Correlations of grain boundary segregation to sintering techniques in a three-phase ceramic

Abstract

The effects of using different sintering techniques (conventional, flash and spark plasma sintering) on grain boundary segregation were investigated in a 3-phase polycrystalline ceramic containing cubic 8 mol% Y2O3 stabilized ZrO2 (YSZ), α-Al2O3 and MgAl2O4. Six types of interfaces for each sintered sample were analyzed for grain boundary chemistry. Using aberration-corrected STEM and EDS, we show Al segregation at YSZ-YSZ boundaries, and Y/Zr segregation at Al2O3-Al2O3, MgAl2O4-MgAl2O4 and MgAl2O4-Al2O3 boundaries. YSZ-MgAl2O4 and YSZ-Al2O3 heterointerfaces, in contrast, do not show elemental segregation. Our results show that the type of segregation at different boundaries does not change with different sintering processes, indicating that elemental segregation mainly depends on initial sample composition, although the amount of segregation can vary. Quantitative analyses reveal that spark plasma sintering (950°C for 5 min, 100 MPa) results in relatively higher average segregation at grain boundaries and heterointerfaces compared to conventional sintering (1550°C for 10 h), suggesting that low temperatures result in higher grain boundary segregation. The average segregation concentrations at the grain boundaries of our flash sintered sample (estimated to reach 1700°C for 6 s) were found to be consistently similar to long-annealed spark-plasma sintered sample (1350°C for 20 h), suggesting that very high temperatures reached during flash process can achieve similar segregation concentrations compared to a SPS sample annealed for several hours at lower temperature. The presence of other phases in multi-phase systems can modify the grain boundary chemistry and segregation compared to segregation in single-phase ceramics.