Mechanical Behavior of PCS-derived Si-C Ceramics
Strengthened using PIP Method
T. Namazu1, H. Kudara1, T. Ishikawa1, K. Yoshiki1, S. Inoue1 and Y. Hasegawa2
1University of Hyogo and 2ART KAGAKU Co., Ltd., Japan

Abstract
Strength evaluation of silicon carbide (Si-C) ceramics fabricated from polycarbosilane (PCS) precursor is described. Si-C ceramics was prepared by firing a green body made of
the mixture of Si-C nano-powders and a PCS solution at 1273 K in N2 gas for an hour. To obtain dense Si-C, the polymer infiltration and pyrolysis (PIP) process was conducted
up to 12 cycles. Si-C ceramics was diced to be rectangle shape, and was subjected to the three-point bending test. Si-C specimens fabricated through PIP processes less than 2cycles showed non-linear force-displacement curves, whereas those through the processes more than 3 cycles showed linear relations and fractured in a brittle manner. The
Young’s modulus of 12-cycles-PIPs specimen was 56 GPa, 22-fold of non-PIP specimen. The bending strength was also increased up to 157 MPa with an increase in the numberof PIP process. The cause of the influence of PIP process on the mechanical characteristics is discussed using a PCS-derived Si-C model.

Experimental ProcedureThree-point bending test equipment for miniature specimen

Porosity of PCS-derived SiC ceramics

Loading jig

30

Porosity P, %Supporting jig

Bending test specimen

Laser displacement
meter

Positioning stage
for specimen
20

Specimen

1 mm
Load cell

Step 1Pre-loading

Specimen
100HolderremovalLoading pin

Specimen
0

2

4

6

8

10

SupportBending
force

12

The number of PIP processes N PIP, times

Relationship between the number of PIP processes and porosity.

1 cm

Porosity was decreased with an increase in the number of PIP
process. After 12-cycles-PIPs, the porosity was 7.5 %, which isonly 6.25 % reduction from 6-cycles-PIPs specimen. This isbecause, in the case of low porosity region, filling a PCS solution
into pores was difficult due to very small pore size or formationof closed pores.

PZT actuator in Al case

1 mmStep 2
Bending test startSpecially developed three-point bending tester for miniature Si-C ceramics along with a schematic of testing procedure.
The tester is able to apply bending force to a specimen in the horizontal direction to easily observe specimen
deformation from its overhead with a CCD camera. The load cell and laser displacement meter have ameasurement resolution of 0.1 N and 10 nm, respectively. Loading and supporting jigs were made of cemented
carbide to restrain elastic deformation of those jigs as small as possible.

Experimental Results & Discussions
The influence of PIP cycles on mechanical characteristics of PCS-derived SiC ceramics
2.0

16

NPIP = 12

1.0

12
12
log(log(1/(1-Pi)))NPIP = 61088

N PIP = 3

66

4

NPIP = 2

NPIP = 0

22

0.0
-1.0
-2.0
-3.0

NPIP = 0

NPIP = 2

NPIP = 6

00

22

44

66

88

10
10

12
14
12 14
Displacement δ, µm

16
16

18
18

-4.0

20
20

0.0

1.0

2.0

3.0

4.0

5.0

Weibull plot of strength in PCS-derived SiC ceramics.

100

180
Polymer-like30
Scale parameter: σβ
Shape parameter: σm

160

BrittleScale parameter σβ, MPa

80
60
40
20

140

Polymer-like

25

Brittle

120

20

100

15

80
60

10

40

5

20

0

0

2

4

6

8

6.0

log( σb, MPa)

Representative bending force-displacement relations.

Young’s modulus E, MPa

NPIP = 12

N PIP = 300

10

The number of PIP processes N PIP, times

Young’s modulus vs. PIP cycles.

12

14

0

Shape parameter σmBending Force F, N

14

0

2

4

6

8

10

12

0
14

Fracture surface of PCS-derived SiC ceramics

The number of PIP processes N PIP, times

Scale and shape parameters vs. PIP cycles.

The Young’s modulus and strength of non-PIP specimen were 2.5 GPa and 9 MPa, respectively. After 12-cycles-PIPs, those values
drastically increased to 68 GPa and 157 MPa. Before PIP, successive brittle fractures of pyrolyzed linkages probably appeared as“apparent” ductility in the force-displacement relations. After PIPs, impregnated PCS was fired many times; consequently, many of
unnecessary atoms included in the PCS vaporized, and the pyrolyzed PCS layer definitely toughened. Therefore, a large number of
PIP process yielded PCS-derived Si-C ceramics having better mechanical characteristics.
T. Namazu, H. Kudara, T. Ishikawa, K. Yoshiki, S. Inoue, and Y. Hasegawa 12th Conference of the European Ceramic Society, ECerS XII, Stockholm, Sweden 2011.

