PHYSICAL LIMITATIONS OF THE INHERENT TOUGHNESS AND STRENGTH IN CERAMIC-CERAMIC AND CERAMIC-METAL NANOCOMPOSITES

Toughening and strengthening effects, addressed as intrinsic material properties, are discussed in the case of ceramic-ceramic and ceramic-metal nanocomposites according to fracture mechanics argument. The theoretical approach followed is conceptually the same as that usually adopted for explaining the mechanical behavior of brittle metals, polymers or other monolithic ceramics, The theory cannot substantiate any tangible synergistic effect by nanosized ceramic dispersoids, it is worked out either in terms of toughness or in terms of strength, It is suggested that the improved mechanical properties, as reported in some ceramic-ceramic nanocomposites, may be related to residual microstresses stored into the material after the sintering process or even be affected by extrinsic factors related to the specimen preparation and/or to the testing procedure. Addition of metallic nanodispersoids seems to be a potentially suitable way to obtain a 30-40% increase in the inherent material strength, although a much larger dispersoid size may be required for gaining better toughness. The present theoretical treatment, although simplified in order to obtain solutions in close form, may provide some general and physically sound directives for the development of ceramic-matrix composites. This study intends to provide a theoretical counterpart to recent empirical practices in the field which are lacking in scientific logic.