Sintering Theory and Practice Questions and Answers

Answer: A

A powder has an enormous surface area and therefore a large surface energy, and the system lowers it by replacing solid-vapour surfaces with solid-solid boundaries. No applied force is needed for the process to proceed at all.

Answer: B

Both are true and the reason gives the thermodynamics: the energy available to be released scales with the surface area, and the diffusion distances are shorter as well, so both the driving force and the kinetics favour fine powder.

Answer: C

Grain boundary and lattice diffusion from the boundary allow the particle centres to approach, which is what closes the pore. Surface diffusion and evaporation-condensation coarsen the neck without bringing the centres any closer.

Answer: D

Surface diffusion and evaporation-condensation move material to the neck without moving the particle centres together, so they coarsen the structure without densifying it. Only transport from the grain boundary produces shrinkage.

Answer: A

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason is the mechanism. Surface diffusion moves material from the particle surface to the neck without moving the particle centres any closer together, so the neck grows and the compact does not shrink. Worse, it blunts the sharp neck curvature that provides the capillary driving force, so the densifying mechanisms - grain boundary and lattice diffusion - have less to work with. It dominates at low temperature, which is why slow heating through the early stage is avoided.

Answer: B

Correct answer: neck formation between adjacent particles

The initial stage is the growth of necks at the points where particles touch, driven by the very high curvature there. Densification in this stage is modest, typically a few per cent, and the particle identities are still distinct. Isolated spherical pores at grain corners belong to the final stage, and a continuous cylindrical pore network to the intermediate one.

Answer: C

Correct answer: a continuous network of cylindrical pores

By the intermediate stage the necks have grown until the pore space is a connected network of roughly cylindrical channels running along the three-grain edges. This is where most of the densification happens, because the pores are still attached to grain boundaries and vacancies have a short path to a sink. The stage ends when the cylinders pinch off into isolated pores.

Answer: D

Once pores are isolated, gas trapped inside them resists further shrinkage and grain growth can carry them into grain interiors where they are almost impossible to remove. Reaching full density from here is the hardest part of the process.

Answer: A

The first two are correct. The third is wrong, since grain growth becomes most pronounced in the final stage and is exactly what makes that stage difficult. Pores separated from boundaries by growing grains are nearly impossible to remove.

Answer: B

Correct answer: True

Once pores close off they contain whatever gas the furnace atmosphere provided. As the pore shrinks that gas is compressed, and its pressure eventually balances the capillary pressure driving the shrinkage - at which point densification simply stops. Prolonged firing then coarsens the pores rather than removing them, which is why the last few per cent of density is so often unreachable in air.