Molecular dynamics study of melting, diffusion, and sintering of cementite chromia core–shell particles

For the additive manufacturing of chromium-oxide coated stainless steel powders, the melting, diffusion, sintering behaviors of the cementite phase in the stainless steel during heat treatment can affect the products’ physical and chemical properties, and thus, are worth studying. In this work, we use ReaxFF reactive force-field-based molecular dynamics simulations to investigate the melting, diffusion, and sintering at the molecular level through an atomistic-scale level representation for the cementite with a Cr₂O₃ coating passivation layer. Additionally, we also compare the Lindemann indices of the Cr₂O₃ cementite core–shell particle with those of a pure cementite particle of the same size, in order to understand the influence of the chromium-oxide layer on the cementite melting. Results indicate that the Cr₂O₃ shell layer does not change the melting temperature of the core–shell particle. However, the Cr₂O₃ shell layer's oxygen atoms diffuse into the core and increase the impurity level in the Fe₃C crystal, destabilizing the cementite structure in the core. We observe that the melting of the core–shell particle starts from the core while, in contrast, the melting of the pure-cementite particle begins from the surface. In addition, a comparison of mean squared displacements of hydrogen (H), oxygen (O), carbon (C), chrome (Cr), and iron (Fe) atoms in the solid (900 K) and liquid (2000 K) phases indicates that the atom mobility in the liquid phase is significantly higher than in the solid phase. H and O atoms in the particle exhibit a higher mobility level than Fe, Cr and C atoms. When the temperature is elevated from 900 K to 2000 K, the O content in the core and C content in the shell rises, while the O content in the shell and C content in the core declines. Furthermore, the sintering simulation shows that there are four stages for the sintering in the liquid phase: the two particles move randomly (Stage 1) until they make the first contact (Stage 2), the shell layers subsequently intermingle (Stage 3), and finally the particle cores fuse (Stage 4). The sintering in the solid phase (900 K) involves the former three stages. However, the temperature of 900 K is not sufficiently high to bring the two particles into Stage 4 during the time scale of our simulations.