Fabrication and wear behaviors of graded Si3N4 ceramics by the combination of two-step sintering and β-Si3N4 seeds

Graded Si3N4 ceramics with sandwich-like microstructure were fabricated by the combination of hot-pressing, spark plasma sintering and β-Si3N4 seeds. Phase compositions, microstructures, mechanical properties, and wear behaviors were investigated. Main α-Si3N4 phase were detected in the outer layers, and only β-Si3N4 phase were observed in the inner layers. The outer layer with ultra-fine equiaxed grains were well bonded to the inner layer with a distinct bimodal grain size distribution. Vickers hardness of outer layer (∼21.2 GPa) was much higher than that of inner layer (∼16.1 GPa), whereas fracture toughness of outer layer (∼3.5 MPa m1/2) was much lower than that of inner layer (∼5.9 MPa m1/2), indicative of the hard surface and tough core. Due to the ultra-fine microstructure and high hardness of outer layer, the graded Si3N4 ceramics exhibited superior wear resistance with low wear rate.