HARD FILM COATINGS FOR HIGH-SPEED ROTARY MEMS SUPPORTED ON MICROBALL BEARINGS Brendan Hanrahan 1,4 , Matthew McCarthy 2 , Josh Balsam 3 , C. Mike Waits 4 , Hugh Bruck 3 , and Reza Ghodssi 1 1 MEMS Sensors and Actuators Laboratory, Dept. of Electrical and Computer Engineering, Institute for Systems Research, University of Maryland, College Park, MD 20742, USA 2 Mechanical Engineering Dept., Massachusetts Institute of Technology, Cambridge, MA 02139, USA 3 Dept. of Mechanical Engineering, University of Maryland, College Park, MD 20742, USA 4 The US Army Research Laboratory, Adelphi, MD 20783, USA Abstract: Titanium Nitride (TiN) and Silicon Carbide (SiC) coatings deposited on the surface of silicon raceways are evaluated in a microturbine. Nanoindentation is employed to study the properties of the hard-thin- film/silicon raceway system and the tribological platform is evaluated through turbine operation curves. TiN films are shown to stay intact over the speeds and forces in the range relevant to future power and energy applications applications, (500-10,000rpm and 10-50mN, respectively), while SiC films wear almost instantaneously. Evaluation of the dynamic friction torque versus normal load relationship between the TiN and bare Si systems suggest the gradual generation of wear debris, comprised of either the raceway or microballs, is negating the benefits of enhanced mechanical properties in TiN. Keywords: tribology, friction, microturbine, microball bearing INTRODUCTION Silicon microfabrication technology has allowed researchers to scale rotary machines such as pumps, generators and motors to continuum and microscale structures for compact power systems. A major challenge in realizing robust power micro- electromechanical systems (PowerMEMS) is providing high-performance mechanical support for surfaces under load in relative motion. Microball bearings have been demonstrated to be viable for micromotors and micropumps across a large speed range [1,2]. In the case of the [2], SiC was applied on the surface of the micromotor to act as low friction, low wear material and thought to enable lower friction operation than bare silicon, however no tribology has been performed and was only limited to operation at low speed/load. Measurements of friction force (or torque), COF, and wear are, by-in-large, limited to the sliding regime for microsystems [3]. Randomly distributed micro rolling contact bearings have been studied in [4]. Ghodssi, et al., first explored the frictional behavior of steel, 285 μm diameter, ball bearings within a microfabricated ball/raceway system [5]. This work presented a statistical analysis of the static COF for bare silicon (μ=0.0560), thin film silicon nitride (μ=0.0290), and chromium (μ=0.0235) micromachined surfaces with steel balls. A microscale analog to a full compliment planar thrust bearing was first demonstrated in a MEMS device in [6]. McCarthy, et al., reported a load and speed dependant dynamic COF values for a planar contact bearing from 0.0005-0.025 [7]. This study aims to illuminate the potential for hard film coatings in a rolling bearing configuration realized within a high-performance rotary microsystem. This work focuses on the enhancement of a previously reported microturbine [7] by using SiC and TiN hard film coatings on the raceways of the hybrid ceramic/metal thrust ball bearings with the improved bearing design from [8]. EXPERIMENT Microturbine Design The microfabricated silicon test device, shown in Fig. 1, consisted of two main components: a radial inflow turbine layer for actuation, and the encapsulated ball bearing. Stator Rotor Turbine Structure Load 1 4 2 5 3 1. Wafer 1: Shallow race, 290μm x 85μm 2. 440C Steel Microball, D= 285μm 3. Wafer 2: Deep race, 290μm x 205μm 4. Hard Film coatings in red 5. Journal release etch Wafer 2 Wafer 1 Plumbing Wafer Fig. 1: Cross section schematic of the microturbine tribology test device with a close-up of the raceway. PowerMEMS 2009, Washington DC, USA, December 1-4, 2009 0-9743611-5-1/PMEMS2009/$20©2009TRF 589