Directed Metal Oxidation Composites

Lanxide A process for making composites of metals with oxides. A molten metal reacts with an adjacent oxidant and is progressively drawn through its own oxidation product so as to yield a ceramic/metal composite. Fibres or other reinforcing materials can be placed in the path of the oxidation reaction and so incorporated in the final product. The Lanxide Corporation was founded in 1983 in Newark, DE, to exploit this invention. In 1990 it formed a joint venture with Du Pont to make electronic components by this process. Variations are Dimox (directed metal oxidation), for making ceramic metal composites, and Primex (pressureless infiltration by metal), for making metal matrix composites. 

This chapter, then, deals primarily with the directed metal oxidation process, although selected examples of stability in metal matrix composites are also discussed briefly. The focus is, of course, on the applications of phase equilibria, and more generally, thermodynamic principles that are applicable to the formation of composites in the presence of molten metals. Because these general principles are the same regardless of whether the end product is an MMC or a CMC, little generality is lost by focusing the discussion on CMCs formed by directed metal oxidation. 

In this section the preparation of ceramic composites by the directed metal oxidation process is described. First, in Section II.A, the aluminum oxide system is used as an example to explain the nature of the process, and a further example, a ZrB2 reinforced ZrC composite, is discussed in Section II.B. 

Fig. 5. The microstructure of zirconium diboride-reinforced zirconium carbide composites produced by directed metal oxidation, (a) A composite prepared with 22 vol % metal, (b) A composite produced with less than 2 vol % metal. 

Typically, the directed metal oxidation process involves the simultaneous reaction of molten metal, e.g., A1 with Oz, and infiltration of the reaction product and metal into a porous preform of the desired reinforcement. The directed metal oxidation process can also form composites in the absence of a reinforcement phase, termed matrix-only growth. Although the former process is more interesting commercially because of the ability to tailor the composite properties and because the product does not show significant preferred orientation, the latter case is simpler conceptually and theoretically. Thus, the thermodynamic discussion will begin with growth in the absence of reinforcements and then cover the additional complications that arise from their presence. 

To illustrate the thermodynamic complexities that arise because of the presence of a molten metal in the directed metal oxidation process, a detailed analysis is presented both with and without the Si metal present. The former analysis is applicable to more traditional ceramic processing such as sintering or hot-pressing of SiC/Si3N4 composites, whereas the latter is applicable to the directed metal oxidation process, or any other composite process where molten Si may be present. 

Fig. 13. Micrograph of a SiC reinforced Si, N4 matrix composite prepared by directed metal oxidation. From Johnson [52].

Using the Zr-B-C phase diagrams, some important features about the process of forming the ZrB2/ZrC/Zr composites by directed metal oxidation are apparent. With a maximum interface temperature of 2300 to... 

A. S. Nagelberg, A. S. Fareed, and D. J. Landini, Production of ceramic matrix composites for elevated temperature applications using the DIMOX directed metal oxidation process. In Processing and Fabrication of Advanced Materials for High Temperature Applications (V. A. Ravi and T. S. Srivatsan, eds.), pp. 127-142. Metallurgical Society, Warrensdale, PA, 1992. 

W. B. Johnson, Reinforced Si3N4 matrix composites formed by the directed metal oxidation process. Ceram. Eng. Sci. Proc. 13(7-8), 573-580 (1992). 

Directed Metal Oxidation Composites from Al-Mg Alloys... 

A variety of CMC systems have been developed and fabricated in the past. These have included aluminum oxide, aluminum nitride and sihcon nitride matrix composites [3-5]. The reinforcement has predominantly consisted of silicon carbide based fibers. Oxide based fibers have also been evaluated over the years as and when they have become available. This chapter reviews the development effort of silicon carbide reinforced aluminum oxide matrix composites fabricated via directed metal oxidation and compares them with those reinforced with oxide fibers. 

The generic process for fabrication of fiber-reinforced aluminum oxide matrix composites by directed metal oxidation includes preforming, fiber-matrix interface coating, matrix growth and removal of residual aluminum. A flow chart with the various processing steps is shown in Fig, 1. 

A.S. Fareed, Ceramic Matrix Composite Fabrication and Processing Directed Metal Oxidation, Handbook on Continuous Fiber Reinforced Ceramic Matrix Composites, ed. R.L. Lehman, S.K. El-Rahaiby Jr and J.B. Wachtman, Jr., ClAC/ACers, 301-324 (1995). 

A.S. Fareed, D.J. Landini, T.A. Johnson, A.N. Patel, PA. Craig, High Temperature Ceramic Matrix Composites by the Directed Metal Oxidation Process, Proc. Hi Tertperature Composites Clinic, SME, (1992). 

G.H. Schiroky, A.S. Fareed, B. Sonuparlak, C.T. Lee, and B. Sorenson, Fabrication and Properties of Fiber-Reinforced Ceramic Composites Made by Directed Metal Oxidation, pp. 151-163, in Flight-Vehicle Materials, Structures and Dynamics - Assessment and Future Directions, Vol. 3, S.R. Levine Jr, ed, ASME, New York, 1992. 

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