OXIDATION OF MOSI2 SIC NANOLAYERED COMPOSITE

The oxidation behavior of a nanolayered MoSi2/SiC composite material w as determined at the temperature range of 400-900 degrees C in wet oxi dation conditions. The samples were produced in the form of thin films using a sputtering technique from two different sources, and a rotati ng substrate holder, onto silicon single crystals and low carbon steel . For comparison, the oxidations of both constituents, MoSi2 and SiC, produced with the same sputtering technique, were measured separately. The microstructure of the MoSi2/SiC samples was determined with high resolution transmission electron microscopy (HRTEM), and the compositi on of the sputtered samples was measured using backscattering (BS) of protons. For quantitative determination of oxidation, the nuclear reac tion O-16(d, p)O-17 was utilized. Oxide layers were also analyzed usin g a secondary ion mass spectrometry (SIMS) and the appearance of the o xidized surface with a scanning electron microscopy (SEM). As expected , the SiC films had both the lowest initial oxidation and steady state oxidation rate. The results show that the oxidation behavior of the M oSi2/SiC nanolayered composite material differs from that of both its constituents and involves a degradation mechanism of its own, resultin g in the highest oxidation during the initial phase of the oxidation. A steady-state oxidation rate was observed after the initial transient phase to be the highest for the metastable C40 structure of the singl e MoSi2 layer. The oxidation rate of the nanolayered structure was ret arded by the SiC layers. No signs of pest disintegration were observed on either of the MoSi2 containing coatings during the steady-state ph ase of the oxidation at 500 degrees C up to 40 h. Our results show tha t the oxidation of nanolayered structures can be only in part explaine d by the oxidation behavior of the constituents and that during the st eady-state oxidation of the nanolayered structure the oxidation rate i s largely determined by the constituent with the lowest oxidation rate and by the layered structure.