Nanoindentation and micro-mechanical fracture toughness of electrodeposited nanocrystalline Ni–W alloy films
Highlights
► Micro-scale cantilevers manufactured by electro-deposition and focused ion beam machining. ► Nanoindenter used to perform micro-scale fracture test on Ni-13at%W micro-cantilevers. ► Calculation of fracture toughness of electrodeposited Ni-13at%W thin films. ► Fracture toughness values lower than that of nanocrystalline nickel.
Introduction
The LIGA (LIGA is a German acronym for lithography, electrodeposition and forming) [1] process for the fabrication of components for micro electro mechanical systems (MEMS) has so far relied mainly on electrodeposited nickel as the construction material. Electrodeposited nickel offers certain advantages: electrodeposition is a well-established industrial technology and it has good toughness in the as-deposited state of 53 MPa √m [2]. However as the conditions MEMS components are expected to operate in become more demanding, its hardness and strength, particularly at high temperature, and its tribological performance may not be adequate. For example Cho et al. [3] have shown that between 20 °C and 400 °C the yield stress of LIGA nickel structures drops by 60% from 370 MPa to 143 MPa. In order to improve the performance of LIGA materials in practical applications a number of electrodeposited alloys have been investigated as potential replacement materials. These include Ni–W [4], Ni–Fe [5], Ni–Co [6], and Ni–Mn [7]. Amongst these, Ni–W alloys have received particular attention as these have the potential to offer useful combination of technologically important properties combined with improved high temperature properties, which are of particular importance in LIGA mould inserts for tools used in hot embossing or injection moulding.
Failure properties of Ni–W alloys have been studied in several micro-scale tensile tests [4], [8]. Haj-Taieb et al. [8] measured a UTS of 0.75 GPa and 1 GPa in Ni5 at.%W and Ni15 at.%W respectively and Yamasaki [4] measured a fracture stress of over 2 GPa in electrodeposited Ni20 at.% W. However none of these studies have attempted to measure the fracture toughness of the samples, partly due to the lack of micro-scale techniques with the ability to do this. Recently, methods have been developed for the determination of fracture toughness of micro-scale specimens. Techniques used for the fabrication of micro-specimens for fracture toughness fall mainly in to two groups: a) focused ion beam (FIB) machining, [9], [10], [11], [12], [13], [14], [15] and b) lithography [2], [16], [17]. Whilst FIB machined cantilevers allow a large degree of flexibility in the materials systems to be studied they are expensive and time consuming to manufacture. It is the objective of the present study to investigate the mechanical properties of electrodeposited nanocrystalline Ni–W alloy film, using lithographically produced micro-cantilevers.
Section snippets
Experimental methods
Ni–W alloy micro-cantilevers were fabricated by electrodeposition on titanium-coated silicon wafers patterned by UV lithography, with FIB machining only being used to produce a sharp pre-crack in each cantilever. Each sample was in the form of a disc of diameter 2 mm, at the centre of which is located a micro-cantilever, Fig. 1. After resist stripping the disc specimen were released by simple tape transfer from the Titanium surface which is possible due to low adhesion. The nominal dimensions of
Nanoindentation
Table 2 shows the average hardness and modulus as a function of depth as measured using the CSM method [21]. Each curve represents the average of 11 tests. The measured hardness is similar to that measured by Schuh et al. [22] (7.2 GPa) and larger than that measured by Haj-Taieb et al. [8] (5.61 GPa) for similar composition samples. Haj-Taieb et al. used micro-hardness testing with a load of 50 g, (≈ 500 mN) whereas the maximum load used during these nanoindentation experiments was 30 mN. Thus an
Discussion and conclusions
The reason for the large spread in fracture toughness values is unclear, but it is thought it may be connected with the sample porosity. Some pores with dimension in the range of micrometres, but with unknown size distribution, were found close to the surface of the samples (Fig. 2c) and Klimenkov et al. [20] have found porosity in the range of 2–40 nm along grain boundaries in a similar material, but no larger porosity is seen on the fracture surface, Fig. 4. The ammonia–citrate bath for Ni–W
Acknowledgement
One of the authors (ASMAH) would like to thank the Alexander von Humboldt Foundation for the financial support. DEJA's research studentship was funded by the EPSRC.
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