EVALUATION OF WEAR RESISTANCE OF SPUTTERED AMORPHOUS SICN
FILM AND MEASUREMENT OF DELAMINATION STRENGTH OF FILM
BY MICRO EDGE-INDENT METHOD
M. Kato, J. Zheng and S. Takezoe
Major of Mechanical System Engineering, Hiroshima University
Higashi-hiroshima, Hiroshima 739-8527
ABSTRACT
SiC and SiCN films were deposited on titanium and tool steel substrates in a RF magnetron sputtering apparatus with a SiC
target using a mixture of argon and nitrogen gas. A micro edge-indent method was newly proposed to evaluate delamination
energy of thin films, and the relationship between the delamination energy and wear strength of the films was examined. The
results show that the micro edge-indent test is effective to evaluate the delamination energy of thin films, and the delamination
energy of the SiC film is larger for Ti substrate than tool steel substrate. The maximum delamination energy of the SiC and SiCN
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films are obtained when the substrate is not heated and the nitrogen gas flow rate is about 1.0cm /min. The wear delamination
cycles of the SiC film decreases with increasing substrate heating temperature, and that of the SiCN film decreases with
increasing nitrogen gas flow rate. The delamination cycles of the films almost correspond with the delamination energy before
wear tests, but they are also affected by friction coefficient and hardness of films.
Introduction
In order to improve wear property of metal substrates, ceramics, such as TiN, TiC, are coated on the substrates by using physical
vapor deposition(PVD) and chemical vapor deposition(CVD) techniques. Among the ceramics, silicon carbide(SiC) has high
vickers hardness[1] next to diamond, C-BN, and superior heat resistance, oxidation resistance.
In a previous research, we reported that amorphous SiC was formed on titanium substrate, and the friction coefficient of the SiC
film was 0.1 which is equal to that of the diamond like carbon(DLC) film[2]. Further decrease in the friction coefficient down to 0.5
was achieved by adding 2.6% titanium to the SiC film formed by simultaneous sputtering of both the SiC and the Ti targets[3]. On
the other hand, nitrogen doped SiC(SiCN) is attractive for electrical and optical materials as the band-gap and transparency of
the film are controlled by the amount of nitrogen dopant, and the wear resistance of the SiCN film is expected to be high as the
film has a probability of containing strong C-N bond[6]. However, delamination of the film occurs when the film receives repeating
sliding contact during wear process. It is therefore very important to improve not only wear resistance of films but also
delamination strength of films. Consequently, employment of a proper evaluation method is required to develop films with high
delamination strength.
Previously, we proposed a “Tensile testing method”, to evaluate the delamination strength of coatings and thin films quantitatively
by applying tensile load to the substrates[7]. However, application of this method is difficult for substrates with poor ductility. In
addition to the “tensile testing method”, we also proposed an “Edge-indent method”, where the delamination strength of coatings
was obtained quantitatively by loading a conical diamond indenter to the surface of coatings near the edge introduced by using a
fine-cutter[8][9].
In order to evaluate the delamination strength of sputtered films, we applied the “Edge-indent method” to the sputtered films,
previously[10]. However, reliable delamination strength of the films could not be obtained as the edge formed by using the
fine-cutter was not sharp enough, and the obtained displacement was not accurate enough. In this research, we newly proposed
a “Micro-edge indent method” where the accuracy of edge processing and displacement measurement were improved up to
sub-micron, and evaluated the delamination strength of SiC and SiCN films. In addition, friction and wear tests of the films were
carried out and the relationship between delamination lifetime and delamination strength of the films was discussed.
Experimental procedures
Sputter coating of SiC and SiCN films
The materials used for the substrates of SiC and SiCN films were commercially supplied pure titanium(JIS: TP340C) with 3mm
thickness and high speed steel(JIS: SKH51) with 1mm thickness. The surface was buff-polished up to a grade of 0.1 m and
ultrasonically cleaned in acetone.
The SiC film was deposited by using a helicon-sputtering apparatus[11][12] as is shown in Fig.1. A SiC disk with 99.9% purity was
used as target material. After placing the substrate in the chamber, the pressure was evacuated to 5×10-4Pa, and the substrate
was heated to 673K for 0.6ks to remove the contaminations on the substrate. Argon gas of 99.99% purity was introduced and the
pressure was maintained at 0.12Pa. Then the SiC film was deposited to a thickness B1 of 1 m by applying a RF power of 100W
to both the RF coil and cathode. The substrate was rotated at a speed of 10r.p.m to obtain homogeneous film thickness.
