STRENGTHENING OF STRUCTURAL CERAMICS BY DIFFERENTIAL CREEP

A NOVEL METHOD FOR STRENGTHENING CERAMICS AND ALLOWING STRENGTH RETENTION TO HIGH TEMPERATURE IS PROPOSED. THE PHASE I RESEARCH WILL DETERMINE THE FEASIBILITY OF INTRODUCING SUBSTANTIAL SURFACE COMPRESSIVE RESIDUAL STRESSES IN ALUMINA, SILICON CARBIDE, AND SILICON NITRIDE USING A DIFFERENTIAL CREEP MECHANISM. THE PHASE II RESEARCHWILL OPTIMIZE PARAMETERS FOR CONTROLLING THE MAGNITUDE AND STRESS DISTRIBUTION ACHIEVABLE BY THIS TECHNIQUE WHICH IS ANALOGOUS TO THERMAL TEMPERING OF GLASS. PRELIMINARY EXPERIMENTS ON TUNGSTEN CARBIDE HAVE SHOWN THAT STRENGTHENING OF OVER 100 MPA WAS ACHIEVED. THE NOVEL CONCEPTS PROPOSED TO CREATE THESE RESIDUAL STRESSES WILL ALLOW THEM TO BE RETAINED UP TO THE TEMPERATURE AT WHICH CREEP IS SUBSTANTIAL. THIS WILL ALLOW STRENGTHENING TO TEMPERATURES ON THE ORDER OF 1300-1600 DEGREES CENTIGRADE FOR THE MATERIALS STUDIED IN PHASE I. THIS IS IN CONTRAST TO OTHER METHODS OF INTRODUCING RESIDUAL STRESSES WHERE THE STRESSES ARE NONEXISTENT ABOVE 1000 DEGREES CENTIGRADE. THESUCCESSFUL COMPLETION OF THE PHASE I RESEARCH WILL DEMONSTRATE THAT THIS TECHNIQUE IS APPLICABLE TO A WIDE VARIETY OF CERAMICS, INCLUDING OXIDES, CARBIDES AND NITRIDES.