Extraordinary Adhesion of Niobium on Sapphire Substrates

Abstract

We have investigated the adhesion between a metal and a ceramic interface, in particular between Nb(110) films and Al 2 O 3 ͑1120͒ substrates. We have tested the adhesion properties by hydrogen loading of the metal film. The resulting strains were measured by in-and out-of-plane lattice parameter measurements. In contrast to the bulk behavior, we find for hydrogen in thin Nb films a highly anisotropic in plane strain. Comparing the in-plane expansion with the free case, we observe a tensile stress exceeding the yield stress of bulk Nb by at least a factor of 10. This unusually large resistance against expansion is due to giant adhesive forces at the metal/ceramic interface.[S0031-9007(97)04843-6] PACS numbers: 68.60.Bs, Understanding adhesion properties of metals on ceramic substrates is one of the most intriguing and important topics of current materials research [1]. High temperature composite materials, corrosion resistant coatings, and electronic packaging rely on the adhesive strength of metal/ ceramic interfaces. Two different models have been brought forth to describe the interfacial bond energy. In one of them the electrostatic image charges between the oxide ions and the metallic surface are made responsible for the bonding [2,3]. Other models favor a hybridization effect between the oxygen p-and the metal d-orbitals, which may lead to a slightly covalent or ionic bond at the interface [4,5]. The latter model requires a well-defined epitaxial relationship at the metal/ceramic interface, whereas the former is independent of specific atomic coordinations. In practice, good adhesion has been observed at both, heteroepitaxial pseudomorphic, as well as amorphous metal/ceramic interfaces. Therefore, a decision in favor of one or the other model is difficult to make.Niobium thin films on sapphire substrates can be considered as a model system for studying adhesion properties between metals and ceramics. The growth of Nb on sapphire has been studied in much detail recently and the epitaxial relation is well understood [6][7][8][9][10][11]. The Nb͞Al 2 O 3 interface can be described by a unique threedimensional epitaxy characterized the following orientational relationships [7-10]:Nb grows on the sapphire substrate almost strain free and forms a highly ordered crystal structure with large structural coherence lengths [12,13]. The residual epitaxial strain which depends on the film thickness is usually less than 0.05%. Small amounts of hydrogen may help to release residual stresses and the crystal structure becomes even more perfect due to a "cold" annealing effect [14]. Furthermore, Nb and sapphire have nearly the same thermal expansion coefficients [15]. Therefore thermal strains are not confused with epitaxial or applied strains.Measuring the adhesion of an epitaxial film is not an easy task. Usually the films are bent together with the substrate and the critical radius of curvature is determined before the film lifts off [16]. We have probed the adhesion properties of the metal/oxide interface in a different ...

Hysteresis and the single-phase metal-insulator transition in switchableYHxfilms

“…Clamping of the film to the substrate on which it is deposited, induces a strong anisotropy in the expansion. 50 However, we also observed the phenomena mentioned in this paper in YH x films deposited on different, flexible ͑polymer͒ substrates. This implies that clamping is not responsible for the large hysteresis observed in YH x .…”

Variations in the Nature of Metal Adsorption on Ultrathin Al2O3

“…The oxygen polarization therefore goes in the opposite direction upon adsorption of Nb and other metals to the left of Mo (which already have more of their core exposed). Together with the ionicbond supplement, this explains the strong binding for Nb at 1 ML recently found in experiment [21]. Thus Nb binds strongly without strong polarization.…”

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“…The behavior of hydrogen in samples with reduced dimensions is one of the more recent areas of research in metal-hydrogen systems [20,158,159,160,161,162,163,164,165,166,167,168,169,170]. The properties of these systems are affected by the proximity of surfaces or interfaces providing additional sites to the DOSE.…”