Wear 352-353 (2016) 188–195

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Tribological performance of ceramics in lubricated ultrasonic motors
Wei Qiu n, Yosuke Mizuno, Kentaro Nakamura
Precision and Intelligence Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan

art ic l e i nf o

a b s t r a c t

Article history:
Received 1 October 2015
Received in revised form
2 February 2016
Accepted 7 February 2016
Available online 12 February 2016

Small ultrasonic motors are driven by the oscillation of piezoelectric transducers. They are primarily applied
to equipment components that require precise movements, such as drivers for auto-focus lenses in cameras,
and actuators in robots and positioning devices. While lubrication has been proven effective in improving theperformance of ultrasonic motors, the friction materials still play a signiﬁcant role. In this study, the suitability of four different engineering ceramics, which included alumina (Al2O3), zirconia (ZrO2), silicon carbide(SiC), and silicon nitride (Si3N4), were investigated in the self-mated conﬁguration. By testing the candidate
materials in the actual ultrasonic motor conﬁguration, the main wear mechanisms of these ceramics wereidentiﬁed under lubricated conditions, with fracture being the most dominant contributor to their wear.
Among the four tested ceramics, ZrO2 was found to be the most promising material as it exhibited themildest wear and a satisfactory level of friction. The obtained results also indicate that the fracture toughnessof ceramics is more critical than their hardness. This study provides guidelines for selecting friction materials
that can be used in long-lived, lubricated ultrasonic motors.
& 2016 Elsevier B.V. All rights reserved.

Keywords:
Ultrasonic motors
Ceramics
Sliding friction
Lubrication
Wear mechanism

1. Introduction
Ultrasonic motors are typically driven by ultrasonic vibration of
the piezoelectric transducers, which is transformed to the output
force through the friction force between the rotor (or slider) and thestator. They are usually several tens of millimeters in size, andpossess some advantageous properties such as high torque at low
speed, high power density, quick response, and precise positioning
capability [1,2]. Compared with electromagnetic motors, ultrasonicmotors have superior output power because of their high power
density, and no necessity of using gear systems. These advantages ofultrasonic motors enable their application as driving solutions incameras (as drivers for auto-focus lenses), robots, and positioning
devices. However, the resulting friction loss and wear of friction
materials (the materials used for the contact between the rotor (orslider) and the stator) lead to low motor efﬁciency and short lifetime, signiﬁcantly limiting their actual implementation in theseﬁelds and restricting their potential application in others.
The friction loss generation can be explained by examining one
particular type of ultrasonic motors, i.e., hybrid transducer-typeultrasonic motors (HTUSMs). In these motors, torsional–longitudinal Langevin transducers are used as stators, and they consist of
torsional lead zirconate titanate (PZT) disks (polarized in the circumferential direction) and longitudinal PZT disks (polarized in the
thickness direction), as shown in Fig. 1. A preload is applied to a rotornCorresponding author. Tel.: þ 81 45 924 5052; fax: þ81 45 924 5091.

http://dx.doi.org/10.1016/j.wear.2016.02.006
0043-1648/& 2016 Elsevier B.V. All rights reserved.

by a coil spring, thus pressing the rotor against the stator. In a
HTUSM, the torsional vibrator generates the output force while thelongitudinal vibrator controls the friction force (Fig. 1). When thetorsional vibrator moves toward the right, the longitudinal vibrator
extends to contact the rotor, transmitting the driving force throughthe friction force between the rotor and the stator. The displacementof the longitudinal vibrator reaches a maximum to provide thehighest stress when the torsional vibrator has the largest rightward
velocity; thus, a 90-degree phase difference is required between thetorsional and longitudinal vibrations to drive the motor. In contrast,
when the torsional vibrator moves to the left, the longitudinal
vibrator shrinks to separate the rotor and the stator. Thus, the rotor
can be rotated in one direction because of inertia. However, this ideal
operating principle is unlikely to be realized if high preloading isapplied to generate sufﬁcient torque. The rotor and the stator cannot
be detached during the negative half cycle of the torsional vibration,
characterized by considerable friction loss and wear of the friction
materials, which is a major reason for the low efﬁciency of ultrasonicmotors; this issue has been extensively analyzed by Nakamura andco-workers [3–5].
To date, lubrication has not been utilized in commercial
ultrasonic motors because an induced slip may lead to a signiﬁcant loss of the output torque and thus a decrease in motor
efﬁciency. However, the preload and the slip between the two
sliding surfaces change instantaneously during the operation ofultrasonic motors because of the alternating vibrations in theloading and driving directions; these instantaneous changes

