Improving the high-temperature oxidation resistance of Zr 2 Al 3 C 4 by silicon pack cementation
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Y.W. Bao, M.S. Li, J.Y. Wang, and Y.C. Zhoua) Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China (Received 18 February 2008; accepted 16 May 2008)
Silicon pack cementation has been applied to improve the oxidation resistance of Zr2Al3C4. The Si pack coating is mainly composed of an inner layer of ZrSi2 and SiC and an outer layer of Al2O3 at 1200 °C. The growth kinetics of silicide coating at 1000–1200 °C obey a parabolic law with an activation energy of 110.3 ± 16.7 kJ/mol, which is controlled by inward diffusion of Si and outward diffusion of Al. Compared with Zr2Al3C4, the oxidation resistance of siliconized Zr2Al3C4 is greatly improved due to the formation of protective oxidation products, aluminosilicate glass, mullite, and ZrSiO4.
I. INTRODUCTION
Zr2Al3C4 is a recently developed ternary layered carbide, which has superior strength, specific stiffness, toughness, and oxidation resistance to ZrC.1–4 Zr2Al3C4 also exhibits good electrical and thermal conductivities, and its thermal expansion coefficient is close to that of ␣-Al2O3.1,3 Typical properties of Zr2Al3C4 are summarized in Table I. In addition, the high degrees of stiffness that remain at elevated temperatures endow Zr2Al3C4 with a high temperature and ultrahigh temperature material (Young’s modulus at 1650 °C is 262 GPa, which is about 73.5% of that at room temperature).5 The oxidation resistance of Zr2Al3C4, however, still cannot meet the requirements for its applications in high temperature and oxidizing atmosphere. For example, the oxidation kinetics of Zr2Al3C4 changes from a parabolic law below 800 °C to a linear law at higher temperatures. In addition, no continuous and protective Al2O3 scales form on Zr2Al3C4 substrate.4 It is well known that pack cementation method has been widely used to improve oxidation resistance of metals, alloys, and C/C composites because of its simplicity, practicality, and low cost.6–9 Recently, pack cementation was used to improve the oxidation resistance of Ti3SiC2based material.10,11 Liu et al.10 prepared a silicide coating on a Ti3SiC2-based ceramic by Si pack cementation. a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0285 J. Mater. Res., Vol. 23, No. 8, Aug 2008
The oxidation resistance of Ti3SiC2 was enhanced, and the parabolic rate constants were reduced by about 2– 3 orders of magnitude due to the protection of silicide coating. Li et al.11 used Al–La cocementation to improve the oxidation resistance of Ti3SiC2-based ceramic. Compared with Ti3SiC2, the parabolic rate constants of the cemented Ti3SiC2 were decreased by two orders of magnitude. Pack cementation, however, has never been used to improve the oxidation resistance of ternary aluminum carbides. For the composition of Zr2Al3C4, formation of zirconium silicides and silicon carbide by Si pack cementation is expected, where zirconium silicides and SiC have good oxidation resistance.12,13 Wei and Wu12 prepared a multilayer co
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