Ceramics offer attractive properties, including good high-temperature strength and resistance to wear and oxidation. However, the major limitation to their use in structural applications is their inherent low fracture toughness, that is, the tendency to break (fracture) and produce low values. Ceramics have low fracture toughness, whereas steels and superalloys have better fracture toughness. Methods currently being used to improve the toughness of ceramics involve incorporation of reinforcing whiskers and fibers; inclusion of a phase that undergoes transformation within the stress field associated with a crack; and cermet technology (ceramic/metal composites), in which the tough metallic component absorbs energy. Improved toughness is attributed to various mechanisms, including crack branching, transformation-induced residual stresses, crack bridging, and energy absorption by plastic flow.
FIGURE M.13 Conceptual architecture of a micro-infiltrated macrolaminated composite.
Another possible approach to improving the toughness of ceramics is via laminated construction. Sophisticated coating techniques have been used to produce laminated micro-structures of ceramics and metals, but the cost of producing a bulk-laminated composite of useful size is prohibitive. Fabrication of micro-infiltrated macrolaminated composites offers an economically feasible approach to produce a variety of cermet/metallic and ceramic/metallic bulk-laminated composites (Figure M.13). The basic architecture of a microinfiltrated macro-laminated composite is a double-layer structure. One layer consists of a soft, ductile material having a low modulus of elasticity, low strength, and high toughness; the second layer consists of a hard, brittle material having a high modulus of elasticity and low toughness. This double layer is repeated as many times as necessary to form the bulk composite; in addition, the brittle material is infiltrated with the ductile constituent.
Advantages
The architecture of’ the microinfiltrated macro-laminated composite offers a compromise to the conventional composite microstructure by providing repeated alternating layers of bulk tough metallic and brittle cermet and ceramic materials. Any crack introduced into the brittle constituent will, upon entering the metallic interlayer, be subjected to a potential crack, stopping higher toughness.
Composition and Fabrication
Fabrication of microinfiltrated macrolaminated composite materials offers a processing route to economical manufacture of large, bulk-laminated composites in which an interpenetrating microstructure is achievable. Use of both ceramic and metal powders in the process opens the door to a potentially wide range of material combinations. The same fundamental processing steps can be used to produce a wide variety of microinfiltrated macrolaminated composites, including ceramic/metal, metal/intermetallic, and ceramic/intermetallic systems.
Various combinations of composite properties, including hardness, strength, ductility, and fracture toughness, are possible by varying the laminate layer thicknesses. Microinfiltrated macrolaminated composites, when used in bulk form, are expected to have properties far superior to those of the individual monolithic constituents of the composite.
Alternative processing routes for microin-filtrated macrolaminated composites are available if the composite constituents have some solubility for one another, such as in the tungsten carbide-cobalt/cobalt (WC-Co/Co) and tungsten-nickel-iron heavy alloy/nickel (W-Ni-Fe/Ni) systems. In general, to obtain a large composite of this type, the best approach is to use the tape-casting process (well known in the ceramic industry) to produce large thin tapes (sheets) of the material.
Consolidation (compaction) helps the material achieve its full density by removing gas voids, porosity, and so forth. An attractive alternative for producing sheets consists of rolling the powder material followed by sintering to the required percent of theoretical density, retaining a certain level of interconnected porosity.
The fabrication steps used to make a W-Ni-Fe/Ni microinfiltrated macrolaminated composite illustrate the process. Tungsten powder is tape cast, and the sheets are sintered to 60 to 70% of theoretical density, which produces totally interconnected porosity. Small amounts of nickel can be used together with tungsten to promote activated sintering and to produce a porous tungsten sheet that can be handled safely. Sheet thickness ranges from 1 to 10 mm.
An 80:20 ratio of nickel and iron powders also is tape-cast and sintered to full density. Sheets of porous tungsten and fully dense nickel-iron alloy material are laid up in alternate layers and heated to about 1475°C, which is about 40°C above the melting point of the nickel-iron alloy. The molten Ni-Fe alloy infiltrates the porous tungsten sheets and takes into solution a fraction of the tungsten. A thin layer of the liquid phase can be retained between the tungsten sheets after the infiltration process is complete. This retention is possible if the tungsten sheet contains porosity levels that are lower than the volume of liquid formed by melting the nickel-iron sheets.
Applications
The design of microinfiltrated macrolaminated composites could prove advantageous in applications involving impact or ballistic penetration. The metallic interlayer would function to hold together damaged portions of the brittle constituent. Thus, the development of ceramic armor having multihit capability becomes possible. Current metal/ceramic composite armor is layered on an extremely macroscopic scale. An approach involving microinfiltrated macro-laminated composites permits optimization of layer frequency and thickness to resist specific threats, as in antipersonnel weapons or armored vehicles.
In addition, high-temperature composites, consisting of a ceramic and intermetallic compound, such as aluminum oxide/nickel alumi-nide (Ni3Al) plus boron, could be fabricated in the form of microinfiltrated macrolaminated composites to yield combinations with new properties. Ultrahigh-temperature composites incorporating high-temperature ceramics and ductile niobium is another potential application to obtain various combinations of high wear resistance and toughness. The hard ceramic outer layer would provide a wear-resistant surface, with toughness provided by the ductile material. A similar concept could lead to a new generation of cutting tools, in which the outermost cutting layer could be made of ultrahard materials suitably layered with a soft, ductile constituent for toughness.
Other potential applications are in heat- and oxidation-resistant, low-density structural components and aerospace parts having low density, high strength and modulus, and good fracture toughness.
Finally, the reinforcement of concrete with carbon fibers is a recently developed technique for the construction of structures such as buildings, bridges, and tunnels. Advantages of carbon-fiber reinforcement include outstanding electromagnetic shielding, high resistance to corrosive environments, light weight, and high mechanical property strength. Two methods are used to achieve the reinforcement: mixing carbon fibers, either chopped or mat, directly into cement (carbon fiber-reinforced concrete); and using carbon fiber/plastic composites in rod or tape form for strengthening the concrete.
