Int. Journ. of Peening Science and Technology, Vol. 1, pp. 221–232
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Evaluation of Surface Properties of Silicon
Nitride Ceramics Treated with Laser Peening
K. Saigusa1, K. Takahashi 1* and N. Sibuya2
1

Faculty of Engineering, Yokohama National University, 79-5 Tokiwadai,
Hodogaya-ku, Yokohama-shi, Kanagawa, Japan
2
Sintokogio, LTD, 71-2 Tsukeda, Nishijo, Oharu-cho, Ama-gun, Aichi, Japan

Laser peening (LP) is a process that has been mainly applied to metals.
However, few studies have applied LP to ceramics. In this study, laser
peening without coating (LPwC) was applied to silicon nitride reinforced
with silicon carbide (Si3N4/SiC). Smooth, laser-peened, and shot-peened
specimens were prepared, and the surface conditions, surface roughness,and residual stresses were investigated. The surface roughness increased
after LP due to laser ablation. When the LP parameters such as power
density and coverage were too high, cracks initiated on the surface. Further, when appropriate LP parameters were selected, a compressive residual stress of up to 230 MPa was introduced on the surface and the depth
was about 50 μm. Although the magnitude of the surface residual stress of
the laser-peened specimen was smaller than that of the shot-peened specimen, LP was able to introduce a deeper compressive residual stress.
Keywords: Laser peening, silicon nitride, ceramics, residual stress,
surface roughness, surface properties

1 INTRODUCTION
Ceramics have excellent properties compared to metals, such as heat shock
resistance, ablation resistance, and corrosion resistance. Therefore, ceramics
are used in various applications. However, ceramic materials are brittle. Thus,
their fracture toughness is much lower than that of metals.
To address this problem, many studies have been conducted with the goalof increasing the strength of ceramics by controlling their microstructures
Corresponding author’s e-mail: takahashi-koji-ph@ynu.ac.jp

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and developing composite materials. In addition, surface treatments have also
been studied. Shot peening (SP) is a technique used to introduce compressive
residual stress by hitting small shot materials against the surface of the target
material. In recent years, the effects of SP on ceramics have been studied byseveral researchers [1–8]. Ito et al. reported that the bending strength of partially stabilized zirconia (PSZ) increased after SP [1]. Moon et al. reported
that a compressive residual stress was introduced by SP on the surface ofsilicon nitride, which improved the apparent fracture toughness [2]. Tanaka et
al. reported that a compressive residual stress was introduced in silicon nitride
by fine-particle peening and ultrasonic peening, resulting in improved apparent fracture toughness [3]. Pfeiffer et al. also reported that the compressive
residual stress introduced by SP increased the static and cyclic load capacitiesof alumina and silicon nitride [4]. Takahashi et al. reported that improvements in the apparent fracture toughness and contact strength of a Si3N4/SiC
composite material were achieved by SP [5]. Takahashi et al. also showedthat the bending strength and abrasion resistance of PSZ increased due to theeffects of the compressive residual stress induced by SP [6, 7]. Shukla
reported that the surface fracture toughness of a zirconia-advanced ceramicincreased by micro-shot peening [8]. However, due to the physical contact
with the shot material during SP, there is a possibility of surface peeling or
chipping, which can result in large decreases in strength.
Therefore, we focused on laser peening (LP) in this study. LP is a technique that utilizes the impact force of plasma generated by the irradiation ofa pulsed laser. LP is used for preventing many forms of damage, such as
fatigue, wear, and stress corrosion cracking (SCC) in metals. For example,
introduction of residual stress and improvement of the fatigue performancesof metal components by LP were reported [9]. Sano et al. investigated theresidual stress improvement in the surface of SUS304 by laser irradiation[10]. In addition, the application of LP to various materials has been studied.
For example, retardation of crack initiation in austenitic stainless steels [11]and improvement in the fatigue life of a titanium alloy [12] were reported.
Moreover, stress corrosion cracking resistance and significant decreases infatigue crack growth rates in aluminum alloy welded joints were reported[13, 14]. Further, introduction of residual stress and improvement of thefatigue strength of a nickel-based alloy were reported [15].
Further, the effects of LP on ceramics have been studied as follows. Akita
et al. reported that compressive residual stresses up to 270 MPa could be
introduced by applying LP to silicon nitride. They also reported that thebending strengths of laser-peened specimens were improved compared tothose of smooth specimens [16]. Shukla et al. reported that LP improved thehardness and fracture toughness of alumina [17] and silicon carbide [18], andthat compressive residual stresses were introduced to Al2O3 armor ceramics[19]. In addition, Shukla and Lawrence applied LP to Si3N4 advanced ceramics and reported an improvement in fracture toughness [20, 21]. Shukla sum-Evaluation of Surface Properties of Silicon Nitride Ceramics

