Reactive crystallization in HfO2 exposed to molten silicates

Abstract

Hafnia is of interest in thermal and environmental barrier coatings, but little is known about its response to molten silicate attack. This article investigates that response using two model silicate melts, compares it with pure ZrO2 and examines the effect of YO1.5 additions. HfO2 was found to form HfSiO4 with acidic melts but undergoes grain boundary penetration in basic melts, which do not exhibit reactive crystallization. The latter can be exacerbated by microcracking resulting from the thermal expansion anisotropy of monoclinic HfO2. Y additions generally degrade the ability to form hafnon (and zircon), and exacerbate grain boundary penetration, especially in HfO2 where Y is present as a fluorite second phase. The fluorite controls grain growth in monoclinic HfO2 and suppresses microcracking, but dissolves faster, especially in basic melts. The results are presented in the context of the relevant thermodynamics and kinetics. The implications for coating applications are discussed.