Elsevier

Materials & Design

Volume 90, 15 January 2016, Pages 942-948
Materials & Design

Effect of temperature on the mechanical behavior of mullite fibrous ceramics with a 3D skeleton structure prepared by molding method

https://doi.org/10.1016/j.matdes.2015.11.043Get rights and content

Highlights

  • Mullite fibrous ceramics with a 3D skeleton structure was prepared by molding method.

  • The mullite fibrous ceramics possessed a low conductivity and a good elasticity.

  • The products sintered at 1500 °C presented mechanical performance degradation.

  • The degradation was caused by the grain growth of the mullite fiber.

Abstract

Fibrous ceramics with the mullite fibers as the matrix and silicon resin as the binder was fabricated by a molding method. The effect of sintering temperature, especially 1500 °C, on the properties of the products, such as porosity, microstructure, mechanical behavior and fracture mechanism, were investigated. This fibrous ceramics exhibited low densities (0.425–0.441 g/cm3), high porosities (81.6–82.3%), low thermal conductivities (0.083–0.089 W/mK), relatively high compressive strength (1.21–1.58 MPa), and high rebound-resilience ratios (70–85%), which made it a promising high-temperature insulation materials. When the sintering temperature increased above 1300 °C, the fracture behavior of the products was mainly attributed to the fibers break and the binders fracture. As the sintering temperature increased to 1500 °C, the grains in the mullite fibers grew up, and the strength of fibers decreased, finally leading to the degradation of the sample mechanical performance. What's important, the compressive strength of the products sintered at 1500 °C was 1.36 MPa and the elastic strain could reach 3.2%, which means that the products can still be put to use in the high-temperature fields.

Introduction

With the rapid development of the aerospace and energy technology, the elastic thermal insulation materials have drawn considerable attention. [1], [2], [3], [4], [5] Fibrous materials with 3D skeleton structures are designed to overcome the brittleness of monolithic ceramics while maintaining their advantageous of excellent mechanical properties. [6], [7], [8], [9], [10], [11] In addition, oxide fibrous ceramics, offering a good thermal and chemical stability against corrosive and oxidative environments, could be promising candidates for use in aerospace fields. [12], [13], [14], [15], [16].

As an inorganic oxide fiber, mullite fibers possessed all of the advanced characteristics of mullite, such as advanced high-temperature stability, low thermal conductivity, good thermal shock resistance and creep resistance, and so on. [17], [18], [19], [20] Therefore, they were extensively employed as the high temperature structure materials. Mullite fibers could be divided into two types: continuous mullite fibers and short mullite fibers. The continuous mullite fibers were usually used as the toughening phase in the ceramic matrix composites (CMCs). [21], [22] As for the CMCs, the strength tested along the continuous fibers could be enhanced. [23] However, the strength tested across the continuous fibers was always ultra-low. [24] Compared with the continuous mullite fibers, short mullite fibers were more easily to be synthesized and were more economical. [25] What's more, the short mullite fibers could form isotropic structural frameworks with isotropic strength and some other good mechanical/high-temperature properties. [26] Therefore, the short mullite fibers are regards as an attractive alternative matrix of the fibrous ceramic composites with 3D skeleton structure for the high-temperature elastic heat-insulation application. In recent years, many efforts have been made to develop high-performance oxide fibrous ceramic composites, using TBA-based gel-casting, freezing-casting, sol–gel, infiltration, molding, and their combined processes. Hereinto, infiltration and molding are believed to be ideal processes to fabricate three dimensional (3D) composites because they can realize the near-net shape forming of complex shape by low cost and simple equipment.

According to the previous studies, the mullite fibrous ceramics with a 3D skeleton structure can be successfully fabricated through the above methods. [12], [13], [16] The mullite fibrous ceramics exhibited excellent thermal stability and mechanical behavior when the sintering temperature was below 1500 °C. The fracture behavior of this material was attributed to the debonding of fiber/binder, binder fracture and fibers break. However, the densities of the products in the previous work were very high (about 0.7–0.8 g/cm3), which could not meet the lightweight demand in some engineering fields. Besides, when the sintering temperature was higher than 1500 °C, the grains in the mullite fibers grew up which had a great influence on the mechanical behavior of the products and this phenomenon was rarely investigated. According to the reports, 1500 °C is a very important operating temperature, in high-temperature industrial and aerospace applications. [27], [28], [29] Therefore, it is very necessary to investigate whether mullite fibrous ceramics can be put to use after heated at 1500 °C.

In this paper, we successfully synthesized a lightweight fibrous ceramics by the molding method, in which the short mullite fiber was used as the matrix and the MK resin (a type of silicon resin) was used as the binder. The green bodies were sintered at 1300, 1400 and 1500 °C in air atmosphere respectively. The influences of the sintering temperature (especially 1500 °C) on the physical, mechanical and thermal properties were investigated. In addition, the influence of the grain growth of mullite fiber sintered at 1500 °C on the fracture behavior of the products was also analyzed.

Section snippets

Materials

Commercially available short polycrystalline mullite fibers (99.5 wt.%, Zhejiang Hongda Crystal Fiber Co., Ltd., China) and MK resin (silicon resin, Wacker Chemical Co., Ltd., Germany) were used as the starting materials. It should be noted that, compared with the silica sol used in our previous work, the MK resin used in this experiment is more affordable and with fewer impurities. The binder was prepared by mixing the MK resin with the iso-propyl alcohol (IPA). All chemicals used in this study

Results and discussion

The sintering temperature plays an important role in the states and properties of both the fiber and silica phase, which will influence the properties of the products. For example, the spherical silica possesses higher melting points than that of the silica sols. The melting point of the binder will greatly affect the bonding strength of the fiber/binder of the sample sintered at the same temperature. In this paper, the effect of sintering temperature (especially 1500 °C) on the properties of

Conclusion

Fibrous ceramics with a 3D skeleton structure were prepared using the mullite fiber as the matrix and MK resin as the binder through the molding method. The unique composites exhibiting low densities of 0.425–0.441 g/cm3, high porosities of 81.6–82.3%, low thermal conductivities of 0.083–0.089 W/mK, relatively high compressive strength of 1.21–1.58 MPa, and high rebound-resilience of 70–85%, were promising high-temperature insulation materials. When the sintering temperature was higher than 1300 

Acknowledgment

This study is supported by the National Natural Science Foundation of China (Project No. 51272171 and No. 51372164) for financial support.

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