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189

Table 1
Physical properties and surface roughness of the ceramics tested in thisexperiment.
Material property

Al2O3

ZrO2

SiC

Si3N4

Vickers hardness (GPa)Fracture toughness (MPa m1/2)
Young’s modulus (GPa)Poisson’s ratio
Bulk density (  103 kg/m3)
Thermal conductivity at 20 °C (W/mK)
Grain size (mm)Average surface roughness Ra (mm)Kernel roughness depth Rk (mm)

19.1
4
390
0.24
3.9
30
4.9
0.12
0.15

14.7
6
200
0.32
6.0
3
0.4
0.12
0.39

30.5
3
410
0.16
3.1
170
2.9
0.11
0.35

16.1
5
290
0.28
3.2
26
1.3
0.11
0.33

Fig. 1. Principle of operation for a hybrid transducer-type ultrasonic motor.

indicate that different lubrication regimes are likely to be
implemented during one cycle of vibration. According to thetheory behind the Stribeck curve [6], the friction coefﬁcient
becomes high either when a small slip occurs between the rotor
and the stator, or at a high preload; both scenarios correspond tothe boundary lubrication regime, during which large output
forces can be achieved. In contrast, the friction coefﬁcientdecreases at large slips or low preloads, corresponding to thehydrodynamic lubrication regime, during which friction losses
can be reduced. Based on this assumption, lubrication wasapplied to HTUSMs in our earlier studies, and it resulted in drastic
improvement in the motor performance under high apparent
contact pressure [5,7].
Friction materials are key components in ultrasonic motors that
greatly affect their performance. More importantly, since the othermajor components including piezoelectric materials are generally
more durable than friction materials, the lifetime of ultrasonicmotors is mainly determined by the friction materials. Studies onfriction materials in dry contact for ultrasonic motors cover a wide
range of categories, with a particular emphasis on ceramics [8–12]
and polymers [1,13–15]. Although some criteria for selecting friction materials for ultrasonic motors have been established [16,17],
the tribological performance of friction materials for lubricatedultrasonic motors has yet to be investigated and may signiﬁcantly
differ from that under dry conditions owing to the presence of
lubricants.
In this study, we used four types of advanced engineering ceramics
as friction materials for lubricated ultrasonic motors. Compared withpolymers, advanced engineering ceramics are characterized by highwear resistance and chemical inertness, which are desirable properties
for ultrasonic motors with lubrication. The goal of this study is toselect reliable friction materials for lubricated ultrasonic motors byinvestigating the tribological properties, especially the primary wear
mechanisms, of ceramics in motors.

2. Materials and methods
2.1. Materials
Alumina (Al2O3, 99.5%), zirconia (ZrO2, 94.0%), silicon carbide(SiC, 99.9%), and silicon nitride (Si3N4, 99.8%) ceramics were tested

Fig. 2. Ceramic dimensions of the (a) stator and (b) rotor sides.

in this study; their physical material properties are summarized inTable 1. All four ceramic materials were sintered at high temperature, and they all possessed crystalline structures. The ceramic
dimensions for the stator and rotor sides are depicted in Fig. 2; thecontact width on the rotor side was  220 mm. The same ceramics
were used for both the stator and the rotor (this is known as theself-mated conﬁguration) during each test. The Vickers hardness ofeach ceramic was measured at a load of 4.9 N with a Vickers
hardness testing machine (HM-102, Mitutoyo Corporation, Kawasaki, Japan).
After sintering, the Al2O3 surface was lapped using a slurry withdiamond powder, while the other ceramic surfaces were groundwith sharpening stones containing diamond particles (40 mm indiameter). The average surface roughness Ra was measured with asurface proﬁlometer (Dektak 150, Veeco Instruments, Inc., Plainview, NY), and very similar Ra values (0.11–0.12 mm) were obtained
for all ceramic types. In addition, the kernel roughness depth Rk(used to eliminate the effect of isolated peaks and valleys) was also
calculated from the Abbott–Firestone curve by following the190

