Thermal evolution of yttrium tetraboride thin films – A candidate for high temperature applications

Abstract

The common drawback of ceramic transition-metal boride films, the inherent brittleness, limits their practical applications under thermal load. Yttrium tetraboride (YB4), having a high melting temperature, chemical inertness, high hardness, and low Young's modulus of elasticity in the bulk form, is a promising candidate for thin films capable of withstanding the demanding mechanical conditions at high temperatures. Here, we report the evolution of the chemical composition, nanostructure, and mechanical properties of sputtered, slightly overstoichiometric YB4.7 thin films after vacuum annealing up to 1300 °C. The as deposited YB4.7 thin films have an amorphous character with hardness of 23.4 GPa and low elastic modulus of 280.7 GPa. Annealing at 800 °C leads to partial crystallization of the films and formation of tetragonal YB4 and cubic YB6 phases in an amorphous matrix. These structural transformations are accompanied by a 12% increase in hardness up to 26.4 GPa at 1000 °C while the elastic moduli remain relatively low by approx. 300 GPa. The cube-corner indents revealed a ductile behavior of the as deposited films, while crack propagation along grain boundaries was observed after annealing at 800 °C. The observed mechanical behavior provides the opportunity to introduce the YB4.7 films into multilayer systems with harder but significantly more brittle transition-metal diborides to reduce crack propagation and achieve better overall thermal shock resistance.