ICMCTF2014 Session E2-2: Mechanical Properties and Adhesion

Metals containing nanoscale growth twins have shown remarkable mechanical performance compared to their nanocrystalline counterparts. In particular, nt-Cu has shown concurrent increases in strength and ductility with decreasing twin spacing, in addition to enhanced mechanical and thermal stability. In order to accurately generalize the performance of nanotwinned metals, it is necessary to synthesize and test other metals containing growth twins. In this study, ultra-fine grained CuAl foils with varying amounts of growth twins were tested in tension and compression, and their mechanical properties and deformation are compared to nt-Cu. Extensive characterization of the twins and their stability under different deformation modes will also be presented.

Concentrated, nanosecond-pulsed laser exposure of oxidizing metals in ambient atmosphere produces highly colored, well-adhered oxide coatings, which are particularly interesting for use as unique authenticity identifiers on welded or sealed components. The combined properties of the oxide-substrate system control the coupled electromechanical behavior and environmental stability of the oxide. Laser-processing parameters dictate oxide thickness, which in turn defines color, residual film stress, and fracture behavior. Oxides grown on stainless steel 304L and CP grade II titanium are residually stressed in tension leading to through-thickness cracking at film thicknesses greater than ~100 nm. Hardness, fracture behavior, and residual stress are highly dependent upon thickness suggesting that internal defects influence mechanical behavior. Conducting nanoindentation reveals an increase in conductance with decreasing oxide thickness, further suggesting the dependence of mechanical behavior on fluctuations in oxide defect density. Additionally, post mortem energy dispersive spectroscopy of oxides on SS 304L exposed to an aggressive environment manifests a Cr-denuded zone in the substrate immediately beneath the oxide, ultimately creating a microstructure that is susceptible to corrosive attack. Furthermore, the fracture behavior and environmental resistance analyses can be combined, by performing high-load, conical indentation in a fluid cell containing various inert and corrosive solutions, to evaluate the coupling of mechanical stresses and environmental effects. The integration of multiple characterization techniques provides a unique approach for defining electro- and chemo-mechanical performance of the oxides in harsh operating environments. This work was partially supported by DTRA Basic Research Award # IACRO 12-2026I and by Sandia National Laboratories, a Lockheed Martin Company for the USDOE NNSA under contract DE-AC04-94AL85000.

In this paper, the creep behaviours of NiTiW thin films at various W contents (2.6-33.6%) were investigated using nanoindentation creep testing method. With W content ranging from 2.6 at.% to 4.5 at.%, the films are strengthened and exhibit much reduced strain rate 6.76×10^-4 s^-1 indicating highest creep resistance. With further increase in W content beyond 4.5 at.% strain rate increases and therefore creep resistance of films decrease gradually. The stress exponents were calculated from the loading curves. The results shows that stress exponent for NiTi was 8.2 and increased to 20.5 for NiTiW (2.6) and 22.9 for NiTiW (4.5) and decreased rapidly to 9.5 increasing the W concentration from 9.1 to 33.6 at %. The mechanism for the room temperature creep is discussed in framework of dislocation dynamics. Grain boundaries play an important role in creep behaviour. Studying the deformation behavious of NiTiW thin films has technological importance because of their various applications in micro and nano-electro-mechanical systems.

Keywords: Thin films, nanoindentation creep, grain boundaries

The indentation test of coated systems allows the analysis of mechanical properties of singular constituents, or of the entire system, including material constitutive behavior and failure properties. Due to the progressive loading and unloading of the indentation, both cohesive and adhesive failures can occur in the coating and at the coating/substrate interface, respectively. In this work, the Finite Element Method (FEM) was applied to develop a numerical model based on a spherical rigid indenter in contact with a coated compliant substrate. The coating behavior was defined based on the properties of a brittle pure elastic material, while the substrate was assumed as a ductile elastic-perfectly plastic material. Both cohesive and adhesive failure models were included in the analyses, allowing the evaluation of failure modes in the coating and/or at the coating/substrate interface. The eXtended Finite Element Method (XFEM) was applied to reproduce the cohesive cracks through the coating thickness, while the Cohesive Zone Model (CZM) was used to evaluate the coating/substrate interfacial crack. The failure modes were analyzed for a range of coating material properties (Young’s modulus, fracture toughness and cohesive crack propagation energy) and coating/substrate interface properties (interface toughness and adhesive crack propagation energy). Results allowed identifying not only different modes of cohesive and adhesive failures, but also the main mode as a function of the constituent properties. Also, the derivatives of the forces acting upon the indenter allowed the characterization of the failure modes, indicating that both cohesive and adhesive failures may generate individual signatures on the load-displacement (P-h) indentation curves.