In situ synthesis of ZrB₂–MoSi₂ platelet composites: Reactive hot pressing process, microstructure and mechanical properties

Abstract

Using elemental Zr, B, Mo and Si as raw materials, partially textured ZrB₂–MoSi₂ composites with in situ platelet ZrB₂ grains were fabricated by reactive hot pressing. Synthesized by a combustion reaction, the in situ formed ZrB₂ phase had unique characteristics to grow up to platelet grains, on the surroundings of Mo–Si–B ternary liquid phase at high temperature. Mechanical properties were dependent on grain size and aspect ratio of the ZrB₂ platelets. The rotation and realignment of the platelet ZrB₂ grains under hot pressing led to a partially textured microstructure, which showed anisotropic mechanical properties on different directions of the as-sintered samples. A roadmap of the reaction process, microstructure evolution and texture formation was given to describe the preparation of partially textured ZrB₂–MoSi₂ composites by reactive hot pressing.

Introduction

ZrB₂-based composite ceramics, as a kind of typical ultra high-temperature ceramics (UHTCs), has attracted much attention in recent years [1]. Although ZrB₂ has a unique combination of excellent properties, yet densification and poor oxidation resistance are major shortcomings which hinder its actual applications [2], [3]. Besides, poor thermal shock resistance is another disadvantage which becomes more salient nowadays [4], [5]. Combining another phase, such as SiC [6], [7], [8], [9] and MoSi₂ [10], ZrB₂-based composite ceramics can be obtained. In ZrB₂-based composite ceramics, not only the above shortcomings can be overcome, but also some performances of the composites can be improved because of the excellent properties of the second phase itself, which make ZrB₂-based composites suitable for practical and potential applications for the hypersonic aerospace vehicles and re-usable atmospheric re-entry vehicles [11], [12].

ZrB₂–MoSi₂ composite is an important kind of ZrB₂-based UHTCs [10], [13], [14]. MoSi₂, another kind of refractory ceramics, is widely used as elevated-temperature structural material owing to its moderate density of 6.24 g/cm³, a high melting point (2030 °C) and excellent oxidation resistance [15], [16]. When using MoSi₂ as a second phase to prepare ZrB₂-based ceramics, densified composite can be obtained at a lower temperature (1750–1850 °C [17], [18], [19]) compared to monolithic ZrB₂ ceramics (2100–2300 °C [10]), due to the plastic character of MoSi₂ which makes it fill the voids of ZrB₂ skeleton to form a pore-free structure. Besides, the oxidation resistance of ZrB₂-based composites can also be improved by the help of the MoSi₂.

ZrB₂–MoSi₂ composite can be obtained by different densification techniques. Sciti et al. have prepared ZrB₂–MoSi₂ composites by pressureless sintering (PLS) [13], [17], [20], hot pressing (HP) [14], [17] and spark plasma sintering (SPS) [14], [17], [21]. On the other hand, Wu et al. used the method of reactive hot pressing (RHP) [18]. It should be noted that when using RHP, ZrB₂–MoSi₂ composites with in situ elongated ZrB₂ grains can be obtained, whose microstructure was different from those prepared by non-reactive process (PLS, HP and SPS) and the mechanical properties could be improved. Furthermore, in situ platelet-reinforced and textured ZrB₂-based ultra high temperature ceramics are prepared by Liu et al. via RHP and successive hot-forging (HF), and the flexural strength after hot-forging have been improved by 52% compared with that before hot-forging, indicating that the platelet ZrB₂ grains is beneficial for the properties of the composites [22]. In situ synthesis is a regular method to prepare boride composites [23], [24], and the appearance of in situ formed platelet ZrB₂ grains in the ZrB₂-based composites is an interesting phenomenon.

Up to now, the growth mechanism of the platelet ZrB₂ grains in the RHPed ceramics is still not clear. Besides, the relationship between the mechanical properties and the microstructure, especially the effect of the aspect ratio of the ZrB₂ platelet grains, also need further investigation. So in the present paper, the reaction process of the Zr–B–Mo–Si system was investigated to make clear the mechanism of the platelet grains growth. The mechanical properties of the obtained partially textured composites with different amounts of MoSi₂ and holding time were studied. Meanwhile, properties on different surfaces of the samples were also discussed.