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TOP > Report & Column > The Forefront of Space Science > 2008 > Ceramics That Do Not Break Even When Cracked ~ Fiber-Reinforced Ceramics and Carbon Composite Materials

The Forefront of Space Science

Ceramics That Do Not Break Even When Cracked ~ Fiber-Reinforced Ceramics and Carbon Composite Materials
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Introduction

It is common knowledge that ceramics and charcoal (carbon) are hard and brittle. We are now studying reliable, fiber-reinforced composite materials that do not fracture even when nailed. The fact that the material can withstand being nailed shows that it can tolerate internal damages. Ceramics and carbon are so brittle that they cannot stand internal damage. The strength of structures made of materials prone to brittle fractures, such as ceramics and glass, diminishes with cracks or surface damage. Meanwhile, in materials with damage tolerance such as metal, a certain degree of surface or internal damage does not impact structural strength.

This paper will discuss methods to add damage tolerance to ceramics or carbon.

Method to add damage tolerance Ceramics and carbon boast excellent properties, such as high resistance to heat and chemicals, which are impossible to realize with metallic materials. These outstanding characteristics arise from the materialsĀEatomic-level micro structure and their atomic binding state. These properties (strong atomic binding force and undeformability), however, cause brittle fractures when destroyed. Now, I will take the case of silicon-carbide (SiC) composite material to reveal the method of adding damage-tolerance to ceramics.

SiC is a covalent crystal of a hardness third only to diamonds and boron carbide. In SiC, plastic deformation does not occur at room temperature. Single SiC, i.e., ceramics, is stable at more than 1,500 deg. C and widely used for cutting tools, etc. In order to add damage-tolerance utilizing SiCís stability, we have to ensure that the entire structure does not break down even if fractures occur inside or on the surface of the ceramics. One effective way to realize this is to make the fracture unit smaller and build the structure from a collection of units. As a result, even ceramics, which are prone to large variations in strength, become highly reliable materials with small variations in strength as a whole. Reducing the size of each fracture unit minimizes the effects of cracks or defects existing inside or on the surface of the unit, eventually raising the ceramicís strength. Previously, SiC- and carbon-made fibers, which are several µ? to dozens of µ? in diameter and boast high strength with large breaking elongation, have been developed by Japanese original technology. By using such fibers, even though individual fibers break in a brittle manner, a bundle of several thousand to tens of thousands of fibers does not fracture in a brittle manner.

Although we can make ropes or cloths with fibers, we cannot build structures with them. To build a structure, we need to solidify the textile with ceramics to create a composite material. Ways to solidify fiber-made textiles with ceramics include: (1) utilizing the reaction caused by gas; (2) dissolving resin, which has Si or C as its skeleton, at high temperature; and (3) dissolving metal (Si) to induce a reaction with the internal carbon. By producing composite materials using one or more of the above methods, we can obtain ceramics with damage tolerance.

Fig. 1 shows the deformation and fracture patterns of fiber-reinforced ceramics and ceramics. The vertical axis shows tension (i.e., stress) imposed per unit area while the horizontal axis shows deformation volume (i.e., distortion) in a dimensionless form. Comparing the two materials, ceramics shows high elasticity and strength, but fractures occur in a brittle manner and the fracture distortion is small. Meanwhile, fiber-reinforced ceramics shows inferior elasticity and strength compared to ceramics, but large fracture distortion. In addition, the fracture pattern arises from small cracks accumulating in the material until breakage. The ceramics (or matrix) used to solidify the fibers in fiber-reinforced ceramics shows the same brittle-fracture behavior as with ceramics. Nevertheless, damage tolerance can be retained if the material is designed so that the ceramic cracks do not sever the fibers. The resulting fracture cross section is shown in Fig. 2, with fibers protruding like a broom head. Without such a design, the fibers break up with the surrounding ceramics, resulting in a flat fracture surface causing a brittle fracture. One factor to determine the performance of composite ceramic materials is to design the boundary face between fiber and matrix to weaken adhesiveness and avoid fiber fractures. It is possible to create a material that is insensitive to notches. In other words, this is a composite ceramic material with maximum damage tolerance where the strength of a notched test piece is equivalent to that of one without notches.


Figure 1

Figure 1. Conceptual illustration of fracture patterns of ceramics and fiber-reinforced ceramics


Figure 2

Figure 2. Fractured surface of carbon-fiber-reinforced carbon, which is destroyed at 2,000 deg. C
Protruding fibers are shown.


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