SiC ceramics model before and after PIPs.

Proceedings of the 12th Conference of the European Ceramic Society – ECerS XII
Stockholm, Sweden - 2011

PIP Process Effect on Mechanical Behavior of PCS-Derived Si-C Ceramics
Takahiro Namazu1, Hiroyuki Kudara1, Tomohiro Ishikawa1, Keisuke Yoshiki1,
Shozo Inoue1 and Yoshio Hasegawa2
Affiliation 1:
Dept. of Mechanical and Systems Engineering
University of Hyogo
Himeji, Japan

Affiliation 2:
R&D Division
ART KAGAKU Co., Ltd.
Tokai, Japannamazu@eng.u-hyogo.ac.jp
produce ceramics MEMS elements by using
polymer precursor method.2 In this method, aprecursor polymer is firstly cast into a micro moldfabricated by UV-thick-photoresist lithography.
Then the cast polymer is pyrolyzed in an inert gas
in order to convert into self-similar shaped ceramics.
However, the polymer-derived ceramics elements
are known to become porous, which provides thereliability of ceramics MEMS structures with anegative effect.
We have fabricated polymer-derived Si-C
ceramics MEMS made from polycarbosilane(PCS).3 To make dense Si-C ceramics element, thepolymer infiltration and pyrolysis (PIP) process was
adopted. In this paper, we focus on investigating themechanical characteristics, such as Young’s
modulus and bending strength, by means of thethree-point bending test. The influence of PIP
process on those properties is discussed on the basisof a PCS-derived Si-C model.

Abstract
Strength evaluation of silicon carbide (Si-C)
ceramics fabricated from polycarbosilane (PCS)
precursor is described. Si-C ceramics was preparedby firing a green body made of the mixture of Si-C
nano-powders and a PCS solution at 1273 K in N2
gas for an hour. To obtain dense Si-C, the polymer
infiltration and pyrolysis (PIP) process was
conducted up to 12 cycles. Si-C ceramics was diced
to be rectangle shape, and was subjected to thethree-point bending test. Si-C specimens fabricatedthrough PIP processes less than 2 cycles showed
non-linear force-displacement curves, whereas
those through the processes more than 3 cycles
showed linear relations and fractured in a brittle
manner. The Young’s modulus of 12-cycles-PIPs
specimen was 56 GPa, 22-fold of non-PIP specimen.
The bending strength was also increased up to 157
MPa with an increase in the number of PIP process.
The cause of the influence of PIP process on themechanical characteristics is discussed using a
PCS-derived Si-C model.

Experimental
As the first step for fabricating PCS-derived SiC ceramics, the 40 %-PCS toluene solution was
mixed with Si-C nano-powder derived from PCS to
make Si-C powder coated with PCS. The volume
ratio of PCS to the powder was 1:10. PCS-coated
Si-C powder containing stearic acid of 2 wt.% was
filled into a mold under a constant pressure of 80
MPa. Then, the green body was removed from themold, and fired at 1273 K in Ar gas. After firing,
the Si-C body was obtained. Since the body had a
lot of pores, it was immersed in a PCS solution tofill it into the pores by means of vacuum degassing.
Then, firing at 1273 K for an hour was performed
again. The PIP process was conducted multiple
cycles (up to 12 cycles) to improve its porosity. The
Si-C body was mechanically cut by using a dicing
saw to form rectangle-shaped structures measuring
1.0 mm×3.0 mm×0.5 mm, which were utilized as
the specimens for the three-point bending test.
Figure 1 shows the relationship between thenumber of PIP process and porosity. Porosity wasdefined as a percentage of pore area to cross-section
prepared by dicing cut. Porosity was decreased withan increase in the number of PIP process. For

Keywords: Polycarbosilane, silicon carbide,mechanical property, strength, polymer infiltrationand pyrolysis process.

Introduction
Silicon carbide (Si-C), which is one of the heat
resistant ceramics, attracts much attention as astructural
material
utilizedforpower
microelectromechanical systems (MEMS). Si-C istypically prepared by chemical vapor deposition(CVD) or sintering.1 Si-C film prepared by CVD is
useful for surface modification of a micro device
because the material shows superior characteristics,
such as small thermal expansion coefficient, lessabrasion, high density, and chemical inertness. Si-C
structure made by sintering has also goodcharacteristics as does that by CVD. In the cases of
CVD and sintering, however, fabricating threedimensional arbitrary shape at the micro scale isvery difficult because Si-C basically has excellent
mechanical strength and chemical inertness. Several
researchers have so far made many efforts to1(4)Proceedings of the 12th Conference of the European Ceramic Society – ECerS XII
Stockholm, Sweden - 2011
30

16

NPIP = 12

Bending Force F, N

Porosity P, %

14
14
20

10

12
12
NPIP = 610
10

88
NPIP = 3

66
4

NPIP = 2

NPIP = 0

22
0

0

2

4

6

8

10

00

12

The number of PIP processes NPIP, times

00

22

4
4

Fig. 1 The number of PIP process vs. porosity.

