Powder Metallurgy Multiple Choice Questions on “Sintering Mechanism – 1”.
1. Which of the following are the main types of material transport mechanisms proposed to be operating during sintering?
a) Evaporation
b) Dislocation movement
c) Evaporation-condensation, viscous flow, plastic flow and diffusion
d) Viscous flow, metal flow
Answer: c
Clarification: A number of material transport mechanisms have been proposed to be operating during sintering. These are, (1) Evaporation and condensation, (2) Viscous flow, (3) Plastic flow, and (4) Diffusion (volume diffusion, grain boundary diffusion, and surface diffusion).
2. _______ is the ‘liquid-like’ movement of individual atoms under stress while _______ is the slip of entire planes of atoms.
a) Plastic flow, viscous flow
b) Volume diffusion, plastic flow
c) Viscous flow, volume diffusion
d) Viscous flow, plastic flow
Answer: d
Clarification: Viscous flow is the ‘liquid-like’ movement of individual atoms under stress while the plastic flow is the dislocation movement or the slip of entire planes of atoms.
3. ________ mechanism operates in a system having materials with a high vapor pressure at sintering temperature.
a) Viscous flow
b) Volume diffusion
c) Evaporation and condensation
d) Plastic flow
Answer: c
Clarification: Evaporation and condensation is a simple mechanism, which operates in a system having materials with a high vapor pressure at the sintering temperatures. Evaporation-condensation is driven by pressure differences created by surface curvature and changes the shape of the particles and pores.
4. The driving force for evaporation-condensation mechanism is given by ________
a) μ-μ0=(γΩ)/r1=RT ln(frac{p_0}{p})
b) μ-μ0=(-γΩ)/r1=RT ln(frac{p}{p_0})
c) μ0-μ=(-γΩ)/r1=RT ln(frac{p}{p_0})
d) μ-μ0=(γΩ)/r1=RT ln(frac{p}{p_0})
Answer: b
Clarification: Based on the Gibbs-Thomson equation, the driving force for the evaporation-condensation mechanism is given by μ-μ0=(-γΩ)/r1=RT ln(frac{p}{p_0}) where μ0 and μ are chemical potentials of initial and final surfaces, R is the universal gas constant, T is the temperature in Kelvin and p and p0 are the partial pressures over the curved (under stress) surface and flat (stress-free) surface respectively, γ is the surface energy and Ω is the atomic volume.
5. The mass transport during evaporation-condensation mechanism occurs from _______ particle surface to the ______ necks.
a) Concave, convex
b) Convex, bi-concave
c) Convex, concave
d) Bi-convex, concave
Answer: c
Clarification: The mass transport during the evaporation-condensation mechanism occurs from the flat surface to the stressed region (or) from the convex particle surface to the concave necks. The rate of transfer of materials depends on the vapor pressure gradient between the two surfaces as well as the rate of evaporation.
6. The equilibrium vapor pressure over the neck _____ particle surface.
a) <
b) >
c) =
d) >>
Answer: a
Clarification: The equilibrium vapor pressure over a concave surface (i.e. neck or pore) is lower compared to that of a convex surface (i.e. particle surfaces), resulting in mass transport along the vapor pressure gradient, from the convex surfaces to the concave necks and the driving force for this mechanism is the surface tension force, which gives rise to stresses in the green compact.
7. Diffusion occurs as a result of _______ gradient.
a) Velocity
b) Vacancy concentration
c) Pressure
d) Momentum