W. Qiu et al. / Wear 352-353 (2016) 188–195

Fig. 3. Scanning electron micrographs of the unworn surfaces of the (a) Al2O3, (b) ZrO2, (c) SiC, and (d) Si3N4 ceramics. The sample tilt was equal to 70°.

procedures reported by Erickson et al. [18]. Comparable Rk values
were obtained for the ZrO2, SiC, and Si3N4 surfaces, while the value
for Al2O3 was much lower (0.15 mm), indicating that the Al2O3
surface was smoother than surfaces of the other ceramics. The
scanning electron micrographs of the unworn surfaces of the tested
ceramics are shown in Fig. 3(a)–(d), conﬁrming that the Al2O3
surface had a smoother appearance than those of the other ceramics, corresponding to the smaller Rk value. In addition, polishing
traces were observed on the Al2O3 surface in random directions,while unidirectional traces appeared on the ZrO2, SiC, and Si3N4
surfaces owing to the different polishing methods. The othermaterial property values listed in Table 1 were obtained from the
supplier.
A high-traction ﬂuid (base oil: synthetic naphthene; additives: zincdithiophosphate (ZDDP), calcium phenate, and organophosphates)with a viscosity of 100 cSt at 40 °C was selected as a lubricant, which
was applied every 2 h to the interface between the stator and the rotor
to maintain sufﬁcient lubrication.
In this work, we utilized HTUSMs to investigate the tribologicalbehavior of ceramics in lubricated contact. A torsional–longitudinal
Langevin transducer with a diameter of 25 mm was used as thestator in the HTUSM, in which two 4-mm-thick torsional PZT disks
and six 1-mm-thick longitudinal PZT disks were clamped together
using metal parts, as shown in Fig. 1.
2.2. Testing procedure
The experiments were conducted at room temperature (20 °C).
A 460 N preload (corresponding to an apparent contact pressure of
30.8 MPa) was applied to the HTUSM, which exhibited high motor
efﬁciency [5]. The motor was driven at a frequency of  23 kHz,
which was close to the resonant frequencies of the torsional andlongitudinal vibrators. The voltage applied to the longitudinal

Fig. 4. Graph illustrating the change in sliding velocity of an ultrasonic motor.

vibrator was maintained at 160 Vrms. The amplitude of the torsional vibration velocity was ﬁxed at  9.0 rad/s, resulting in arotational speed of one revolution (rev) per second (1 rev/s or1 rps) for the rotor. In general, the sliding speed of an ultrasonicmotor varies because of the alternating vibration (Fig. 4): its value
is low during the driving period (when the torsional vibration
velocity exceeds the rotational speed of the rotor), and it becomeshigh when the stator moves in the direction opposite to that of therotor. A high preload and a relatively low rotational speed wereselected in our experiments because ultrasonic motors are generally employed in high-torque applications with low speeds. Eachtest was run for 100 h, corresponding to 360,000 motor rev and a
total sliding distance of 24.31 km.
Owing to the difﬁculty in measuring the friction force during
motor operation, the friction coefﬁcient was estimated from thefalling curve of the motor rotational speed when the input power
was switched off [19]. In dry contact, the rotational speed of an
ultrasonic motor decreases linearly after the power is switched off.
The retarding torque of ultrasonic motors TR (the torque without

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191

ultrasonic vibration) can be calculated as
TR ¼ J

dΩ
Ω0
¼J ;
dt
tf

where J, Ω, Ω0, and tf represent the moment of inertia, rotational
speed, no-load speed, and fall time (the time lapse from thehighest speed (no-load) to zero speed), respectively. Meanwhile,
TR can also be expressed asT R ¼ μFr;
where m, F, and r represent the friction coefﬁcient, preload, andcontact radius, respectively. Hence, the friction coefﬁcient without
ultrasonic vibration m can be obtained from

μ¼

J Ω0
:
t f Fr

According to the Stribeck curve, m depends on the relative
velocity of the contacting surfaces under lubrication. The boundary lubrication regime is activated at high preloads and low rotational speeds, especially under pure sliding conditions between
two parallel surfaces, which was the case in these experiments.
After the power was switched off, the sliding speed decreased
from 68 mm/s (at a rotational speed of 1 rps) to 0 under a 460 N
preload. The obtained falling curves of the rotational speed werecharacterized by linear proﬁles, indicating that the m value wasconstant, and the boundary lubrication regime was initiated during deceleration.
Friction measurements were performed after the motor ran for
1,000; 10,000; 50,000; 100,000; 150, 000; 200,000; 250,000; 300,000;
and 350,000 rev. After the wear test, the worn surfaces of both thestator and the rotor sides were studied using scanning electron
microscopy (SEM). Chemical compositions of the worn surfaces werealso analyzed by energy-dispersive X-ray spectroscopy (EDS).

