OXIDATION RESISTANCE OF SI_3N_4 CERAMICS MODIFIED WITH BORON AND TRANSITION METAL DIBORIDES

摘要

Silicon nitride ceramics are one of the most promising materials for high-temperature structural applications in oxidizing environments. However, the use of Si_3N_4 ceramics at very high temperatures is limited by oxidation and reactions with combustion atmosphere. The effect of transition-metal diborides (CrB_2, TaB_2, and ZrB_2), oxides (Cr_2O_3, Ta_2O_5, and ZrO_2), and BN was studied on the oxidation behavior and microstructure of the oxidized layer of Si_3N_4 ceramics containing 5 wt. % Y_2O_3 and 2 wt. % Al_2O_3 as sintering aids. The ceramics were hot-pressed at 1825℃ and 20 MPa in He for 1 hour. The oxidation behavior of the ceramics was characterized after furnace heating at 1200 - 1600℃ in air as a function of the composition and structure of oxidized surface layer. The presence of Ta and Zr compounds, as well as BN did not improve the oxidation resistance of the baseline composition. Only the introduction of CrB_2 or Cr_2O_3 led to an increase in the oxidation resistance of Si_3N_4 ceramics up to 1550℃. The oxidized baseline Si_3N_4 ceramics exhibited phase separation in the surface glass with the formation of yttria-rich matrix phase and silica-rich droplets. The presence of Cr_2O_3 catalyzed in-situ crystallization of Y_2O_3 · 2SiO_2 in the surface glass. The high concentration of Y_2O_3·2SiO_2 crystals on the surface of ceramics during material exposure to oxidizing atmosphere provided effective oxidation protection. The highest oxidation resistance was shown by the ceramics containing less than 5 vol. % CrB_2.