Sintering Theory and Practice Questions and Answers

Answer: A

This is hot enough for appreciable diffusion but well below melting, which is the whole point of the powder route. Quoting the ratio in kelvin rather than celsius matters, since the ratio is meaningless on a scale with an arbitrary zero.

Answer: B

The melting point is about 2,320 kelvin, two thirds of which is roughly 1,550 kelvin, or about 1,275 degrees Celsius. Real alumina is sintered somewhat hotter than this, so the rule is a guide rather than a specification.

Answer: C

A fraction of a temperature only means anything on an absolute scale, since celsius has an arbitrary zero. Applying the rule in celsius gives an answer several hundred degrees wrong. Absolute temperature is required whenever a ratio is taken.

Answer: D

Correct answer: increase density but also coarsen the grains

Both processes are thermally activated and both run for as long as the part is held hot, so extra time buys density at the price of grain size. Because strength falls as grains coarsen, the two effects work against each other, and beyond a point the coarsening wins. The whole art of sintering practice lies in separating the two - through fast firing, pressure assistance or grain growth inhibitors.

Answer: A

The applied stress adds to the surface energy driving force and allows full density at a lower temperature and shorter time. The penalty is that only simple shapes can be made and the cycle produces one part at a time.

Answer: B

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

Both are true and the reason is why the drawbacks are accepted. Applied pressure adds to the capillary driving force, so full density is reached at a lower temperature and in less time - and since grain growth depends far more steeply on temperature than densification does, the grains stay fine. For a ceramic whose strength depends on grain size, that microstructure justifies producing one simple shape at a time.

Answer: C

The external stress supplements the surface energy driving force, so less thermal activation is needed. Lower temperature in turn means less grain growth and a finer final microstructure. Strength and grain size move together in a dense ceramic.

Answer: D

Correct answer: passing a pulsed current through the die and powder

A pulsed direct current is passed through the graphite die and, in a conducting powder, through the compact itself, so the heat is generated within the sample rather than conducted in from outside. Heating rates of hundreds of degrees per minute are possible, and full density can be reached in minutes - far too quickly for the grains to coarsen appreciably.

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 principle behind fast firing. Grain boundary migration has a higher activation energy than the diffusion that densifies, so raising the temperature quickly and holding briefly favours densification over coarsening. Passing rapidly through the low-temperature range where non-densifying surface diffusion dominates helps for the same reason.

Answer: B

Correct answer: about 14 per cent

Shrinkage is three-dimensional, so the linear figure is the cube root of the volume ratio: \[ 1 - \left(\frac{0.60}{0.96}\right)^{1/3} = 1 - (0.625)^{1/3} = 1 - 0.855 = 0.145, \] about 14 per cent. The 36 per cent offered is the volumetric shrinkage, \( 1-0.625 \), which is what results from forgetting the cube root.