Nano-Impact (Fatigue) Characterization of As-Deposited Amorphous Nitinol Thin Film

Abstract

This paper presents nano-impact (low cycle fatigue) behavior of as-deposited amorphous nitinol (TiNi) thin film deposited on Si wafer. The nitinol film was 3.5 µm thick and was deposited by the sputtering process. Nano-impact tests were conducted to comprehend the localized fatigue performance and failure modes of thin film using a calibrated nano-indenter NanoTest™, equipped with standard diamond Berkovich and conical indenter in the load range of 0.5 mN to 100 mN. Each nano-impact test was conducted for a total of 1000 fatigue cycles. Depth sensing approach was adapted to understand the mechanisms of film failure. Based on the depth-time data and surface observations of films using atomic force microscope, it is concluded that the shape of the indenter test probe is critical in inducing the localized indentation stress and film failure. The measurement technique proposed in this paper can be used to optimize the design of nitinol thin films. Keywords

Cyclic Nanoindentation and Nano-Impact Fatigue Mechanisms of Functionally Graded TiN/TiNi Film

“…In the current investigation, the nanoindentation tests were done in a force control mode with a fixed maximum load of 100 lN, which corresponds to an RID of 0. 16 [32,33,45,46]. The values of hardness (7.8 ± 4 GPa) and modulus (83 ± 42 GPa) measured in this investigation are, however, much lower than those observed for TiN coatings of similar thicknesses deposited by unbalanced magnetron sputtering [48], where typical hardness and modulus values were 22.8 ± 3.4, and 276 ± 41 GPa, respectively.…”

“…The evolution of surface impact response was recorded in situ by monitoring the changes in the positions of the indenter (depth vs. time). This technique was adapted to understand the mechanisms of nano-impact (low-cycle fatigue) failure [32,33] of the constraint TiN/TiNi bilayer which could be different from those of free-standing films due to constraint effect of the substrate, stress gradient effect and existence of residual stress. The failure was defined as a sudden change in depth amplitude with time or number of impacts.…”

“…In contrast to a few reports about the nanoindentation of shape memory thin films primarily focusing on the shape memory effects, pseudo-elastic behaviour, or mechanical properties, etc. [31][32][33][34][35][36], this work sheds new light on the multipleloading nanoindentation (low-cycle fatigue) and nano-impact (relatively high-cycle fatigue) performance of functionally graded TiN/TiNi films. The multiple-loading nanoindentation tests are useful to measure hardness and elastic modulus versus depth in the cases of thin-film and thick coatings, whereas, the multicycle tests are performed at a constant load to investigate the dynamic indentation response to study localised fatigue behaviour.…”

Advanced Nanomechanical Test Techniques

“…With spherical probes at low impact forces so that plasticity is suppressed, a backward depth evolution can be observed (i.e. the on-load probe depth decreases with continued impacts) [105,106]. This atypical behaviour has been confirmed by post-test AFM imaging of impact scars showing volume uplift and has been interpreted as due to interfacial delamination without (or before) the accompanying fracture that results in the increase in depth.…”

Surface characteristics and electrochemical behaviour of sputter-deposited NiTi thin film

“…Among different methods of thin film formation, the magnetron sputtering has become the preferred technique because of important specific advantages such as easy control of the deposition rate, low levels of impurities, and capability to produce thin films with various morphologies and crystallographic structures. So far, structural, mechanical and electrical characteristics of the sputter-deposited Nitinol thin films have widely been investigated [19][20][21][22][23][24]. Corrosion behaviour is of particular importance to determine the biocompatibility of Nitinol devices in medical applications.…”