Coating and Surface Treatment Processes

In this tutorial, you will learn about the different surface treatment and coating processes prevalent in the industry and why they are important. You will understand which process is applicable for which kind of materials and what are their advantages and limitations.

Contents:

  1. Why do we Need Treatment Processes?
  2. What is Galvanic Action?
  3. The Method of Electroplating
  4. What is Anodizing?
  5. Plasma Spray Coatings
  6. Coating with Chemicals
  7. Surface Hardening Processes
  8. What is Vapor Deposition?

Why do we Need Treatment Processes?

Before deployment in the field, any part needs to be protected to maximize efficiency during application. Surface coating are any mixture of film-forming materials, pigments, solvents, and other additives that, when applied to a surface and cured or dried, forms a thin film that is practical and often aesthetic.

  • Paints, drying oils and varnishes, synthetic crystalline coatings, and other substances whose primary function is to protect an object’s surface from the environment are examples of surface coatings.
  • In mechanical applications where mating elements move against each other under extreme loads, such as shaft/bushing pairs, the mating surfaces of each are typically coated or treated to increase the hardness of the mating surfaces and hence increase wear resistance.
  • Coatings produce an excellent finish with very low surface roughness values, resulting in the least amount of friction between the components.
  • Another common reason for requiring a surface treatment on a component is to protect it against corrosion or chemical damage when utilized in a chemically hostile environment.

What is Galvanic Action?

A galvanic cell can be formed when a coating of one metal is placed on a different metal. To varying degrees, all metals are electrolytically active, and if sufficiently varied in their electrolytic potential, they will form a battery in the presence of a conductive electrolyte such as saltwater or even tap water.

  • Combinations of metals at opposite ends of the scale, such as aluminum and copper, will corrode severely in an electrolyte or even in a wet atmosphere.
  • The two metals form anode and cathode in a conducting media, with the less noble metal functioning as the anode.
  • The flow of self-generated electrical current causes material loss from the anode and material deposition on the cathode. The less noble metal eventually fades away. When two metals sufficiently enough apart in the galvanic series are present in an electrically conductive medium, this problem develops.
  • Therefore, not only coatings but also fasteners and mating components must be designed of metal combinations that will not cause this problem.
  • The following table lists some common materials in an order of decreasing noble behavior
Material
Highly Inert Platinum
Gold
Graphite
Titanium
Silver
Copper
Tin
Lead
Iron/Steel
Aluminum Alloys
Least Inert Cadmium

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The Method of Electroplating

Electroplating is a process which involves deliberate creation of a galvanic cell between two metals, causing transfer of metal ions from anode to cathode. The part to be plated is the cathode and the plating material is the anode.

The following diagram shows the electroplating process.

Electroplating Apparatus
  • The two metals are placed in an electrolyte bath and a direct current is applied from anode to cathode.
  • Ions of the plating material are driven to the plating substrate through the electrolyte and cover the part with a thin coating of the plating material.
  • Worn parts can also be repaired by plating on a coating of suitable material, then machining to dimension.
  • Steels, nickel, and copper-based alloys are readily electroplatable.
  • Chrome plating also offers an increase in surface hardness to HRC 70, which is above that obtainable from many hardened alloy steels by conventional methods.

What is Anodizing?

Anodizing is an electrochemical process that changes the surface of a metal into a hard, corrosion-resistant, and often decorative anodic oxide finish.

  • Aluminum, magnesium, and titanium can be anodized.
  • When anodizing a metal like Aluminium, the oxide originates from the Aluminium substrate itself and is made of Aluminium Oxide only.
  • The aluminum, as an anode, attracts oxygen ions are released from the electrolyte to combine with the aluminum atoms at the surface of the Aluminium part being anodized.
  • Hard anodizing provides a thicker coating than conventional anodizing and is used to protect the relatively soft aluminum parts from wear in abrasive contact situations.

Plasma Spray Coatings

A variety of very hard ceramic coatings can be applied to steel and other metal parts by a plasma-spray technique. The application temperatures are very high, which severely limits the choice of substrate used.

The following diagram shows the plasma spray coating apparatus.

