Like traditional ceramics, advanced ceramics are densified from powders by applying heat—a process known as sintering. Unlike traditional ceramics, however, advanced powders are not bonded by the particle-dissolving action of glassy liquids that appear at high temperatures. Instead, solid-state sintering predominates. In this process, matter from adjacent particles, under the influence of heat and pressure, diffuses to “neck” regions that grow between the particles and ultimately bond the particles together. As the boundaries between grains grow, porosity progressively decreases until, in a final stage, pores close off and are no longer interconnected.

source : www.britannica.com