66

10 12
14
88 10
12 14
Displacement , m

16
16

18
18

20
20

Fig. 3 Bending force-displacement relations.
Positioning stage
for specimen

Laser displacement
meter

Bending test specimen

100
1 mm

Young’s modulus E, MPa

Load cell
Specimen

Specimen

1 cm

Loading pin

PZT actuator in Al case

Support1 mm

Fig. 2 Three-point bending test apparatus.
example, the porosity of Si-C ceramics after the
first PIP process was approximately 25 %, which
decreased to 8 % after 6-cycles-PIPs. Further PIP
processes, however, did not bring drastic
improvement. After 12-cycles-PIPs, the porosity
was 7.5 %, which is only 6.25 % reduction from 6cycles-PIPs specimen. This is because, in the caseof low porosity region, filling a PCS solution intopores was difficult due to very small pore size or
formation of closed pores.
Figure 2 shows photographs of three-point
bending tester for microscale Si-C ceramics
specimen. The tester is able to apply bending force
to a specimen in the horizontal direction to easily
observe specimen deformation from its overhead
with a CCD camera during bending. The load celland laser displacement meter have a measurement
resolution of 0.1 N and 10 nm, respectively.
Loading and supporting jigs were made ofcemented carbide to restrain elastic deformation of
those jigs as small as possible. The distance
between supporting points is 2.4 mm. The radius ofcurvature in supporting and loading jigs is 0.3 mm.

80

Polymer-like

Brittle

60
40
20
0

0

2

4

6

8

10

12

14

The number of PIP processes NPIP, times

Fig. 4 Relationship between the number of PIP
process and Young’s modulus.
force-displacement relation which slope is verysmall. The bending force hardly increases evenwhen bending displacement increases until failure.
No obvious yield point is seen. 2-cycles-PIPs
specimen shows a similar trend as does non-PIP
specimen. The maximum bending force is seen at
the displacement of 1.5 m, and then the force
gradually drops to failure. At the beginning ofbending test for 3-cycles-PIPs specimen, the forcedisplacement relation is linear. The slope isdefinitely larger than that of non- and 2-cycles-PIPs
specimens. The bending force drastically drops at
the deflection of 3.2 m, but the force does notreach to 0 N. By virtue of rapid crack propagation
into the specimen, the first fracture occurred, but
the entire specimen did not fracture completely.
After the first large fracture, probably lots of small
fractures happened sequentially at the vicinity of
the crack tip. 6- and 12-cycles-PIPs specimensshow the same behavior that the force-displacement
relation is almost linear until failure. At the
deflection of 4.4 m, bending force immediately
drops to 0 N, indicating that brittle failure happened
abruptly. From the bending test results describedabove, it is considered that “apparent” ductilebrittle deformation boundary would exist in Si-C
ceramics produced through 2- or 3-cycles-PIPs.

Results and Discussion
Figure 3 shows representative bending forcedeflection relations of Si-C specimens preparedthrough the PIP processes of 0, 3, 6, and 12 cycles.
All the specimens were bent to failure at room
temperature. Humidity was not controlled during
the test. The loading speed was kept constant to be
0.167 m/s. Non-PIP specimen shows non-linear2(4)Proceedings of the 12th Conference of the European Ceramic Society – ECerS XII
Stockholm, Sweden - 2011180

30
Scale parameter: 
Shape parameter: m160
Scale parameter , MPalog(log(1/(1-Pi)))

1.0
0.0
-1.0
-2.0
-3.0

NPIP = 0

NPIP = 2

NPIP = 3

2.0

3.0

4.0

5.0

25

Brittle

120

20

100

15

80
60

10

40
0

NPIP = 6

1.0

Polymer-like

5

20

NPIP = 12

-4.0
0.0

140

0

2

4

6

8

10

12

0
14

The number of PIP processes NPIP, times

6.0

log(b, MPa)

Fig. 5 Weibull distribution of bending strength.

Fig. 6 Relationship between the number of PIP
process and Weibull parameters.

