We have performed classical molecular dynamics simulations of amorphous Si-C-N materials. The dependence of the local order and of the microstructure on the chemical composition was investigated. Our simulations show that for a stoichiometric nitrogen/silicon ratio equal to or higher than 4/3, the amorphous ceramic separates into different amorphous domains, namely C-rich, SiN-rich, and SiC-rich phases. Below this ratio, the material is composed of mixed structures, homogeneously spread within the material. For a very particular composition range, we found that carbon atoms crystallize into monoatomic graphitic layers surrounding the SiN-rich domains.