Improving the flexural-strength-to-density ratio in alumina ceramics with the addition of silicon nitride

Alumina (Al2O3) and silicon nitride (Si3N4) ceramic compositions were studied as prospective materials for ballistic ceramics. Alumina ceramics with contents of 0, 5, 10 and 15 wt% of silicon nitride studied in order to improve their flexural strength/density ratio. Samples were molded by uniaxial pressing and sintered at 1600 degrees C. Retraction, density, water absorption and mass loss of the samples were determined, as well as their hardness, mechanical strength and fracture toughness. Ceramics produced in the presence of Si3N4 exhibited an interior with a very different aspect from their external surface. X-ray diffractometry showed that most compositions presented two phases, alumina and mullite, while the interior of the 15 wt% composition presented four phases: alumina, mullite, silicon nitride and X-SiAlON. Samples with 10 wt% of silicon nitride displayed the highest fracture toughness and the best flexural strength. Furthermore, this sample achieved a density approximately 21% lower than that of the control samples, providing the best flexural strength/density ratio, roughly 53% higher than that of alumina without Si3N4. Thus, these ceramics show great potential for utilization in advanced ceramic protections, where weight is a major limiting factor.