Fabrication and properties of reactively hot pressed ZrB_2-SiC ceramics

摘要

ZrB_2-SiC ceramics with relative densities >99 percent were fabricated by 'in situ' reactive hot pressing from ZrH_2, B_4C and Si. The reaction was studied using two processes, (1) powder reactions at temperatures from 1150 to 1400 deg C and (2) reactive hot pressing between 1600 and 1900 deg C. The products from the reaction of a 2ZrH_2:1B_4C:lSi molar mixture were ZrB_2, SiC, ZrO_2 and ZrC. Modification of the composition to 2ZrH_2:1.07B_4C:1.16Si resulted in the elimination of the undesired ZrO_2 and ZrC phases. The final composition was approximately ZrB_2-27 vol percent SiC with no undesired phases detected by X-ray diffraction, and only low concentrations of B_4C detected by scanning electron microscopy. Elimination of the undesired phases was accomplished by removing surface oxides through chemical reactions at elevated temperatures. Reactively hot pressed samples consisting of ZrB_2 with 27 vol percent SiC had a Young's modulus of 508 GPa, a flexure strength of 720 MPa, a fracture toughness of 3.5 MPa m~(1/2) and a Vickers' hardness of 22.8 GPa.