Effect of ball milled Zr/Al/ZrB_2 composite powders on microstructure and toughening of ZrB_2-SiC/Zr-Al-C composite ceramics sintered by spark plasma sintering

摘要

ZrB_2-20 vol% SiC-30 vol% Zr-Al-C (ZSA) composite ceramics were fabricated by using graphite powders, SiC powders and ball milled Zr/Al/ZrB_2 composite powders through spark plasma sintering. The ball milling process resulted in forming Zr/Al coating layer covered ZrB_2 powders and introducing 0 into the composite powders despite of Ar protective atmosphere. The Zr/Al coating layer reacted with graphite powders to in situ form layered Zr-Al-C grains in ZSA composite ceramics during the sintering process. As increasing milling time longer than 1 h, the great increase in the introduced 0 included by Zr/Al/ZrB_2 composite powders leaded to an insufficiency in Al due to the formation of Al_2O_3, which resulted in a remarkable phase transition from Zr_2Al_4C_5 to Zr_3Al_4C_6 in the ZSA composite ceramics. The more sufficient mixing and combination of Zr, Al and ZrB_2 achieved by increasing milling time leaded to the more uniform distribution and the longer slenderness ratio of Zr-Al-C grains in the ZSA composite ceramics, which made the major contribution to the toughening of ZSA composite ceramics. The optimal milling time for Zr/Al/ZrB_2 composite powders was 4 h and the corresponding ZSA composite ceramic showed the maximum value of fracture toughness, 5.96 ± 0.41 MPa m~(1/2), which is about 15% higher than that of the ZSA ceramic sintered using un-milled powders. The longer milling time of 6 h leaded to much more Al_2O_3 and few layered Zr-Al-C grains in the ZSA ceramic that exhibited the minimum fracture toughness. The fracture mode is a mixture of inter- and intra-granular fractures, and the toughening mechanisms are the crack deflection and crack bridging that result from the existed Zr-Al-C grains with laminated microstructure.