Powder Metallurgy Quiz on “Sintering of Multicomponent System – 2”.
1. Which of the following condition should be satisfied for complete penetration of liquid into the grain boundaries?
a) γGS < 0.5γGB
b) γLG < 0.5γGB
c) γGB < 0.5γLS
d) γLS < 0.5γGB
Answer: d
Clarification: A measure of penetration by the liquid between the two grains is given by the dihedral angle. Complete penetration into grain boundary takes place when the liquid-solid interfacial energy is less than one half of the grain boundary energy i.e. γLS < 0.5γGB.
2. For liquid phase to sinter successfully, the contact angle should be _______
a) more than 90°
b) less than 90°
c) equal to 90°
d) equal to 0°
Answer: b
Clarification: For the liquid phase to sinter successfully, the contact angle should be less than 90°. When the contact angle is 0°, the liquid phase completely wets the solid particles. In the case of the non-wetting liquid phase (contact angle≥90°), partial exudations from the compact will occur in the form of droplets during sintering.
3. Initial particle size must be small for liquid phase sintering.
a) True
b) False
Answer: a
Clarification: For liquid phase sintering to be effective, the initial particle size must be small for liquid phase sintering. Small particles have higher surface energy/unit volume and so the driving force for sintering is large because liquid phase sintering involves transport of materials from surfaces of higher energy to surfaces of lower energy.
4. ______ dihedral angle, will hinder further densification.
a) Negative
b) Zero
c) Positive
d) Infinite
Answer: c
Clarification: If the dihedral angle is positive, then grain boundaries may appear between the particles and an aggregate of two or more grains will be established. This situation will lead to the formation of a rigid skeleton, which we hinder further densification.
5. In liquid phase sintering, ______ stresses are developed at the contact point of two particles when the dihedral angle is zero.
a) Bending
b) Shear
c) Tensile
d) Compressive
Answer: d
Clarification: When the dihedral angle is zero, a negative pressure gradient is developed because of the small negative radius of curvature of the liquid droplet. At the contact point, compressive stresses are developed. This stress leads to increased dissolution of the material at the contact points and their movement away from contact points cause the center of the particles to come together giving rise to densification.
6. ______ amount of liquid phase must be present to penetrate the adjoining grains.
a) Minimum
b) Sufficient
c) Maximum
d) Unlimited
Answer: b
Clarification: Sufficient amount of liquid phase must be present to penetrate the adjoining grains. However, the amount of liquid phase must be kept as minimum as possible, to avoid deformation of the compact.
7. Liquid phase sintering is advantageous over solid phase sintering due to ______
a) Temperature
b) Microstructure
c) Kinetics
d) Economy
Answer: c
Clarification: Liquid phase sintering promotes enhanced kinetics because of the presence of the liquid phase. The capillary attraction due to wetting allows rapid densification, reduction in interparticle friction leads to a faster rearrangement of particles, and presence of liquid phase aids in material flow and better packing.
8. The greater the ______ more will be the driving force for the liquid flow stage in the liquid phase sintering.
a) Wetting
b) Temperature
c) Pressure
d) Diffusion
Answer: a
Clarification: The greater the extent of wetting the greater will be the driving force for the liquid flow stage in the liquid phase sintering. Moreover, densification increases with the increasing amount of liquid phase in this process. The extent of wetting can be increased by lowering the liquid-solid interfacial energy by diffusion or absorption method at the interface.
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