The effect of sintering on the mechanical properties of freestanding, plasma-sprayed ZrO2-8 wt% Y2O3 thick thermal barrier coatings (TBCs) was determined after annealing at 1316 degrees C in air. Mechanical properties of the TBCs, including flexure strength, modes I and II fracture toughnesses, constitutive relations, elastic modulus, and microhardness, were determined at ambient temperature as a function of annealing time ranging from 0 to 500 h. In addition, some physical properties such as density and phase stability were also determined. Mechanical and physical properties increased significantly in 5-100 h and then reached a plateau above 100 h. An exception to this was the monoclinic phase that increased monotonically without forming a plateau. Annealing resulted in healing of microcracks and pores, and in grain growth, accompanying densification of the TBC's body because of the sintering effect. However, an inevitable adverse effect also occurred such that the desired lower thermal conductivity and good strain-tolerant capability, which makes the TBCs unique in thermal barrier applications, were degraded upon annealing. A phenomenological model was proposed to assess and quantify all the property variables in response to annealing in a normalized scheme.