We report on the microstructural, mechanical and biological properties of novel quaternary TiZrNbSn (in at.%) coatings obtained by magnetron sputter-deposition on Si substrates and Ti disks. In particular, the influence of Nb content on the phase composition and superelastic response of as-deposited Ti-22Zr-xNb-3Sn films are examined. Three compositions were selected corresponding to Nb content x = 10, 17 and 20 at.%. The structure of the films containing 10 and 17 at.% Nb consists of a mix of alpha '' and (3 phases in different proportions while the film with higher Nb content (20 at.%) possesses a single (3 structure. Their superelasticity was studied using a nanoindenter equipped with a spherical tip of 50 mu m diameter. It revealed that the film containing 17 at.% Nb exhibited the highest superelastic behavior with the largest indentation depth recovery ratio values all over the broad range of maximum penetration depths investigated as compared to the values measured for the two other films with 10 and 20 at.% Nb. The biocompatibility tests showed that Ti-22Zr-xNb-3Sn coatings involve high cytocompatibility and do not induce haemolysis, similarly to reference materials (Ti and NiTi).

Grain size effect on superelasticity was investigated in Ti-20Zr-3Mo-3Sn alloys for the same crystallographic texture and for a broad grain size range from 7 mu m to 180 mu m. The dependence of superelasticity on grain size was studied by spherical nanoindentation. Results displayed the same tendency as that observed during tensile tests. The small-sized beta grains favored the martensitic transformation, and exhibited the greater superelasticity. It is confirmed that spherical nanoindentation is capable of characterizing superelasticity and is sensitive enough to probe grain size effect in superelastic alloys. It is also shown that beta grain size evidently affects indentation hardness and modulus.

Nanoindentation was combined with electron backscatter diffraction (EBSD) to investigate the crystallographic anisotropy of indentation responses of individual grains in the superelastic Ti2033 alloy. The high statistic of measurements from grains oriented over a large range of crystallographic directions facilitated a fundamental study covering the entire stereographic triangle of the beta phase. For a highlighted visual effect, results were presented as inverse pole figure distribution maps of depth recovery, work recovery, indentation modulus and hardness. Results showed that the pronounced anisotropy behaviors of indentation modulus, depth and work recovery ratios resulted from the anisotropy in Young's modulus and in compressive lattice distortion between the bcc structure of beta phase and the orthorhombic structure of the stress-induced alpha" martensite phase, respectively. However, orientation dependence was gone when plasticity occurred. The orientation-independency in indentation hardness was due to the various slip systems that might be activated within bcc crystals. (c) 2021 Elsevier B.V. All rights reserved.

A new Ti-16Zr-13Nb-2Sn superelastic film incorporating β-stabilizing and highly biocompatible elements was elaborated by magnetron sputtering. The morphological, crystallographic and microstructural characteristics of the obtained films were studied by scanning electron microscopy, atomic force microscopy, X-ray diffraction, and transmission electron microscopy. Superelastic response of the film was investigated at local scale by nanoindentation using both spherical and Berkovich indenters. The sputter-deposited film revealed nanograined β microstructure with preferential growth orientation along [110] direction and excellent superelastic recovery at room temperature. Special attention was paid to the indenter geometry influencing reliable evaluation of the superelastic nature of the film. Evolution of the deformation mechanisms during nanoindentation at increasing depths was rationalized by the calculated representative strain beneath the indenting tips and is discussed in this work.

Intrinsic stresses in vapor deposited thin films have been a topic of considerable scientific and technological interest owing to their importance for functionality and performance of thin film devices. The origin of compressive stresses typically observed during deposition of polycrystalline metal films at conditions that result in high atomic mobility has been under debate in the literature in the course of the past decades. In this study, we contribute towards resolving this debate by investigating the grain size dependence of compressive stress magnitude in dense polycrystalline Mo films grown by magnetron sputtering. Although Mo is a refractory metal and hence exhibits an intrinsically low mobility, low energy ion bombardment is used during growth to enhance atomic mobility and densify the grain boundaries. Concurrently, the lateral grain size is controlled by using appropriate seed layers on which Mo films are grown epitaxially. The combination of in situ stress monitoring with ex situ microstructural characterization reveals a strong, seemingly linear, increase of the compressive stress magnitude on the inverse grain size and thus provides evidence that compressive stress is generated in the grain boundaries of the film. These results are consistent with models suggesting that compressive stresses in metallic films deposited at high homologous temperatures are generated by atom incorporation into and densification of grain boundaries. However, the underlying mechanisms for grain boundary densification might be different from those in the present study where atomic mobility is intrinsically low.