223

marized the results of LP of ceramics in recent years [22]. Wang et al. applied
LP to Al2O3 ceramics and reported that compressive residual stresses of up to671 MPa were introduced on the surface with a depth of about 1.2 mm [23].
Wang et al. also reported that compressive residual stresses of 850~900 MPa
were generated on the surface of Al2O3 ceramics by LP, and were retained on
the surface after annealing for 10 h at 1100 ºC and 1300 ºC [24]. Xing et al.
found that femtosecond LP improved the wear resistance of Al2O3/TiC cutting tools [25].
As mentioned above, many studies have applied LP to metals. However,
the application of LP to ceramics has been limited because of the concerns
regarding the formation of micro-cracks under the strong LP conditions
required to introduce large compressive residual stresses. Therefore, when
LP is applied to ceramics, it is necessary to investigate the relationshipbetween LP and the surface conditions.
In this study, we applied LP to silicon nitride reinforced with silicon carbide (Si3N4/SiC) and investigated its surface properties such as the surface
roughness and residual stress distribution. Finally, we determined the appropriate LP conditions for Si3N4/SiC.
2 EXPERIMENTAL PROCEDURES
2.1 Materials and specimens
In this study, we used silicon nitride reinforced with silicon carbide (Si3N4 /SiC). The samples were prepared using a mixture of Si3N4 with 20 wt% SiC
powder and 8 wt% Y2O3 as a sintering additive. The mixed powders weredried and subsequently hot-pressed at 1850 °C and 35 MPa for 2 h in a N2
atmosphere. Detailed methods for preparing the test materials were reported
previously [26]. The hot-pressed plates were then cut into test specimens
measuring 3 mm × 4 mm × 40 mm. One face of each specimen was groundwith a grindstone and polished with a 6 μm diamond slurry to a mirror-likefinish. The arithmetic average surface roughness (Ra) was 0.025 μm. One of
the edges of the specimen was chamfered. This specimen is denoted as the“smooth specimen.”
2.2 Laser peening process
Laser peening without coating (LPwC) was applied to the smooth specimens
in water. Table 1 lists the conditions of the LP process. The second harmonicof a Q-switched Nd:YAG laser (λ = 532 nm, SAGA, Thales, France) wasused. The pulse duration (t) was 6.2 ns and the repetition rate was 10 Hz. The
spot diameter of the laser (D) was fixed as 0.5 mm. The pulse energy (Ep)indicates the energy contained per pulse of the laser. The irradiation density(Np) indicates the number of pulses irradiated per unit area. The peak power

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TABLE 1
Conditions of laser peening.

