Extreme hardness via nanoscale confinement effects in ultra-low density carbon matrix nanocomposites

Polymer-derived pyrolytic carbons (PyCs) are often desirable for low density high-temperature structural applications when they can be reinforced with high stiffness and strength fibers to address brittleness. Nanofibers, such as aligned carbon nanotubes (A-CNTs), can be an ideal reinforcement owing to their high mass-specific properties, and while modeling suggests that properties of A-CNTs at high volume fractions (i.e., 10–30 vol%) could yield significant enhancements, it is unknown how CNT confinement influences processing, as such materials have never been synthesized. Here, we report process development demonstrating the first successful fabrication of fully infused, void-free A-CNT carbon matrix nanocomposites (A/C-NCs) via polymer infiltration pyrolysis (PIP) that establishes a platform to study nanofiber-reinforced polymer-derived ceramics. The size of the average A/C-NC graphitic crystallites increases as CNT vol% increases up to 30 vol%, and the inter-layer separation of the PyC graphitic crystallites decreases, evidencing a nanoscale confinement effect observed in concert with increased mechanical performance. Vickers microhardness testing in the axial CNT direction of A/C-NCs shows agreement with prior data for low vol% CNTs in PyC and mechanical modeling, where specific hardness increases from ∼3.3 GPa/(g/cm3) for PyC to ∼6.7 GPa/(g/cm3) for 30 vol% A/C-NCs, demonstrating A/C-NCs as an advantaged superhard lightweight material. [Display omitted] •Fabrication of pyrolytic carbon composites with 1-30 vol% aligned carbon nanotubes.•Vacuum- and capillary-assisted infiltration of phenolic resin enable full infusion.•Four polymer infiltration pyrolysis cycles decrease porosity and increases density.•Graphitic crystallites in matrix are longer and denser with nanotube reinforcement.•Specific microhardness increases up to 30 vol% carbon nanotubes, rivaling diamond.