7

Corrosion of Composites

Materials

The whole is most always better than the sum of the parts.

ANONYMOUS

7.1 INTRODUCTION

Although the term composite historically meant any product made from a combination of two (or more) materials, the modern meaning is less broad in scope. In general, a composite is manufactured in an attempt to obtain the best properties of two materials or at least to capture a specific property of each material that is potentially better in the composite. It is also possible for the composite to have a particular property that neither component exhibited individually. According to Holmes and Just [7.1], a true composite is where distinct materials are combined in a nonrandom manner to produce overall structural characteristics superior to those of the individual components. Although, in a very broad sense, products such as glazed

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ceramic tile, enameled metal, and ceramic coated metal (e.g., thermal barrier coatings) could be considered composites, they will not be considered as such here. Only those materials where a substantial intermixing of the different materials exists on a microscopic scale will be considered composites.

The concept of composite materials is not a new idea and is definitely not limited to ceramics. Nature has provided us with several excellent examples of composite materials. Wood is a composite of cellulose fibers contained in a matrix of lignin. Bone, another example, is composed of the protein collagen and the mineral apatite. In all these materials, the result is a product that is lighter and stronger than either of the components individually. Because of this, they can be used in more severe environments, e.g., space exploration. A list of the more desirable properties of a composite is given in Table 7.1. In a very broad sense, all engineering materials are composites of one kind or another.

The matrix and the reinforcement, quite often fibrous, provide two different functions. The reinforcement is most often a discontinuous phase whether it be a fibrous material or a particulate material. It is important that the reinforcement be discontinuous, especially if it is a ceramic, so that cracks

TABLE 7.1 Desirable Properties of Composites

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will not be able to propagate through it. The matrix must not damage the reinforcement and it must transmit any stresses to the reinforcement. Thus the adhesion of the matrix to the reinforcement is of prime importance for mechanical integrity and is the region of greatest importance related to corrosion. Since it is necessary to have weak interfaces to maximize toughness (i.e., resistance to crack propagation), the development of optimum fiber/matrix interfaces is quite difficult. To obtain these optimum characteristics, it is sometimes required to coat the reinforcement fibers with various materials to obtain the proper debonding, sliding, and/ or reaction characteristics. Fibers that do not debond do not enhance toughening and lead only to increased brittle fracture of the composite [7.2–7.7].

A recent development in composites is that of a nanosized second phase or reinforcement material. The second phase particles are generally less than 300 nm and are present in amounts equal to 1–30 vol.%. These new composites unfortunately have been called nanocomposites.

Before going into the specifics of corrosion of composite materials, a few words must be said about those materials that have been called cermets. Historically, the term cermet was derived to cover those materials composed of cobalt-bonded tungsten carbide and used as cutting tools. Since cermets contain both ceramics and metals, some confusion has existed in the literature as to an exact meaning. The term, however, has been used to cover a broad list of materials. It appears that the ceramic community confines cermets to essentially cutting tool materials, whatever the matrix or reinforcement, whereas the metals community confines cermets to only those materials with a metal matrix. Since the broader concept of composites includes those materials called cermets, only the term composite will be used in the discussion below.

The actual corrosion of composite materials quite often begins with reaction of the reinforcement material and especially with any interface material (called the interphase) used to coat the reinforcement for debonding. One property

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that exacerbates this is a mismatch in thermal expansion coefficients between the reinforcement and the matrix, leading to microcracks. These microcracks allow the ingress of corrosive gases (e.g., oxygen). Courtright [7.8] has given the value of 10-12 g O2/ cm sec for the limit of oxygen ingress that causes nonoxide fiber deterioration. Microcracks are also quite often a product of sample preparation techniques, and thus great care must be used in cutting and grinding/polishing samples for testing. If the composite is cut or machined, any exposed fiber reinforcement will be susceptible to attack by the environment. Because of this inherent problem, protective coatings are often applied to the exterior surfaces. Actually, the whole corrosion process of composite materials is not unlike that of other polyphase ceramic materials where the grain boundary phase is the first to corrode. A complete understanding of all the phases that make up the microstructure of the composite must also be known for an accurate interpretation of any corrosion. For example, Munson and Jenkins [7.9] reported that their samples were actually attacked internally by molten metal from a small amount of free aluminum present as a residue during the manufacture of Dimox™* (a melt-infiltrated alumina). Actually, a large amount of the literature on composites is concerned with an evaluation of the internal reactions that take place among the various reinforcement, interphase, and matrix materials. The time-dependent loss of strength due to the corrosive nature of moist environments at room temperature is a major concern for composites containing glass or glass-ceramics as either the matrix or the reinforcement [7.10]. As temperatures are increased, the concern shifts toward oxidation problems associated with nonoxide materials. See the discussions in Chap. 5, Sec. 5.2.2, Nitrides and Carbides, and Chapter 8, Properties and Corrosion, for more details of oxidation and its effects upon the properties of nonoxides.

* DIMOX™ (directed metal oxidation) is the name given to composites manufactured by a process developed by Lanxide Corp., Newark, DE in 1986.

Copyright © 2004 by Marcel Dekker, Inc.