Abstract
The common drawback of ceramic transition-metal boride films, the inherent brittleness, limits their practical applications under thermal load . Yttrium tetraboride (YB 4 ), 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 YB 4.7 thin films after vacuum annealing up to 1300 °C. The as deposited YB 4.7 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 YB 4 and cubic YB 6 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 YB 4.7 films into multilayer systems with harder but significantly more brittle transition-metal diborides to reduce crack propagation and achieve better overall thermal shock resistance . • Amorphous, overstoichiometric YB 4.7 thin films with hardness of 23.4 GPa • Chemical, structural and mechanical properties after annealing up to 1300 °C • Nanocomposite character of YB 4 /YB 6 nanograins embedded in a disordered matrix • Hardness increase by 11% after vacuum annealing at 1000 °C • Formation of YB 6 phase at higher temperature
Published Version
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