Specimens

Specimen 1

Specimen 2

Specimen 3

Pulse
energy,E p [mJ]

Spot
diameter, D
[mm]

24

194

401

Power
density,
G [GW/
cm2]

Irradiation
density,Np [Pulse/
mm2]Overlapping
pitch
[mm]

Overlap
ratio[%]

Coverage, Cv
[%]

16

0.25

50

314

2

0.5

16

33

51

0.14

72

1000

153

0.081

84

3000

255

0.063

87

5000

16

0.25

50

314

51

0.14

72

1000

153

0.081

84

3000

255

0.063

87

5000

16

0.25

50

314

51

0.14

72

1000

153

0.081

84

3000

255

0.063

87

5000

density (G), which is the light intensity per unit area, can be calculated by thefollowing equation using the pulse energy, Ep, and spot area, Ap (= πd2/4):
G=

Ep
Ap t

(1)

Three power densities, G = 2, 16, and 33 GW/cm2, were selected.
Figure 1(a) shows the peening area of the specimen, and Figure 1(b) shows
the peening pattern and laser track in the laser-peened area. Figure 1(c) showsthe experimental setup of LP. The specimens were submerged in a water bath,and then processed by LP. The laser was rastered over the specimens in the y(width) direction followed by shifting in the x (longitudinal) direction in one
layer. The pitches of the laser spot were ranged from 0.063 to 0.25 mm, andthe corresponding overlap ratios were 50% to 87%. The pitch is the distance
between two laser-irradiated spots. In Figure 1(c), the pitch in the x directionis the same as the pitch in the y direction.
The coverage, Cv, which is the amount of overlap per unit area, is calculated by the following equation proposed by Sano et al. [10] using the irradiation density, Np, and spot area, Ap:
Cv = Np Ap(2)

Evaluation of Surface Properties of Silicon Nitride Ceramics

FIGURE 1
Schematic diagram of peening method applied to the specimen.

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Here, Cv = 314, 1000, 3000, and 5000% were selected. In this LP, we constructed
1 path. In this study, shot-peened specimens were also prepared for comparison.
In SP, the shot diameter was 300 μm and the shot pressure was 0.2 MPa [5].
2.3 Measurement of surface condition, surface roughness,and residual stress
Surface observations and surface roughness measurements of the specimens
were carried out using an optical microscope and a stylus-type surface roughness-measuring machine, respectively. For the surface roughness measurements, three random points on the surface of each test specimen were
measured in the longitudinal direction, and an average value was calculated.
The Ra was used as a parameter for the surface roughness.
In addition, the surface residual stresses of the laser-peened specimens in
the longitudinal direction were measured by the cos α method using an X-ray
residual stress-measuring device. The residual stress was evaluated at threepoints and an average was calculated. In this study, the (212) diffraction peakof Si3N4 was measured using the Cr-Kα characteristic X-ray line.
Furthermore, to evaluate the residual stress distribution in the depth direction, sequential polishing in the depth direction was performed. During polishing, the surface of the specimen was mirror-finished by buffing with adiamond slurry with a particle diameter of 1 µm. The amount of polishing
was measured based on the change in the thickness of the specimens before
and after polishing using a micrometer.

3 RESULTS AND DISCUSSION
3.1 Surface observations and surface roughness
The surface of each test specimen was observed. Figure 2 shows the optical micrographs of the surface of each specimen. There was no chippingon the edge of the smooth specimens, as shown in Figure 2(a). The
micrographs in Figure 2(b) and (c) show an enlarged view of the surface
on which LP was applied. The specimens that were laser peened with apower density of 16 GW/cm2 and coverage of 1000% exhibited a partially dark and discolored surface, while the edges remained similar tothat of the smooth specimens, as shown in Figure 2(b). However, surface
cracks were observed on the surface of the samples laser peened with apower density of 16 GW/cm2 or more and coverage of 3000% or more, as
shown in Figure 2(c). Chipping and peeling were clearly observed at theedges of the shot-peened specimens due to the physical contact of theshot material, as shown in Figure 2(d).
Figure 3 shows the calculated Ra of each test specimen. The Ra of the laserpeened specimens increased with increasing coverage and power density. The
asterisk mark in Figure 3 shows that surface cracks occurred due to LP.

