Restriction of Phase Transformation in Carbon Nanotube-Reinforced Yttria-Stabilized Zirconia

Abstract

The present research aims to investigate the effect of multi-walled carbon nanotube (MWNT) reinforcement on the mechanism of transformation toughening in zirconia matrix, and consequently, its fracture toughness. Monoclinic zirconia (un-doped ZrO2), partially stabilized zirconia (3 mol pct yttria-stabilized zirconia (3 mol pct YSZ)), and fully stabilized cubic zirconia (8 mol pct YSZ) with and without 6 vol pct MWNT-reinforced nanocomposites were processed via multi-stage spark plasma sintering. Phase analysis of powders, and sintered and crushed pellets performed using X-ray diffraction reveals the absence of any phase transformation in monoclinic ZrO2, 8 mol pct YSZ and their MWNT-reinforced nanocomposites upon application of stress by means of crushing. However, a significant decrease in the stress-induced phase transformation (81.1 pct metastable tetragonal phase retained with 6 vol pct MWNT reinforcement when compared to that of 68.4 pct tetragonal phase in 3 mol pct YSZ) is observed in the crushed pellet samples of partially stabilized zirconia upon 6 vol pct MWNT reinforcement. Transmission electron microscopy has been utilized for complementary phase analysis. Evaluation of mechanical properties indicates enhancement in fracture toughness (~23.6 to 26.4 pct) with the incorporation of 6 vol pct MWNT. Isolation of the net toughening contribution suggests that MWNT toughening mechanisms are ~3.8 times more effective than transformation toughening in enhancing the fracture toughness of YSZ/MWNT nanocomposites.