Oxidation behavior of ytterbium silicide in air and steam
Introduction
Silicon carbide (SiC)-fiber reinforced SiC matrix (SiC/SiC) composites, which are classified as ceramic matrix composites (CMCs), have been proposed as a candidate material for the high-temperature sections of aero gas-turbine engines [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10]]. In a combustion environment, components are subjected to severe heat cycles in a high-pressure water-vapor atmosphere. Recently, environmental barrier coating (EBC) systems have become a key technology, and many related studies have been carried out to prevent the recession of SiC-based composites, as caused by the evaporation of the oxidation product (silica; SiO2). In currently available components, the Si is contained in an EBC-SiC/SiC system as a bond coat (BC) layer [[1], [2], [3], [4], [5], [6], [7], [8], [9],[11], [12], [13]] to reduce the thermal expansion mismatch between the topcoat layer of EBC system and the substrate.
The Si–BC layer also oxidizes to form SiO2 (cristobalite) as a result of oxygen penetrating transverse cracks in the topcoat layer, or due to the diffusion of oxygen within the EBC layer. A continuous cristobalite layer can then evolve during heat exposure. The formation of SiO2 is a critical problem in current Si-BC layers, because the transformation from β to α cristobalite also leads to a 4.5% decrease in the volume [4], which causes severe cracking. These cracks combine such that the topcoat layer delaminates.
To overcome the above-mentioned problems, some researchers have investigated the potential of rare earth (RE-) silicides [[13], [14], [15], [16], [17]]. The melting points of RE-silicides depend the atomic ratio of the RE-element to Si. Among RE-silicides, ytterbium (Yb)-silicide a potential BC candidate material because stable phases with higher melting point than Si exist (see Fig. 1) and oxides formed from Yb–Si intermetallics are Yb-silicates, which are oxide coating of EBCs. Zhu [13] found that a BC layer based on silicides and ternary intermetallics exhibited good environmental durability in a combustion environment. Another recent study [15] also showed that severe oxidation occurred in Si-rich Yb–silicide after exposure to heat in air, with formation of ytterbium oxide (Yb2O3), ytterbium mono- and disilicates (hereafter denoted YbMS and YbDS, respectively). However, the fundamental phenomena occurring at elevated temperatures in air, as well as in a water-vapor atmosphere, are still not understood.
The objective of the present study was to understand the effect of the atomic ratio of Yb to Si on the oxidation mechanisms. Firstly, we fabricated Yb-silicides with different atomic ratios (Yb:Si = 5:3 and 3:5). Then, changes in morphology and thermogravimetric behavior of these Yb-silicide were evaluated up to 1200 °C in dry air and steam. The environmental effects at elevated temperatures were also examined. The degradation mechanism for both silicides was discussed.
Section snippets
Materials
We fabricated Yb-silicide with atomic ratios of Yb3Si5 and Yb5Si3. As raw materials, Yb (purity: 3 N, Mitsui Kinzoku Trading Co., Ltd, Tokyo, Japan) and Si chunks (purity: 4 N, Kojyundo Chemical Corp., Saitama, Japan) were used. An arc-melting facility (NEV-AD03, Nissin Giken Corp., Saitama) was used to form the Yb3Si5. To synthesize a single phase of Yb3Si5, the melting was repeated three times, with Yb metal being added each time. For Yb3Si5, the details of the fabrication procedure can be
Microstructure of as-sintered Yb-silicides
XRD profiles of both samples are shown in Fig. 2(a). It confirms that the sintered compacts are mostly composed of Yb3Si5 and Yb5Si3, although weak diffraction peaks from Yb2O3 and Si appear. The microstructures of both silicides after sintering are also shown in Fig. 2(b) and (c). There are some pores at grain boundaries, and the porosities of Yb3Si5 and Yb5Si3 re 29.0% and 16.6%, respectively, as measured by analysis of the SEM photographs. The apparent densities, ρa, of the as-sintered Yb3Si5
Environmental effects on the oxidation behavior of both silicides
Fig. 6 summarizes the thicknesses of the oxidized region for both Yb-silicides during heat exposure in air-H2O and steam. The thickness of the oxidized region was defined as the penetration depth of the O signal (see Figs. 3 and 4) and decreased with increasing H2O partial pressure. These results implied that the oxidation of both silicides strongly depending on the partial pressure of oxygen.
Fig. 7 (a)-(d) show typical optical microscope images for both silicides after the test in steam. These
Conclusions
In the present study, the degradation behavior of two kinds of Yb–Si silicides with different compositions were examined in dry air, air-H2O, and steam. The effects of the phase composition and oxygen partial pressure on the oxidation behaviors of both silicides were examined, and the following conclusions were summarized:
- 1.
The oxidation behavior of the Yb–Si system strongly depended on its Si (Yb) content, which changed during oxidation because of the preferential oxidation of Yb in silicides.
CRediT authorship contribution statement
Toshihisa Miyazaki: Investigation, Writing - original draft. Syo Usami: Investigation. Yutaro Arai: Investigation, Data curation. Toru Tsunoura: Resources. Ryo Inoue: Conceptualization, Data curation, Writing - original draft, Writing - review & editing. Takuya Aoki: Resources. Ryuji Tamura: Resources. Yasuo Kogo: Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The authors would like to thank K. Sato and H. Yokogawa (NETZTH Japan), for their experimental support for TG-DTA in steam. A part of this work was conducted in the Research Hub for Advanced Nano Characterization, The University of Tokyo, supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The authors also thank Mr. S. Ohtsuka and Mr. M. Fukugawa of the University of Tokyo for their help in conducting the elemental analysis.
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