Effect of ball milled Zr/Al/ZrB2 composite powders on microstructure and toughening of ZrB2–SiC/Zr–Al–C composite ceramics sintered by spark plasma sintering

Abstract

ZrB2-20vol% SiC-30vol% Zr–Al–C (ZSA) composite ceramics were fabricated by using graphite powders, SiC powders and ball milled Zr/Al/ZrB2 composite powders through spark plasma sintering. The ball milling process resulted in forming Zr/Al coating layer covered ZrB2 powders and introducing O 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 1h, the great increase in the introduced O included by Zr/Al/ZrB2 composite powders leaded to an insufficiency in Al due to the formation of Al2O3, which resulted in a remarkable phase transition from Zr2Al4C5 to Zr3Al4C6 in the ZSA composite ceramics. The more sufficient mixing and combination of Zr, Al and ZrB2 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/ZrB2 composite powders was 4h and the corresponding ZSA composite ceramic showed the maximum value of fracture toughness, 5.96±0.41MPam1/2, which is about 15% higher than that of the ZSA ceramic sintered using un-milled powders. The longer milling time of 6h leaded to much more Al2O3 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.