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TABLE 7.2 Acronyms Used in the Discussion of Composites

With the advancement of the development of composites, there is an increasing number of acronyms with which one must contend. To aid the reader, a list is given in Table 7.2 of the most common acronyms.

7.2 REINFORCEMENT

7.2.1 Fibers

Various types of materials have been used as the fibrous reinforcement. These include various glasses, metals, oxides,

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nitrides, and carbides either in the amorphous or crystalline state. The surface chemistry and morphology of fibers is very important in determining their adherence to the matrix. Fiber internal structure and morphology determines the mechanical strength. A tremendous amount of literature is available that discusses the degradation of mechanical properties as temperatures are increased in various atmospheres; however, there is very little interpretation of any corrosion mechanisms that may be involved. Although many composites are classified as continuous-fiber-reinforced, some composites contain fibers that are actually not continuous but of a high aspect ratio (i.e., length-to-width). The actual matrix material will determine the aspect ratio required to obtain a certain set of properties. Thus the term “high aspect ratio” is a relative term.

Boron fibers can generally be heated in air to temperatures of about 500°C without major strength deterioration. Above 500°C, the oxide that formed at lower temperatures becomes fluid increasing the oxidation rate and drastically reducing the strength [7.11]. Galasso [7.11] discussed the benefits of coating boron fibers with either SiC or by nitriding the surface. The SiC coating was more protective than the nitride with strength retention even after 1000 hr at 600°C in air. Boron carbide

(B4C) is stable to 1090°C in an oxidizing atmosphere, whereas boron nitride is stable to only 850°C.

Carbon or graphite fibers have been used since the early 1970s as reinforcement for composites. Strength loss due to oxidation occurs at temperatures above 500°C in air. An interesting structural feature of carbon fibers is that they have a relatively large negative axial thermal expansion coefficient.

Glass fibers generally are used as reinforcement for composites that are to be used at low temperatures (i.e., <500°C) due to the softening of glasses at elevated temperatures. These composites are generally of the polymer matrix type and are used for marine or at least moist environments. It is well known that glass is attacked by moist environments with the specific mechanism dependent upon

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the pH (see Chap. 6). It has been shown by Metcalfe and

Schmitz [7.12] that borosilicate glass fibers when exposed to moist ambient environments developed surface tensile stresses caused by exchange of alkali for hydrogen sufficient to cause failure.

A large portion of the CMC today contains SiC fiber reinforcement. This is mainly due to the excellent properties of SiC—low reactivity to many matrix materials, its strength at elevated temperatures, and its oxidation resistance. It is this latter property (i.e., oxidation resistance) that generally causes deterioration in these materials. SiC will oxidize readily when heated to temperatures greater than 1000°C. As discussed in

Chap. 5, Silicon Carbide, page 223, at low partial pressures of oxygen, active corrosion takes place with the formation of gaseous products of CO and SiO. At higher partial pressures, passive oxidation occurs with the formation of CO and SiO2 that may be protective if cracks do not form. The formation of cracks is dependent upon the heat treatment and whether the oxide layer is crystalline or amorphous. These reactions generally result in the decrease of fiber strength. Nicalon™ fiber*, being formed by the pyrolysis of organometallics, actually contains some remnant oxygen (~9%) and carbon

(~11%) that will affect the subsequent oxidation of the fiber. Two different grades of Nicalon™ fiber have been examined by various investigators [7.13–7.15]. Clark et al. [7.13] reported these fibers to exhibit weight losses of 13% and 33% after being treated in argon at 1400°C. Both grades of fiber gained weight (on the order of 2–3%) when treated in flowing wet air at 1000°C, 1200°C, and 1400°C. As-received Nicalon™ fibers have protective sizing (i.e., polyvinyl acetate) on their surfaces. When heated in air, this sizing will burn off at temperatures between 250°C and 500°C. At temperatures above about 1250°C, the SiCxOy amorphous phase contained in these fibers decomposed to SiO and CO [7.16].

* Nicalon™, Nippon Carbon Co., Tokyo, Japan.

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Titanium nitride (TiN) resists attack from iron or nickel aluminides better than does SiC and thus is a better reinforcement for these metal alloy matrix composites [7.17].

7.2.2 Fiber Coatings or Interphases

Protective coatings (also called interphases) such as graphite or BN, in addition to providing proper debonding and pullout [7.18,7.19], are used to provide some degree of oxidation resistance [7.20,7.21] for fibers such as SiC. Bender et al. [7.21] concluded that the BN protects the SiC fiber from the matrix since BN will not react with SiO2, which is generally present on the surface of the fibers. Boron nitride-coated mullite, carbon, and SiC fibers were tested in a mullite matrix with varying degrees of success by Singh and Brun [7.22].

Boron nitride-coated SiC fibers have shown a slight improvement over carbon-coated fibers with an increase of about 100–200°C in composite embrittlement (see Sec. 7.3 for a discussion concerning embrittlement) temperatures [7.23]. Since some matrices are grown in situ, techniques to coat fibers become problematic. A combination coating of BN and SiC was developed by Fareed et al. [7.24] to eliminate the undesirable reaction of molten aluminum in contact with Nicalon™ fibers forming alumina and aluminum carbide during the directed metal oxidation method (at 900–1000°C) of forming an alumina matrix. When used alone as a coating, BN oxidation inhibited complete oxidation of the aluminum. In combination with SiC, however, Fareed et al. believed that any oxidation of BN led to the formation of boria glass that acted as a sealant to any microcracks, thus minimizing oxygen ingress and protection of the composite. The SiC outer coating protected the BN inner coating during growth of the matrix. Ogbuji [7.25] reported that the BN first oxidized to B2O3, which then dissolved some of the SiC fiber and matrix forming a borosilicate liquid. If any moisture were present, the boria may be volatilized by hydrolysis releasing B(OH)4 gas. This reaction resulted in a silica residue that cemented the fibers together embrittling the composite.

Copyright © 2004 by Marcel Dekker, Inc.