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-thinfilm/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

Brendan Hanrahan

C Mike Waits

Hugh Bruck

Josh Balsam

Matthew Mccarthy

Reza Ghodssi

CiteSeerX

 
 
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, 20090-9743611-5-1/PMEMS2009/$20©2009TRF 589
 
 
 
The bearing is encased by two bonded silicon 
layers with an asymmetrical raceway deep-reactive-
ion etched into each layer. The bearing depths are 
offset so that the balls do not contact the silicon bond 
interface during operation. Before bonding, the 
raceway surfaces were coated with either SiC or TiN 
films, or left bare. A journal is etched completely 
through each wafer to create a rotor that is supported 
only by the microball bearings. After bonding, turbine 
blades and vanes are etched into the top structure with 
a height of 250 μm, simultaneously reaching the 
journal etch and releasing the turbine side of the 
device. Similarly, the thrust-side release structure is 
etched into the backside of the bonded test device, 
completely releasing the rotor. A complete fabrication 
schematic can be found in [1]. The packaging of the 
test device allowed for turbine flow and a thrust 
plenum to be operated independently, allowing for the 
decoupling of thrust and actuation loads as described 
in [7].  
 
Hard Film Coating 
The improvement of raceway hardness, 
considered to be the principle mechanical property of 
interest, through the use of thin films is the focus of 
this study. TiN and SiC thin films were chosen 
because of their promising mechanical properties and 
their compatibility with microfabrication techniques.  
SiC and TiN films are sputtered onto a bare 
silicon substrate after successive O2 plasma and Ar 
plasma cleaning steps. The SiC films were sputtered 
to a thickness of 250nm from a SiC target in an Ar 
atmosphere. The chamber pressure is held at 2.5 
mTorr and the DC sputtering source is operated at 1 
kW without any external substrate heating. These 
conditions most likely produced an amorphous film, 
although the morphology was not studied. TiN films 
were deposited in a reactive sputtering process. First, 
a thin Ti adhesion layer was deposited at 5 mTorr, and 
1kW, in pure Ar. Next, TiN was sputtered from a Ti 
target in a N2 plasma at 2-5 mTorr and 1 kW after a 
30 min pre-sputter that nitrides the surface of the 
target. TiN sputtered in this manner typically has a 
columnar grain structure. The total film thickness of 
the Ti/TiN stack is 250nm. 
Fig. 2 shows the load/displacement curves 
obtained for the three materials systems in a 
nanoindentation study. A Hysitron Triboindenter was 
used to probe the mechanical properties of the films 
and substrate. Table 1 summarizes the mechanical 
properties of interest for all three materials systems, 
extracted from the curves shown in Fig. 2. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Table 1: Summary of mechanical properties obtained 
from nanoindentation.  
Properties Si 250nm TiN 250nm SiC 
Stiffness (GPa) 169 155.9 127.1 
Hardness (GPa) 13.4 15.7 15.5-17.7 
 
Testing Methodology 
Friction torque measurements were taken for each 
material system using spin-down deceleration tests at 
variable normal loads. First, the thrust plenum 
pressure is set to provide normal load (5-50mN) to the 
bearings. The independent turbine flow then actuates 
the released rotor structure via the deep-etched turbine 
structures. Because of leakage through the journals, 
the normal force on the bearing during operation is the 
sum of the thrust pressure and actuation pressure at 
the journal. After the turbine has equilibrated to a 
steady rotational speed, the actuation flow is cut-off 
and the turbine is left to decelerate under only the 
pressure of the thrust plenum. Friction torque over the 
speed range of 1,000-10,000 rpm, or dynamic friction 
torque (DFT), is obtained from the second derivative 
of a power law fit to the position data from the spin-
down. A more in-depth description of this 
methodology can be found in [6].  
Turbine operation curves were taken for each of 
the three materials systems. For these tests, rotation 
speed was measured as a function of input power 
provided by the turbine flow. All three materials 
systems were tested after an initial run in. Bare Si and 
TiN turbines were also tested after every 500,000 
revolutions to determine if wear had significantly 
altered the operating conditions of the turbines.  
 
DISCUSSION 
Silicon Friction Torque 
A power-law relationship between DFT and 
normal load was presented for a silicon high-speed 
microball bearing supported microturbine in [7]. The 
Fig. 2: Characteristic load/displacement curves 
obtained for Si, SiC and TiN  
 
590
 
 
empirical power law relationship implies that friction 
torque approaches zero as normal load becomes zero. 
In this work, a linear relationship of the form (1): 
 
D=f*L+v, (1) 
  
where D is the DFT, f is a constant representing the 
linear dependence of DFT on normal load, L is the 
normal load in mN and v is the load independent 
component of DFT. The value v is significant because 
it represents the theoretical minimum friction torque 
in an encapsulated microball bearing turbine for a 
perfectly thrust-balanced system. Fig. 3 illustrates the 
DFT values from spin-down tests taken at successive 
normal loads for a silicon raceway tested through 
progressive levels of run-in (fresh, 1 M revolutions, 
and 2 M revolutions). 
 
 
Fig. 3: DFT relationship from 10mN-50mN normal 
load in a silicon raceway/steel ball microturbine 
system.  
 
Wear, or lack thereof, can be indirectly observed 
in the consistent values of DFT through the lifetime of 
the device. Future microball bearing supported 
MEMS devices can utilize the empirical, linear DFT 
relationship over this range of operating loads. Bare 
silicon raceways with the modified bearing design 
from [8] exhibit consistent behavior up to, and more 
than likely beyond 2M revolutions of run in, at typical 
MEMS operating loads. This makes the simple 
fabrication of an uncoated silicon/steel bearing 
attractive to applications that can accept current levels 
of friction torque where minimal wear can be 
tolerated.  
 
