Discussion
Assertion (A): Surface diffusion can be actively unhelpful during sintering.
Reason (R): It rounds the necks and reduces the driving force without producing any densification.
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.
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