Role of Carbon Nanotubes Reinforcement and Processing Parameters on Mechanical Properties of Alumina Based Nanocomposites

Abstract

Along with other oxide ceramics, alumina is an important and widely used industrial material. Its applications include prostheses and dental implants used as bio-medical replacements, wear- resistant components and speedy cutting tools, thermal and electrical insulations and coatings for high temperature use. The effectiveness of alumina for such uses is credited to its excellent corrosion resistance, high hardness, good electrical and thermal insulating properties and high compressive strength. However, regardless of its excellent potentials and properties, its use for structural applications has significantly been limited due to its low-fracture toughness and low- fracture strength. The potential use of carbon nanotubes reinforced ceramic nanocomposites for various engineering applications has unlocked an interesting area of research. In the current work, two kinds of sintering routes, namely pressureless and spark plasma sintering are used for the synthesis of multiwalled carbon nanotube reinforced alumina matrix nanocomposites. The characterization of the resulted nanocomposites is carried out and their comparison with the sintering behavior of monolithic alumina is presented. Two types of composites were prepared by using pressureless and spark plasma sintering techniques both contained 1, 2 and 3 wt% of as-received and functionalized carbon nanotubes. The mixing and dispersion of carbon nanotubes in alumina was done by a novel technique of gas purging sonication. Varying percentages of carbon nanotubes in the composites were compacted using a uniaxial press followed by pressureless sintering at 1600°C in flowing argon with a dwell time of 15 minutes and spark plasma sintering at 1400°C under a pressure of 60 MPa for a holding time of 10 minutes. Pressureless sintered nanocomposites with 1 wt% carbon nanotubes gave 98.5% relative density with no degradation of carbon nanotubes. Moreover, it also resulted in an increase in fracture toughness from 8.1% and 9.4% and Young’s modulus by 5% and 7% when compared to as-received and functionalized carbon nanotube nanocomposites respectively with respect to pure alumina. This investigation has shown that the densification can be achieved without degradation of carbon nanotubes at elevated temperatures in the carbon nanotube-alumina nanocomposites sintered by conventional route. vWell-dispersed carbon nanotube-reinforced alumina nanocomposites have been synthesized successfully having a high density by spark plasma sintering. At 1 wt% functionalized carbon nanotubes addition in alumina, a near full density is achieved that contributes to the improvement in mechanical properties of the nanocomposites. On addition of 1wt% CNTs, fracture toughness values increased by approximately 18.6% and 14% for functionalized and as-received CNT-alumina nanocomposites respectively. However further addition of CNTs up to 3 wt % slightly decreased the hardness and the fracture toughness. Young’s modulus was improved by 6.5% for functionalized and 4% for as-received CNT-alumina nanocomposites over monolithic alumina. Average grain size of monolithic alumina is observed as 2.0 ± 0.5 μm while that of 1wt% CNT-alumina nanocomposite was less than 1 μm. The well-dispersed carbon nanotubes within the alumina matrix enhanced the pullout resistance, bridged the gaps between cracks and held up the crack propagation by using elasticity that lead to improved fracture toughness.