# Powder Metallurgy Questions and Answers – Sintering Theory – 1

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This set of Powder Metallurgy Multiple Choice Questions & Answers (MCQs) focuses on “Sintering Theory – 1”.

1. Sintering involves _____ types of system.
a) Unary
b) Binary
c) Single component andmulticomponent
d) Ternary

Explanation: Sintering may involve (i) single component system, wherein shrinkage is a major factor or (ii) multi-component system, involving more than one phase, where several processes like solid solution formation and liquid phase formation may also occur in addition to densification.

2. _______ is the major material transport mechanism in sintering of the single component system.
a) Inter-diffusion
b) Self-diffusion
c) Lattice-diffusion
d) Volume-diffusion

Explanation: Self-diffusion is the major material transport mechanism in sintering of single component system and the driving force resulting from a chemical potential gradient due to surface tension and capillary forces between particles.

3. Which of the following are the stages of sintering, according to Sauerwald theory?

Explanation: According to Sauerwald theory, there are two stages in sintering- adhesion which occurs during heating due to atomic attraction, and recrystallization consists of diffusion of atoms between adjacent grains.
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4. Which of the following changes do not occur on during recrystallization?
a) Pore shrinkage
b) Phase changes
c) Grain shrinkage
d) Precipitation

Explanation: Recrystallization includes changes in the microstructure, phase changes, grain growth, precipitation and pore shrinkage.

5. _______ requires mass transport.
a) Pore expansion
b) Grain shrinkage
c) Densification

Explanation: Densification requires mass transport and hence sintering is carried out at temperatures where material transport due to diffusion is appreciable so that the purpose of sintering is achieved.

6. The process of sintering will stop when _______
a) dE=γSSdASSSVdASV=0
b) dE=γSSdASSSVdASV>0
c) dE=γSSdASSSVdASV<0
d) dE=γSSdASVSVdASS=0

Explanation: The process of sintering will stop when the overall change in the free energy of the system (dE) becomes zero i.e. dE=γSSdASSSVdASV=0 where γSS and γSV are the surface energies of the solid-solid interfaces (in N-m) and solid-vapor interfaces respectively, dASSand dASV are the interfacial area of solid-solid and solid-vapor interfaces (in sq.m).

7. The driving force for sintering is reduction in ________
a) Internal energy
b) Surface tension
c) Surface energy
d) Entropy

Explanation: In sintering, the driving force is the overall decrease in the surface free energy of the compacts. This occurs by replacing the high-energy solid-vapor (of free energy γSV) interfaces with the low energy solid-solid interfaces (of free energy γSS). This surface energy reduction results in the densification of the compact.

8. The initial free energy of the compact before sintering must be _______
a) Positive
b) Negative
c) Zero
d) Infinite

Explanation: Initially, the free energy of the system (compact) (dE) must be negative, that is, the free energy of solid-solid interfaces must have lower free energy than the solid-vapor interface. Thus, during sintering, the newly formed solid-solid interfaces (i.e. grain boundaries) replace the existing solid-vapor interfaces, thereby lowering the free energy of the system.

9. To achieve densification during sintering without grain growth, the γSS must be _____, while the γSV must be _____
a) Zero, maximized
b) Maximized, zero
c) Zero, minimized
d) Maximized, minimized

Explanation: To achieve densification during sintering without grain growth (i.e. a fine-grained material), the γSS must be maximized, while the γSV must be zero (i.e. driving force must be zero for grain growth). Such conditions can be achieved by altering the conditions of sintering by doping or by the use of a suitable sintering environment to maximize the surface free energy.

10. It is desirable that sintering stops when ______
a) $$\frac{dA_{SV}}{dA_{SS}}$$is close to zero
b) $$\frac{dA_{SS}}{dA_{SV}}$$ is close to zero
c) γSS >> γSV
d) $$\frac{-dA_{SV}}{dA_{SS}}$$is close to zero

Explanation: In sintering, the major objective is to eliminate the porosity and obtain a dense compact. It is, thus desirable that the sintering stops when the values of $$\frac{-dA_{SV}}{dA_{SS}}$$ is close to zero, (or γSS << γSV).

11. Sintering can be considered as an irreversible thermodynamic process.
a) True
b) False

Explanation: Sintering is an irreversible thermodynamic process because the change in surface free energy during sintering is irreversible.

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