Oxidation of MoSi 2 /SiC nanolayered composite
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Oxidation of MoSi2ySiC nanolayered composite J-P. Hirvonen European Commission, Joint Research Centre, Institute for Advanced Materials, I-2120 Ispra, Italy
P. Torri and R. Lappalainen Department of Physics, University of Helsinki, FIN-00014 University of Helsinki, Helsinki, Finland
J. Likonen VTT Chemical Technology, FIN-02044 VTT, Finland
H. Kung, J. R. Jervis and M. Nastasi Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (Received 16 December 1996; accepted 12 December 1997)
The oxidation behavior of a nanolayered MoSi2ySiC composite material was determined at the temperature range of 400–900 ±C in wet oxidation conditions. The samples were produced in the form of thin films using a sputtering technique from two different sources, and a rotating 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 MoSi2ySiC samples was determined with high resolution transmission electron microscopy (HRTEM), and the composition of the sputtered samples was measured using backscattering (BS) of protons. For quantitative determination of oxidation, the nuclear reaction 16 Osd, pd17 O was utilized. Oxide layers were also analyzed using a secondary ion mass spectrometry (SIMS) and the appearance of the oxidized 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 MoSi2ySiC nanolayered composite material differs from that of both its constituents and involves a degradation mechanism of its own, resulting 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 single MoSi2 layer. The oxidation rate of the nanolayered structure was retarded by the SiC layers. No signs of pest disintegration were observed on either of the MoSi2 containing coatings during the steady-state phase of the oxidation at 500 ±C up to 40 h. Our results show that the oxidation of nanolayered structures can be only in part explained by the oxidation behavior of the constituents and that during the steady-state oxidation of the nanolayered structure the oxidation rate is largely determined by the constituent with the lowest oxidation rate and by the layered structure.
I. INTRODUCTION
Among intermetallic compounds and silicides, molybdenum disilicide has a unique position. It has a high melting point (2020 ±C), its high temperature oxidation and corrosion resistance are excellent, and it has a useful brittle-to-ductile transition at a temperature range of 1000 to 1350 ±C.1,2 Furthermore, MoSi2 has good thermal and electrical conductivities. Its processing is relatively easy compared to its closest competitors, silicon-based ceramics. In
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