Towards the growth of stoichiometric chromia on pure chromium by the control of temperature and oxygen partial pressure

The possibility to tune the semiconducting properties of chromia scales thermally grown on pure chromium as a function of temperature and oxygen partial pressure p(O2) has been demonstrated in the present paper. While at 800 °C and a p(O2) of 10−14 atm, a single n-chromia is observed, at 900 °C and a p(O2) of 10−12 atm, a duplex n- and p-chromia is obtained. Between these two situations, a stoichiometric chromia exhibiting insulating properties could be identified at 850 °C and a p(O2) of 10−13 atm. In every case, the morphology of the oxide scale is duplex and seems to depend only on the growth direction: equiaxis grains for an inward (anionic) growth and columnar grains for an outward (cationic) growth. The nature of point defects, linked to the n or p semiconducting character, governs the oxide growth but surprisingly, does not seem to have any influence on the oxide scale morphology. Finally, the control of the growth of an insulating stoichiometric chromia layer should permit to optimize its protective character.