316

Chapter 7

may occur. French commented that reaction (7.11) was not appreciable below 1400°C.

One of the early investigations into improving the oxidation of carbides was that by Accountius et al. [7.75]. They attempted to form glassy protective layers on TiC, B4C, and SiC.

7.4 METAL MATRIX COMPOSITES

Aluminum is probably the most common metal used for the production of metal matrix composites. This is due mostly to its low density, excellent mechanical properties, and low melting point that aids in fabrication. The addition of zinc creates an alloy with improved strength, wear resistance, and corrosion resistance. Since aluminum has a relatively low melting point, other metals have been used as the matrix when increased service temperatures are required. See Table 7.3 for a list of some of the metals used and their service temperature limits.

The stress corrosion cracking in a 3.5% NaCl solution of a pure aluminum matrix composite reinforced by alumina borate whiskers (Al18B4O33) was studied by Hu et al. [7.76]. Crack propagation was predominantly along the whisker/matrix interface. This was attributed by Hu et al. to dissolution of the matrix surrounding the whiskers as evidenced by pits that formed at the crack tips.

A SiC-coated graphite fiber/Al alloy matrix composite was developed to overcome the reaction of graphite with aluminum

TABLE 7.3 Matrix Metals and Their Upper Limits of Temperature

Service

Copyright © 2004 by Marcel Dekker, Inc.

Corrosion of Composites Materials

317

above 500°C forming A14C3 [7.77]. This phase formed as hexagonal plates with subsequent degradation of fiber strength. In addition, Al4C3 is hygroscopic causing further deterioration. A silicon enrichment on the outer surface of the fibers inhibited the formation of Al4C3; however, it allowed improvements of only short duration.

Although there is not much on corrosion, the book by Taya and Arsenault [7.78] contains a lot of information about the properties and behavior of MMC.

Galvanic corrosion (see Chap 2, page 25 for a discussion on galvanic corrosion) between the matrix metal and the reinforcement fibers can occur, especially when in contact with aqueous solutions. This has been reported by Trzaskoma [7.79] for magnesium alloy matrix-graphite fiber composites.

The seawater corrosion of SiC/Al was found to be more resistant than graphite/Al by Aylor and Kain [7.80]. This was attributed to a lack of a galvanic driving force between the SiC and the aluminum matrix, although both composites exhibited similar mechanisms of corrosion—essentially pitting of the metal matrix around the reinforcement material.

The reactions of hydrogen and SiC reinforced titanium-based matrix composites have been described by Nelson [7.37]. Since these reactions are ones involving various metallic phases rather than the SiC fiber or the interface, the details will not be given here. However, as reported by Nelson, the solubility of hydrogen in metals was problematic and, in some cases, formed metal hydrides (generally at higher temperatures) that led to mechanical degradation.

The stability of TiN toward reaction with various metals such as iron and nickel aluminides has been reported to be better than SiC by Nolan et al. [7.81]. Thus TiN would be the better choice as the reinforcement for these metal matrix composites.

Since the intermetallic Niand Ti-aluminides have service temperatures limited to about 1200°C, molybdenum disilicide has been investigated for applications where greater temperatures may be reached. One of the major drawbacks of

Copyright © 2004 by Marcel Dekker, Inc.

318

Chapter 7

MoSi2 is its oxidation resistance. Cook et al. [7.82] investigated the incorporation of 30 vol.% TiB2, ZrB2 HfB2, and SiC as a reinforcement in hopes of developing a composite of greater oxidation resistance than the base MoSi2. Specimen were exposed to isothermal testing at 800°C, 1200°C, 1400°C, and 1500°C for 24 hr in air, in addition to a thermal cycle consisting of 55 min at 1200°C or 1500°C and then 5-min ambient cooling with subsequent reheating. All the boride-containing materials exhibited a greater deterioration than the silicon carbide-containing composite, although none exhibited a greater oxidation resistance than the base MoSi2. See Sec. 5.2.3 on borides for a discussion of the oxidation of these materials.

Although not generally thought of as metal matrix composites, a relatively new class of materials called fibrous monolithic ceramics [7.83] actually may contain a metal as the matrix that surrounds cells of a fibrous polycrystalline ceramic. One example of such a material investigated by Baskaran et al. [7.84] contained fibrous polycrystalline alumina cells surrounded by nickel. The nickel cell boundary thickness varied from 1 to about 15 µm. Oxidation at 1200°C for 10 hr initially formed NiO that subsequently reacted with the alumina forming NiAl2O4. The formation of the aluminate was thought to provide protection toward additional oxidation.

7.5 POLYMER MATRIX COMPOSITES

Two publications by ASTM discuss the environmental effects upon polymeric composites [7.85,7.86]. The largest amount of composites produced is probably of this type reinforced with glass fibers, called glass-reinforced plastics, polymers, or polyesters (GRP). Degradation in aqueous environments generally occurs by fiber/matrix debonding. Since glass fibers are attacked by moisture, which drastically reduces their strength, glass fibers are given a protective coating.

Graphite/carbon fiber/epoxy composites (CFRP) have seen some recent use in marine environments. In many cases, they are generally used in contact with metals. In a seawater

Copyright © 2004 by Marcel Dekker, Inc.

Corrosion of Composites Materials

319

environment, the graphite fibers act as the cathode for accelerated galvanic corrosion of the metals.

Electrochemical impedance spectroscopy was used by Wall et al. [7.87] to monitor the damage in graphite fiber/ bismaleimide composites in contact with aluminum, steel, copper, and titanium immersed into aerated 3.5 wt.% NaCl solution. Decomposition. of the bismaleimide polymer was thought to occur by the action of hydroxyl ions, which break imide linkages. The production of hydroxyl ions occurred through the following reaction:

(7.12)

at the surface of the graphite fibers. They concluded that the corrosion concentrated at the fiber/matrix interface was caused by cathodic polarization and was dependent upon the overpotential and the cathodic reaction rate. Oxidation of the matrix and fibers was thought to be the cause of ablation of the composite.

Aylor [7.88] reported increased galvanic action (i.e., initial current level) with increased amounts of fiber exposure for a graphite fiber/epoxy composite in contact with either HY80 steel or nickel aluminum bronze subjected to seawater at ambient temperature for 180 days. Even when no fibers were exposed to the environment galvanic corrosion occurred. This phenomenon was attributed by Aylor to the absorption of moisture through the epoxy to the fibers. The galvanic current determined during the tests was found to display several distinct regions. These have been identified by Aylor as:

Region I—activation of surface Region II—film formation

Region III—reduction of active surface areas

Region IV—buildup of calcareous deposit on composite

These regions were attributed to localized differences in active anodic and cathodic areas, which could also be affected by the stability of the films formed on the surfaces of the metal and composite. The calcareous deposit on the surfaces of the

Copyright © 2004 by Marcel Dekker, Inc.