Densification and mechanical properties of spark plasma sintered Si3N4/ZrO2 nano-composites

Fully dense Si3N4-ZrO2 nano-composites, where ZrO2 content varied from 0 to 30 vol%, were produced via Spark Plasma Sintering (SPS) at 1600 °C under the pressure of 30 MPa for 10 min. Si3N4, Y2O3-stabilized ZrO2 (YSZ), and Al2O3 (sintering aid) were used as starting powders and mixed with high energy planetary ball mill. The obtained nano-powders with an average particle size of 45 nm were employed for the sample preparation. XRD patterns of the sintered nano-composites showed that the entire ZrO2 constituent was successfully retained in tetragonal crystal structure, while neither monoclinic ZrO2 nor detrimental ZrN/ZrON phase was formed during the SPS process. Homogenous distribution of the tetragonal ZrO2 particles in Si3N4 matrix was observed by SEM analysis. By considering shrinkage behavior of the nano-composites as a function of sintering temperature and time, it was found that the addition of Y2O3-doped ZrO2 into the initial Si3N4 powder can noticeably facilitate densification process, decreasing sintering temperature of monolithic Si3N4 ceramic from 1600 °C to 1420 °C through 30 vol% YSZ utilization. The evaluation of mechanical properties indicated that the hardness of the Si3N4-base composites declined from 16.6 to 13.2 GPa, whereas the fracture toughness improved from 5.8 to 7.1 MPa m1/2 by increasing ZrO2 content from 0 to 30 vol%, which were explained based on the in-situ formation of β-Si3N4 and the stress-induced phase transformation from tetragonal to monoclinic ZrO2, respectively. Friction coefficient (μ) and specific wear rate (Kw) of Si3N4-ZrO2 nano-composites were estimated being in the range of 0.59–0.67 and 2.46 × 10−6-3.41 × 10−6 mm3/N m, respectively, demonstrating promising characteristics for tribological applications.