Nanostructural effects beyond Hall-Petch: Towards superhard tungsten carbide

Abstract

Industrial application of superhard materials (Vickers hardness, HV > 40 GPa) such as diamond and cubic boron nitride is limited by high costs and complex routes of synthesis. Tungsten carbide (WC) is a common industrial material valued for its hardness, but falls well short of qualification as a superhard material even in its less common but harder binderless form (HV ∼ 26 GPa). Importantly, recent efforts have demonstrated the potential for alternative materials, such as WC, to achieve similar hardness to diamond and cubic boron nitride via microstructural refinement. However, despite recent advances in sintering technology, even the smallest grained binderless WC (< 100 nm) has failed to achieve HV values above 33 GPa. In this work, multiple hardening mechanisms are exploited through a unique sintering approach proving WC as a candidate superhard material. Environmentally Controlled – Pressure Assisted Sintering (EC-PAS) is utilized to produce > 99 % dense, binderless nanocrystalline WC ceramics with hardness as high as 39 GPa. The unprecedented WC hardness is attributed to the combined effects of small average crystallite size and, importantly, deformation-induced nanoscale intragranular defects including stacking faults. The demonstration of the superposition of multiple hardening mechanisms provides a new avenue to improve hardness of ceramics beyond traditional Hall-Petch hardening, yielding new classes of superhard materials.