Hardness and oxidation resistance of the perovskite-type RRh 3B xC 1− x (R = Y, Sc)

Abstract

Perovskite-type RRh 3B and RRh 3C (R = Y, Sc) form a continuous solid solution, RRh 3B x C 1− x , in the range of 0 ≦ x ≦ 1 with cubic structure (space group: Pm3 m, Z = 1). The values of the microhardness of YRh 3B x C 1− x for x = 0, 0.25, 0.50, 0.75 and 1.00 are investigated as 4.4 ± 0.1, 4.9 ± 0.1, 5.5 ± 0.2, 6.4 ± 0.2 and 7.5 ± 0.15 GPa, respectively. On the other hand, the values of the microhardness of ScRh 3B x C 1− x for x = 0, 0.25, 0.50, 0.75 and 1.00 are 4.5 ± 0.2, 6.1 ± 0.2, 7.4 ± 0.2, 8.9 ± 0.2 and 9.6 ± 0.1 GPa, respectively. Thus, the microhardness of RRh 3B x C 1− x continuously becomes larger with increasing boron content. The oxidation onset temperatures of YRh 3B x C 1− x for x = 0, 0.25, 0.50, 0.75 and 1.00 are 604, 631, 655, 687 and 978 K, respectively. On the other hand, the oxidation onset temperatures of ScRh 3B x C 1− x for x = 0, 0.25, 0.50, 0.75 and 1.00 are 674, 675, 695, 725 and 753 K, respectively. Thermogravimetric analysis of the phase indicates that the oxidation onset temperature also increases with boron content. Thus, it appears that both mechanical strength and chemical stability of the RRh 3B x C 1 −x phase essentially depend on its boron content. Ab initio calculations have been performed to obtain the equilibrium lattice constants and the bulk moduli. The calculated lattice constants are in excellent agreement with experimental results.