Transformation toughening of glass ceramics
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Transformation toughening of glass ceramics D. R. Clarke and B. Schwartz IBM, Thomas J. Watson Research Center, Yorktown Heights, New York 10598 (Received 5 March 1987; accepted 28 July 1987) The utilization of transformation toughening has hitherto been restricted to increasing the fracture resistance of polycrystalline ceramic materials. Although a number of investigators have attempted to extend the concept to toughening glasses and glass ceramics with tetragonal zirconia, no successful reports have been published. It is argued that the approaches employed are inevitably limited primarily because they do not take into account the necessity of nucleating the tetragonal-to-monoclinic transformation away from the crack tip itself. By concentrating on the nucleation event and using standard ceramic processing techniques, it has been demonstated that transformation toughening can be used to increase the toughness of glass-ceramic materials, and this approach is illustrated by increasing the fracture toughness of a cordierite glass ceramic. The use of transformation toughening1 has become a powerful method of increasing the fracture resistance of a wide variety of ceramic materials.2'3 Originally used to toughen zirconia ceramics and alumina, the use of transformation toughening has been accepted rapidly as a rational strategy for toughening all sorts of ceramics including oxides, nitrides, and carbides. In all these materials, marked increases in toughening have been achieved when the processing conditions have enabled the tetragonal form of zirconia to be retained in the microstructure. However, there is one class of materials in which, despite the incorporation of tetragonal zirconia, true transformation toughening has not been reported, namely glasses and glass ceramics. Previous workers who have attempted to toughen glasses and glass ceramics by the use of zirconia have done so by precipitating the zirconia from the glass phase. There is considerable precedent for this approach, since this is the manner in which zirconia, one of the traditional nucleating aids in the manufacture of glass ceramics,4 is known to be formed in its tetragonal form. However, it is argued here that this approach suffers from a number of limitations if zirconia is incorporated for the purposes of transformation toughening of the materials. It is conjectured that perhaps the most important of the limitations is that the precipitated zirconia has a morphology ill suited for nucleation of the tetragonal-monoclinic transformation away from the crack tip itself. Nucleation is a necessary step in order to develop a zone of transformation having an appreciable extent around the crack. (Existing theories predict that the increase in toughness is proportional to both the square root of the zone size and the concentration of tetragonal particles in the zone that can be transformed, see below.) J. Mater. Res. 2 (6), Nov/Dec 1987
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In this preliminary contribution we describe an alternative approach to transformation to
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