Fabrication of short SiC fiber reinforced SiC matrix composites with high fiber volume fraction

https://doi.org/10.1016/j.fusengdes.2015.12.060Get rights and content

Abstract

Short fiber reinforced SiC/SiC composites with high fiber volume fraction were fabricated by NITE process. The effects of fiber length and sintering condition on microstructure and mechanical properties were investigated. Although, high sintering temperature promotes the densification of SiC matrix but leads to the degradation of F/M interface. The flexural strength of SF-SiC/SiC with long fiber length shows higher flexural strength compared to those of SF-SiC/SiC with short fiber length. SF-SiC/SiC composites with complicated shape have been successfully fabricated by pseudo-HIP.

Introduction

Continuous SiC fiber reinforced SiC matrix (SiC/SiC) composites are one of promising structural materials for nuclear energy and aerospace application due to their excellent properties such as light weight, high strength, reliability under high temperature, low after heat, low radioactivity and so on [1]. In recent, it is encouraged to use SiC/SiC components by the commercialization of high crystalline and high purity SiC fibers as well as the development of their innovative fabrication process, like Nano Infiltration and Transient Eutectic (NITE) process [2], [3], [4]. NITE-SiC/SiC composites are consisted of well densified SiC matrix and well aligned continuous SiC fiber with thin pyrocarbon (PyC) coating as fiber/matrix (F/M) interface. Due to its dens microstructure and high mechanical properties, it is considered as one of nuclear grade SiC/SiC composites [5].

In the other hand, we must carefully discuss on the anisotropic properties of SiC/SiC composites for their practical use. Because, the thermo-mechanical properties of continuous fiber reinforced composites strongly depend on fiber reinforcing architecture. In case of fusion reactor environments, thermal gradient inside the material occurred by a high heat flux. This thermal gradient gives differential swelling under irradiation, as well as differential thermal expansion, resulting in the complicated stress field inside the material [6], [7].

Thus, the evaluation of off-axial mechanical properties of SiC/SiC composites is one of the important technical issues for fusion application. Actually, it is well known that the specimen with unidirectional fiber (UD) reinforcing architecture shows quite low strength in non-fiber direction.

In the other hand, short fiber reinforced composites, there is no need to consider about the anisotropy due to their randomly aligned fiber architecture. Moreover, it provides many advantages, such as high productivity of practical components with large sizes and complicated shapes, shorter process time and lower cost.

Although, there are many advantages of short fiber as mentioned above, but only limited data concerned on short fiber (SF) SiC/SiC composites have been reported [8]. The production of homogeneous SF-SiC/SiC composites with high fiber volume fraction (Vf) is quite difficult, because of the entanglement of short fibers. As a result, Vf of SF-SiC or C/SiC was usually under 40% and it may be difficult to obtain sufficient effects of fiber reinforcing [8], [9], [10].

The purpose of this work is the process development of high Vf SF-SiC/SiC composites based on NITE process. The effects of fiber length and sintering conditions on the microstructure and mechanical properties of SF-SiC/SiC were investigated. Near net shaping of SF-SiC/SiC composites with complicated shape was also performed by pseudo-hot isostatic press (P-HIP) method.

Section snippets

Experimental procedures

Highly crystallized SiC fiber, Cef-NITE™ (GUNZE LIMITED, Japan), has been used as a reinforcing fiber. Fiber diameter is approximately 10 μm. Number of fiber filaments per bundle is 800. As F/M interface, thin PyC coat (thickness < 500 nm) has been formed on fiber surface by conventional chemical vapor deposition (CVD) method.

SiC powder, small amount of oxide additives and SiC based polymer binder were used as raw materials for SiC matrix. In this study, cut prepreg sheet has been used for the

Experimental results and discussions

Fig. 3 shows the appearance of SF2 specimens as an example of SF-SiC/SiC composites fabricated in this work. The unique architecture of randomly aligned short fibers, which are prepared by cutting of prepreg sheet, can be confirmed from the naked eye observation of plate surface.

Summary

Based on NITE process, the fabrication method for SF-SiC/SiC composites with high Vf has been discussed in this study. We have succeeded in fabricating of SF-SiC/SiC composites with High Vf (approx. 45 vol%) by using of cut prepreg sheets. It was revealed that high sintering temperature promotes the densification of SiC matrix but leads to the degradation of F/M interface. SF-SiC/SiC composites with long fiber length shows higher flexural strength (135 MPa) than those of shorter one. It was also

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