Coated Raceway Performance 
TiN and SiC film coatings are sputtered on to 
silicon raceways of identical geometry to those 
previously discussed. The mechanical properties of 
these films are determined through nanoindentation of 
the film-substrate system. Hardness is increased over 
bare silicon by 17% and 32% for TiN and SiC coated 
raceways, respectively. The higher hardness of the 
coated systems should be more resistant to plastic 
deformation from a contact load compared to bare Si, 
leading to lower friction. 
Operating curves of each microturbine/coated 
raceway system are taken as an initial metric and 
periodic assessment of turbine performance. Input 
power, the product of flow and pressure drop across 
the turbine, is measured within the packaging. Turbine 
speed is measured in a tachometer style with a high-
rate optical displacement sensor. Fig. 4 shows the 
operation curves taken with less than 10,000 
revolutions for the three materials systems.  
Fig. 4: Initial operation curves taken for each 
material system.  
 
The SiC coated raceway system wore almost 
immediately upon testing and caused the turbine to 
behave erratically. Visual inspection of the SiC 
raceway surfaces revealed minimal fracture of the 
film on the surface, but rather a smooth worn interface 
that was within the estimated contact area of the balls. 
This suggests that the mechanism of wear took place 
within the film, rather than at the film/substrate 
interface. Film properties, rather than adhesion 
properties dominated the failure mechanism of this 
film, suggesting that crystalline 3C-SiC with higher 
harness and stiffness, but similar adhesion to silicon 
could be a candidate for future study. 
TiN coated raceways were tested using spin-down 
friction testing in the same manner as described in the 
Silicon Friction Torque section. Fig. 5a compares the 
DFT of TiN and Si raceways with little run-in (within 
the range of 5,000-10,000 revolutions). In contrast, 
Fig. 5b compares the DFT of TiN and Si raceways 
after 2M revolutions of run-in. Initally TiN coated 
races perform almost identically to bare Si systems. 
591
 
 
At 2M revolutions the TiN raceways begin to exhibit 
erratic behavior when compared to the consistent 
performance of the bare Si raceways. Inspection of the 
raceway after testing revealed localized delamination 
of the TiN coating over ~ 25% of the bearing surface 
within the contact area of the ball. Fatigue at the 
interface of the film/substrate system could lead to 
sub-surface crack initation and growth, causeing a 
gradual delamination of the film. The film adhesion 
properties could be tuned to avoid fatigue induced 
delamination, although the friction of the film from 
the onset of testing showed no significant 
improvement over bare silicon. This suggests that 
other phenomena, such as ball-on-ball sliding friction, 
may dominate over rolling friction dictated by 
raceway mechanical properties. The ball packing and 
surface properties present an interesting avenue to 
reduce system friction as opposed to tuning the 
properties of the raceway. 
 
Fig. 5: (a) DFT values for bare Si and TiN coated 
systems. Both systems follow the linear 
relationship between DFT and normal load. (b) 
DFT values for raceways after 2M revolutions of 
run-in. Si raceways remain consistent, while the 
TiN coated systems begin to exhibit erratic 
behavoir.  
 
CONCLUSION 
A linear DFT/normal load relationship was 
established for bare Si and TiN coated raceways 
within fabricated microturbines.  The linear 
relationship (Eq. 1) accounts for load independent 
contributions to friction torque and establishes a 
theoretical minimum operating torque. Friction testing 
was repeated and the DFT values re-evaluated at 
various intervals of turbine wear up to two million 
revolutions (fig. 3,5b). Despite having a superior 
hardness, the TiN coated turbines exhibited similar 
values of friction torque for the range of normal loads 
and film thickness tested. An increasingly erratic 
behavior of the TiN turbines with increased run-in 
suggests the microball/raceway system is wearing and 
generating debris.  Further testing will investigate the 
other potential contributions to friction within the 
coated raceway systems, such as ball-on-ball sliding. 
A greater understanding of the wear mechanisms 
present in both the coated and bare-Si systems, 
including a study of the ball wear, will help elucidate 
the fundamental mechanisms dictating the tribological 
properties of the microball bearing platform.  
 
 
ACKNOWLEDGEMENTS:  
The authors would like to thank the Maryland 
Nanocenter for the use of fabrication facilities. 
 
 
REFERENCES 
[1] C. Mike Waits, M. McCarthy, and R. Ghodssi, 
2009 A Microfabricated Spiral-Groove 
Turbopump Supported on Microball Bearings 
accepted for publication  in JMEMS, July 2009. 
[2] M. McCarthy, C. M. Waits, M. I. Beyaz, and R. 
Ghodssi, 2009 A Rotary Microactuator 
Supported on Encapsulated Microball Bearings 
Using an Electro-pneumatic Thrust Balance 
JMM  19 9 1-7 
[3] Wang W, Wang Y, Bao H, Xiong B and Bao M 
2002 Friction and wear properties in MEMS 
Sens. & Actrs. A 97/98 486–91 
[4] U. Beerschwinger, R.L. Reuben and S.J. Yang, 
1997 Frictional study of micromotor bearings. 
Sens. & Actrs. A 63 229–241 
[5] R. Ghodssi, D. Denton, A. Seireg, and B. 
Howland 1993 Rolling friction in a linear 
microactuator J. Vac. Sci. & Tech., A 11 803–7  
[6] C.M. Waits, N. Jankowski, B. Geil, and R. 
Ghodssi 2007 MEMS Rotary Actuator using an 
Integrated Ball Bearing and Air Turbine Tech. 
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[7] M. McCarthy, C. M. Waits, and R. Ghodssi, 
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Hard Film Coatings for High-Speed Rotary MEMS Supported on Microball Bearings

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Hard Film Coatings for High-Speed Rotary MEMS Supported on Microball Bearings

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