Stress development in thin yttrium films on hard substrates during hydrogen loading

Abstract

Polycrystalline (0002)-textured yttrium (Y) films of 50 500 nm thickness on sapphire substrates were loaded electrolytically with hydrogen (H). The stresses which build up in these films were measured in situ using curvature measurements. The results are compared to the behavior of bulk Y-H. A linear elastic model is used to predict the behavior of clamped thin films. Basic properties of the bulk Y-H phase diagram and elastic constants resemble the measured values of the thin films. Compressive stress builds up during H-loading in the alpha-Y phase and in the (alpha-Y+beta-YH2) two-phase field, showing An initial stress increase of -1.3 GPa per hydrogen concentration X-H (compressive stress). While bulk Y-H samples are known to show a contraction in the beta-YH2 phase during H loading, thin films show no evidence for such a contraction during the first loading cycle of the film. The stress remains constant in the bulk beta-phase concentration range (DeltaX(H) = 0.1 H/Y). This is attributed to the narrow beta-phase field (DeltaX(H)= 0.02 H/Y) of the thin film during the first loading.. Only samples which have been kept at a hydrogen concentration of about 1.5 H/Y for weeks show tensile stress in the concentration range of the bulk beta phase. Amazingly a stress increase of about + 0.5 GPa/X-H (tensile stress) is measured in the beta + y two-phase field. This is attributed to the smaller in-plane nearest-neighbor distance in the y phase compared to the beta phase. In the gamma-phase field compressive stress is built up again, compensating the tensile stress. It increases by - 1.3 GPa/X-H. In total, the net stress in Y-H films remains comparably small. This could be a reason for the good mechanical stability of such Y-H switchable mirrors during H cycling. (C) 2003 American Institute of Physics.