Research
Kyriakos Komvopoulos
Professor Komvopoulos is the Director of the Surface Mechanics and Tribology
Laboratory (SMTL), which occupies 733 ft² on 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 mechanical
characterization of thin films, plasma, ion-beam and laser-aided thin film
deposition, surface engineering and reliability of MEMS devices, tribology and
mechanics in magnetic recording, analytical and finite element contact
mechanics studies, fatigue and fracture of coatings, high-temperature
formulated lubricants for automotive applications, and nano-/micro-scale
behavior of polymers and biomaterials. This research program is mainly funded
through grants from NSF, DARPA, Sandia National Laboratories, NIST, CML,
Chevron Chemical Co., and the UC Berkeley Research Board.
SMTL is equipped with RF sputtering facility (Perkin-Elmer Randex), ion beam
film deposition system, high-vacuum system for polymer plasma treatment and
MEMS monolayer deposition, multi-probe environmental chamber (MMR
Tech-nologies, Inc.) for stiction and reliability testing of various MEMS
devices, AFM and STM (Nanoscope II, Digital Instruments), surface force
microscope (Triboscope, Hysitron, Inc.) for nano-mechanical/tribological
studies, scratch tester with acoustic emission capability, pin-on-disk tribometer,
ball-on-flat tribometer (Falex Inc.) with in situ electrical contact resistance
capability, and a computational facility consisting of an IBM RS6000-540
system, an NEC 386/20 PC, a Dell-Pentium 586 station, two Pentium I
workstations, six Intel-Pentium III stations, and two Dell-Pentium 4 Dimension
8200 stations.
The main thrust areas in SMTL are: (a) contact
mechanics, (b) MEMS, (c) thin films, (d) biopolymers, and (e) lubrication.
Project summaries accompanied by selected results of the various projects,
currently in progress in MSTL, are given below.
A. Contact Mechanics
1.Contact Stress Analysis of Elastic Layered Media with Fractal Surface Topographies
2. Finite Element Contact Analysis of Elastic-Plastic Layered Media
3. A Generalized Contact Mechanics Analysis of Elastic-Plastic Spherical
4. Molecular Dynamic Analysis of Indentation and Sliding Contact
B. MEMS
5. Adhesion Forces and Dynamic Friction at MEMS Interfaces
6. Fatigue of Polycrystalline MEMS Devices
7. Dynamic Analysis of MEMS Resonators
8. Stress Analysis and Crack Propagation Simulations in Polycrystalline Silicon
9. Design and Fabrication of Electrical Contacts for
IC Probing at the Wafer Level
C. Thin Films
10. Effect of Plasma Conditions in Low-Pressure Radio-Frequency Discharges on Thickness and Surface Roughness of Thin Carbon Films (Single and Multilayer)
11. Femtosecond Laser Aperturless Near-Field Surface Nanomachining Assisted by Scanning Probe Microscopy
D. Biopolymers
12. Tribology of Total Joint Replacements
13. AFM and SFG Vibrational Spectroscopy Studies of Biopolymers
E. Lubrication
14. Tribological Behavior of Steel Surfaces Lubricated with Gear Oil Containing Sulfide, Phosphate,and Metal Deactivator Additions
Summaries of Ongoing Research Projects
1. Contact Stress Analysis of Elastic Layered Media
With Fractal Surface Topographies
Knowledge of the contact
stresses generated when two rough surfaces come into contact play a critical
role in understanding most mechanisms of friction and wear. A
two-dimensional contact model was developed for elastic layered media with
rough surfaces characterized by fractal geometry. Contact pressure profiles and
stresses are computed based on the distribution of real asperity microcontacts.
Numerical results for the contact load and real contact area are presented as a
function of fractal dimension, elastic properties, and maximum surface
interference. Results for the surface tensile stresses that may cause surface
cracking have been obtained in terms of the maximum surface interference and
friction coefficient. The dependence of subsurface stress state on the modulus
ratio, maximum surface interference, fractal dimension, and friction
coefficient is also examined in light of the evolution of von
Mises equivalent stress in the layered media. This work is currently
extended to the thermomechanical analysis to include frictional heating.
