Research

 

Professor Komvopoulos is the director of the Surface Mechanics and Tribology Laboratory (SMTL), which occupies 733 ft2 of the fifth floor of the Department of Mechanical Engineering at the University of California at Berkeley (5119 Etcheverry Hall). The main research activity in SMTL is on thin-film processing and characterization, laser materials processing, surface engineering and fatigue in MEMS, tribology of magnetic record-ing media, surface contact mechanics, fatigue, and tribological properties of biomaterials. This research program is mainly funded through grants from NSF, DARPA, CML, and the University Research Board.

SMTL is equipped with a Perkin-Elmer Randex RF sputtering facility, AFM and STM Digital Nanoscope II instruments, a Hysitron surface force microprobe apparatus for nano-tribological studies, a scratch tester, a pin-on-disk tribometer, a ball-on-flat Fallex tribometer, excimer and pulsed Nd-YAG lasers, and a computational facility consisting of an IBM RS6000 Model 540 system, a DEC 3000 Model 300LX workstation, two INTEL-Pentium 586-166DX stations, one DELL-Pentium 586 station, and an NEC 386/20 PC.

Summaries of Ongoing Research Projects

1. Effect of Microfabrication, Environment, and Cyclic-Loading on the Long-Term Durability of Thin-Film Structures
The objective of this research is to investigate the effects of fabrication process parameters (e.g., doping, etching, and annealing), microstructure characteristics (e.g., grain size), type and amplitude of loading (e.g., pure bending versus multi-axial), and environmental conditions (e.g., temperature and humidity) on the mechanical behavior and endurance of thin-film structures. Microstructure and mechanical property characterization is performed at the microscale using state-of-the-art surface imaging and surface force microprobe instruments, a custom-made vacuum multi-probe station specifically designed to simulate typical micromachine operating conditions, and various microanalysis techniques. Special thin-film structures are tested under controlled loading and environmental conditions to examine changes in the material behavior under cyclic loading and different temperature and relative humidity levels. Results from this work are expected to yield insight into the effects of various intrinsic and extrinsic parameters on the long-term mechanical properties of thin-film structures.

2. Stiction in Microelectromechanical Systems (MEMS)
The objective of this research is to study adhesion and microtribological behavior relevant to micromachines. The test vehicles for this study are micromechanical devices designed and fabricated with friction and wear testing in mind. High static friction (stiction) is investigated by testing friction couples relevant to micromachines. Passive microstructures exhibiting a wide range of stiffness and surface roughness are used to gain further insight into the magnitude of the stiction force as a function of surface roughness and surface energies produced from different texturing and chemical treatment processes. Emphasis is given to the study of high friction leading to sticking during operation resulting from capillary, van der Waals, and electrostatic force effects. Atomic scale friction behavior is studied analytically by performing molecular dynamic simulations of normal and sliding contact to analyze the dependence of friction on lattice parameters, direction and speed of scanning, and temperature.

3. Deposition, Microstructure, and Nanomechanical Properties of RF Sputtered Ultra-Thin Amorphous Carbon Films
Basic research is performed on the dependence of the composition, microstructure, topography, and nanomechanical properties of ultra-thin carbon films on plasma deposition conditions and the nanotribological properties of these films. Nanometer-thick (e.g., 10-100 nm) amorphous carbon (a-C) and nitrogenated amorphous carbon (a-CNx) films possessing different chemical compositions and amounts of sp2 and sp3 carbon-bond hybridization are synthesized under controlled RF sputtering conditions. The processing parameters that are studied are the RF power, chamber pressure, substrate bias voltage, and deposition rate. The structural phases and chemical compositions of the a-C and a-CNx films are studied by XPS, XAES, and Raman spectroscopy. The surface topographies are characterized by AFM, and the nanomechanical properties (e.g., hardness, friction, and wear) are investigated with a surface force microprobe apparatus interfaced with an AFM. Emphasis is on the identification of optimum film processing parameters and the study of microscale tribophenomena. The results of this research program are expected to enhance the technological basis for the design of anti-friction, wear-resistant interfaces for leading-edge technologies, such as high-density magnetic recording.

4. Fatigue of Polysilicon MEMS Devices
In view of recent rapid developments in micromachine devices, there is a pressing need to obtain information about the dynamic material behavior at scales relevant to MEMS. The significance of such information is of paramount importance to the development of reliable MEMS devices capable of performing sensing, actuation, and computing functions in a robust manner. To increase the reliability and longevity of micromachines, it is essential to accurately determine the material response to both static and dynamic loading conditions. The present understanding of fatigue at the MEMS scale and, more importantly, the effect of processing parameters, geometry, loading, and environmental conditions is sparse. This research is expected to bridge this gap by generating the necessary knowledge that will lead to increased productivity and significant broadening of the application range of MEMS devices.

This research program is seeking to develop novel microstructures for fatigue testing under conditions typical of most MEMS devices. The main goals of this research are:

(a) Develop a new microstructure design appropriate for performing multiaxial fatigue testing at the MEMS scale.

(b) Determine new experimental procedures for fatigue testing under controlled environmental conditions resembling those in MEMS applications.

(c) Identify a statistically relevant experimental scheme for fatigue testing by controlling the amplitude during resonance to account for changes in compliance due to fatigue damage.

(d) Study the dependence of fatigue life on specimen size, etch type, operation temperature, anneal time, and relative humidity, and the effects of different processing conditions on the elastic modulus and residual stress in polysilicon using resonant microstructures processed on the same wafer as the fatigue specimens.