Figure 4 shows the relationship between thenumber of PIP process and the Young’s modulus of
Si-C ceramics. The Young’s modulus wascalculated from the slope of force-displacement
relation in the beginning of the test. The modulus of
non-PIP specimen is found to be 2.5 GPa on
average. This value is very low as with a polymer,
which would be caused by low density andinsufficiency sintering. With an increase in thenumber of PIP process to 3 cycles, the modulus
remarkably increases to 45 GPa. This indicates thatthe specimen can rapidly harden by conducting a
few PIP processes. After 3-cycles-PIPs processes,
the increment in Young’s modulus gradually
decreases with increasing the number of PIP
process. The mean value for 12-cycles-PIPs
specimen is 56 GPa, approximately 22 times of thatfor non-PIP specimen. The maximum value of 68
GPa was obtained, which is about one-sixth of the
Si-C bulk value. Compared with PCS-derived Si-C
fibers, the value is about 0.36 times. The difference
would be caused by poor crystallinity and/or low
density of the Si-C body. And also, in the PCSderived Si-C body fired at 1273 K, carbon-richcomposition is typically obtained, and it might havegiven rise to low Young’s modulus.
Figure 5 shows the Weibull plot of bending
strength for Si-C ceramics. Total 56 specimens
were subjected to the bending test. The strength
data of all the specimens under respective PIP
conditions were lined up on Weibull probability
plotting paper, and could be approximated byrespective straight lines. This indicates that 2parameter Weibull distribution function isappropriate for the data fitting.4 The slopes ofrespective lines are closely related to the variabilityof strength for each specimen, and tend towards
vertical with an increase in the number of PIP
process.
Figure 6 shows the relationships between thenumber of PIP process, scale, and shape parameters.
The closed and open plots are indicative of the scaleand shape parameters of Weibull, respectively. The

scale parameter of non-PIP specimen is found to beabout 9 MPa, which is quite lower than the bulk
strength.5 The parameter rapidly increases withincreasing the number of PIP process as does theYoung’s modulus. After 6-cycles-PIPs, theparameter drastically increases to 140 MPa, which
is approximately 15 times higher than that in nonPIP specimen. The strength increase is probably
associated with a decrease in porosity. Multiple PIP
processes can definitely reduce the size of poresthat is thought to play a role as stress riser, and also
can provide the formation of strong PCS-derived
Si-C linkages between Si-C particles because of
sintering time period accumulation. After 12cycles-PIPs, however, the strength shows another
10 % increase only from the strength of 6-cyclesPIPs specimen. This implies that, in low porosity
region after several PIP processes, a change ofporosity affecting strength was small. The
maximum strength value obtained was 157 MPa,
which is only 8.3 % of the strength of Si-C fiber.6
On the other hand, the shape parameter of non-PIP
specimen is calculated to be 2.6, which increases to
14 with increasing the number of PIP process to 12
cycles. Therefore, the number of PIP process haslarge influence on the improvement of specimen
strength and its scatter.
Figures 7 (a)-(c) show scanning electron
microscope (SEM) photographs of the fracturesurface of Si-C ceramics produced through the PIP
processes of 0, 6, and 12 cycles, respectively. In Fig.
7 (a), non-PIP specimen possesses irregular roughsurface indicating that there are a lot of pores inside
the body. The fracture surface shows a similarirregularity to side surface, so that fracture origin
and river pattern indicating crack propagation
direction cannot be found. Meanwhile, as shown in
Figs. 7 (b) and (c), the specimen’s side surface isrelatively smooth, co mp ared with non -PIP
specimen. The surface flatness was brought about
by conducting multiple PIP processes. These
specimens have visible river pattern on theirfracture surfaces. The river pattern starts from a3(4)Shape parameter m

2.0Proceedings of the 12th Conference of the European Ceramic Society – ECerS XII
Stockholm, Sweden - 2011(a) NPIP = 0

River pattern

Bending force(b) NPIP = 6(c) NPIP = 12River pattern

Bending force
Bending force
Fracture
originFracture
originFig. 7 SEM photographs of fracture surface of PCS-derived Si-C ceramics.
Impregnated & fired PCS

Conclusions

Si-C powder

We fabricated Si-C ceramics micro-elements
derived from PCS precursor polymer, and evaluatedtheir mechanical characteristics by means of threepoint bending test. The produced Si-C body was
immersed into a PCS solution, and then firing at
1273 was performed. This PIP process was carriedout up to 12 cycles, in order to improve their poor
porosity. A large number of PIP process yielded
PCS-derived Si-C ceramics having bettermechanical characteristics.

PIP
processes

Void

Weak

StrongFig. 8 Schematic of PCS-derived Si-C ceramics
specimens before and after multiple PIP processes.
specimen surface and proceeds to the inside. This
indicates that the fracture origin would have existed
at the vicinity of specimen surface where themaximum tensile stress generated during thebending test.
The mechanism can be explained using the Si-C
ceramics model before and after multiple PIP
processes, as illustrated in Figure 8. After the first
PIP process illustrated in the left figure, PCSderived Si-C nano-powders were coated once with a
PCS solution, which was fired once. Although the
Si-C body was fired, it was weak. This is because,
compared with that of Si-C particles, the rigidity of
the coating layer is thought to be very low though
the organic precursor was fully pyrolyzed. In
addition, because PCS definitely shrunk during thefiring, many voids were produced in the coating
layer. One weakest PCS linkage between Si-C
particles where the maximum tensile stress wasproduced was firstly broken by applying external
force, and then other linkages around the failed link
successively fractured one by one even whenapplied force was small. The successive brittle
fractures of pyrolyzed linkages probably appeared
as “apparent” ductility in the force-displacement
relations. On the other hand, after several PIP
processes, as shown in the right figure, theimpregnated PCS was fired many times;
consequently, many of unnecessary atoms includedin the PCS vaporized, and the pyrolyzed PCS layer
definitely toughened. By multiple impregnations
and firings, the number of voids decreased andthose sizes were reduced. Once a crack initiated, it
probably propagated soon into strong Si-C body,and finally the entire body would have been
fractured.