3. Results
The worn Al2O3 surfaces on the rotor and stator sides are illustrated in Fig. 5(a)–(c). On the rotor side, Al2O3 grains were removed
from the lower layer of the surface where microfractures wereobserved. No signs of wear debris agglomeration were detected on
the surface, suggesting that the wear debris was transported fromthe contact area or directly dissolved by the lubricant. Similarmorphology was observed in a small fraction of the worn Al2O3
surface on the stator side (Fig. 5(b)). However, the majority of thesurface was characterized by extremely mild wear and traces leftfrom the initial polishing, as shown in Fig. 5(c). The worn surface
proﬁle for the stator side was also examined (Fig. 6(a)). Since thesurface area on the rotor side was smaller than that on the statorside, a groove should appear on the stator side if the wear volume
was large. However, no distinct grooves were observed on the worn
Al2O3 surface, indicating that its wear was relatively mild. The
obtained EDS results showed the presence of elemental P, Zn, andCa from the lubricant on the worn Al2O3 surfaces (Table 2).
The worn ZrO2 surfaces on both the stator and rotor sides werecharacterized by extremely mild wear, with no distinct differences
between the unworn and worn surfaces (Fig. 7(a) and (b)). The initialpolishing traces were clearly visible, while no grooves were observedon the worn surface on the stator side (Fig. 6(b)). Only elemental Zn
from the lubricant was detected on the worn ZrO2 surfaces.
The worn SiC surface on the rotor side, shown in Fig. 8(a)–(c),exhibited two types of surface morphology, where both microfractures and smooth regions were observed in a large area. Surface
fractures were detected on most of the middle part, with small areasof smooth surfaces nearby along the two sides of the wear scar (Fig. 8(a) and (b)). In addition, some areas of the worn surface were mostly

Fig. 5. (a) Scanning electron micrograph of the worn Al2O3 surface on the rotor
side. Scanning electron micrographs showing (b) the fractured region and (c) themild wear region on the worn Al2O3 surface on the stator side. The sample tilt was
equal to 70°. The arrows indicate the sliding directions.

covered by a smooth layer containing agglomerations of the wear
debris (Fig. 8(c)). The debris layer was found to be strongly attachedto the wear track since it could not be removed by ultrasoniccleaning. The surface appearance on the stator side was slightly
different, exhibiting very mild wear in a very small region of thesurface. The Rk values for the fractured and smooth regions on thestator side were 5.24 and 0.08 mm, respectively. No elements fromthe lubricant were detected on the worn SiC surface, while therelated O content slightly increased.
Most of the outer grains of the worn Si3N4 surface on the statorside were polished and covered by a thin surface layer, althoughsome initial polishing traces could still be observed (Fig. 9(a)). The

192

W. Qiu et al. / Wear 352-353 (2016) 188–195

Fig. 6. Worn surface proﬁles of the (a) Al2O3, (b) ZrO2, (c) SiC (fractured region), and (d) Si3N4 (polished region) ceramics on the stator side.

Table 2
Chemical composition of the unworn and worn surfaces of four different ceramics obtained by EDS.
Material

Al2O3
ZrO2
SiC
Si3N4

Detected elements
Unworn surface

Worn surface

Al, O
Zr, O, Y
Si, C, O
N, O, Si, Al, Y

Al, O, P, Zn, Ca
Zr, O, Y, Zn
Si, C, O
N, O, Si, Al, Y, Zn, Ca

Fig. 7. Scanning electron micrographs of the worn ZrO2 surface on the (a) stator side and (b) rotor side. The sample tilt was equal to 70°. The arrows indicate the sliding
directions.

