Processing, characterization and mechanical properties of alumina-based nanocomposites

The present study focuses on improving the fracture toughness of nanocrystalline alumina by incorporating second phases---specifically niobium and carbon nanotubes. Ceramics have many properties that lend themselves well to load bearing and armor applications. Chemical inertness, high hardness and strength, low wear rates and low densities are examples of these properties that warrant potential substitution of metals and their alloys. In this study, nanocrystalline alumina was investigated based on its impressive elevated temperature properties and high hardness. Despite these promising structural properties, pure nanocrystalline alumina has low fracture toughness (∼2.5 MPa*m1/2) and is thus limited to non-structural applications.; Alumina-based nanocomposites reinforced with niobium and/or carbon nanotubes (CNT) were fabricated by advanced powder processing techniques and consolidated by spark plasma sintering (∼1200°C, 4 min). Raman spectroscopy revealed that single-walled carbon nanotubes (SWCNTs) begin to break down at sintering temperatures above 1150°C. Nuclear magnetic resonance (NMR) showed that, although thermodynamically unlikely, no Al4C3 was formed in the CNT-alumina nanocomposites. Thus, the nanocomposite is purely a physical mixture and no chemical bond was formed between the nanotubes and matrix.; In addition, in-situ 3-pt and standard 4-pt bend tests were conducted on niobium and/or carbon nanotube-reinforced alumina nanocomposites in order to assess their toughness. Although stable crack growth was not achieved in the 3-pt bend testing, average fracture toughness vales of 6.1 and 3.3 MPa·m 1/2 were measured for 10 vol%Nb and 10 vol%Nb-5 vol%SWCNT-alumina, respectively. The 4-pt bend testing measured average intrinsic fracture toughness of 2.95, 2.76, 3.33 and 3.95 MPa·m1/2 for alumina nanocomposites containing 5 vol%SWCNT, 10 vol%SWCNT, 5 vol%DWCNT and 10 vol% Nb, respectively.; Although nanocrystalline alumina will never be able to compete with its microcrystalline counter part in terms of fracture toughness, its nanocomposite form does have a niche in small components and devices requiring high hardness and conductivity---perhaps in the IC industry. Adding SWCNTs to nanocrystalline alumina increases the electrical conductivity 13 orders of magnitude without degradation of intrinsic fracture toughness and with a very small decrease in hardness.