Investigating the mechanism of formation of Ca-rich Ca[sbnd]Yb apatite that forms from the Yb-silicate–CMAS reaction at 1300°C

Abstract

The formation mechanism and structure of Ca-rich CaYb apatite, which forms from the corrosion of Yb-silicate based coatings with CMAS at 1300°C in air for 48 h was investigated. X-ray photoelectron spectroscopy (XPS) was used to determine the bond type and oxidation states of the apatite. XPS data showed that the Si2p peaks are related to silicates, while the Ca2p and Yb4d are linked to apatite structures. The mechanism of growth for the apatite is proposed using the Wagner-Hauffe oxidation principle and the Kröger-Vink notations. The formation mechanism of the apatite-structure reaction phase is by substitutional diffusion, which is governed by the movement (formation and destruction) of point defects within the Yb2O3 lattice, particularly oxygen vacancies and electron holes. When Ca2+ substitutes Yb3+ in its lattice position, this results in the formation of anionic vacancies, which leads to more Ca2+ diffusion. The diffusion process continues until vacancies can no longer form to achieve electroneutrality, which leads to the newly formed Yb–Ca–Si being saturated with cations. Thus, based on this investigation, it is assumed that a CaYb silicate oxyapatite coatings can suppress the vacancy formation mechanism by containing high Ca and Si content which can thus decrease the ionic diffusion from CMAS melt into the EBC lattice. This can be the premises of a more CMAS-resistant coating family.