The structural and morphological modifications induced by the carbonization stage upon 3C-SiC heteroepitaxial films grown on (111) and (100) oriented silicon substrates have been investigated. The crystalline quality of the films is strongly dependent on the carbonization parameters (propane flow rate and duration of carbonization). The (111) heteroepitaxial films coalesce more rapidly and present a lower dependence on the carbonization conditions than (100) films. By comparing the evolution of the interfacial defects (voids) density with existing models, we show that this is related to the initial mechanisms occurring during the carbonization stage. The twin defects densities on (111), (100) and (211) films are also investigated and the role of the only carbonization stage on their formation is studied.