2. Finite Element Contact Analysis of Elastic-Plastic
Layered Media with Patterned Surfaces
A
three-dimensional finite element analysis of a sphere (rigid or deformable)
indenting and sliding over an elastic-plastic layered medium with a patterned
surface consisting of equally spaced rectangular pads was conducted in order to
investigate the effect of surface patterning on the contact pressure
distribution and subsurface stress-strain field. Sliding simulations were
performed for constant surface interference and lateral displacements
approximately equal to two times the pad width. Three complete loading cycles
were simulated in order to assess the effect of repeated sliding on the
stresses in the first (hard) layer and plastic strain in the underlying (soft)
layer. The close similarity of the plastic
zones in each pad confirms that interaction of the stress fields of each
pad was found to be negligible and deformation is only a function of the pad
geometry and material properties of each layer. Thermomechanical simulations of
normal and sliding contact of an elastic-plastic layered medium with a
patterned surface and a deformable sphere with properties identical to those of
the first layer are performed to elucidate the effect of frictional heating at
the sliding surface on contact deformation. The objective is to obtain
solutions for the temperature distribution along the surface and the maximum
temperature variation and evolution of subsurface plasticity in terms of Peclet
number. The likelihood of thermal cracking in the wake of microcontacts during
sliding is another objective of this work.
3. A
Generalized Contact Mechanics Analysis of Elastic-Plastic Spherical Indentation
The objective of this research is to elucidate the
deformation behavior of an
elastic-plastic half-space indented by a rigid sphere, during loading and
unloading. Emphasis is given to the effect of material properties, in terms of
reduced elastic modulus-to-yield strength ratio, on the deformation behavior. A finite element
model of frictionless indentation is developed and verified
by comparing the numerical results, in the elastic deformation regime, to the
Hertz theory. The analysis yields dimensionless constitutive relations for the
normal load, contact
area, and mean
contact pressure during loading for a wide range of material properties and
for interference distances ranging from the inception of yielding to the
initiation of fully plastic deformation. The boundaries
between elastic, elastic-plastic, and fully plastic deformation regimes are
determined in terms of interference distance, mean contact pressure, and
reduced elastic modulus-to-yield strength ratio. Relations for the hardness
and corresponding interference distance versus elastic-plastic material
properties and truncated contact radius are introduced, and the shape of
the plastic zone and maximum
equivalent plastic strain are interpreted in the context of finite element
simulation results. The unloading response of the spherical indenter is also
analyzed to evaluate the validity of basic assumptions in traditional
indentation approaches used to measure the hardness
and reduced
elastic modulus of materials. An
alternative approach for determining the reduced elastic modulus, yield
strength, and hardness of materials is proposed based on the obtained results.
4. Molecular Dynamic Analysis of
Indentation and Sliding Contact
Dynamic response of an FCC substrate under contact loading is studied
using molecular dynamic simulations. Two-body inter-atomic potentials
(Lenard-Jones and Morse potentials) are used to characterize the interaction
between atoms. Indentation
of the FCC substrate by a rigid tip shows that in case the tip and the
substrate are of the same material the interfacial adhesion is relatively
strong, while when the tip and substrate materials are different the adhesion
is much weaker. Sliding
between a diamond tip and an FCC copper-like substrate is also simulated. It has been found that for a prism tip the
friction coefficient is sensitive to the tip substrate interference, while for
pyramidal tip it is relatively insensitive. For prism-shaped tip, the friction
coefficient decreases as the tip bottom size increases. Sliding with the
tip-edge at the front produces lower friction coefficient than sliding with the
tip-face at the front. Stresses can be
defined from the traction vectors passing a plane segment. Using this stress
definition, the phenomena of surface tension in a single crystal solid was
verified. The stress field in an elastically deformed FCC substrate in contact
with a rigid diamond tip was obtained, and it was found that the
distribution of the von Mises stress is similar to that predicted by
continuum mechanics with some difference caused by surface tension effect.
5. Adhesion Forces and Dynamic Friction at MEMS
Interfaces
The objective of this research is to study adhesion
and microtribological behavior relevant to micromachines. Emphasis is given to the study of high
friction leading to stiction during operation resulting from capillary, van der
Waals, and electrostatic forces. 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 energy resulting from different
texturing and chemical treatment processes.
The passive devices are an adaptation of the more classical
microcantilevers, where the transition from stuck and free-standing
configuration is used to determine the surface energy.
