Original Article
Incorporation of BN-coated carbon fibers into ZrB2/SiBCN ceramic composites and their ablation behavior

https://doi.org/10.1016/j.jeurceramsoc.2019.12.031Get rights and content

Highlights

  • Thin BN layer passivated carbon fibers and prevented them from reacting with the matrix.

  • Diffusion coefficient decreased, and the thermal expansion coefficient increased in comparison to ZrB2/SiBCN composites without carbon fibers.

  • ZrSiO4 formed from ZrO2 and SiO2 reaction in the central ablation region would prevent samples from further ablation-related damages.

Abstract

ZrB2/SiBCN composites containing carbon fibers coated with BN were prepared using combination of sol-gel and spark plasma sintering (SPS) techniques. Thermal shock and ablation behavior of these composites were thoroughly analyzed. Sintered composites contained ZrB2, SiC and BN(C) crystalline phases. Thin BN layer passivated carbon fibers and prevented them from reacting with the matrix. With the addition of carbon fiber, diffusion coefficient decreased, and the thermal expansion coefficient increased in comparison to ZrB2/SiBCN composites without carbon fibers. Ablation tests showed no crack formation after carbon fibers were added to the composites. Ablation center exhibited loose and porous network structure. ZrSiO4 formed from ZrO2 and SiO2 reaction in the central ablation region. Presence of ZrSiO4 prevented samples from further ablation-related damages.

Introduction

Ablation resistance is essential for high-temperature ceramics used in defense and aerospace industries, in which parts made from ceramic materials are often exposed to high-temperatures. Rapid temperature increase during high-temperature ablation can cause catastrophic damage to brittle materials and to weaken their thermal shock resistance [[1], [2], [3], [4]]. Therefore, thermal shock resistance is a critical parameter, which demonstrates structural integrity and intrinsic brittleness of a material in high-temperature environment [[5], [6], [7], [8]].

Carbon-fiber (CF) reinforced ceramics attracted significant attention over the last five decades because of their high strength to weight ratio, high rigidity, excellent mechanical properties and chemical resistance [9,10]. Such materials have great potential in applications related to automotive, civil engineering, aerospace, etc. CF-reinforced ceramics demonstrate improvements over conventional high-temperature ceramic materials (e.g. brittleness and low reliability) [11,12]. During more recent decades, SiC/SiBCN composites reinforced with CFs became frequently used as high-temperature structural materials because of their excellent thermal and mechanical strength, low density, chemical stability and stiffness in comparison to other commonly used metal and alloys [13,14]. Mechanical properties (e.g. fracture toughness) of CF-reinforced SiC/SiBCN composites can be controlled and enhanced by adjusting the properties of the interface between the matrix and CFs, which can help to relieve interfacial interactions and to improve toughening mechanisms through crack deflection, crack arrest, etc. [15,16]. Properties like these are usually achieved by coating fibers with thin protective layers (e.g. BN, SiC or carbon) using ex-situ chemical vapor deposition or liquid phase techniques. However, these techniques are expensive [17,18]. SiBCN ceramic composites strengthened by CF with and without SiC coating and prepared using precursor impregnation and pyrolysis (PIP) technique exhibited a non-brittle fracture behavior with excellent high-temperature flexural strength and superior creep resistance [19,20]. Hot pressed CF/SiBCN ceramic composites possess very good effective thermal shock resistance. However, their anti-ablative properties are unsatisfactory [21,22]. Our previous study demonstrated successful incorporation of high-temperature ZrB2 polymorph into a SiBCN matrix using a combination of spark plasma sintering and sol-gel process. Addition of ZrB2 enhanced mechanical properties of SiBCN as well as its densification. Ablation off these SiBCN-based ceramic composites was also significantly improved especially comparing to unmodified SiBCN material [[23], [24], [25]].

In this work, we added CFs to ZrB2/SiBCN composite matrix to reduce cost and weight of the this potential spacecraft composite without compromising material performance. Thin BN layer protected CF from reacting with the matrix. Comparison of ablation behavior and thermal shock of ZrB2/SiBCN with and without CFs helped us to establish a primary ablation mechanism.

Section snippets

BN coating on carbon fiber

Carbon fibers (purchased from National University of Defense Technology) were pretreated in acetone for the removal of the oxide scale and density up-regulation in a ceramic matrix. Boric acid and urea nitrogen at a ratio of 1:3 were mixed in deionized water and ethanol. After 3 times of liquid impregnation and ultrasonic concussion, carbon fibers were introduced into a tubular furnace at 1000 °C under N2 for 1 h.

Preparation of bulk ceramic composites and amorphous SiBCN powder

The SiBCN amorphous powder was first compounded by mechanical alloying with raw

Influence of BN coating

XRD of the CF/SZB15 and CF/BN/SZB15 composites sintered below 2000 °C at 40 MPa for 5 min demonstrated presence of ZrB2, BN(C) and SiC phases (see Fig. 1). Absence of ZrO2 suggests its reaction with B2O3 and carbon at these conditions. Minimum difference was observed between XRD patterns of CF/SZB15 and CF/BN/SZB15. Carbon fiber and BN phases were quite hard to identified from the XRD pattern.

Microstructure of the as-prepared ZrB2/SiBCN composites reinforced with CFs with and without BN coating

Conclusions

ZrB2/SiBCN composites reinforced with carbon fiber coated with BN coating were successfully prepared using combination of sol-gel and spark plasma sintering (SPS) methods. Thin BN layer protected carbon fiber from reacting with the matrix. Thermal shock resistance of the composites was noticeably enhanced as thermal diffusion coefficients increased, and thermal expansion coefficient decreased. Ablation properties of CF/BN/SZB15 composites were obtained by their exposure to the oxyacetylene

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.

Acknowledgments

The authors give thanks to the National Natural Science Foundation of China for support (NSFC, Grant no. 51802213). The authors also thank Richard M. Laine (Dept. Of Materials Science and Engineering, University of Michigan, USA) for the help of language use.

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