Tribological and thermomechanical behaviour of alumina-based hybrid nanocomposites

This study investigated the influence of carbon nanotubes (1–5 wt.%) into 30 wt% cBN (Ni coated)-Al₂O₃ composite. Hybrid nanocomposites were prepared via spark plasma sintering (SPS) at 1400 °C to target the optimum properties and innovative solutions to self-lubricating ceramic cutting inserts. The hybrid nanocomposite prepared with the lowest reinforcement (1 wt.% CNTs) showed the highest thermomechanical and tribological properties due to the better distribution of CNTs and interfacial bonding provided by the nickel coating on hard cBN particles, leading to better densification. The wear rate and coefficient of friction (COF) reduction are attributed to the self-lubrication provided by a small quantity of hBN, Ni, and CNTs in the alumina matrix. The 1 wt.% composition shows a high hardness value (H_n ≈ 25.5 GPa) and high thermal conductivity (≈31.9 W/m⋅K) with a coefficient of thermal expansion (CTE) of ≈6.1 ppm⋅K⁻¹. However, incorporating a higher CNT content reduced grain growth, interfacial bonding strength, and phase transformation from the more challenging phase (cBN) to the softer phase (hBN). Interestingly, the COF was significantly reduced to ∼0.28 with increased CNTs (wt.%). Furthermore, the increased CNT content significantly enhanced the elastic modulus (E_r ≈ 617 GPa). It yielded an ultralow CTE (≈4.4 ppm⋅K⁻¹), which can be attributed to the densification and inherent nature of CNTs. This comprehensive analysis offers invaluable insights into the design and optimisation of hybrid composite materials for environment-friendly machining applications. Hybrid compositions were designed for hard-to-machine materials (i.e., titanium, superalloys) under dry conditions to be environmentally friendly and economically viable.