References
1. S. R. Shah, and R. Raj, Mechanical properties ofa fully dense polymer derived ceramic made by anovel pressure casting process. Acta Materialia, 50(2002), 4093-4103.
2. D-H. Lee, K-H. Park, L-Y. Hong, and D-P. Kim,
SiCN ceramic patterns fabricated by soft
lithography techniques. Sensors and Actuators A,
135, (2007) 895-901.
3. T. Ishikawa, T. Namazu, K. Yoshiki, S. Inoue,and Y. Hasegawa, Polycarbosilane-Derived SiliconCarbide MEMS Component Fabricated by Slip
Casting with SU8 Micro Mold. Proc. ofMicroelectromech. Syst., MEMS 2010, (2010), 416419.
4. T. Namazu, Y. Isono, and T. Tanaka, Evaluationof Size Effect on Mechanical Properties of Single
Crystal Silicon by Nanoscale Bending Test Using
AFM. J. Microelectromech. Syst., 9, 4, (2000), 450459.
5. A. M. Hurtado, and Z Alkan, Studies on the
Sintering Behaviour of SiC with Polycarbosilane
Binding Agents. Ceram Int., 19, 5, (1993), 327-331.
6. R. J. Day, V. Piddock, R. Taylor, R. J. Young,and M. Zakikhani, The Distribution of Graphitic
Microcrystals and The Sensitivity of Their Raman
Bands to Strain in SiC Fibres. J. Materials Science,
24, (1989), 2898-2902.

4(4)Fabrication of PCS-Derived Si-C Ceramics MEMS using
Micro Slip Casting Technique
H. Kudara1, T. Namazu1, K. Yoshiki1, S. Inoue1 and Y. Hasegawa2
1University of Hyogo and 2ART KAGAKU Co., Ltd., Japan

Background
Power MEMS is one of the significant concerns in the aerospace and energy harvesting fields. In operation of power MEMS device, temperature is typically very high, so that structural
materials in the device should possess good heat resistance and mechanical characteristics. Single or poly crystal silicon (SCS/PCS) is commonly used as a structural material in MEMS
because MEMS originated from semiconductor industry. SCS and PCS are easily formed with micro or nano scale accuracy, but both materials do not have heat resistance enough to be
admitted in power MEMS. In recent years, SiC, one of the heat resistant ceramics, is being used as the structural material in power MEMS. SiC is typically prepared by chemical vacuum
deposition or sintering. However, fabricating three-dimensional arbitrary shape of SiC on the micro scale is very difficult due to its superior mechanical strength and chemical inertness. To
develop reliable power MEMS device for commercialization, a new fabrication technique for microscale SiC ceramics element should be developed.

Experimental Procedure
PCS-derived SiC ceramics
Heattreatment

1273 K
1273K,Ar
in inactive gas
SiC ceramics

Polycarbosilane

Fabrication of SiC ceramics MEMS

Polycarbosilane (PCS), which is known as one ofultrahigh molecular weight polymers, is used as aprecursor for SiC fiber. PCS that consists of Si, C,
O, and H atoms is combined by heatdecomposition of tetramethylsilane. By heating at
1273 K in an inactive gas, PCS is pyrolyzed andchanged into SiC ceramics with high ceramics
yield. In this case, the ceramics yield istheoretically 84 %, and the volume constriction
yield is about 1.8 %.

pressurization

SU-8 sheet
WC plate

depressurization(a) SU-8 sheet lamination on WC plate(d) Slip casting

UV

Construction model of PCS precursor for SiC.

UV

mask

dissolve

Fabrication of SU8 micro mold on porous WC plate
SU8 sheet
WC plate

1273K
Hot plate(b) Photolithography for SU-8 mold(e) Firing at 1273K in N2 gas(c) Development

Completion(d) Post-Exposure-Bake(a) SU-8 sheet lamination on WC plate

SU8 layer
SU8 micro mold(a) WC plate with polish surface(b) WC plate with blast surface

Porous WC plate with the porosity of 11.1%(e) Development(b) Pre-bake

UV
UV

Process flow for fabricating microscale SiC ceramics parts.

mask(a)SEM photograph of the surface with
polish finishing(b) SEM photograph of the surface
with blast finishing(c) Photolithography for SU8 mold

SEM photograph of WC plate with porosity of 11.1%.
WC plate used for a filter during casting is made bypowder sintering, and it includes many pores.