obtained Rk value was 0.21 mm, which is slightly lower than that for
the unworn surface. In addition, several grooves indicating more
severe plastic deformation were detected in some regions of theworn surface, from which the initial surface layer was completely
removed (Fig. 9(b)), resulting in the Rk value of 0.82 mm. The worn
Si3N4 surface on the rotor side exhibited a similar appearance, while

W. Qiu et al. / Wear 352-353 (2016) 188–195

Fig. 8. (a) Scanning electron micrograph of the worn SiC surface with both fractured and smooth morphologies on the rotor side. Magniﬁed views of the area
inside the (b) fractured region and (c) smooth region. The sample tilt was equal to70°. The arrows indicate the sliding directions.

more grooves were formed along the sliding direction and they
were much longer than those on the stator side (Fig. 9(c)). Elemental Zn and Ca from the lubricant were detected on the worn
Si3N4 surfaces, and the related O content increased.
The dependence of the friction coefﬁcient on the number of revolutions is shown in Fig. 10. The plotted data represent averaged values of
10 measurements. The friction forces of the motors with the SiC andSi3N4 ceramics were relatively stable, leading to small changes in theirfriction coefﬁcients (ranging from 0.13 to 0.15). In contrast, the friction
coefﬁcients of the Al2O3 and ZrO2 ceramics increased after initial
operation and then stabilized after 50,000 rev (0.17–0.19 for Al2O3; 0.19–
0.21 for ZrO2) until they ﬁnally decreased to levels similar to those of
the SiC and Si3N4 ceramics after 250,000 rev.

193

Fig. 9. Scanning electron micrographs of the worn Si3N4 surface: (a) mild wear region;(b) region with a groove on the stator side; (c) region with a groove on the rotor side.
The sample tilt was equal to 70°. The arrows indicate the sliding directions.

4. Discussion
The worn surfaces of three ceramic types (all but ZrO2) on thestator and rotor sides were characterized by different appearances,possibly because the stressed volume on the stator side was conﬁned
by the surrounding unstressed material during motor operation as aresult of the difference in material shapes. The worn Al2O3, SiC, andSi3N4 surfaces exhibited both mild and severe wear regions, probably
because of the non-uniform preloads produced by the coil spring. In
general, uniform preloads are not required for HTUSMs since theirmechanical characteristics are not signiﬁcantly affected by the uniformity of preloads (in contrast to surface acoustic wave motors [20]).
However, non-uniform preloads caused high stresses in some areas on

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W. Qiu et al. / Wear 352-353 (2016) 188–195

rougher (Rk ¼5.24 mm), indicating that hydrodynamic lubrication was
not realized even after the dynamic preload reached its minimum. The
stable friction coefﬁcient indicates that severe wear was generated
within a short time. No elements from the lubricant were detected on
the worn surfaces, which indicate that the tribochemical reaction did
not occur.
Several grooves were obtained on the worn Si3N4 surfaces on boththe stator and rotor sides because the wear debris was dragged along
the surfaces during sliding and grooves were formed, as described inan earlier study [24]. Thus, the wear debris of Si3N4 could not be easily
removed from the contact zone and induced more severe wear on thesurfaces, which does not satisfy the requirements as a friction material
for ultrasonic motors [16]. The tribochemical reaction probably
occurred because of the increased O content and the presence ofelemental Zn and Ca on the worn surface.
Fig. 10. Measured dependence of the friction coefﬁcient on the number of revolutions for various ceramic materials.

