Oxidation of MoSi₂/SiC nanolayered composite

Abstract

The oxidation behavior of a nanolayered MoSi 2 ͞SiC 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, MoSi 2 and SiC, produced with the same sputtering technique, were measured separately. The microstructure of the MoSi 2 ͞SiC 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 O͑d, p͒ 17 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 MoSi 2 ͞SiC 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 MoSi 2 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 MoSi 2 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.

Oxidation of nanocrystalline Mo–Si–N and nanolayered Mo–Si–N/SiC coatings

“…An example of a depth profile is pre-sented in Fig. 20 Both single-layer coatings and all multilayers formed a well-adherent smooth oxide on the surface. Strong reduction of carbon and nitrogen in the oxide layer is evident.…”

“…5 At higher temperatures MoO 3 is volatile and a protective SiO 2 layer is formed on the surface. [19][20][21] Nanolayered (multilayers with the sublayer thickness on the nanometric scale) and nanocrystalline materials may possess many unique properties, which are fundamentally different and often superior compared to those of materials with coarser microstructure. While accelerated oxidation is generic to all forms of MoSi 2 , pesting has been concluded to result from accelerated oxidation within the preexisting microcracks.…”

Mechanical properties, stress evolution and high-temperature thermal stability of nanolayered Mo–Si–N/SiC thin films

“…The thermal expansion coefficient of MoSi 2 is also more closely matched to metals, thus it is easier to join to metals. 7,8 In these studies, a partial breakdown of the layered structure was observed after 1 h vacuum anneal at 800°C. [1][2][3] In spite of its many promising properties, there are still some limitations to large scale use of MoSi 2 in applications.…”