Kinetics of zircon formation in yttria partially stabilized zirconia as a result of oxidation of embedded molybdenum disilicide

Abstract

Recently MoSi₂ sacrificial particles embedded in yttria partially stabilized zirconia (YPSZ)have been proposed as attractive healing agents to realize significant extension of the lifetime of the thermally loaded structures. Upon local fracture of the YPSZ, the embedded healing particles in the path and in the vicinity of the crack react with the oxygen atoms transported via the crack and first fill the crack with a viscous glassy silica phase (SiO₂). The subsequent reaction between this freshly formed SiO₂ and the existing tetragonal ZrO₂ of the YPSZ leads to the formation of rigid crystalline zircon (ZrSiO₄), which is key in the crack-healing mechanism of YPSZ based materials. The isothermal kinetics of the self-healing reaction and the mechanism of zircon formation from the decomposing MoSi₂ and the surrounding YPSZ were assessed via X-ray diffraction (XRD). The obtained results revealed that at 1100 °C the reaction between amorphous SiO₂ and YPSZ is completed after about 10 h. For a more accurate determination of the kinetics of the self-healing reaction, bilayer samples of YPSZ – MoSi₂ (with and without boron addition)were annealed in air over a temperature range of 1100–1300 °C. This led to the formation of a MoSi₂/amorphous (boro)silica/zircon/YPSZ multi-layer, which was investigated with scanning electron microscopy (SEM)and electron probe X-ray microanalysis (EPMA). Kinetic modeling of the growth of zircon and silica or borosilicate layers showed that zircon growth was dominated by the diffusion of Si⁴⁺ in zircon whereas the growth of the silica or borosilicate layer was controlled by oxygen diffusion. Moreover, a significant increase in the rate of ZrSiO₄ formation was observed due to the presence of B in the MoSi₂ particles.