GAS PRESSURE SINTERING AND CONTAINERLESS HIP'ING OF SI(3)N(4)/SIC COMPOSITES

THIS PROPOSAL DESCRIBES AND R&D PROGRAM, THE OBJECTIVE OF WHICH IS TO INITIATE DEVELOPMENT OF SILICON NITRIDE/SILICON CARBIDE WHISKER COMPOSITES FABRICATED BY GAS PRESSURE SINTERING AND CONTAINERLESS HOT ISOSTATIC PRESSING (HIP). THE TARGET MATERIALS WILL BE BASED ON CERAMIC MATRICES FORMED FROM STARTING COMPOSITIONS OF SI(3)N(4) + OR </4 WT% Y(2)O(3) AND REINFORCED BY RANDOMLY ORIENTED SILICON CARBIDE WHISKERS (SIC(W). A PRINCIPAL AIM OF THE STUDY WILL BE TO DETERMINE THE FEASIBILITY OF SINTERING THESE MATERIALS TO HIGH DENSITY, USING GAS PRESSURE SINTERING AND CONTAINERLESS HIP. THE PRODUCT MATERIALS SHOULD POSSESS GOOD HIGH TEMPERATURE STRENGTH AND OXIDATION RESISTANCE, OWING TO THE SMALL AMOUNTS OF SINTERING AIDS USED IN THE STARTING COMPOSITIONS, WHILE THE SIC(W) SHOULD ENHANCE THE RESULTANT FRACTURE TOUGHNESS. TO PRECLUDE OR MITIGATE THE POSSIBILITY OF HIGH TEMPERATURE REACTIONS OCCURRING BETWEEN THE SIC(W) AND N(2) OR OTHER REACTANT SPECIES PRESENT IN THE MATRIX (E.G., SIO, MOLTEN SILICATES) DURING FIRING, THE SIC(W) WILL BE COATED WITH INERT THIN FILMS OF BORON NITRIDE, USING CHEMICAL VAPOR DEPOSITION TECHNOLOGY, PRIOR TO THEIR INTRODUCTION INTO THE STARTING POWDER MIX. BY SUPERIMPOSING CONTAINERLESS HIP'ING ON SINTERING, IT SHOULD BE POSSIBLE TO ELIMINATE OR MARKEDLY REDUCE RESIDUAL POROSITY, THEREBY MINIMIZING FLAWS, AND HOMOGENIZE THE PROPERTIES OF THE MATERIALS.