Hot-press sintering of aluminum nitride nanoceramics

The increasing interest in nanostructured ceramics and their applications highlights the need to understand the hot-press sintering of nanoscale AlN powders. We use molecular dynamics simulations to investigate the hot-press sintering of AlN nanoceramics and to clarify the underlying sintering mechanisms. We consider samples with 32 nanoparticles with diameters 8, 12, and 16 nm, arranged in a face centered cubic supercell: samples AlN-8, AlN-12, and AlN-16. Sintering simulations are performed at $T=1900\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ under 1 GPa for 6 ns. An additional simulation is performed for a sample with 8 nm sized nanoparticles at a lower pressure of 0.1 GPa, namely, sample AlN-8-0.1. After 6 ns, densifications of 99%, 96.2%, 95.6%, and 93.2% are achieved for samples AlN-8, AlN-8-0.1, AlN-12, and AlN-16, respectively. Analysis shows that the fast densification process is rooted at the high diffusivity of nanoparticles surface atoms. The AlN-8 sample undergoes intense microstructural evolution during the sintering process from 3 to 6 ns, resulting in a wide distribution of grain sizes from 4 to 15 nm and a larger, 11 nm average grain size. A slower grain growth process is observed in the AlN-8-0.1 sample from $\ensuremath{\sim}4.0\phantom{\rule{0.28em}{0ex}}\mathrm{ns}$. These results indicate a change in the densification mechanism from surface diffusion to grain boundary migration and relaxation of grain boundaries and triple junctions, resulting in a two-stage sintering process, i.e., initially the sample experiences a fast densification, which is followed by intense microstructural evolution. The densification mechanism crossover occurs at 98.7% and 95% densification for the AlN-8 and AlN-8-0.1 samples, respectively. The results indicate that the onset of the second stage depends on a densification threshold, which can be delayed by applying higher external pressure. Sintering of the AlN-16 sample indicates the presence of structural phase transformation at the nanoparticles contact points, which reach over 12 GPa of local pressure during the 1 GPa compression. These results provide atomistic insights into the hot-press sintering of nanoscale ceramics, highlighting the intrinsic swift densification and microstructural evolution processes.