Complete

Process flow for fabricating SU8 micro mold.
The 250µµm thick micro mold of gear patterns was
fabricated on porous WC plate.

(a)50 µm thick SU8 sheet lamination onto WC plate to form a 250 µm thickSU8 layer(b), (c) UV-thick lithography for fabrication of SU8 micro mold(d) Micro slip casting into the mold. To fill up the mold with the slips,
pressurization and depressurization are conducted from the top and backof WC plate, respectively(e) Cast body releasing. Fired at 1273 K in N2 gas for an hour.

Experimental Results & Discussions
The ratio of crack

The SiC micro gears by slip casting16

The top side of SiC micro gear

1mm

1mm

1mm

Crack percentage in surface area , %1mm

Polish
14
Blast

12
10
8
6
4
2
0
1

1mm
(b) 3 cycles pressurization

1mm

1mm

(c) 6 cycles pressurization

SiC micro gears fabricated by micro slip casting using WC plate with porosity of 11.1%.
The shape of produced parts in the number of pressurization cycles during casting was 2, at least, gear. Most of cogs fractured. In
the case of 3 cycles pressurization, the shape of gears is better than that prepared in the number of pressurization cycles was 2. The
number of cogs surviving increases with an increase in the number of pressurization cycles. After 6 cycles pressurization, all of thecogs are alive, and the MEMS gear was able to be finely fabricated.

H.Kudara, T. Namazu, K. Yoshiki, S. Inoue, and Y. Hasegawa

3

4

5

6

7

The number of pressurization vs. crack percentage insurface area.

12th Conference of the European Ceramic Society-ECerS Ⅻ Stockholm, Sweden 2011

Crack percentage in surface area , %1mm(a) 2 cycles pressurization

2

The number of pressurization , time

The back side SiC micro gear

Porosity of WC plates , %Porosity of WC plates vs. crack percentage in surfacearea.
The crack percentage in surface area decreases with anincrease in the number of pressurization cycles, andgradually increases with an increase in the porosity.

Proceedings of the 12th Conference of the European Ceramic Society – ECerS XII
Stockholm, Sweden - 2011

Fabrication of PCS-Derived Si-C Ceramics MEMS using Micro Slip
Casting Technique
Hiroyuki Kudara1, Takahiro Namazu1, Keisuke Yoshiki1, Shozo Inoue1
and Yoshio Hasegawa2
Affiliation 1:
Dept. of Mechanical and Systems Engineering
University of Hyogo
Himeji, Japan

Affiliation 2:
R&D Division
ART KAGAKU Co., Ltd.
Tokai, Japannamazu@eng.u-hyogo.ac.jp

Abstract
A new fabrication technique for silicon carbide(Si-C) microelectromechanical systems (MEMS) is
described. Slip casting using UV-thick photoresist
SU8 micro mold was carried out for fabrication of
three-dimensional Si-C MEMS parts. Ultrahighmolecular weight polycarbosilane (PCS) was usedas the precursor. Si-C nano powder was firstly
mixed with a PCS solution, and then the slips were
cast into SU8 micro mold fabricated on porous
tungsten carbide (WC) plate. Firing at 1273 K was
conducted for SU8 evaporation and PCS pyrolysis
simultaneously. We have succeeded in producing
Si-C ceramics micro gears using the “µ slip
casting” technique. The influences of WC plate
porosity and the number of casting cycles on the
shape of the produced Si-C MEMS are discussed.
Keywords: Polycarbosilane, silicon
MEMS, slip casting, UV-LIGA.

Heattreatment

1273 K
1273K,Ar
in inactive gas
Polycarbosilane

SiC ceramics

Fig.1 Construction model of PCS precursor for SiC.
The technique that we have developed is the
combination of UV thick-photoresist lithography
and conventional slip casting. In this paper, thetechnique is firstly described. Then, the effects of
WC plate porosity and the number of pressurization
during casting on the shape of the produced SiC
MEMS parts are discussed.

carbide,Experimental TechniqueIntroduction

Precursor polymer
Polycarbosilane (PCS), which is known as oneof ultrahigh molecular weight polymers, is typically2
being used as a precursor for SiC fiber. PCS thatconsists of Si, C, O, and H atoms is combined byheat decomposition of tetramethylsilane. As
illustrated in Fig.1, by heating at 1273 K in an
inactive gas, PCS is pyrolyzed and changed into
3
SiC ceramics with high ceramics yield. In this case,the ceramics yield is theoretically 84 %, and the4volume constriction yield is about 1.8 %. In thiswork, PCS was employed as a precursor for
microscale SiC ceramics parts.

