Sintering Theory and Practice Questions and Answers

Answer: C

Correct answer: False

Volumetric shrinkage is roughly three times the linear figure for small strains, and exactly \( 1-(1-\varepsilon_L)^3 \) in general. Confusing the two is the standard error in mould design: sizing a die for 14 per cent when the volume change is 36 per cent, or the reverse, produces a part far from the intended dimensions.

Answer: D

Correct answer: lower strength than a fully dense one

Eight per cent porosity is eight per cent of pores acting as stress concentrators, and strength falls roughly exponentially with porosity - a common approximation is \( \sigma = \sigma_0 e^{-bP} \) with \( b \) around 4 to 7. At eight per cent porosity that is a loss of a third or more of the strength, which is why the final few per cent of density is worth chasing.

Answer: A

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

Both are true and the reason justifies the cost. A ceramic fails from its largest flaw, and a pore is a flaw with a stress concentration around it, so the strength is governed by the worst pore rather than by the average density. That is why hot pressing, hot isostatic pressing and controlled atmospheres are used for structural ceramics and not for, say, a firebrick, where porosity is harmless or even wanted.

Answer: B

Hydrogen and oxygen dissolve in some lattices and can diffuse out, so the pore is not held open by trapped gas. This is one route by which fully dense transparent ceramics are made. Sodium vapour lamp envelopes are the best known application.

Answer: C

Correct answer: 1 and 2 only

Statements 1 and 2 are correct: hot pressing applies uniaxial pressure at temperature, and that pressure permits full density at a lower temperature than pressureless sintering. Statement 3 is wrong, and it is the technique’s principal limitation - a uniaxial die can only produce simple shapes such as discs and plates, one at a time. Hot isostatic pressing exists to lift exactly that restriction.

Answer: D

Correct answer: True

A green part shrinks by roughly a seventh in each linear dimension on firing, so any feature machined to a finished size before sintering will come out undersized. Green machining is done to oversized dimensions calculated from the expected shrinkage, and anything needing close tolerance is ground after firing instead - expensively, since a dense ceramic is very hard.