Other test vehicles for this study are active
microdevices designed and fabricated with friction and wear testing in
mind. High static friction (stiction) is
investigated by testing friction couples relevant to micromachines. The active microdevices are also used to
study the effect of surface roughness, environmental conditions, mechanical
properties, and loading conditions, commonly encountered in MEMS devices, on
the electrical resistance formed between two contacting surfaces. The
experimental work is accompanied by theoretical
modeling, where the surface roughness is characterized by fractal
geometry using AFM measurements of the microdevice interfacial
microtopographies.
The lubrication efficacy of different self-assembled monolayers (SAM), such as OTS and FDTS, and
monolayers, such as DDMS and 1-octadecane, under various environments is
the most recent activity in this project.
A
vapor phase deposition chamber using inductively coupled RF plasma is used
to deposit the previous layers. Dynamic
friction testing is used to determine the endurance of the lubricant layers in
terms of contact load and environmental conditions (e.g., temperature and
pressure).
6. Fatigue of Polycrystalline 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 under 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 that have been accomplished are:
Future studies include:
7. Dynamic
analysis of MEMS resonators
Dynamic characteristics of MEMS devices are of great
importance, especially for inertia measurement applications. In this research,
the dynamic behavior of specially designed fatigue resonators is analyzed.The
finite element method (FEM) is employed to extract the natural frequency and
simulate the dynamic response
of the fatigue resonators, which requires excessive
computational time to reach steady state. Therefore, a more efficient
analytical method with reasonable accuracy suitable for dynamic behavior
analysis of various MEMS devices was employed. Analytical results were found to
be in good agreement with FEM results. With the aid of this analytical method,
the nonlinear dynamic behavior of the fatigue resonators was analyzed and the
effects of damping ratio, loading, and
geometry on the dynamic behavior were interpreted.
8. Stress
analysis and crack propagation simulations in polycrystalline silicon
Although FEM
dynamic analysis can yield accurate results for both the resonant amplitude and
the equivalent stress distribution in the fatigue devices, it requires a prohibitively
long computational time to reach steady state. Based on the simple analytical
method proposed in the dynamic behavior response analysis, a computationally
efficient method (i.e., equivalent static analysis) was introduced to produce
an equivalent stress distribution field for the resonant amplitude equivalent
to that obtained from the FEM dynamic analysis. A comparison
of the equivalent stress contours clearly shows that the result
obtained from the equivalent static analysis is essentially identical to that
of the FEM dynamic analysis.
Polysilicon, the common structural material in MEMS,
is often assumed to be homogenous and isotropic. However, as the characteristic
size of MEMS devices approaches the diameter of the polysilicon grains, such
simplification is no longer appropriate due to the inherent inhomogeneity and
anisotropy of polysilicon at this scale. In this work, a Poisson
VoronoiDiagram (PVD) is used to simulate the polycrystalline structure.
FEM models with the polycrystalline
structure (represented by PVD) incorporated in the critical region (beam-anchor
region) were constructed and the difference in stress due to different material
property was studied. Based on these FEM models, crack propagation within one
grain is simulated for the {100} texture. Crack
propagation simulations show that the crack growth path is
independent of the orientation angle of surrounding grains and that the crack
propagates almost straight within the grain. However, once the crack tip
reaches the grain boundary, crack
propagation along the grain boundary may occur (transgranular fracture) since
the maximum effective stress intensity factor along the grain boundary, which
probably exhibits lower fracture toughness, is comparable to that predicted for
crack growth within the neighboring grain (intragranular fracture).
9. Design and Fabrication of Electrical Contacts for
IC Probing at the Wafer Level
The semiconductor industry currently tests integrated
circuits (IC) at the wafer level in order to avoid delays and wasting resources
due to packaging of bad dies. This testing has traditionally been performed
with tungsten cantilever probes on peripheral Al-Cu or Al-Cu-Si bond pads. The
geometry of the cantilever probe is such that the probe tip scratches the bond pad
when the die is driven towards the probes. This scrubbing action breaks the
native oxide film that covers the bond pad and permits good electrical contact.
