Silicon nitride (Si3N4) based ceramics attract the attention of manufacturers and researchers due to their high strength to weight, fracture toughness to weight, and hardness to weight ratios enabling them as promising sources for the creation of state-of-the-art macrocomposite protective plates used in the arms industry. In this work, sintering mode-related physical and mechanical properties of fine-grained Si3N4–10 wt%Y2O3 ceramics were studied to reveal an optimum combination of strength, fracture toughness, and hardness. Initial Si3N4 and Y2O3 powders with high specific surface areas were synthesized by the plasma-chemical method. In the process of synthesis of Si3N4 powder, Y2O3 powder was introduced into the plasma jet forming thus a powder mixture. The content of Y2O3 powder in all the studied compositions was 10 wt% which provided the preparation of relatively dense materials by conventional sintering at the set temperatures in a nitrogen atmosphere. Based on the results of mechanical tests and fracture surface analysis, the effect of fine-grained microstructure of Si3N4–Y2O3 ceramics on the protective properties of metal-ceramic macrocomposite plates was substantiated. The ceramic material sintered at 1700 °C for 5 h exhibited an optimal combination of porosity, hardness, strength, and fracture toughness. However, the highest strength and fracture toughness were found in the ceramic sintered at 1700 °C for 2 h, in which the plasticizer content was 2 wt%. Peculiarities of fracture surface morphology of Si3N4–Y2O3 ceramics were also studied. It was shown that nanometric silicon nitride particles are mostly agglomerated in the ceramic materials sintered at 1700 °C for 2 h and 5 h ensuring thus the high-energy consuming fracture micromechanism in the fracture toughness test. Such mechanical behavior evidences that Si3N4–Y2O3 ceramic material sintered in the appropriate mode is promising for application in protective metal-ceramic macrocomposite layers.