Power MEMS is one of the significant concerns
1in the aerospace and energy harvesting fields. In
operation of power MEMS device, typicallytemperature rises to several hundred degC, so thatthe structural materials should possess good heat
resistance and mechanical characteristics. Single
and poly crystal silicon are used in conventional
MEMS devices, but they are not suitable forstructural material in power MEMS. In recent years,
SiC, which is one of the heat resistant ceramics, isbeing used as the structural material in power
MEMS. SiC is known to have excellent heat
resistance, mechanical characteristics,and chemical
inertness. However, fabricating three-dimensional
arbitrary shape of SiC on the micro scale is very
difficult due to its superior mechanical strength andhardness. To develop reliable power MEMS
devices, the development of a new fabrication
technique for microscale SiC ceramics element isrequired.
The objective of this work is to propose µ slip
casting technique for producing SiC micro elements.

“µ slip casting” techniqueFig. 2 shows a process chart for fabricating SiC
ceramics micro parts. At first, SU8 sheet of 50 µm
thick is laminated in five times onto WC plate so as
to form a 250 µm thick SU8 layer. Then UV-thicklithography is performed for fabrication of SU8
micro mold on the plate. The fabrication detail is
described in the following section. The slips include
SiC nano powder and a PCS solution for 10:1 are

1(4)Proceedings of the 12th Conference of the European Ceramic Society – ECerS XII
Stockholm, Sweden - 2011

Fig.3 Porous WC plate with the porosity of 11.1%

Fig.2 Fabrication process of SiC-MEMS ceramics
parts.
mixed in xylene and isopropyl alcohol (IPA). The
slips are cast into the mold. During casting, to fillup the mold with the slips, pressurization anddepressurization are conducted from the top andback of WC plate, respectively. This process isrepeatedly performed up to 12 times in order toinvestigate the influence of the number ofpressurization process on the shape of the cast body.
Total amount of slops is 600 µl, which was kept
constant. After the casting, the cast body is fired at1273 K in N2 gas for an hour to vaporize
unnecessary ingredients in PCS to produce solid
SiC. We are able to release the SiC ceramics bodyafter firing because SU8 is dissolved at around 723
K.

Fig.4 Fabrication process of SU-8 micro mold.
bake (PEB) temperature fixing;328K and time
period, exposure time period, developing
temperature and time fixing; 1h period, andlaminating temperature.

Results and Discussion
Fabrication of SU8 micro mold
Table 1 lists a summary of SU8 micro moldfabrication on WC plates. The circle, triangle andsquare plots in the table indicate non-delamination
area of more than 90%, 90-50%, and less than 50%,
respectively. Higher lamination temperature andlonger exposure time period using lower porosity
WC plate is found to be better for precisely
fabricating SU8 micro mold. Those trends are
represented in Fig. 5. As shown in Fig. 5(a), SU8
mold pattern is completely laminated after UVLIGA process when it was produced through ideal
experimental condition. As the condition somewhat
deviates from the ideal one, the mold is partly
delaminated from WC plate, shown in Fig. 5(b). In
the case of the worst condition, SU8 mold iscompletely delaminated and released from WC
plate as shown in Fig. 5(c). Those phenomena areprobably related to stress relaxation of SU8 sheet.
In this work, the optimum experimental condition

SU8 micro mold fabricated on WC plate
As is well known, WC is representative
cemented carbide material with a high melting pointand high strength. WC plate used for a filter during
casting is made by powder sintering, and it includes
many pores. We prepared seven kinds of WC plateswith different porosity and surface finishing. Fig. 3
representatively shows 11.1%-porosity WC plateswith polish and blast surfaces.
SU8 micro mold is fabricated onto those WC
plates. Fig. 4 shows a process chart for fabricating
SU8 micro mold. It is well known that SU8 isdelamination-easy (i.e., easily removable orseparable) material from a substrate. In addition,
surface roughness and its inhomogeneousness cause
expose-light dispersion, which leads to low
accuracy patterning. To obtain SU8 micro moldwith superior shape and without delamination,
fabrication conditions were optimized. The
fabrication parameters were employed as follows:
pre-bake temperature fixing;368K, post-exposure-2(4)Proceedings of the 12th Conference of the European Ceramic Society – ECerS XII
Stockholm, Sweden - 2011

