Sintering Theory and Practice Questions and Answers

Answer: C

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

Both are true and the reason is exactly why the atmosphere is chosen. A vacuum leaves nothing to be trapped. Hydrogen, and helium, are small and soluble enough to diffuse out through the lattice as the pore shrinks, so the pressure never builds. Nitrogen and argon do neither, which is why firing in air or argon leaves residual closed porosity that no amount of extra time will remove.

Answer: D

Correct answer: separates pores from boundaries and reduces strength

While a pore stays attached to a grain boundary it has a short diffusion path to a vacancy sink and can be removed. If the boundary sweeps past and leaves the pore inside a grain, that path becomes the whole grain radius and the pore is effectively permanent. The result is a coarse-grained body with trapped porosity - both of which reduce strength, which is why grain growth inhibitors such as magnesia in alumina are added.

Answer: A

A grain boundary is a fast path for the diffusion that carries material into the pore, so a pore that stays on a boundary shrinks. Once the boundary sweeps past and leaves the pore behind, removal becomes extremely slow.

Answer: B

Both are true and the reason gives the mechanism: pore removal depends on boundary contact, so the aim is to keep boundaries and pores moving together rather than to stop the boundaries entirely. Boundaries that stop moving cannot carry pores to the surface either.

Answer: C

It slows boundary movement so that pores are not left behind, which is what makes fully dense translucent alumina possible. The quantity involved is a fraction of a per cent and its discovery was a landmark in ceramic processing.

Answer: D

Keeping boundaries and pores together means the pores stay on the fast diffusion paths that remove them. The magnesia addition to alumina is the classic demonstration of the principle. Very small dopant additions can transform what a ceramic can achieve.

Answer: A

Correct answer: wets the particles and lets them rearrange rapidly

A wetting liquid pulls the particles together by capillary action and lubricates their rearrangement into a denser packing, and that rearrangement alone can accomplish much of the densification within minutes. Beyond it, solution-reprecipitation through the liquid transports material far faster than solid state diffusion, since diffusion in a liquid is orders of magnitude quicker.

Answer: B

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

Both are true and the reason gives the cause. Feldspar melts at around 1100 to 1200 degrees Celsius and forms a viscous glass that wets the quartz and clay particles, so porcelain matures near 1300 degrees. Pure alumina has no such phase and must be sintered in the solid state at 1600 degrees or more. The liquid is what makes the difference, not the fineness.

Answer: C

Correct answer: False

Liquid phase sintering densifies faster and at a lower temperature, but it leaves a glassy phase at the grain boundaries, and that phase is what fails first: it softens at high temperature, causes creep under load, and is often the route for chemical attack. Where high-temperature strength or corrosion resistance matters, solid state sintering is preferred despite its cost. Which is better depends entirely on what the ceramic must do.

Answer: D

The first two are correct. The third reverses the effect, since a glassy boundary phase softens at temperature and is precisely what limits creep resistance in such materials. Silicon nitride for engine parts is designed to minimise that phase.