# Powder Metallurgy Questions and Answers – Sintering Theory – 2

This set of Powder Metallurgy Questions and Answers for Experienced people focuses on “Sintering Theory – 2”.

1. Solid-state sintering can be broadly divided into _____ stages.
b) Adhesion, Densification, and spheroidization of isolated pores
d) Initial bonding, and spheroidization of isolated pores

Explanation: Solid-state sintering is a complex process, which may be broadly divided into three stages, (i) Adhesion without shrinkage, (ii) Densification and grain growth stage, and (iii) Elimination of the last isolated and rounded pores.

2. In final stage of solid-state sintering, _____ become isolated and are no longer _______
a) Grains, connected
b) Grains, interconnected
c) Pores, open
d) Pores, interconnected

Explanation: In the final stage of solid-state sintering, pore channel closure occurs and the pores become isolated and are no longer interconnected. The residual individual pores are located either at the grain boundaries or within the grains. Porosity decreases more quickly in the vicinity of the grain boundary and densification proceeds at a very slow rate.

3. The first stage of solid-state sintering, includes _______ of vacancies and ________ between surface regions with different radii of curvature.
a) Surface diffusion, volume diffusion
b) Volume diffusion, surface diffusion
c) Lattice diffusion, surface diffusion
d) Plastic flow, surface diffusion

Explanation: The first stage of sintering includes volume diffusion of vacancies from the neck region to the convex portion of the system remote from the neck and surface diffusion between surface regions with different radii of curvature.

4. In which stage of solid-state sintering the pores are interconnected and pore shape is irregular?
a) 1st
b) 2nd
c) 3rd
d) 4th

Explanation: During the first stage of solid-state sintering, rapid neck formation and neck growth take place in the powder compact. This effect a greater extent of bonding within the sintered mass and thus during this stage the pores are interconnected and the pore shape is irregular.

5. In the second stage of sintering, with sufficient neck growth, the pore channels become more _______ in nature.
a) Spherical
b) Cylindrical
c) Cubical
d) Linear

Explanation: In the second stage of sintering, with sufficient neck growth, the pore channels become more cylindrical in nature. During this stage, pore rounding may also occur, without any shrinkage. With continued sintering, these cylindrical pore channels become unstable, gradually pinch off and close.
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6. The ________ gradient is high for small neck size leading to faster sintering.
a) Curvature
b) Linear
c) Volume
d) Velocity

Explanation: The interfacial energy is the driving force for the 2nd stage of solid-state sintering. The curvature gradients near the necks are responsible for the mass flow during this stage. The curvature gradient is high for small neck size leading to faster sintering. With sufficient time at the sintering temperature, the pore eventually becomes rounded. As the neck grows, the curvature gradient decreases and the sintering rate also decreases.

7. The driving force for the various stages of solid-state sintering are ________ and ________
a) Lattice diffusion and capillary force
b) Surface tension forces and diffusional flow
c) Surface tension forces and plastic flow
d) Lattice diffusion and surface tension force

Explanation: The driving force for the various stages of solid-state sintering are surface tension forces and diffusional flow. In the first stage, necks are formed at the contact points between the particles, which continue to grow. During the second stage of sintering, pore channel closure, as well as shrinkage, occurs and in the final stage, the formation of isolated pores occurs.

8. ________ is the mechanism by which bulk transportation can take place in solid-state sintering.
a) Surface diffusion
b) Plastic flow
d) Evaporation-condensation

Explanation: Material transport during sintering can occur through the gas phase, liquid phase or solid phase. Plastic flow, viscous flow, grain boundary, and volume diffusion are the mechanisms by which bulk transportation can take place in solid-state sintering.

9. Which of the following is not a mechanism of by which surface transportation occur during solid-state sintering?
a) Plastic flow
b) Volume diffusion
c) Viscous flow
d) Surface diffusion

Explanation: In solid-state sintering, mass transportation can take place either by surface transportation, grain boundaries or bulk transportation. Surface transportation can occur through diffusion, adhesion or evaporation-condensation or by surface diffusion.

10. The following diagram represents the sintering on a microscopic scale.

a) True
b) False

Explanation: The above diagram depicts sintering on a microscopic scale. (i) particle bonding is initiated at contact points. (ii) contact points grow into necks. (iii) pores between particles are reduced in size. (iv) grain boundary develops between particles in place of necked regions.

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