Fig. 5 Typical SU8 mold fabrication results.
was determined as: laminating temperature, 348K;
exposure time period, 75s; PEB time perild,12h.
SiC ceramics MEMS parts
Fig. 6 shows representative examples of the SiC
ceramics micro gears produced using µ slip casting
technique. All the gears shown in the figure wereobtained using WC plate with the porosity of 11.1%.
When the number of pressurization cycles during
casting was 2, many of cogs are found to fracture.
The number of cogs surviving increases with anincrease in the number of pressurization cycles.
After 6 cycles pressurization, all of the cogs arealive, and the MEMS gear was able to be finely
fabricated. The influence of the number ofpressurization on on the percentage completion of
MEMS gear fabrication is thought to depend on theexistence of cracks in the cast body. Any apparent
cracks were not seen in the top surface of MEMS
gear regardless of the number of pressurization
cycles, whereas in the back surface the number ofcracks decreased with increasing the pressurization
cycles. The number of cracks would likely be
related to casting speed and the amount of slips
used in one time.
Fig. 7 shows the relationship between thenumber of pressurization cycles and the crack-tosurface area ratio. The ratio was defined as the ratioof crack portion to surface area of gear, and thecrack portion was determined using binary format
image method. In the case of 2 pressurization cycles
using polished and blasted WC plates with theporosity of 11.1%, the ratio value is found to be
13.3% and 9.7% on average, respectively. Blasted
WC plate is able to provide crack-less SiC MEMS
parts. This is because surface morphology of theblasted plate was more uniform than that of thepolished plate. The uniformity is important for slip
casting because the difference in casting speed on
the plate causes crack generation. Both the ratio
values proportionally decrease with an increase in
the number of pressurization cycles. At 6 cycles
pressurizations, the crack ratio values for polishedand blasted WC plates castings were 6.3% and2.9%, respectively, which are approximately 7.0%
and 6.8% reductions from those at 2 cycles. The
reason why many pressurization cycles yielded lesscracks in the cast body is probably attributed to thedifference in casting speed between small and large
SiC particles. During casting, large particles settle
down faster than small ones, so that large SiC

Fig. 6 SiC micro gears fabricated by micro slip
casting using WC plate with the porosity of 7.9%.

Fig. 7 Relationship between the number ofpressurization and the ratio of crack using WC platewith the porosity of 11.1%.
particles exist at around the back surface. This
yields large gaps between large particles. The large
gaps probably lead to large crack generation. In thecase of many pressurization cycles, the thickness ofcast layer is thin. So, because the difference insettling down speed between small and large
particles is small, small particles can intrude into
gaps between large particles; therefore it is difficult
to generate large cracks.
Fig. 8 shows the crack ratio as a function of
WC plate prosity. Those data indicate the results of
6 cycles pressurization experiments. The crack ratio
value at the polished plate porosity of 7.9% is foundto be 5.3%, which gradually increases with anincrease in the porosity. The largest value was 7.8%,
which was brought about using the WC plate of
23.9%-porosity. The increase trend is probably
derived from casting speed increase. Using thecoarse plates, it would appear that the difference incasting speed between small and large SiC particles
differ greatly. In addition, course WC plate makes3(4)Proceedings of the 12th Conference of the European Ceramic Society – ECerS XII
Stockholm, Sweden - 2011
MEMS. The technique includes slip casting and UV
thick-photoresist lithography. The slips consisted of
100 nm diameter SiC powders and a PCS solution,
and were cast into SU8 micro mold fabricated on
porous WC plate. We obtained optimum
lithography conditions to achieve SU8 micro moldwithout delamination Using fabricated SU8 micromold, we showed that larger number ofpressurization cycles and the use of low-porosity
WC plate were important for SiC ceramics microparts without large cracks.

References
Fig. 8 Relationship between WC plate porosity andcrack percentage at 6 cycles pressurizations.

1. K. Takahashi, MEMS Technology and Space
Propulusion. JSASS, 52 (2004), 262-266.

its surface morphology rough, so that casting speed
would vary from place to place. From those twothings, the porosity increase in WC plate is thoughto yield the number of cracks on its plate. On theother hand, in experiments using blasted WC plates,
the crack percentages in surface area of gear stayed
constant without respect to porosity. This implies
that smaller difference in casting speed from place
to place can produce Si-C MEMS parts with lesser
cracks. Therefore, the uniformity of surface
morphology on WC plate is significant for crackless Si-C MEMS parts.
.

2. Y. Hasegawa, New curing method forpolycarbosilane with unsaturated hydrocarbons andapplication to thermal stable Si-C fibre. Composites
Science and Technology, 51, 161-166, 1994.
3. Y. Hasegawa and K. Okamura, Synthesis ofcontinuous silicon carbide fibre; Part 3 Pyrolysis
process of polycarbosilane and structure of the
products. J. Mater. Sci., 18, 3633-3648, 1983.
4. Y. Hasegawa, Ultrahigh molecular weight ofpolycarbosilane for SiC/SiC. Proc. of The 55th
Polymer Conf., 3003-3004, 2006.

Conclusions

5. D. Akagi, K. Takada and H. Ukita, Fabrication ofThree Wing Optical Mixers and Its Application to a
Liquid Moxing in a Microchannel. MSS-05-30,
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We proposed “µ slip casting” technique formicroscale SiC ceramics parts used for power4(4)