorative colour, electrical insulation, or the ability to
function in high-temperature environments. Enamels may possess varying degree resistances to alkali, acids,
detergents, cleaners, water
• Typical applications: plumbing, fixtures, cookware, jewellery etc. they may be used for protective coating on jet engine components.
• Multiple coats may be used, with the first or ground coat
Chemical Conversion Coatings
• The surface of the metal is chemically treated to
produce a non-metallic, nonconductive surface that can impart a range of desirable properties.
• The most popular types of conversion coatings are chromate and phosphate.
• Phosphate coatings
• Formed by immersing metals (usually steel or zinc) in baths where metal phosphates (iron, zinc, and manganese
phosphates) have been dissolved in solutions of phosphoric acid.
• Uses:
• To precondition surfaces to receive and retain paint or enhance the subsequent bonding with rubber or plastic.
• Phosphate coatings are usually rough and can provide an excellent surface for holding oils and lubricants.
• Corrosion resistance is provided by a phosphate layer impregnated with wax or oil.
Chemical Conversion Coatings
• Chromate conversion process
• Usually involves immersion in a chemical bath.
• Aluminium, magnesium, zinc, and copper, cadmium and silver can all be treated by a chromate process.
• The surface of the metal is converted into a layer of
complex chromium compounds that can impart colours ranging from bright clear through blue, yellow, brown, olive, and black.
• Most of the films are soft and gelatinous when they are formed but harden upon drying.
• They can be used to:
•
Impart exceptionally good corrosion resistance
Anodizing
• Anodizing thickens the natural oxide film resulting in a heavy oxide film of controlled thickness having very high hardness.
• Aluminium anodizing is the electrochemical process by which aluminium is converted into aluminium oxide on the surface of a part. This coating is desirable in specific applications due to the following properties:
• Increased corrosion resistance
• Increased durability / wear resistance
• Ability to be coloured through dying
• Electrical insulation
• Excellent base or primer for secondary coatings
The Anodizing Process
• Pre-Treatment Cleaning: An alkaline detergent is used to remove accumulated contaminants and light oils.
• Rinsing: Multiple rinses follow each process step.
• Etching (Chemical Milling): Etching in caustic soda (sodium hydroxide) prepares the aluminium for anodizing by chemically removing a thin layer of aluminium.
• Desmutting: Rinsing in an acidic solution removes unwanted surface alloy constituent particles not removed by the etching process.
• Anodizing: Aluminium is immersed in a tank containing an electrolyte having a 15% sulfuric acid concentration. Aluminium is made the anode and the tank is the cathode. Electric current is passed through the electrolyte, causing the negatively charged anions to migrate to the anode where the oxygen in the anions combines with the aluminium to form
aluminium oxide (Al2O3).
• Coloring may be done using organic dyes o produce stable and durable films.
• Sealing: Proper sealing of the porous oxide coating is absolutely essential to provide
Powder Coating
• It is a variation of electrostatic spraying. It uses solid particles for coating.
• Thermoplastics can also be used, but thermosetting powders are most common.
• As against conventional methods, several coats, such as primer and finish, can be applied and before baking.
• Advantages:
• Overspray powder may be collected and reused.
• Modern powder technology can produce a high-quality finish with superior surface properties and usually at a lower cost than liquid painting.
• Lower energy requirements.
• However,
• The process is not-suitable or large objects or heat-sensitive objects.
• Airborne particles are a health hazards.
• It is not easy to produce film thickness less than 0.03 mm.
Hot Dipping
• Prior to coating, cleaning of rust, scale, oil, paint and other surface contaminants is required.
• A metal substrate is immersed in a molten bath of a second metal to coat it.
• Post coating treatments like slow cooling, quenching, conversion coating and painting may be needed.
• They have excellent long term corrosion resistance property when properly sealed. The coating provides a shield to the base material and acts as a
sacrificial anode. Coatings of metals higher in the electromotive series than the basis metal will corrode in preference to the basis metal.
• Thickness of coating dependents on the process and coating type. It could range from 0.01 to 0.13 mm.
• The most common substrate materials for hot-dip metallic coating are cast iron and steel.
Hot-Dip Coating
• Hot-dip galvanizing
• It is the most widely used method of imparting corrosion resistance to steel. Zinc acts as a sacrificial anode, protecting the underlying iron.
• The products are cleaned to remove oil, grease, scale, and rust, fluxed by dipping into a solution of zinc ammonium chloride and dried. Next, the article is completely immersed in a bath of molten zinc. The zinc and iron react metallurgically to produce a coating that consists of a series of zinc–iron compounds and a surface layer of nearly pure zinc.
• Typical coating thickness 0.5 and 3.0 oz/ft2
• Thickness of coatings depends on the time of immersion and speed of withdrawal. Thinner layers can be produced by incorporating some form of air jet or mechanical wiping as the product is withdrawn.
• Zinc-galvanized sheet can be heat treated with a zinc–iron alloy coating. The 10% iron content adds strength and makes for good corrosion and pitting/chipping resistance.
• The process is not suitable for large parts and parts that may get damaged when exposed to high temperatures.
• Tin coatings can also be applied by immersing in a bath of molten tin with a covering of flux material. However, because tin is expensive and coatings thick (in hot
dipping), most tin coatings are now applied by electroplating.
• Terne is an alloy of 15 to 20% tin and the remainder lead. This material is cheaper than tin and can provide satisfactory corrosion resistance for many applications.
Painting
• Wet or liquid paints and enamels are the most widely used finish on manufactured products
• Most commercial paints are synthetic organic compounds that contain pigments and dry by polymerization or by a combination of polymerization and
adsorption of oxygen. Water is the most common carrying vehicle for the pigments.
• Heat may sometimes be used to accelerate the drying,
• Paints are used to:
• Provide protection and decoration
• Fill or conceal surface irregularities
• Change the surface friction, or modify the light or heat absorption or radiation characteristics.
• Painting is a complex system that includes the substrate material, cleaning and other pre-treatments (such as anodizing, phosphating, and various conversion coatings), priming, and possible intermediate layers. The method of application is another integral feature to be considered.
• Generally, two coats are required.
Paint Application Methods
• Dipping
• It is a simple and economical when all surfaces of the part are to be coated.
• Suitable for applying prime coats and painting small parts.
• Not suitable for situations where only some of the surfaces require painting or where a very thin, uniform coating is needed, as on automobile bodies.
• Results in a wavy surface with a final drop of paint attached to the lowest drip point.
• Spray painting
• The most widely used paint application process because of its versatility and the economy in the use of paint.
• A very thin film can be deposited at one time, usually less than 0.001 in. So many coats may be required
• Electrostatic deposition:
• A DC electrostatic potential is applied between the atomizer and the workpiece.
• The atomized paint particles assume the same charge as the atomizer and are
therefore repelled. The oppositely charged workpiece then attracts the particles, with the actual path of the particle being a combination of the kinetic trajectory and the electrostatic attraction.
• Overspraying and air borne particle emissions can be reduced by 60 to 80%.
• Part edges and holes receive a heavier coating than flat surfaces.
• Recessed areas receive a reduced amount of paint, and a manual touch-up may be