(e) Examine the effect of wet etching on grain size and defect density on polysilicon surfaces.

(f) Perform analytical and FEM modeling of fatiguing specimens based on phenomenological observations obtained from the experimental studies.

5. Surface Contact Mechanics
Finite element analysis of half-space media with and without surface layers subjected to normal and tangential surface traction are conducted to illustrate the significance of the layer thickness, magnitude of contact stresses, interfacial friction, and mechanical properties of the layer and substrate media on the elastoplastic deformation behavior and subsurface cracking. Early FEM work was focused on the 2D (plane strain) elastic and elastoplastic deformation of indented layered media. More recent work has been on the axisymmetric and 3D normal and sliding contact problems, including the effects of material hardening and asperity spacing. In particular, 3D FEM simulations have demonstrated the effect of strain hardening on the residual stress field arising in thin-film (10-30 nm thick layers) media subjected to repetitive normal loads well above the yield point. Results showing the magnitude and location of the maximum von Mises equivalent stress, the equivalent plastic strain, the maximum (principal) tensile stress, and the stress amplitude within a load/unload cycle were obtained as a function of normal load and number of loading cycles. The conditions leading to shakedown and ratcheting have been studied for both homogeneous and layered media. FEM modeling of subsurface cracking in homogeneous elastic and elastoplastic media has yielded solutions for the stress intensity factors, the crack tip sliding and opening displacements, and the crack growth direction in terms of the position of the surface traction, surface and crack face friction, and location/size of subsurface crack. FEM simulations dealing with multiple asperity contacts showed the effect of neighboring asperities on the elastic stress field of homogeneous half spaces subjected to normal loading. Results demonstrated that deviati-ons from the classical Hertz solution could be encountered, depending on the indentation depth and spacing of the asperities. Current simulations involve elastic-plastic contact of real surfaces characterized by fractal geometry.

6. Tribology of Total Joint Replacements
In a total knee joint replacement, a metallic femoral component articulates against an ultra-high molecular weight polyethylene (UHMWPE) tibial component. The life of this orthopedic component is limited by the generation of polyethylene particles causing implant loosening and chronic pain associated with osteolysis. Reduction of wear debris is critical to extending the life of joint replacements, and basic understanding of the fundamental wear mechanisms is essential to achieving this goal. The accumulation of plastic deformation coupled with texture development of the polyethylene crystal lamellae are considered to be the prime reasons for the formation of wear debris by delamination wear. This is supported by recent work in our laboratory focused on the structural evolution of UHMWPE occurring due to the effects of contact stresses and different sliding speeds. Wear tests performed with polished CoCr alloy pins and UHMWPE flat disks lubricated with bovine serum at a range typical of physiological pressures and relative speeds and transmission electron microscopy have revealed that lamellae alignment parallel to the sliding direction may occur even at low contact pressures. This is an early warning of delamination wear, a damage process leading to excessive wear debris formation. Ongoing research is focused on the plasma-assisted modification of the surface chemistry and microstructure of medical-grade UHMWPE and testing of the surface topography and friction and wear properties using an atomic force microscope and a pin-on-disk apparatus under contact conditions resembling those at artificial knee replacements. The ultimate objective is to determine the effect of the plasma chemistry and process conditions on resulting surface microstructures and chemical state and, in turn, on the tribological properties.

7. Analysis of Stiction at the Head-Disk Interface
High adhesion forces are often encountered at the head-disk interface (HDI) of disk drives due to the development of capillary attractive forces. Permanent adhesion of the slider onto the disk surface (referred to as stiction) may occur when the total interfacial force is excessive. In this research, 2D and 3D fractal topography descriptions are incorporated into an elastic-plastic contact mechanics analysis of asperity deformation. Contact simulations are performed to reveal the contribution of capillary and asperity deformation forces to the total interfacial force for various material systems (e.g., carbon/carbon and carbon/alumina) and different mean surface interference distances (or contact loads). A pronounced effect of surface roughness, chemical state (hydrophilicity), and relative humidity on the stiction force at the HDI has been demonstrated. Results for the magnitude of the stiction force have been obtained for sliders of different size and crown at various humidity levels. In addition to the analytical work, the apparent friction force and electric contact resistance at the magnetic head-disk interface have been investigated for textured and untextured disks lubricated with perfluoropolyether films of different thicknesses. Our work shows that the initial stick time (representing the time between the application of a driving torque and the initiation of interfacial slip) can be determined based on the initial rise of the friction force and the abrupt increase of the electric contact resistance. It has been observed that relatively thin lubricant films yield very short initial stick times and low static friction coefficients. However, for a lubricant film thickness comparable to the equivalent surface roughness, relatively long initial stick times and high static friction coefficients occurred. The peak value of the apparent friction coefficient is low for thin lubricant films and increases gradually with film thickness. In view of the variations of the initial stick time, static friction coefficient, and peak friction coefficient with the lubricant film thickness and surface roughness, a physical model of the lubricated interface was introduced. This model accounts for the effects of the lubricant coverage, effective shear area, saturation of interfacial cavities, limited meniscus effects, and the increase of the critical shear stress of thin liquid films associated with the solid-like behavior exhibited at a state of increased molecular ordering.

 

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Send comments to slavin@euler.me.berkeley.edu Latest update: October 8, 1998