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 $T=1900\mathrm{K}$ 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 $\sim 4.0\mathrm{ns}$. 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.

Figure 1

Illustration of the hot-press sintering of AlN simulated by molecular dynamics. (a) Illustration of the simulation model composed of 32 nanoparticles of 8 nm diameter, arranged in a face centered cubic supercell. Different colors indicate different nanoparticles. (b) Densification during the sintering process for different pressures and particle sizes.

Figure 2

Illustration of the of AlN supercells and their corresponding pores after hot-press sintering for 6 ns. [(a) and (e)] $p=1.0\mathrm{GPa}$, $d=8\mathrm{nm}$. [(b) and (f)] $p=0.1\mathrm{GPa}$, $d=8\mathrm{nm}$. [(c)and (g)] $p=1.0\mathrm{GPa}$, $d=12\mathrm{nm}$. [(d) and (h)] $p=1.0\mathrm{GPa}$, $d=16\mathrm{nm}$. Different colors indicate different grains.

Figure 3

Mean square displacement (MSD) and diffusion coefficient of aluminum and nitrogen atoms during the sintering of the 8 nm nanoparticle system under different pressures. (a) Cross-section view of the three nanoparticle regions used in the calculations. (b) MSD and (c) diffusion coefficient of Al atoms initially in the different regions shown in (a) for different pressures, and corresponding (d) MSD and (e) diffusion coefficient for N atoms.

Figure 4

Bond angle distributions of different layers in the 8 nm nanoparticle system under 1 GPa before and after sintering. (a) First shell before sintering, (b) first shell after sintering, (c) core region before sintering, and (d) core region after sintering.

Figure 5

Cross-section view of the AlN-8 sample before sintering, showing Al-Al and Al-N bonds by yellow and red sticks, respectively. Regions 1 and 2 in (a) highlight the bonding structure at the nanoparticle surface and contacting region, respectively. (b) and (c) shows a zoomed view of regions 1 and 2 indicated in (a).

Figure 6

Evolutions of the grain size distribution for the 8 nm nanoparticle systems during sintering at 1.0 and 0.1 GPa. Blue and red solid lines indicate the total number of grains at 1.0 and 0.1 GPa, respectively. (a) $t=0.0$, (b) 1.5, (c) 3.0, (d) 4.5, (e) 5.3, and (f) 6.0 ns.

Figure 7

Grain size evolutions for the 8 nm nanoparticle systems sintered under (a) 1.0 and (b) 0.1 GPa. Each colored line represents one of the initial 32 grains, the black thick line represents the weighted average grain size, and the red thick line represents the average grain size in (a) and (b). The relationship between reduced density and weighted average grain size is shown in (c).

Figure 8

Densification during the sintering process for different pressures and particle sizes. Models for early stage densification are shown by dashed lines while models for late-stage densification are shown by continuous lines.

Figure 9

Cross-section view of the AlN-16 sample after sintering for 6 ns. (a) Cross-section view of the crystalline structure with atoms colored by the phases they form. (b) Spatial distribution of Al-Al bonds for the cross-section shown in (a). The blue dashed circles in (b) highlight the location of Al-Al wrong bonds inside the grains. Such in-grain wrong bonds are indicative of the presence of interstitial-vacancy pairs.

Figure 10

Structural phase transition for the 16 nm sized nanoparticle system sintered under 1.0 GPa. (a) Local pressure distribution before sintering. (b) Crystalline structure after sintering for 6 ns. (c) Evolution of the volume fraction of rocksalt and wurtzite structures in the sintering system.