Effect of heat treatment on the structure and mechanical properties of zirconia crystals partially stabilized with samarium oxide

The effect of high-temperature treatment in different media on the phase composition, microhardness and fracture toughness of (ZrO₂)_1-х(Sm₂O₃)_х crystals with x = 0.02÷0.06 has been studied. The crystals have been grown using direction melt crystallization in a cold skull. The crystals have been heat treated at 1600 °C for 2 h in air and in vacuum. The phase composition of the crystals has been studied using X-ray diffraction and Raman scattering. We show that samarium cations enter the ZrO₂ lattice mainly in a trivalent charge state and do not change their charge after air or vacuum annealing. The as-annealed phase composition has changed in all the test crystals except for the (ZrO₂)_0.94(Sm₂O₃)_0.06 composition. After air or vacuum annealing the (ZrO₂)_1-x(Sm₂O₃)_x crystals with 0.002 ≤ x ≤ 0.05 contain a monoclinic phase. The (ZrO₂)_0.94(Sm₂O₃)_0.06 crystals contain two tetragonal phases (t and t´) with different tetragonality degrees. After air or vacuum annealing of the (ZrO₂)_0.94(Sm₂O₃)_0.06 crystals the lattice parameters of the t and t´ phases change in opposite manners, suggesting that the tetragonality degree of the t phase increases whereas the tetragonality degree of the t´ phase decreases. The microhardness and fracture toughness of the as-annealed crystals depend on the Sm₂O₃ concentration in the solid solutions. The formation of the monoclinic phase in the (ZrO₂)_1-х(Sm₂O₃)_х crystals with 0.037 ≤ x ≤ 0.05 significantly reduces the microhardness and fracture toughness of the crystals. Annealing of the (ZrO₂)_0.94(Sm₂O₃)_0.06 crystals triggers more efficient hardening mechanisms and thus increases the fracture toughness of the crystals. We show that air or vacuum annealing of the (ZrO₂)_0.94(Sm₂O₃)_0.06 crystals increases the fracture toughness of the crystals by 1.5 times as compared with that of the as-grown crystals.