The present study is devoted to the problem of enhancing fracture toughness of ZrO₂ ceramic materials through the formation of composite structure by addition of Al₂O₃ and NiO particles. In this paper, we analyzed the general and distinguished features of microstructure of both composite materials and its effect on fracture toughness of materials. In this paper, we used the XRD, SEM, and EDS methods for determination of granulometric, phase, and chemical composition of sintered materials. The peculiarities of dependence of fracture toughness values from dopant concentration and changing the Y³⁺ amount in zirconia grains allow us to assume that at least two mechanisms can affect the fracture toughness of ZrO₂ ceramics. Crack bridging/deflection processes with the “transformation toughening” affect the K_1C values depending on the dopant concentration. Crack deflection mechanism affects the K_1C values when the dopant concentrations are low, and transformation toughening affects the K_1C values when the dopant concentrations begin to have an impact on microstructure reorganization–redistribution of Y³⁺ ions and formation of Y³⁺-depleted grains with high ability to phase transformation.