SYNTHESIS AND MICROSTRUCTURE OF TiC/TiN POROUS CERAMIC COMPOSITES PROCESSED VIA HYSYCVD/DIRECT NITRIDATION

摘要

The effect of varying oxygen content in the nitrogen atmosphere during successive stages of vapor infiltration on the microstructural evolution and physical properties of TiC/TiN porous ceramic composites was investigated. Composites were processed via the hybrid precursor system chemical vapor deposition (HYSYCVD) and direct nitridation (DN) routes. TiC_p porous preforms were infiltrated in subsequent stages: S1-1 and S1-2 at 1300°C for 70 min in high purity nitrogen (HPN); S2 at 1350°C for 120 min in ultra high purity nitrogen (UHPN). The composites were characterized by XRD, SEM, He picnometry, volume-measurement approach and immersion in Hg. The study was complemented with thermodynamic analyses using the FactSage program and databases. Results show that the presence of O_2, NH_3 and silicon-fluoride gaseous species throughout consecutive stages of processing affect the amount, morphology and distribution of TiN formed. Formation of TiN is ascribed to a combined effect of DN and HYSYCVD mechanisms.