4H-SiC seeds have been treated at high temperature (1650-1900degreesC) under Ar or N-2 in a sublimation like graphite crucible with SiC powder at the hotter part. It was found that the surface morphology is significantly altered by the nature of the atmosphere. N-2 gives a smoother surface for the low temperature range (<1700degreesC) whereas at higher temperature it degrades compared to Ar. Thermodynamical calculations performed on the Si-C-N (Ar) system show that N-2 plays an important role on the gas phase chemistry of decomposition of SiC by forming gaseous species of nitrides. It is found that the gas phase is C rich when N-2 is used at low temperature whereas it becomes Si rich at a temperature higher than 1700degreesC. The theoretical results correlate well with the graphitisation of the SiC powder.