The driving force of sintering is the reduction of the total interfacial energy of a powder, either by reducing the product gdA where g is the interfacial energy and A is the interface area, or by reducing Adg.  Densification, which reduces interfacial energy (dg) and coarsening, which reduces interfacial area (dA) are intimately coupled during sintering. Practitioners favor densification while trying to limit coarsening, especially to preserve the initial nanometric size of ceramic powders. A fast heating rate, coupled with a specific thermal cycle combining high and low temperatures is an efficient method to favor densification over grain growth. Recently, the two- step sintering technique has been demonstrated to be effective to densify Al₂O₃ ceramic nanopowders [1]. Here, we use the Discrete Element Method with our in-house code dp3D to explore the reasons for the suppression of grain growth in the two-step sintering of a-alumina. We adapt a model that couples densification and grain-growth with material parameters originating from the literature [2]. The simplifying assumptions and the inherent limitations of the model are discussed. In particular, the proposed grain growth model is detailed with the conditions necessary for the initiation of each mechanism (surface diffusion and grain boundary migration). Large initial packings with up to 400 000 particles and a controlled size distribution are simulated as many particles disappear with coarsening [3].  We first validate the model for non-isothermal conditions, demonstrating the beneficial effect of high heating rates. The evolution of density, densification rate and grain size with temperature are critically compared to experimental data [1]. The simulations of two- step sintering are then presented and the origin of the lack of grain growth is discussed. Our results suggest that the cessation of grain growth in the second step is explained by a large increase of the activation energy of grain boundary mobility for a transition temperature of 1100^? C.
 
[1] Yang, H., Li, L., Li, Y., Shen, B., Kang, Y., Zhao, L., Li, J., Dong, Y., & Li, J. (2021). Unveiling exceptional sinterability of ultrafine α-Al2O3 nanopowders. Journal of Materiomics, 837–844.
[2] Paredes-Goyes, B., Jauffres, D., Missiaen, J.-M., & Martin, C. L. (2021). Grain growth in sintering: a discrete element model on large packings. Acta Materialia, 218, 117182.
[3] Paredes-Goyes, B., Venkatesh, A. M., Jauffres, D., & Martin, C. L. (2022). Two-step sintering of alumina nano-powders: A discrete element study. Journal of the European Ceramic Society, 43, 501–509.