Temperature and load dependent mechanical properties of pressureless sintered carbon nanotube/alumina nanocomposites

Multiwalled carbon nanotube (MWCNT)/alumina (Al2O3) nanocomposites having different CNT contents have been fabricated by wet-mixing and pressureless sintering in Argon at 1700 °C. Depending on indentation load, the highest improvement of 15–34% in hardness was achieved in 0.3 vol.% MWCNT/Al2O3 nanocomposite. Indentation size effect (Mayerʹs exponent = 1.753) on hardness was the most prominent in 1.2 vol.% MWCNT/Al2O3 nanocomposite due to presence of clustered CNTs, non-uniform interface and porous microstructure. ∼34% increase in fracture toughness was achieved in 0.3 vol.% MWCNT/Al2O3 nanocomposite than pure Al2O3 (∼3.84 MPa m0.5). High MWCNT loaded nanocomposites had reduced R-curve sensitivity because of increased matrix grain refining effect and reduced matrix compressive residual stress. High temperature flexural strength results indicated that strength retention of nanocomposites up to 1100 °C in ambient was much better compared to pure Al2O3. To predict structure–property relationship in nanocomposites, detailed microstructural and fractographic studies were also performed.