* 1994 Highlight * Center for Advanced Materials (CAM): Ceramic Science Program * * Materials Sciences Division, Lawrence Berkeley National Laboratory *

LBNL Develops Tough Silicon Carbide Ceramic L.C. De Jonghe

A research group under the direction of Lutgard C. De Jonghe has developed a processing method for achieving stronger and tougher silicon carbide composites.

The new method involves the preparation of ceramic composites in which silicon carbide platelets, suspended in solution, are coated in a carefully controlled process. In the high temperature processing that follows this procedure, the platelet coating acts as an effective barrier layer, modifying and enhancing the mechanical properties of the material.

The high density ceramic composites prepared by the De Jonghe group consist of alpha silicon carbide platelets in a beta silicon carbide matrix. The platelets are isolated from the matrix by an alumina coating about 2um thick. The coating prevents the alpha silicon carbide platelets from assisting in the transformation of the beta to alpha silicon carbide, a transformation that would lead normally to the loss of the identity of the platelets during the high temperature processing. The alumina layer around the platelets has additional benefits: it prevents the platelets from bonding directly to the matrix and dramatically increases the resistance to fracture of the ceramic by causing cracks to travel around each platelet instead of cracking straight through them.

SiC-SiC composite ceramics prepared in this way are 2-3 times tougher than similar materials processed by conventional methods. The simultaneous increase in the fracture strength and fracture toughness of silicon carbide ceramics is significant. Silicon carbide ceramics have some of the best high temperature characteristics of ceramics, but their extreme brittleness has hindered their practical application. In addition, the increased strength and toughness are necessary to allow exploitation of other advantageous properties of silicon carbide, such as high oxidation resistance, in demanding high temperature structural applications. This work should give materials scientists a better understanding and control of the processing, not only of silicon carbide ceramics, but of many advanced materials.

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L.C. De Jonghe (510. 486-6138), J. Cao, B. Dalgleish and A. Garg at the Center for Advanced Materials, Ceramics Science Program, Materials Sciences Division (510. 486-4755), Lawrence Berkeley National Laboratory.

L.C. De Jonghe, T. Mitchell, W.J. Moberly-Chan and R.O. Ritchie, "Silicon Carbide Platelet/Silicon Carbide Composites," Jour. Amer. Ceramics Soc. (1994) .

MSD Index: De Jonghe.

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