ICMCTF 2022 Session H2-2-WeM: Advanced Mechanical Testing of Surfaces, Thin Films, Coatings and Small Volumes II

Abnormal grain growth can occur in polycrystalline materials with only a fraction of grains growing drastically to consume other grains. Here we report abnormal grain growth in ultrafine grained metal in a rarely explored high-cycle loading regime at ambient temperature. Abnormal grain growth is observed in electroplated Ni microbeams with average initial grain sizes less than 640 nm under a large number of loading cycles (up to 10⁹) with low strain amplitudes (< 0.3%). Such abnormal grain growth occurs predominantly in the family of grains whose <100>orientation is along the tensile/compressive loading direction. Micromechanics analysis suggests that the elastic anisotropy of grains dictates the thermodynamic driving force of abnormal grain growth, such that the lowest strain energy density of the <100>oriented grain family dominates grain growth. These results are compared to other recent investigations of cyclic-induced grain growth in metals. In order to establish the general applicability of abnormal grain growth in metals controlled by the elastic anisotropy of single-crystal grains, a high-throughput technique to perform systematic characterization of cycle-induced grain growth in ultrafine grained metallic films with varying degrees of elastic anisotropy is presented.

PVD deposited superlattice structures enable the simultaneous enhancement of hardness and fracture toughness of thin ceramic coatings – evading the strength-ductility trade-off dilemma [1]. While a deeper understanding of this effect has been gained for transition metal nitrides (TMN) [2], hardly any knowledge is yet available for diborides (TMB₂). Here we show that superlattices can—similarly to the nitrides—increase both mechanical properties of diboride coatings. For this purpose, we developed non-reactively sputtered TiB₂/WB₂ and TiB₂/ZrB₂ superlattice coatings, the former is characterized by a high difference in shear modulus (ΔG ~112 GPa), and the latter features a high lattice mismatch (Δa ~0.14 Å) of the participating layer materials.

Nanoindentation, as well as in-situ microcantilever bending tests, yield a distinct increase in hardness (up to 45.5 ± 1.3 GPa) for the TiB₂/WB₂ system but no increase in fracture toughness. Contrary, TiB₂/ZrB₂ shows no increase in H, while K_IC increases by ~ 20% up to 3.70 ± 0.26 MPa∙m^1/2. Similar behavior is observed for cube-corner-based fracture toughness evaluation, however, under the influence of corresponding residual compressive stresses. X-ray diffraction studies show a preferred (001) orientation for most of our coatings, the only exception thereby are the TiB₂/WB₂ superlattices with a bilayer period Λ > 9 nm, where we observe increasing (101) orientation with an increasing bilayer period. Furthermore, the number of satellite peaks and their intensity hints towards sharp interfaces, later confirmed by our HR-TEM studies. These results are discussed and complemented by an extensive literature review.

Keywords: Hard Coatings, Diborides, Physical Vapor Deposition, Micromechanical Testing, Fracture Toughness

[1] R. Hahn, M. Bartosik, R. Soler, C. Kirchlechner, G. Dehm, P.H. Mayrhofer, Scr. Mater. 124 (2016) 67–70.

[2] R. Hahn, N. Koutná, T. Wójcik, A. Davydok, S. Kolozsvári, C. Krywka, D. Holec, M. Bartosik, P.H. Mayrhofer, Commun. Mater. 2020 11 1 (2020) 1–11.

Microcomponents, such as microchips, actuators and sensors, are often based on metallic thin films that must endure cyclic thermomechanical loading during their lifetime. The mechanical properties of these thin films are usually different from bulk materials and so are their thermomechanical fatigue mechanisms. For this reason, an advanced bulge setup was used to cyclically load gold thin films of 150 nm thickness at temperatures in the range 25 °C – 100 °C. The stress-controlled experiments highlight the significance of the interface character for the fatigue lifetime. The presentation will discuss the fatigue properties and damage mechanisms of gold thin films – freestanding and with brittle sublayer - as a function of the microstructure, temperature and stress amplitude.