Spark Plasma Sintering Of Translucent Polycrystalline Silicon Nitride Ceramic With AlN/MgO/La₂O₃as Composite Additives

In the field of high temperature engineering, Si3N4ceramics have been widely usedas structural materials, but there are less research on the relationship between the dopantsand the optical properties. In this study, transparent polycrystalline Si3N4ceramics withexcellent mechanical and optical performances have been prepared by Spark PlasmaSintering (SPS), while α-Si3N4particles are as raw materials and AlN/MgO/La2O3arecomposite additives. The density, phase composition and morphology of composite Si3N4ceramics were characterized by Archimedes’ method, XRD and SEM. The effects of SPSparameters (such as sintering temperature, holding time, heating rate, sintering pressure)on the performances have been investigated in order to obtain the optimal sinteringprocess of transparent polycrystalline Si3N4ceramics with good properties. On this basis,the hardness, toughness, friction, wear and optical performance of Si3N4ceramics havebeen studied. Finally, with the best sintering process, the role of La2O3as additives on thefinal performance in sintering process is researched through comparing the density,structure and performance of polycrystalline Si3N4ceramics with AlN/MgO dopants andwith AlN/MgO/La2O3dopants. The results show that:In all process parameters, sintering temperature and time are the most importantfactors on sintering characteristics of polycrystalline Si3N4ceramics. When the designedtotal amount of addition is6wt.%with the additive ratio of AlN/MgO/La2O3=3:1:1, theoptimal sintering process parameters of SPS-sintered polycrystalline Si3N4ceramics withhigh density are T=1700℃, t=5min, v=75℃/min, P=30MPa, respectively. The vickershardness and fracture toughness of Si3N4ceramics samples are18.5±0.26GPa and8.42±0.15MPa·m1/2, respectively. The linear transmittance at900nm of0.2mm thickpolycrystalline Si3N4ceramics is35%.The balls-disc dry friction and wear properties of SPS-sintered AlN/MgO/La2O3doped polycrystalline Si3N4ceramics have been studied. The friction coefficient withSi3N4-Si3N4material for grinding increased sharply at the beginning, and then graduallydecreased, and finally achieve to a relatively stable state. When the load is6N and sliding speed is0.4m/s, the friction coefficient between the grinding materials is the lowest (0.47),and the domain wear mechanism is adhesive.With and without1wt.%La2O3additives, the relative density of composite Si3N4ceramics increase from98.51%to99.86%, the open porosity rate decreases from0.49%to0.17%, Vickers hardness decreases from19.32±0.29GPa to18.53±0.26GPa, fracturetoughness increases from6.97±0.09MPa·m1/2to8.42±0.15MPa·m1/2, and the lineartransmittance at900nm improves from26%to35%. Therefore, AlN/MgO/La2O3is aneffective additive for high-density transparent polycrystalline Si3N4ceramic with excellentmechanical and optical properties.