Deposition of the SiCN film was carried out by using the same apparatus as the SiC film. After the substrate was heated, SiCN
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film of 1 m thick was deposited at the conditions of an argon gas(99.99% purity) flow rate Q Ar of 18cm /min, and a nitrogen
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gas(99.9998% purity) flow rate QN2 ranging from 0 to 10cm /min under a pressure of 0.12Pa.
Figure 1 Schematic diagram of RF magnetron sputtering apparatus
Micro edge-indent test
Figure 2 Micro edge-indent test method (a)indent apparatus, and (b) load-displacement curves
Figure 2 shows schematic of the micro edge-indent method. As is shown in Fig.2(a), a groove of 5 or 10 m wide and 4-5 m deep
was etched on the surface of the film by using a focused ion beam apparatus(FIB:SMI 9200D, Seiko Instruments Inc.), and a
conical diamond indenter(apex angle =120° and radius of curvature of the apex r=1.4 m) was indented at a position with a
distance of x from the edge of the groove at a constant loading speed of 10mN/s until the load reaches a maximum load Pmax
of1N by using a micro indenter(MZT4, Akashi Co.). The apex angle of the indenter and a ratio of the curvature radius of the apex
of the indenter to the film thickness are chosen the same values used for the edge-indent test[8] to guarantee geometrical
similarity.
Figure 2(b) illustrates typical load-displacement curves(P-h curves) obtained by the micro-edge indent test. With increasing
displacement, the load increases monotonically. When the load reaches a critical value, Pd, (Ⅰ)the displacement increases
discretely or (Ⅱ) the gradient of the curve decreases with increasing displacement due to delamination of film. Finally, the load
reaches a preset value of Pmax. From the P-h curve, delamination energy of the film is calculated from the following equation[8]:
h0
Ed
0
Pdh
h0 B1
0
P dh
2
,
2
S
tan
1
S
,
x2
(1)
where, S is the delamination area, h0 is the displacement of the indenter at delamination, 2θ is the apex angle of the delamination
area when the shape of the delamination area is approximated to an isosceles triangle. The delamination area S excludes the
area of the indentation mark at delamination, Si. However, the area of the indentation mark after the indent test, Si,max is larger
than Si as the indenter continues pressing the specimen until the displacement reaches h max. In this research, S i is assumed to
2
be (h 0/hmax) Si,max. The second term of the left side in the eq.(1) means the energy used to deform the substrate when the
displacement is larger than the film thickness, B1.
Hardness HUT and elastic modulus E1 of the films are measured by using the micro indentater with vercovich indenter(elastic
constant Ei＝1141GPa, poison ratio i=0.07). The maximum indentation depth hmax=0.1 m, which is about 1/10 of the B1. HUT is
defined as the following equation:
HUT
1.75
Pmax
2
hmax
,
(2)
and E1 is calculated from the gradient of the load-displacement curve measured at unloading[13],[14].
Wear test
Friction and wear tests of the SiC and SiCN films were carried out by using a ball on disk type testing machine, as is shown in
Fig.3[2]. The specimen was set on the turn disk, and the disk was rotated at a sliding velocity of 0.1m/s, A SiC ball with 10mm
diameter was pressed on the specimen at a constant load P(2.94 or 5.88N). Wear sectional area was measured by using a
contact needle type roughness meter.
Figure 3 Schematic drawing of ball-on-disk wear test machine
Results and discussion
Crystal structure, hardness and elastic constant of SiC and SiCN films
X-ray diffraction analysis of the SiC film without substrate heating, heated at 573K, 873K and the SiCN film with nitrogen flow rate
of 0.1-10cm3/min deposited on glass substrate was carried out under a condition of an incident angle of 1°. Diffraction patterns
were shown in Fig.4, where diffraction patterns of SiC target and sintered Si3N4 are shown as reference. No clear peak but the
broad peak around 2 =36° corresponding to the main peak from the SiC crystal is observed, which suggests that the SiC film is
amorphous. The intensity increases with increasing substrate heating temperature due to progress of crystallization of the SiC
film.
For the SiCN film of QN2=0.1 and 1.0cm3/min, diffraction pattern of the SiCN film is similar with that of the SiC film. When the
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nitrogen gas flow rate Q N2 is 10cm /min, broad peaks at 2 =36° and 27° are observed, and the pattern is similar to that of the
sintered Si3N4. The result of the electron probe analysis shows that the atomic composition of Si:C:N is 33:15:52, which implies
that the SiCN film consists of amorphous SiCN with very fine Si3N4 crystal.
Figure 5(a) and (b) show the influence of substrate heating temperature T on hardness HUT and elastic modulus E1 of the SiC
film. HUT and E1 increase with increasing T, and approaches almost the same values as those of SiC target at T=873K.
Figure 4 X-ray diffraction patterns of SiC and SiCN films
Figure 5 Effect of substrate heating temperature on
(a) hardness and (b) elastic modulus of SiC films
Figure 6 Effect of N2 gas flow rate on (a) hardness and
(b) elastic modulus of SiCN films.
The effect of nitrogen gas flow rate QN2 on hardness HUT and elastic constant E1 is shown in Fig.6(a) and (b). HUT and E1
decrease with increasing QN2. This suggests that no increase in strong C-N bond[6] but weak Si-N bond. The difference of HUT
and E1 between the SiCN film and the SKH51 substrate is smaller than that between the SiC film and the Ti substrate.
Delamination strength of films by micro edge-indent method
The micro edge-indent test was applied to the SiC film(B 1= m) on Ti substrate, and the SiCN film(B 1=1 m) on SKH51 substrate.
Figures 7 and 8 show load displacement curves and optical micrographs after the micro edge-indent test. For both the SiC and
SiCN films, displacement increases with increasing load, and discrete increase in displacement is observed when the load
reaches a critical value.
Figure 7 Load-displacement curves and optical micrographs
of SiC films after micro edge-indent tests
Figure 8 Load-displacement curves and optical micrographs
of SiCN films after micro edge-indent tests
Figure 9 Relationship between substrate heating temperature
and delamination energy of SiC film
Figure10 Relationship between N2 gas flow rate and
delamination energy of SiCN film
The delamination energy of the SiC film calculated by using eq.(1) is shown in Fig.9. The Ed of the SiC film decreases with
increasing substrate heating temperature. For the un-heated SiC film, delamination occurs only when the distance between the
indentation point and the edge is small, which indicates the delamination strength of the film is high.
Figure 10 shows the relationship between nitrogen gas low rate QN2 and delamination energy E d of the SiCN film(SKH51
3
3
substrate). Ed increases with increasing QN2, and decreases when QN2 exceeds 1cm /min. The Ed at Q N2=0cm /min is smaller
than that of the un-heated SiC film, which means that the delamination strength of the SiC film on Ti substrate is larger than that
between the SiC film on SKH51 substrate.
The above-mentioned results indicate that the micro edge-indent method is effective to evaluate the delamination strength of
films quantitatively.
Wear property of SiC and SiCN films and relationship between delamination life and delamination energy of films
The relationship between the number of rotation cycles N and wear sectional area S, friction coefficient , wear depth hw were
measured for the SiCN films. Figure 11 shows an example. Rapid increase in S and is observed at a critical number of rotation
cycles(shown as D in Fig.11) due to film delamination. The before delamination is very low. The effect of substrate heating
temperature T on wear property of the SiC film reported in the previous paper showed that became large with increasing T[3].
Examples of wear scars in the SiC and SiCN films observed during the wear test are shown in Fig.12. Semicircular cracks occur
at a certain number of rotation cycles followed by partial delamination of the films. With increasing the number of rotation cycles,
both the length and number of the cracks increase and delamination area extends. As is pointed by a white arrow in Fig.12, we
can observe black areas indicating optical interference of the SiCN film with optical transparency[5].
Figure 11 Effect of N 2 gas flow rate on wear sectional area, friction coefficient and wear depth of SiCN films
Figure12 Surface views of wear scar in SiC and SiCN films
Figure 13 shows the relationship between substrate heating temperature T and the number of rotation cycles when the
delamination of the SiC film is observed for the first time N d. The Nd of the SiC film decreases with increasing T, and this trend is
the same as that of the relationship between T and Ed(Fig.9). The effect of nitrogen gas flow rate Q N2 on the number of rotation
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cycles at delamination Nd for the SiCN film is shown in Fig.14. Nd is independent of QN2 at a range less than 1cm /min. When QN2
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is larger than 1cm /min, Nd decreases with increasing QN2 and reaches about 1/40 of that for the SiC film(QN2=0 cm3/min) at
3
QN2=10 cm /min. This trend is almost the same as that shown in Fig.10.
From above results, we can understand strong correlation between delamination strength of the films before wear tests and
delamination life of the films. In general, the delamination life of films is affected by not only the delamination strength of films but
the friction coefficient and wear property of films, as the delamination of films occurs due to repeating shear stress caused by
friction force between the ball and the film[2][3]. The reason why the delamination life of the SiCN film at Q N2=1.0cm3/min is
3
3
almost the same as that at QN2=0 and 0.1cm /min(Fig.14) despite that the Ed at Q N2=1.0cm /min is higher than that at QN2=0 and
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0.1cm /min(Fig.10) is as follows: N d is decreased by the increase in the shear stress with increasing QN2 due to the increase in
friction coefficient as is shown in Fig.11.
Figure13 Relationship between substrate heating temperature Figure14 Relationship between N2 gas flow rate and number
and number of rotation cycles at delamination for SiC film
of rotation cycles at delamination for SiCN film
Decrease in delamination energy by friction and wear
In order to evaluate the influence of repeating sliding friction on delamination strength quantitatively, the micro edge-indent test
was applied on wear scars of the SiC and SiCN films. The surface view after the micro edge-indent test is shown in Fig.15. The
SiC film with B1=1 m on Ti substrate was wear tested until partial delamination(Fig.15(a), N=1.1×10 4 cycles, remaining film
thickness:1.5 m), and the micro edge-indent test was applied around the delamination region to evaluate the delamination
energy of this point. However, the obtained delamination energy has insufficient reliability, since no discrete increase in the
displacement is observed obviously in the load displacement curves.
On the other hand, for the SiC film with B1=1 m on SKH51 substrate(Fig.15(b)) wear tested for N=4.5×10 3cycles (remaining film
thickness:0.6 m), delamination of the film occurs. The delamination energy of the film is shown in Fig.10 as a ○ symbol. The
delamination energy is smaller than that of the film before the wear test, which means that the delamination energy of the SiC film
on SKH51 substrate decreases with repeating sliding friction.
From this research, it is clarified that the micro edge-indent method is effective for evaluation of the delamination life of films, and
furthermore, measurement of the delamination energy of films at local area, such as the wear scar.
Figure 15 Surface views after micro edge indent tests at wear scar of SiC film on (a)Ti substrate(N=1.1×104) and (b)SKH51
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substrate(N=4.5×10 )
Conclusions
SiC and SiCN films were deposited on titanium and tool steel substrates in a RF magnetron sputtering apparatus with a SiC
target. A micro edge-indent method was newly proposed to evaluate delamination strength of the thin films, and the relationship
between the delamination energy and wear strength of the films was examined. The results are as follows:
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(1) Amorphous SiC and SiCN films are formed on the substrate when the nitrogen gas flow rate is less than 10cm /min, and
crystallization proceeds with increasing substrate heating temperature.
(2) Elastic modulus and hardness of SiC film increase with increasing substrate heating temperature, and approach to those of
the SiC target(crystalline), and those of the SiCN film slightly decreases with increasing nitrogen gas flow rate.
(3) The micro edge-indent method is effective to evaluate the delamination energy of thin films, and the delamination energy of
the SiC film decreases with increasing substrate heating temperature, and that of the SiCN film increases with increasing
nitrogen gas flow rate and reaches a maximum at a nitrogen gas flow rate of 1.0cm3/min.
(4) The delamination energy at the beginning of partial delamination of the SiCN film is smaller than that before the wear test.
(5) When the number of rotation cycles reaches a critical value, delamination of film occurs, and the amount of wear increases
rapidly. The delamination cycles of the films almost correspond with the delamination energy before wear tests.
Acknowledgments
This research was supported by a Grant-in-aid for Scientific Research (18560134) from Japan society for the promotion of
science.
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