Role of carbonaceous reinforcements on mechanical properties and micro-scratch behaviour of Y₂O₃ stabilized ZrO₂

An efficient 3 mol% Y2O3 stabilized ZrO2 (3YSZ) based composite was developed for structural application. The role of graphene nanoplatelets (GNP) and multi-walled carbon nanotubes (MWNT) on evolution of phases and microstructural features in the 3YSZ-based composites that affect the mechanical and tribological properties of composites have been investigated in the present study. Here, the spark plasma sintered 3YSZ composites with 1, 2 and 4 wt% of MWNT and GNP reinforcement were synthesised. Sintered composites retained the tetragonal ZrO2 (t-ZrO2, high-temperature phase) along with the formation of ZrC with the addition of GNP/MWNT, which marginally improves the mechanical properties of the composite. The marginal enhancement in hardness and elastic modulus with GNP and MWNT reinforcement is attributed to the synergistic effect of grain refinement, densification, phases and distribution. The composite with 2 wt% of MWNT showed marginal improvement in the indentation fracture toughness (7.2 MPa m0.5) compared with monolithic 3YSZ by crack deflection, MWNT bridging & pull-out mechanism. Further, the average scratch toughness of the 3YSZ-GNP/MWNT based composite is measured as 1.99-2.15 MPa m0.5 derived from progressive load scratching. The scratch toughness with 1.5-3.3 times lower than the indentation fracture toughness is attributed to the elastic recovery during scratching and non-uniform distribution of phases in the composites. Further, the optimum value of hardness, elastic modulus, indentation fracture toughness, and scratch toughness of 2 wt% of MWNT reinforced 3YSZ composite is found suitable for structural applications.