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Coating Techniques - Coggle Diagram
Coating Techniques
Anodizing aluminium
Aluminum anodizing is an electrochemical process which an aluminium oxide layer is chemically built on the surface of the aluminium metal.
Working steps
- Aluminium part immersed in acid electrolyte
- Apply electrical current, DC, ~12V
- Aluminium part is the anode (+), lead as cathode
- Switch on the power, occurs electrolysis and chemical reaction
- Porous aluminium oxide layer grows on the aluminum.
- Dye goes into pores
- Place in boiling water to seal pores
Principle
when the current is applied, the water in the electrolyte breaks down and oxygen is deposited at the anode, this oxygen combines with Al to form oxide on the surface.
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Clear anodizing- the process resulting in formation of translucent clear film. It produced in sulfuric acid solutions then sealed in hot water with good corrosion and moderate wear resistance that can apply in medical field.
Hard anodizing- the process resulting in formation of high density coating with large cells and small pores with low temperature and acidity that can apply with durable and abrasion resistance protective layer.
Purpose of anodizing
Provide better adhesion for primer.
Increase corrosion and wear resistance
Hardening
Color-cosmetic effect
Refractory ceramic coating- layer of ceramic applied to substrate surface using very high temperature exposure.
Characteristics
Low thermal conductivity and expansion
High oxidation resistance
Good wear resistance
Surface erosion resistance
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Organic coating
- Binders -use to form the continuous film that
adheres to the substrate.
- Volatile Components - make the coating fluid enough for application, and evaporate during and after application.
- Pigments - to provide color and opacity to the coating film.
- Additives - to modify some property of the coating.
Technique used
(Brushing, Fluidized bed coating, Spray coating, Electrophoresis)
Dip coating- is a robust technique for coating glass and plastic sheets with uniform, thin films.
Pull the substrate from the solution of the solution of the coating material after immersed in the solution, a thin layer forms after solvent evaporate.
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Metal deposition
Immersion deposition
Metal ions in solution deposits at the cathode, in the absence of external power source. Spontaneous reaction, driven by the electrode potential.
Limitation
Deposit - porous and poorly adherent.
Rate deposition declines with time
Surface needs catalytically activated.
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Electroless deposition- only one electrode, no external source, involves oxidation of a soluble reducing agent
Characteristics
- Must have an autocatalytic substrate
- Constant deposition rate with time
- Uniform deposit thickness
- Baths require good analytical control
- Baths have a short lifetime
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Chemical conversion coating- thin protective surface layer that is created by chemical reaction between metal and a chemical solution.
Principle
- Original rough surface (alkaline cleaning-rinsing-acid cleaning-rinsing prior to coating)
- Conversion coating formation (dipping in a chemical solution)
- Conversion coating removal by media (rinsing)
- Conversion coating re-forms
- Burnish removes conversion coating, producing a bright finish.
- Rinsing and drying.
Process flow
Alkaline cleaning, rinsing, acid cleaning, rinsing, conversion coating, rinsing and drying
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Porcelain enamel coating -highly durable glass permanently fused to metal under extremely high temperature.
Characteristics
- Opaque
- Smooth, hard can't burnt surface
- High durability, high quality aesthetics
- High wear, abrasion, temperature and corrosion resistance
- Lubricity, flexibility and thickness
Processes
- Mixture and preparation of frit
Unfired enamel mixture mixed and smelted at higher temperature.
- Preparation of substrate
The substrate must be cleaned to remove all the remnants of chemical.
- Enamel coating method
Applied by dipping, slushing, brushing, spraying or bepowdering. Can be either applied as dry powder or slurry suspension.
4.Firing
Coated metal out into oven to improve the adherence.
Pros & cons
Advantages- color retention, surface finish, design flexibility, durable and mom-stick, UV and weather resistance.
Disadvantages- rusts easily when chipped and poor heat conduction
Applications
- Non-stick cookware
- Bedpan
-Chemical reaction vessel
-Strove
Deposition coating - transfer of atomic level and to reducing friction, improving hardness, wear and oxidation resistance.
Physical vapour deposition
Deposition of a material in the vapour phase onto a solid in a vacuum via evaporation and usually no chemical reactions take place.
Principle
- A material is converted to the gas phase by evaporation that relies on thermal energy or sputtering on an atomic scale.
- Gas (Material melts and vaporizes) hits the samples to be coated.
- Movement to the samples and condensation on the surface of the sample.
Pros & cons
Advantages- harder and more corrosion resistance than electroplating coating, excellent abrasion resistance and durable.
Disadvantages- difficult to coat undercut, slow rate of deposition and high capital cost
Applications
Semiconductor devices, food packaging, metalworking and fabrication tools.
Chemical vapour deposition- chemical process used to high-purity, high-performance solid materials or coatings.
Principle
a) Transport of reactants by forced convection to the deposition region.
b) Transport of reactants by diffusion from the
main gas stream to the substrate surface.
c) Adsorption of gaseous reactants onto the heated wafer surface.
d) Chemical deposition and other surface (heterogeneous) reactions take place at the gas solid interface. Produce deposits and by-product
e) Deposits diffuse along the surface forming the crystallization centre and growth of the film. Desorption of by-products from the surface.
f) Transport of by-product by diffusion.
g) Transport of by-products by forced convection away from the deposition region.
Pros & cons
Advantages- versatile, highly density and pure materials, uniform films with good reproducibility and econimical.
Disadvantages- chemical and safety hazards, difficult to deposit multi-component materials.
Applications
Semiconductors and related devices, dense structural parts and optical fibres.