As IC's have become ever more complex, the number of I/O and power and ground
lines have also increased. The traditional peripheral layout of bond pads does
not lend itself well to a very large number of pads as the larger number
necessitates a larger dice footprint on the wafer. The semiconductor industry
has for some time now used "flip-chip" packaging to address this
problem in high pin count devices. However, the array arrangement of eutectic
Sn-Pb solder bumps used in "flip-chip" packaging excludes the use of
the tungsten cantilever probes due to the geometrical constraint when attempting
to access the innermost bumps. Vertical probes are used instead, and these are
designed to have vertical compliance and some wiping action. A few vertical
designs have received commercial acceptance; but each technology has its
limitations; and none can yet declare victory. In this study, the vertical
contact problem is investigated in an attempt to better understand the effects
of temperature, mechanical and electrical loading, tip shape, and contacting
materials on contact resistance, damage to the bump/pad, and oxidation,
deformation, and wear of the probe tip. In particular, the performance of
precious metal coatings (such as Ir, Rh, Au, Pt, Pd, and C) deposited onto
silicon wafers by ion beam assisted deposition and probes of Be-Cu base
material are examined. The research comprises deposition of the coating
material onto probe tips, microstructure characterization, and electric contact
resistance and mechanical property evaluation to determine the optimum coating
deposition conditions.
10. Effect of plasma conditions in
low-pressure radio-frequency discharges on thickness and surface roughness of
thin carbon films
Characteristics of low-pressure rf discharges in pure Ar atmospheres and
the effect of plasma conditions on
the thickness and roughness of ultrathin a-C films deposited on Si(100)
substrates by rf
sputtering system are investigated experimentally. The
observed characteristics of capacitive rf discharges at low
working pressures (<10 mTorr) are interpreted in terms of energy balance and
sheath capacitance considerations. The rf sputtering deposition system has to
be tuned in order to efficiently deliver energy from the source to the
discharge, and better control the discharge process for a-C film deposition. It
was found that the quality of the a-C films depends strongly on the
rf sputtering process conditions. The film
thickness, measured directly from TEM images,
depends linearly on the sputtering rate. The energetic particles bombarding on
the film surface (rejected C atoms and Ar+ ions) significantly
improves the film’s surface
roughness (rms surface roughness < 0.2 nm) with increasing
particle kinetic energy up to ~ 200 eV.
11. Femtosecond laser aperturless near-field surface
nanomachining assisted by scanning probe microscopy
In this project conducted
jointly with Professor C. Grigoropoulos, ultra-short pulsed-laser radiation is
used for precision materials processing and surface nano-/micro-modification.
Controllable surface nanomachining can be achieved by femtosecond laser pulses
through local field enhancement in the near-field of a sharp
probe tip. Nanomachining of thin gold films has been demonstrated by
coupling 800-nm femtosecond laser radiation with a silicon tip in ambient air.
Results illustrate the flexibility of this scheme to produce various
nanopatterns, such as multiple line nanogrid
structures, nanocraters,
and nanocurves.
The present process provides an intriguing means for massive nanofabrication
due to the flexibility in the substrate material selection, high spatial
resolution of ~10 nm (not possible with standard nanomachining techniques), and
fast processing rates achievable through simultaneous irradiation of multiarray
tips.
12. 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 is commonly used. 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 is considered to be
the prime reason 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. Pin-on-disk
Wear tests performed with polished CoCr alloy disk and UHMWPE flat pins
lubricated with bovine serum at a range typical of physiological pressures and relative speeds. Typical coefficient
of friction of plots for rough and smooth CoCr surfaces are in ranges of
0.12~0.14 and 0.06~0.08, respectively.
Transmission electron microscopy has 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. Scanning
Electron Microscope (SEM) pictures of wear tested UHMWPE pin surface show
formation of fibrils and ripples (or folds), typical of early wear mechanisms.
Ongoing research, funded by NSF, 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 reciprocating pin-on-disk
apparatus under contact conditions resembling those at artificial knee replacements.
Recent cytotoxicity results indicate that treatment with plasmas of Ar, C3F6,
CH4, NH4, and hex-amethyldisiloxane (HMDSO) produce
surface moieties that are nontoxic to the cells, as compared to negative and
positive controls of Latex and silicone, respectively. Hemolysis studies
demonstrated that the overall effect of these plasma treatments on the cells
examined is marginal.
This early research has led to a recent US patent
(#6, 379, 741) The ultimate objective of this research is to determine the
effects of different plasma chemistries and process conditions on the resulting
surface microstructures and chemical state and, in turn, on the tribological
properties and biocompatibility of medical-grade polyethylene. A
nanoindentation and atomic force microscopy determine localized changes in
surface mechanical properties due to plasma surface treatments. A
customized plasma chamber, which can be used to study in-situ plasma
characteristics evaluation, has been designed and is currently under
construction.
