Coating and Deposition Processes Flashcards
Principal and reasons for Coating and Deposition Processes?
Principal reasons for coating
Improve resistance to wear, fatigue, erosion or indentation
Improve resistance to corrosion and oxidation
Reduce and control friction of surfaces
Increase electrical conductivity or electrical resistance
Rebuild worn or eroded surfaces during service
Enhance product appearance
What are the typical classification of coating processes?
- Plating processes
- Conversion coating
- Physical vapour deposition
- Chemical vapour deposition
The principle/method behind Electroplating?
A cathode workpiece is plated with a different metal (anode) which is transferred through an electrolyte solution
Electroplating Process Mechanism?
Rack Plating
Barrel Plating
Strip Plating
Brush Processing
What are the two electrolyte solutions?
Strong acids or cyanide solution
Name three coating metals and their purpose?
Nickel Copper - Often used as intermediate layers providing bright finish and smooth surfaces, corrosion resistant and electrical conductivity
Chromium - Corrosion protection, wear resistance, decorative
Hard Chroming- corrosion protection, wear resistance
Zinc - Barrier corrosion protection for steel
Tin - barrier corrosion protection and increase of solderability
Gold- decorative, thermal and electrical conductivity
Advantages of Electroplating
- No size limitation of the workpiece
- Simple and complex shapes
- Basic tooling requirements; some costs for jigs and fixtures
- Medium labour costs
Suitable for both, one-off and high volume production
Limitations of Electroplating
- Significant health and safety requirements
- Strict environmental controls are essential
- Complex shapes may require altered geometries
- Significant preparation of the workpiece required (chemical cleaning and degreasing)
-Moderate cycle times
Some post-processing (fine polishing) required
What is electroless plating and how is it different form electroplating?
Electroless plating - Deposition of metal onto the workpiece surface in a solution containing ions of the plating metal by chemical reaction only.
Difference - Difference is electroplating uses an external power source compared to electroless plating where the material reacts to the solution
What is electroforming ?
- Metal is electro-deposited onto a mandrel (pattern) using the principle of electroplating producing a coated shell (final workpiece)
What are the coating metals of electroforming?
Nickel, copper, silver, gold and soft materials
What are some the applications of Electroforming?
Fine moulding tools for lenses, plates for printing
Biomedical equipment
Electro-optics and electronics equipment
Decorative applications – fittings, tableware
Jewellery, sculptures
What is Hot Dipping - Galvanising?
Hot dip galvanizing is the process of coating iron or steel with a layer of zinc by immersing the metal in a bath of molten zinc at high temperatures
Process of Galvanising?
Cleaning and degreasing process – workpiece is cleaned with caustic acid, hydrochloric acid (pickling bath) and final wash
Dipped in zinc-ammonium chloride (flux) to prepare for galvanising
Galvanised by dipping in a bath of molten zinc (450°C) or centrifugal galvanising for more uniform and even coating
Applications of Galvanising?
Architectural steelworks, bridges, walls
Agricultural hardware
Automotive chassis
Furniture
What is Conversion Coating or Phosphate coating?
Thin film of oxide, phosphate or chromate is formed on a metallic surface by chemical or electrochemical reaction
What are the two categories within conversion coating?
- Phosphate and chromate coating – by chemical reaction only
- Anodising – oxide coating by electrochemical reaction
What are the few reasons for conversion coating?
Corrosion protection and wear reduction
Preparation for painting
Increased electrical
resistance of surfaces
Decorative finish
What is method and principle of Phosphate coating?
Surface of the metal is chemically treated to produce non-metallic, non-conductive surfaces
Coating is formed by immersing the base metal (usually zinc, steel) in a bath where phosphate salts (zinc, magnesium, calcium) have been dissolved in solutions of phosphoric acid
What is method and principle of Chromate coating?
Surface of the metal is chemically treated to produce non-metallic, non-conductive surfaces
Coating is formed by immersing the base metal (aluminium, copper, magnesium, zinc, cadmium, silver) in a bath where chromate salts have been dissolved in solutions of chromic acid
Anodising what is the principle and method behind it?
Workpiece surface (aluminium, magnesium, titanium) is converted to a hard and porous oxide layer by applying a DC current while being submerged in an electrolytic solution
What is Anodising?
Workpiece surface (aluminium, magnesium, titanium) is converted to a hard and porous oxide layer by applying a DC current while being submerged in an electrolytic solution
What are the applications of anodising ?
- Automotive aluminium components
- Furniture and architectural shapes
- Consumer electronics
What is the principle behind Physical Vapour Deposition(PVD)?
Material is converted into its vapour phase in a vacuum chamber and condensed onto a substrate surface as a very thin film
What is the process mechanism of PVD?
Vaporization of the material from a solid source assisted by high temperature vacuum or gaseous plasma.
Transportation of the vapor in vacuum or partial vacuum to the substrate surface.
Condensation onto the substrate to generate thin films.
What are the two most common PVD processes?
- Vacuum Evaporation
- Sputtering
Process Mechanism of PVD (Sputtering)
Energising inert gas (e.g. argon) by means of an electric field to form a plasma
Bombardment of the cathodic coating material with argon ions (Ar+)
Difference between Vacuum Evaporation and Sputtering (PVD)?
Sputtering causes a gaseous or cloud around the material compared to vacuum.
Application of PVD?
Electronic components – electrical connections on printed circuit boards; electromagnetic interference (EMI) or radio frequency shielding
Decorative coatings on plastics and metals – trophies, automotive trim, toys, jewellery
Automotive light reflectors – heat and light deflection
Antireflection coating on optical lenses
Coating on cutting tools and injection moulding tools
PVD Advantages?
Capable of coating substrate with metals, alloys, and refractories
Preserving substrate metallurgy
Applicable for coating all tooling
Fine particle deposition (PVD evaporation)
Very good adhesion properties (PVD sputtering better than evaporation processes)
Reasonable quantities per batch (PVD evaporation)
PVD Limitations ?
High level of cleanliness required
Parts require fixturing and orientation in line-of-sight process (PVD evaporation)
Slower deposition rates with PVD sputtering compared to PVD evaporation
Difficult process control with PVD sputtering compared to PVD evaporation (traces of gas can be found in the coated film)
Chemical Vapour Deposition CVD
Chemical decomposition of gas constituents from interacting gas mixtures form a solid film on the substrate surface
What is the process involved in CVD?
Reactant gases are introduced into a heated chamber (reactor)
Heating of substrate (deposition temperature 250-2,000°C)
Chemical reaction of the gases lead to layer deposition
Advantages of CVD?
Capability to deposit refractory materials at temperatures below their melting points
Excellent adhesion
Deposition does not require a vacuum
Large quantities per batch
Short reaction times reduce stresses on substrate
CVD Limitations
Corrosive and toxic elements require special closed chamber pumping and disposal equipment
High temperatures can affect substrate metallurgy
Expensive deposition material
Poor material utilisation
