Effect of a fiber coating on the fabrication of fiber reinforced metal-matrix composites

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Abstract

The surface state of the reinforcements for composites plays an important role in the fabrication process and in determining the properties of metal-matrix composites. This article explores the effect of a reinforcement coating on the fabrication process and the properties of metal-matrix composites. For a carbon-fiber-reinforced aluminum composite system, it is difficult to obtain a uniform distribution of the carbon fibers in the aluminum matrix and to control the fabrication process parameters because of poor wettability. For example, the fabrication pressure must be set properly, otherwise a higher pressure will cause fiber fracture, whilst a lower pressure will cause holes in the matrix. When coating the fibers with SiC by a polycarbosilane solution process, the wettability between the fibers and the aluminum is improved. Consequently, the fabrication process can be controlled easily and the properties of the composites improved substantially.

Introduction

Metal-matrix composites (MMCs) with the properties of their components have the most potential application as structural and functional materials in future. Unfortunately, the application of MMCs in practice is limited by the high cost of the fabrication process and associated equipment as well as of the raw materials such as fibers. The squeeze-casting process, as is known, behaves with the characteristics of high productivity and providing high-quality products. If the process is used to fabricate MMCs, the cost caused by the fabrication process will decrease obviously. On the other hand, the contact time between the reinforcements and the molten metal at high temperature has to be short in order to control harmful reactions at the interface. The squeeze casting process is fast. Therefore, the process will have to be studied widely for fabricating MMCs [1].

There are few published papers about the casting process of carbon-fiber-reinforced aluminum composites. Sample and co-workers carried out the pressure casting of carbon-fiber-reinforced aluminum composites, which had an ultimate tensile strength of 310 MPa and a fiber fraction of 4–50 vol.% [2]. Cheng reported that the properties can be improved by the addition of SiC particles to the fiber preform, where the fiber volume fraction is controlled to within 25–59 vol.%. The ultimate tensile strength of low-modulus carbon-fiber-reinforced aluminum composites achieved to 412 MPa and for high-modulus carbon-fiber-composites the strength reached 811 MPa [3]. The reason for the large difference for the low- and high-modulus fiber system is that the high modulus had low reactivity and the low modulus fiber had high reactivity, with the molten aluminum. Therefore the control of harmful reactions at the interface is very important in the molten casting process. This paper focuses on the coating on carbon fibers and on the process parameters and properties of the composites as well as on the relationship between them.

Section snippets

Experimental procedure

PAN-based carbon fibers, having a density of 1.76 g cm−3, a strength of 3000–4000 MPa, a Young's modulus of 220 GPa and a mean diameter of 6.5 μm, were produced in China and used in the present research.

Modification of the carbon fibers was carried out to investigate the behavior of the coating and its effect on the interfaces and the properties of the composites. The carbon fibers were coated with SiC by polycarbosilane (PCS) solution processes [4]. The coating thickness was controlled to

The microstructure of the composites

Optical observation of the composites showed that the distribution of the carbon fibers in the aluminum depended on the preform, the temperature of the molten aluminum, the die speed and the pressure. Under the appropriate temperature and pressure, the die speed must also be controlled to obtain a uniform distribution of the carbon fibers in the aluminum. Fig. 1 shows that the distribution of the fibers in the aluminum for the same temperatures and pressure but for different die speeds. When

Conclusions

(1) The SiC coating on carbon fibers improved the distribution of the fibers in the matrix and the operation of the fabrication process.

(2) Based on the device used, the optimized temperature and penetration pressure can be estimated to obtain a CF/A1 composites with high strength.

Acknowledgements

The main work was supported financially by the National Nature Science Foundation of China and the National Key Laboratory of Corrosion Science, Institute of Corrosion and Protection for Metals, Academia Sinica.

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