Mechanical properties and phase-transformation behavior of carbon nanotube-reinforced yttria-stabilized zirconia composites

The mechanical properties and phase transformation behavior of carbon nanotube (CNT) reinforced 3 mol% yttria-stabilized zirconia (3YSZ) composites prepared by spark plasma sintering are reported for CNT contents between 0 and 7 wt%. CNTs are shown to improve the fracture toughness of the material by a combination of fiber pull-out and crack-bridging toughening mechanisms. Stress release by these mechanisms results in a smaller proportion of the YSZ matrix transforming from the tetragonal to monoclinic phase, reducing the microcracking that would otherwise result from cumulative volume expansion associated with this well-known phase transformation process. This produces a material with high fracture toughness without compromising hardness, thereby providing superior mechanical reliability. The high fracture toughness (7.0 MPa m^1/2) of the 7 wt% CNT/3YSZ composite makes it an attractive material for high-impact, high-temperature applications such as environmental barrier coatings.