Abstract: Ternary layered MAX phase materials have excellent corrosion and oxidation resistance. However, their applications are limited by low hardness yet poor crack resistance, due to weak M-A metallic bonding and poor crack resistance stemming from their extremely high plastic anisotropy with ultrahigh c/a ratio (>4). In this work, we demonstrate significant improvements in both hardness and crack resistance when the grain size of MAX phases is reduced to nanoscale. Nanocrystalline Cr₂AlC MAX coatings with grain size ranging from 0 to 100 nm were successfully fabricated using a controllable PVD-based two-step bottom-up strategy. Remarkable improvements are achieved in both hardness and toughness, with hardness (15.5 GPa)/record-high strength (8.53 GPa) and toughness/plasticity peaking at a grain size of 15.8 nm near the critical value. Such unusual hardening-toughening effect at nanoscale stems from homogeneous deformation mode transitions with synchronous Hall-Petch hardening. Transmission electron microscopic observations proved that both pyramidal and prismatic slip, which are unlikely to operate at microcrystalline regime at room temperature, are completely active at nanocrystalline regime, unlocking the key 〈c〉-axial plasticity. As grain size further decreases approaching the critical value, a dynamic grain refinement-induced secondary sub-shear banding mechanism is triggered, which further extends the homogeneous deformation stage. These findings provide a simple route to fabricate advanced MAX phase corrosion-protection coatings with superior mechanical properties for extreme condition applications.

Key words: Nanocrystalline Cr₂AlC MAX phase, Hall-Petch relationship, Hardness, Toughness, Deformation mechanisms