Evaluation of Surface Properties of Silicon Nitride Ceramics

(a) Smooth (Non-LP) specimen(b) Laser-peened specimen(G = 16 GW/cm2, Cv = 1000%)(c) Laser-peened specimen(d) Shot-peened specimen(G = 16 GW/cm2, Cv = 3000%)
FIGURE 2
Optical micrographs of the surfaces of the specimens.

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FIGURE 3
Surface roughness (Ra) of each surface-treated specimen.

3.2 Surface residual stress
Figure 4 shows the residual stresses measured at the surfaces of the specimens. Compressive residual stress was introduced by LP. The maximum
compressive residual stress was about 280 MPa. The specimens that werelaser peened at 16 and 33 GW/cm2 showed much larger compressive residual
stresses than those peened at 2 GW/cm2. The asterisk mark in Figure 4 shows
the occurrence of surface cracks. Surface cracks were observed on the sample
when the LP parameters such as power density and coverage were too high.
3.3 Residual stress distribution from the surface in the depth direction
Figure 5 shows the residual stress distribution for each coverage. Generally,
the compressive residual stress and the depth of the crossing point (i.e., thepoint at which the residual stress changes from compression to tension) tend
to increase as the value of G increases. However, when the Cv values were314% and 1000%, the compressive residual stress values saturated at G = 16
GW/cm2. The amount of heat input increases as the value of G increases.
Therefore, it was expected that the compressive residual stress would become
small. When the Cv was 314%, the compressive residual stress values were
smaller than those at other Cv values.
When SP was applied to Si3N4/SiC, the compressive residual stress on thesurface was about 1500 MPa and the introduction depth was about 40 µm, asEvaluation of Surface Properties of Silicon Nitride Ceramics

229

FIGURE 4
Surface residual stress at each power density and coverage.

shown in Figure 6. The residual stress of the shot-peened specimen greatly
decreased and approached 0 MPa as the depth exceeded 20 µm. On the other
hand, the laser-peened specimen retained the compressive residual stress evenat a depth of 40 µm. From this observation, it was concluded that although thelaser-peened sample had a smaller surface residual stress than the shot-peened
sample, the compressive residual stress could be introduced deeper below thesurface. The optimal LP conditions were found to be a G value of 16 GW/cm2
and a Cv of 1000%. This is because the surface compressive residual stress was
the largest among the specimens with no surface cracking after LP.

4 CONCLUSIONSLPwC was applied to Si3N4/SiC composite materials to increase the surface
strengths of the ceramics. The influences of LP on the surface conditions,surface roughness, and distribution of residual stress in the depth direction
were investigated.
(1) Surface roughness increased by LP due to laser ablation. When the LPparameters such as power density and coverage were too high, surface
cracks appeared on the sample.

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FIGURE 5
Residual stress distributions for Si3N4/SiC samples after LP.

FIGURE 6
Comparison of residual stress distributions of laser-peened and shot-peened specimens.

Evaluation of Surface Properties of Silicon Nitride Ceramics

231(2) Compressive residual stresses up to 230 MPa were introduced by LP
without peeling or chipping of the surface.
(3) The depth of the compressive residual stress was about 50 µm. The surface compressive residual stress of the laser-peened specimen was
smaller than that of the shot-peened specimens. However, the depths ofcompressive residual stress in laser-peened specimens were deeper thanthose of the shot-peened specimens.
Nomenclaturet Pulse duration (ns)
D Spot diameter of the laser (mm)Ep Pulse energy (mJ)Np Irradiation density (mm-2)
G Power density (GW/cm2)
Ap Spot area (mm2)Cv Coverage (%)
Ra Arithmetic average surface roughness (μm)
Greek symbols
λWavelength of pulse laser (nm)σResidual stress (MPa)

Acknowledgment
The study was supported in part by a grant for Scientific Research (B)(16H04231) from the Ministry of Education, Culture, Sports, Science, andTechnology of Japan.

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