Plasma Coating Apparatus
  • The coatings as sprayed have a rough “orange-peel” surface because of the process finish, which requires grinding or polishing to obtain a fine finish.
  • The main advantage is that the final surface has extremely high hardness and chemical resistance.
  • The ceramic coatings are very brittle and can be chipped or scratched under mechanical or thermal shock.

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Coating with Chemicals

A chemical coating is produced by electrochemical or chemical reactions of metals, giving a covering layer that contains the metal compound formed. The surface is coated with the chemicals which interacts with the surface to provide a protective layer.

  • It offers economical and easy protection against corrosion and a surface suitable for powder coating and painting. It preserves electric conductivity, in contrast to anodizing whichHere’s the rest of the HTML code. It preserves electric conductivity, in contrast to anodizing which produces non-conductive coatings.
  • Unlike anodizing, chemical conversion coatings do not require electric current, making production more cost-effective. It can be colored or clear, depending on the preference.
  • A chemical conversion coating is also known as chromating or alodining depending on the type of chemical used. Chromating deposits a metal chromate on the surface while alodining involves coating with Alodine, resulting a highly durable coat.
  • The most common chemical treatments for metals range from a phosphoric acid wash on steel that provides limited and short-term oxidation resistance, to paints of various types designed to give more lasting corrosion protection.

Surface Hardening Processes

When a part is large or thick, it is difficult to obtain uniform hardness within its interior through conventional hardening processes. An alternative way is to harden only the surface, leaving the core soft. This also avoids the distortion associated with quenching a large, through-heated part.

  • If the steel has sufficient carbon content, its surface can be heated, quenched, and tempered as would be done through hardening.
  • Carburizing is a process where heat is applied to low-carbon steel in a carbon monoxide gas atmosphere, causing the surface to take up carbon in solution.
  • Nitriding is a process where heat is applied to low-carbon steel in a nitrogen-gas atmosphere and forms hard iron nitrides in the surface layers.
  • Cyaniding is a process where heat is applied to the part in a cyanide salt bath at about 800°C, and the low-carbon steel takes up both carbides and nitrides from the salt.
  • Flame hardening passes an oxyacetylene flame over the surface to be hardened and follows it with a water jet for quenching.
  • Induction hardening uses electric coils to rapidly heat the part surface, which is then quenched before the core can get hot.
  • Large machine parts such as cams and gears are examples of elements that can benefit more from case hardening than from through hardening, as heat distortion is minimized and the tough, ductile core can better absorb impact energy.

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What is Vapor Deposition?

The vapor deposition methods utilize vaporization and condensation processes for making thin-film layers. The desired metal is first evaporated and condensed onto the target surface. Vapor deposition methods divide into two groups, namely chemical vapor deposition (CVD) and physical vapor deposition (PVD).

  • PVD method can create ultra-thin films from the micrometer range. The ionization or atomization process can be done by physical evaporation of the substance or plasma sputtering.
  • In the physical evaporation method, the metal substance is first evaporated at high temperatures from 1000 to 2000 °C and condensates under a highly pressurized vacuum environment.
  • In the plasma sputtering method, the coating material is emitted from the surface by accelerating different ions to the surface of the substance.
  • The CVD technique also uses the gaseous phase of the desired metal substance which can be condensed onto the target base metal surface.
  • In the CVD technique, the substrate is heated at temperatures over 850 °C. This high-temperature process restricts the material selection because in this process only materials with high melting temperature can be used.

Key Points to Remember

Here is the list of key points we need to remember about “Coating and Surface Treatment Processes”.

  • Surface treatment and coating is an important step before any component is deployed in the field.
  • Galvanic Action involves the creation of a potential difference between two metals resulting in the wearing away of one of the metals.
  • Electroplating is an application of Galvanic Cells to coat one metal onto another to act as a surface coating.
  • Anodizing is a common treatment process for aluminum and its alloys and involves the deposition of an oxide substrate on the surface.
  • Plasma spray coating involves the deposition of powder-based coatings like ceramics on a surface with the aid of a Plasma arc which melts the powder before application.
  • Chemical coatings are a cost-effective way to preserve surfaces and provide a wide range of coating options depending on the surface.
  • The surface Hardening process involves artificially hardening the top layer of components, typically made of carbon steels, to reduce wear during application.
  • Vapor Deposition is a unique way of providing high accuracy, thin and functional coatings for components by depositing vaporized substrate on the material.

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Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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