13. AFM and SFG Vibrational Spectroscopy Studies of
Polymers
This research program,
performed jointly with Professor G. A. Somorjai from the UC Berkeley Chemistry
Department, aims to provide new insight into the surface nanomechanical
properties and molecular behavior of various polymeric materials subjected to
different stress/strain conditions. The work includes the following two main
activities:
(a) Molecular surface
structure and chemical composition of very thin polymer films
The study of the surface
structure and composition of very thin polymer films is the goal of this
research. Copolymer (poly(methylmethacrylate), PMMA) and block copolymer
(poly(methylmethacrylate)-polystyrene, PMMA-PS) are examined to obtain a basic
molecular-level understanding of composition and molecular orientation at these
polymer surfaces. Ultra-thin polymer films are prepared from dilute polymer
solutions. The solutions are spun cast on different, smooth, solid substrates
(glass, silicon, and metal surfaces) to produce polymer films of varied
thickness. Film topography is examined by AFM, and film thickness is determined
from ellipsometry. The effect of annealing on the surface
molecular composition and structure is studied using SFG. Particular
emphasis is placed on the effects of film thickness and molecular weight on the
surface chemical composition.
(b) Effects of polymer
chain and reversible deformation on molecular surface structure of polyurethane
copolymer
The surface molecular
structure and the deformation mechanisms of two polyurethane (PUR)
short-segmented copolymers are studied by sum frequency generation (SFG)
vibrational spectroscopy. These polyurethanes differ only in the length of
their hard segment, where the molecular weight of the hard segment in one
composition (710 g/mol) is twice as much as in the other (1450 g/mol). Surface
deformation is induced by cyclically stretching the PUR films to a macroscopic
elastic elongation. The surfaces are investigated by monitoring the SFG
spectra, which gives information related to the
backbone methylene orientation. Results for both PUR compositions indicate
that the upward orientation of the methylene group that contribute to the SFG
signal increases with elongation and decreases upon relaxation. The surface
of PUR with shorter hard segments exhibits irreversible deformation at the
molecular level resulting in a composition similar to that of the PUR with
longer hard segments that is only elastically deformed after three stretching
cycles. Based on the obtained results, two methods for producing similar surface
compositions of PUR block copolymers can be proposed: (a) macroscopic elastic
cyclic stretching and relaxation of the polymer or (b) increasing the molecular
weight of the hard segment in the copolymer chain.
14. Tribological Behavior of Steel Surfaces Lubricated with Gear Oil
Containing Sulfide, Phosphate, and Metal Deactivator Additions
The objective of
this research, funded by Chevron Oronite Co., is to evaluate the tribological
behavior of a full-formulated lubricant using various additives including
sulfide, phosphate, and metal deactivator (oxidation inhibitor) in a base oil
at low (~32 oC) and elevated (~100 oC) temperature
regimes. Additives, such as the ones used in this study, have been used
extensively to improve wear-resistance and reduce friction under high loads in
many lubricated components, such as automotive gears and bearings.
A
ball-on-disk tribometer is used to obtain the friction coefficient and
electrical contact resistance (ECR) responses. The ECR is used as a method to
detect the presence of an insulating anti-wear tribofilm by producing a contact
voltage response (see figures for the steady-state
friction responses for the various lubricants tested under boundary
lubrication conditions and an example of a
typical ECR response for the phosphate lubricant in base oil at elevated
temperatures indicating the presence of a tribofilm.) Surface profilometry is
performed to evaluate the tribofilm’s wear resistance in terms of the disk
wear rate for the various lubricants and scanning
electron microscopy (SEM) images of the disk wear surface are obtained to
determine the dominant wear mechanisms under various lubrications at elevated
temperatures. SEM images revealed the presence of adhesive and/or abrasive wear
and evidence of tribofilm formation.
Results from the
various tests demonstrate a strong dependence of temperature and type of
additives on the tribological behavior of the different oil blends. While the
addition of additives reduced the coefficient of friction at elevated
temperatures slightly, the effect on the wear resistance at both low and
elevated temperatures was pronounced compared to the base oil. However, the
wear resistance of the individual formulations decreased at elevated
temperatures, suggesting possibly a weaker tribofilm strength and/or attachment
to the metal surfaces at high temperatures.
Work in
progress involves friction/wear testing, ECR, profilometry, SEM, XPS, and
nanoindentation methods to characterize the composition and nanomechanical
properties of the produced anti-wear tribofilms.
