Manufacturing of metal-based microparts: Fabrication strategies and surface engineering applications Yang Mu a , Ke Chen a , Bin Lu a , W.J. Meng a, ⁎, G.L. Doll b a Department of Mechanical Engineering, Louisiana State University, Baton Rouge, LA 70803, USA b University of Akron, Akron, OH 44325, USA abstract article info Available online 19 September 2013 Keywords: Micromanufacturing Replication Compression molding Roll molding Conformal coatings Interfacial shear strength A wide range of industrial applications, current and anticipated, drive the process of miniaturization of thermal and chemical devices. In certain cases, metal-based microsystems enjoy advantages of performance, cost, or both, over silicon-based counterparts. The critical bottleneck restricting the use of metal-based microsystems in actual appli- cations has often been the lack of effective and economical manufacturing methods for metal-based microparts. In this paper, our recent efforts in fabrication of metal-based high aspect ratio microscale structures through various replication strategies, such as compression molding and roll molding, are summarized. The need for engineering surfaces of microscale replication tools through conformal coating deposition is illustrated. Two attempts to exper- imentally measure shear strength of coating/substrate interfaces are described. An estimate of the limiting shear strength of coating/substrate interfaces is achieved through examination of transverse coating cracks induced by substrate tensile loading. Experimental data obtained from two series of TiN/Cr/steel and TiN/Ti/steel specimens are presented. Issues associated with this approach are discussed. Direct failure of coating/substrate interfaces has been induced through compression loading of micro pillars containing inclined coating/substrate interfaces. Preliminary experimental results obtained from TiN/Ti/Si micro pillars are presented. © 2013 Elsevier B.V. All rights reserved. 1. Introduction The global microelectromechanical system (MEMS) market has grown from its infancy in early 1980s [1] to be worth about ten billion U.S. dollars in 2011 [2]. So far, major categories of commercialized MEMS devices, including pressure sensors, accelerometers, gyroscopes, and deflectable optical mirrors, have been fabricated from Si-based ma- terials following protocols developed for integrated circuit processing industries [3]. Although metal-/alloy-based microsystem products are much rarer at present, their unique collection of physical properties enables construction of certain microdevices with either performance advantages over Si-based counterparts or no Si-based analogs at all. Piezoelectric Pb(Zr, Ti)O 3 (PZT) thin films have been vapor deposited directly onto 50 μm thick Ti substrates [4]. Using the PZT/Ti material combination, MEMS scanner mirrors have been built with increased frac- ture resistance as compared to Si-based counterparts [5]. Free-standing microscale structures made of electrodeposited Co–Ni alloys have been demonstrated, with soft magnetic properties suitable for magnetic field actuation [6]. Micro grippers and other microscale actuators are enabled by the use of shape memory alloys in microsystems [7]. Wings for micro aerial vehicles were fabricated out of the Ti–6Al–4V alloy because of its mechanical properties [8]. The incorporation of metals/alloys into microsystems enables different device configurations/designs, and metal-based microsystem products hold promise technologically for improved performance or expanded functionality and commercially for additional applications and associated revenue streams. In addition to microscale actuators and sensors, the continued trend towards miniaturization of electronic and medical devices demands efficient manufacturing of small metal-based parts, including miniatur- ized connector pins, sockets, screws, springs, etc. [9]. To satisfy such demands, there are ongoing efforts focused on extending macroscale metal forming technologies down to the microscale [9,10]. Studies of micro bending [11], micro extrusion [12], and micro die upsetting [13] have all been conducted. Fluid flow within microchannels furnishes another category of appli- cations for microsystems. Tuckerman and Pease demonstrated in 1981 that liquid flow within enclosed microchannel structures leads to signifi- cantly increased rates of liquid–solid convective heat transfer [14]. The potential application of microchannel heat exchangers to improve cooling of high-performance microelectronic modules motivated intense studies of heat transfer in microchannel geometries [15]. Although initial studies were carried out on Si-based microchannels due to fabrication conve- nience [14,16], heat transfer in metal-based microchannel structures has always been of interest because of their higher bulk thermal conduc- tivities and increased mechanical robustness [17]. In addition, high- aspect-ratio, enclosed, Ni-based microchannel structures have been utilized successfully to build micro gas chromatograph columns [18]. Var- ious metal-based microchannel structures have been used as building blocks for micro chemical reactors [19]. Surface & Coatings Technology 237 (2013) 390–401 ⁎ Corresponding author. 0257-8972/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2013.09.014 Contents lists available at ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat