Micro-Fracture Testing of Ni-W Microbeams Produced by Electrodeposition and FIB Machining
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0983-LL08-07
Micro-Fracture Testing of Ni-W Microbeams Produced by Electrodeposition and FIB Machining David Armstrong1, Abdul Haseeb2, Angus Wilkinson1, and Steve Roberts1 1 Materials Department, Oxford University, Parks Road, Oxford, OX1 3PH, United Kingdom 2 Bangladesh University of Engineering and technology, Dhaka, Bangladesh
ABSTRACT Electrodeposited nickel-tungsten alloys are being considered as a candidate material for components for microelectromechanical systems (MEMS) fabricated by the LIGA (German acronym for lithography, electrodeposition, and forming) technology. In spite of having a useful range of properties including; hardness and strength, better tribological and chemical resistance and improved high temperature resistance as compared with the conventionally used electrodeposited Ni, these alloys possess certain brittleness. In this study, the fracture toughness of Ni-17.5 at%W alloy microcantilever beams (dimension: 60µm x 20µm x 14µm) fabricated by UV lithography and electrodeposition and notched by focused ion beam machining is investigated. Load was applied to the beams using a nanoindenter, which also allowed accurate positioning of the sample. Fracture toughness was calculated from the fracture load assuming a linear elastic behaviour. The Ni-W alloy beams were found to possess a mean fracture toughness of 2.97 MPa √m. The fracture toughness of Ni-W alloy is found to be higher than that of Si – another important MEMS material, but considerably lower than that of electrodeposited nickel and nickel base alloys. INTRODUCTION Electrodeposited nickel has so far been the dominant material in LIGA (a German acronym for lithography, electrodeposition and forming) fabrication of components for microelectromechanical systems (MEMS). Electrodeposited Ni has its own advantages in that its processing by electrodeposition is a well established industrial technology and it has good toughness in the as-deposited state. However its hardness and strength, particularly at high temperature, and its tribological performance may not adequate in many microsystems applications. Researchers are therefore directing their efforts to incorporate alloy additions into nickel during electrodeposition in order to improve the performance of LIGA-MEMS in practical applications. A number of electrodeposited alloys are currently under investigation, e.g., Ni-W, Ni-Fe, Ni-Co, Ni-Mn etc. Among these, Ni-W alloys have received particular attention as these have the potential to offer useful combination of technologically important properties combined with improved thermal resistance. This combination is particularly important in LIGA mould inserts. Studies revealed that electrodeposited nanocrystalline Ni-W alloys can possess an asdeposited hardness of about 700 VHN, which can further be improved by heat treatment [1]. These alloys were found to have a tensile strength of 1930 MPa in the as-deposited state [2] and as high as 2337 MPa in the de-gassed condition [1]. Electrodeposited Ni-W alloys are also
reported to have improve
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