Structural Studies of Nanocrystalline Thin Pd Films Electrochemically Doped with Hydrogen

Abstract

Abstract. Hydrogen absorption in Pd causes a significant volume expansion. In free-standing bulk Pd, the hydrogen-induced volume expansion is isotropic. However, the situation becomes more complicated in thin Pd films. Contrary to bulk samples, thin films are clamped to an elastically stiff substrate, which prevents in-plane expansion. Hence, the volume expansion of a thin film is strongly anisotropic because it expands in the out-of-plane direction only. Internal stresses introduced by absorbed hydrogen may become so high that detachment of a film from the substrate is energetically favorable and buckles of various morphologies are formed. In the present work, we studied hydrogen-induced buckling in a nanocrystalline thin Pd film deposited on a sapphire substrate. Slow positron implantation spectroscopy (SPIS) was employed as a principal tool for the characterization of defects and investigation of defect interactions with hydrogen. SPIS studies were combined with X-ray diffraction and direct observations of buckling by light microscopy. It was found that buckling of thin Pd film occurs at hydrogen concentrations x H > 0.1 and is accompanied by a strong increase of dislocation density. IntroductionHydrogen dissolved in interstitial sites in a host metal lattice causes a volume expansion, which is isotropic in free standing bulk samples. However, in thin films, in-plane expansion is hindered by clamping of the film to the substrate. This makes hydrogen-induced expansion strongly anisotropic: the in-plane expansion is suppressed, while the out-of-plane expansion is remarkably larger than in a free standing bulk metal. As a consequence, high compressive bi-axial in-plane stresses up to several GPa occur in thin films loaded with hydrogen [1]. These hydrogen-induced stresses grow with increasing hydrogen content and may cause local or global detachment of the loaded film from the surface, which results in formation of buckles with various morphologies [2][3][4]. Understanding of hydrogen-induced buckling is highly important since this process may cause catastrophic adhesion failure in many thin film systems or coatings exposed to hydrogen.Hydrogen-induced buckling occurs when the stored elastic strain energy overcomes the adhesion energy to the surface. Since the formation of buckles represents an irreversible change of film shape (i.e. plastic deformation) one can expect that new defects are introduced during buckling process. Although the morphology of buckles was investigated in a number of works [2][3][4], there is still a lack of information about defect evolution in buckled films. In the present work slow positron implantation spectroscopy (SPIS) was employed for characterization of development of defects in thin Pd films loaded with hydrogen. Defect studies by SPIS were combined with X-ray diffraction (XRD) and direct observations of buckles by light microscopy.