the stator side, resulting in the formation of regions characterized bysevere wear.
The ZrO2 ceramic exhibited the mildest wear (compared to theother tested ceramics), and the worn and unworn surfaces were
almost indistinguishable because ZrO2 had the highest fracture
toughness among the four ceramics. In addition, according to thewear map of ZrO2, severe wear usually occurs either at high sliding
speeds or high temperatures [21,22]. In a HTUSM, high sliding
speeds arise only during the negative half cycle of the torsional
vibration, corresponding to a low dynamic preload due to thelongitudinal vibration, while the presence of the lubricant reduces
the interfacial temperature. Moreover, the friction coefﬁcient ofZrO2 after lubrication was maintained within an appropriate range(0.15–0.2) in our study: the initial value of the friction coefﬁcient
was 0.13; it increased to 0.2 after 50,000 rev and ﬁnally decreased
to 0.15 after 300,000 rev. At the present time, we do not have any
experimental data to explain such complex behavior. However,since the surface topography was not signiﬁcantly changed, thevariation in friction coefﬁcient was due to neither the change incontact pressure nor the tribochemical reaction.
The unworn Al2O3 surface was the smoothest one among thetested ceramics because of the unique polishing method used for
Al2O3, which was beneﬁcial for achieving complete hydrodynamic
lubrication that reduced friction loss and wear when the stator moved
in the direction opposite to that of the rotor movement. Hence, thewear on the stator side was mild except for the microfractures
observed in small areas. However, the fracture toughness of Al2O3 wasrelatively low, and the stress on the rotor side was not adequately
conﬁned by the surrounding material, resulting in the development of
more intensive surface microfractures. The friction coefﬁcient was
initially 0.14; it then increased to 0.19 after 100,000 rev, and ﬁnallydecreased to 0.15 after 250,000 rev, showing a trend similar to that for
ZrO2. The initial increase in friction coefﬁcient was caused by the
microfractures, which enhanced the local pressure of the contact
asperities.
SiC exhibited the most severe wear among the tested ceramics on
both the rotor and stator sides, which was mainly due to its low
fracture toughness (although the hardness of SiC was the highest). The
SiC wear mechanism is similar to that for the transition zone shown in
the SiC wear transition diagram where microfractures are dominant[23]. The friction coefﬁcient obtained for SiC in our study was highly
stable, indicating that no chemical reaction products were formed.
Tribo-oxidation may have occurred, but it is believed to have a minor
inﬂuence on friction and wear since the temperature rise due to thefriction heating was not signiﬁcant under lubricated conditions.
Although the worn surface was ﬂattened in some areas (especiallyaround the edges of the contact area), most of it became much

5. Conclusions
Friction materials are essential components in ultrasonic motors
in both dry and lubricated contact, which greatly affect the motor
performance and lifetime. The tribological behavior of four types ofadvanced engineering ceramics was investigated for an actual
ultrasonic motor lubricated with a high-traction ﬂuid. The following
major conclusions were obtained from the experimental results:(1) The main wear mechanism can be described by mechanicalfracture of the tested ceramics in lubricated ultrasonic motors.
(2) The ZrO2 ceramic exhibited the mildest wear among the tested
ceramics, indicating that ceramics possessing high fracture
toughness are more suitable for lubricated ultrasonic motors. In
contrast, since the SiC ceramic exhibited the most severe wear, it
indicates that ceramic hardness plays a less important role thanfracture toughness.
(3) The Al2O3, SiC, and Si3N4 ceramic surfaces on the rotor side
experienced more severe wear than those on the stator side.
Non-uniform preloading induced by a coil spring was one of thereasons for different surface morphologies on the stator side.
(4) Smooth surfaces facilitate hydrodynamic lubrication that
reduces friction loss and wear and hence improves the longterm performance of a motor, provided that the friction
coefﬁcient is maintained at an appropriate level (0.15–0.2).
We anticipate that the obtained results will provide adequate
information for selecting appropriate friction materials for lubricated ultrasonic motors in the future.

Acknowledgments
This work was supported by the Japan Society for the Promotionof Science (JSPS) KAKENHI Grant 15K13896. The work of W. Qiu was
supported by a Grant-in-Aid for JSPS Research Fellows (No.
26011842). The authors are indebted to Prof. Koshi Adachi (Divisionof Mechanical Engineering, Tohoku University) and Dr. Satoshi
Momozono (Department of Mechanical and Aerospace Engineering,Tokyo Institute of Technology) for their insightful discussions, Dr.
Tso-Fu Mark Chang, Dr. Takashi Nagoshi, and Dr. Masaki Tahara(Precision and Intelligence Laboratory, Tokyo Institute of Technology)
for their assistance in SEM and EDX analyses and Vickers hardness
test, and Dr. Kunio Nishioka (Semiconductor and MEMS Processing
Center, Technical Department, Tokyo Institute of Technology) for his
assistance in the measurements of surface proﬁles. They also
appreciate Idemitsu Kosan Co., Ltd., for providing the lubricant andthe staff members of the Precision and Manufacturing Center

W. Qiu et al. / Wear 352-353 (2016) 188–195(Technical Department, Tokyo Institute of Technology) for machining
the metal components of the motors used in the experiment.

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