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QUESTION 3 [30 Marks] a) After a part is manufactured, some of its surfaces may require surface treatments in order to improve its properties and characteristics. State five methods that can be done in case hardening process for a metal. Describe in details one of case hardening process with the aid of schematic diagram of the process chosen. (15 Marks) b) One of the surface treatment method is physical vapour deposition (PVD) process. There are three types of PVD processes such as vaccum deposition, sputtering and ion plating process. Analyse the differences between the sputtering and ion plating process using a schematic diagram and describe the process in details. State two applications can be made from each process. (15 Marks)

Answer 1

a)

Case hardening is a process of making the outer surface of a metal component harder while allowing the inside softer.

Processes-

• Flame or Induction Hardening
• Nitriding
• Carburizing
• Cyaniding
• Carbonitriding
• Ferritic Nitrocarburising

Nitriding

Nitriding involves heat treating to diffuse nitrogen inside the surface of a metal to achieve case hardened surface.

Types of Nitriding -

• Gas Nitriding

In this, the workpiece is heated to a particular reactive temperature and exposed to an atmosphere containing nitrogen-rich ammonia gas.

Ammonia Gas Workpiece(Heated)

• Salt Bath Nitriding

In this, the nitrogen donating medium is nitrogen-containing salt in the liquid phase. Then the heated workpiece is dipped inside the liquid salt medium for a bath.

• Plasma Nitriding

​​​​​​​It is also called ion nitriding or glow-discharge nitriding. In this the nitrogen donating gas medium is excited by inducing huge electric fields that ionize the gas molecules and help in diffusion into metal workpiece.  

Electric Discharge Ammonia Gas Medium Workpiece(Heated)

b.)

Vacuum Deposition

It is a process to deposit layers of a material atom by atom or molecule by molecule onto a target surface. The process operates at very low pressure far below atmospheric pressures. The condensed particles of source material can be generated in various ways like thermal evaporation, sputtering, cathodic arc vaporization, laser ablation.

Applications-

• Tribological Coatings
• Semiconductor Films

Electric Discharge Vapour Cloud Workpiece

Sputtering

In sputter deposition atoms from a target material ejected by the collision of charged gas particles from sputtering medium and gets deposited to the substrate.

Applications-

• Film Deposition
• Etching

Substrate Argon Argon Gas Gas Inlet Outlet Charged Ar ions Target

Ion Plating

Ion plating uses periodic bombardment of the substrate and deposit film by atomic-scale energetic particles. The energetic particles used for bombardment are ions of inert gas.

Applications-

• Optical Coatings of high densities
• Hard Coatings on tools