Atom insertion into grain boundaries generates compressive intrinsic stress in polycrystalline thin films

We present a detailed experimental study on Mo1-xSix thin films, an archetypal alloy system combining metallic and semiconductor materials. The correlations between structure and elastic response are comprehensively investigated. We focus on assessing trends for understanding the evolution of elastic properties upon Si alloying in relation to the structural state (crystalline vs amorphous), bonding character (metallic vs covalent), and local atomic environment. By combining picosecond ultrasonics and Brillouin light scattering techniques, a complete set of effective elastic constants and mechanical moduli (B, G, E) is provided in the whole compositional range, covering bcc solid solutions (x < 0.20) and the amorphous phase (0.20 < x < 1.0). A softening of the shear and Young moduli and a concomitant decrease of the Debye temperature is revealed for crystalline alloys, with a significant drop being observed at x similar to 0.2 corresponding to the limit of crystal lattice stability. Amorphous alloys exhibit a more complex elastic response, related to variations in coordination number, atomic volume, and bonding state, depending on Si content. Finally, distinct evolutions of the G/B ratio as a function of Cauchy pressure are reported for crystalline and amorphous alloys, enabling us to identify signatures of ductility vs brittleness in the features of the local atomic environment. This work paves the way to design materials with improved mechanical properties by appropriate chemical substitution or impurity incorporation during thin-film growth.

Using a multi-beam optical stress sensor, the real-time stress evolution during the early growth stages of a large class of sputter-deposited metal (Me) films is studied with monolayer sensitivity. For high-mobility fcc (Ag, Au, Pd) metals, a typical compressive-tensile-compressive (CTC) behavior is observed, characteristic of a Volmer-Weber growth mode. A correlation between the homologous temperature (T-s/T-m), tensile stress peak position, grain size and steady-state compressive stress in the post-coalescence stage is presented. For low-mobility bcc (Mo, W, Ta) metals (T-s/T-m <= 0.10) deposited on a-Si, kinetic limitations result in a 2D growth mode highly influenced by interfacial effects. The film force is initially dominated by change in surface stress, which scales with the surface energy difference Delta gamma = gamma(Me) - gamma(a-Si). For both Mo and W, a stress transient is observed in the 2-4 nm range, followed by the development of unexpectedly large tensile stress, ascribed to a phase transition towards their equilibrium alpha-Mo and alpha-W structure. Such transient is not evidenced during Ta growth for which a compressive stress regime is steadily established and related to the growth of its metastable beta-Ta structure. For all low-mobility metals, the final stress regime is controlled by the energetics of the incoming species and intrinsic mechanical properties of the material. (c) 2013 Elsevier Ltd. All rights reserved.

Elastic properties of Mo1−xSix alloy films with 0≤x≤1 have been investigated by the Brillouin light scattering technique thanks to the analysis of the surface acoustic waves in the film on substrate: the so-called Rayleigh surface wave and the Sezawa waves. A transition from crystalline to amorphous state is observed for a Si content, x=0.19. This transition is accompanied by different modifications of the elastic constants, namely, C11, C33, C13 and C44 that are necessary with C12 to define the effective hexagonal symmetry for the textured <110> polycrystalline alloys. A pronounced softening of the shear elastic C44 constant is observed, from 110 to 60GPa when x varies from 0 to the critical threshold composition x=0.19. The longitudinal C33 constant estimated in this work from the BLS measurements shows also a softening from 425 to 315GPa. An intrinsic consequence of the high supersaturation of MoSi alloys is the development of an important lattice instability. In the amorphous state, the evolution of the elastic properties, that can be defined by the two independent constants C11 and C44=(C11−C12)/2, exhibits two distinct behaviors depending on the electronic properties and the interatomic bonding of the amorphous alloys.

In situ stress measurements during two-dimensional growth of low mobility metal films on amorphous Si were used to demonstrate the impact of interface reactivity and phase transformation on stress evolution. Using Mo1-xSix films as examples, the results show that the tensile stress rise, which develops after the film has become crystalline, is correlated with an increase in lateral grain size. The origin of the tensile stress is attributed to the volume change resulting from the alloy crystallization, which occurs at a concentration-dependent critical thickness.

Large tensile stresses (up to 3GPa) were previously observed in low-mobility metallic Mo1−xSix films grown on amorphous Si and they were ascribed to the densification strain at the amorphous-crystalline transition occurring at a critical film thickness. Here, we focus on the influence of the nucleation conditions on the subsequent stress build-up in sputter-deposited Mo0.84Si0.16 alloy films. For this purpose, growth was initiated on various underlayers, including amorphous layers and crystalline templates with different lattice mismatch, and the stress evolution was measured in situ during growth using the wafer curvature technique. Tensile stress evolutions were observed on amorphous SiO2 and (111) Ni underlayers, similarly to the stress behaviour found on amorphous Si. For these series, the films were characterized by large in-plane grain size (~500nm). However, on a (110) Mo buffer layer, a different stress behaviour occurred: after an initial tensile rise ascribed to coherence stress, a reversal towards a compressive steady state stress was observed. A change in film microstructure was also noticed, the typical grain size being ~30nm. The origin of the compressive stress source in the metastable Mo0.84Si0.16 alloy grown on (110) Mo is discussed based on the stress evolutions measured at varying deposition rates and Ar working pressures, as well as in comparison with stress evolutions in pure Mo films.