INCREASE IN CRITICAL STRESS FOR CRACK-HEALING OF
SI
3N
4/SIC CERAMICS BY SHOT PEENING
K. Takahashi 1, S. Nakagawa 2, and T. Osada3
1
Division of Materials Science and Engineering, Yokohama National University, 79-5, Tokiwadai, Hodogaya, Yokohama, 240-8501, Japan – e-mail: ktaka@ynu.ac.jp
2
Graduate Student, Yokohama National University, 79-5, Tokiwadai, Hodogaya, Yokohama, 240-8501, Japan
3
Cooperative Research and Development Center, Yokohama National University, 79-5, Tokiwadai, Hodogaya, Yokohama, 240-8501, Japan – e-mail:tosada@ynu.ac.jp
Keywords: Ceramics, Crack-healing, Shot peening, Residual stress, Critical stress
ABSTRACT
The effects of shot peening on the critical stress for crack-healing of Si3N4/SiC composite ceramics were investigated. Shot peening was carried out on a Si3N4/SiC composite with a surface crack by using yttrium-stabilized zirconia beads. Shot-peened specimens were crack-healed at 1100 °C for 5 h under a tensile stress (app) in the range 0–500 MPa. The results clearly showed that the surface cracks in shot-peened specimens were healed completely for app 450 MPa. Thus, the critical stress for crack-healing (C
app) was determined to be 450 MPa. This Capp value was 2.3 times that for an unpeened specimen. Furthermore, the C
app values for both specimens showed a proportional relationship with the bending strength of the cracked specimen c (Capp ≈ 0.64c). Therefore, it can be concluded that the increase in C
app of the peened specimen is due to the compressive residual stress and the consequently increase in c.
1. INTRODUCTION
Some structural ceramics have a self-crack-healing ability [1]. The utilization of this ability in ceramic components can afford the following great merits: (a) increase in reliability, (b) reduction in inspection, machining, polishing, and maintenance costs, and (c) increase in the lifetime of the ceramics.
However, the reliability of structural ceramics is considerably reduced by crack initiation during operation because their fracture toughness is not high. Thus, the reliability and lifetime of ceramic components for high-temperature applications such as micro-gas-turbine blades can be increased by healing cracks under service conditions, e.g., tensile stress and high temperatures.
The crack-healing behavior of Si3N4/SiC under stress at 1100 and 1200 °C in air was investigated [2]. It was found that a surface crack of 100 m, which reduced bending strength about 50%, could be healed completely in just 0.5–1 h even under cyclic stress at 1100 and 1200 °C in air. These results indicate that Si3N4/SiC has an excellent crack-healing ability. In this study, the critical stress for crack-healing was as high as 200 MPa. The reliability of ceramics can be increased by further increasing the critical stress.
Takahashi et al. reported that shot peening on the surface of Si3N4/SiC induces compressive residual stress [3]. This compressive residual stress prevents crack
propagation. In this study, the effects of shot peening on the critical stress for crack-healing of Si3N4/SiC composite ceramics were investigated.
2. MATERIALS
Hot-pressed SiC-reinforced Si3N4 containing 20 wt% SiC particles and 8 wt% Y2O3 as a sintering additive was used for preparing the specimens. The details of the material fabrication process are given in the literature [2,3]. The hot-pressed Si3N4 was cut into test specimens measuring 2 mm × 4 mm × 20 mm. Semi-elliptical surface cracks 100 m in surface length and 45 m in depth were induced at the center of the tension surface of the specimens by using a Vickers indenter. Shot peening (SP) was then carried out using a direct-pressure peening system. For this, commercial zirconium oxide (ZrO2) shots with a diameter of 300 m were used. The peening pressure and peening time were 0.2 MPa and 30 s, respectively. The specimens subjected and not subjected to SP are hereafter denoted as “SP” and “non-SP,” specimens, respectively.
3. METHODS
Crack-healing was carried out under constant bending stress by using an in-situ observation apparatus shown in Fig. 1. This apparatus consisted of an infrared image furnace, a three-point bending device, and an optical microscope equipped with a CCD camera. The pre-cracked specimens were crack-healed under a constant tensile stress (app) in the range 0–500 MPa at 1100 °C for 5 h in air.
After the crack-healing process, the bending strengths of the specimens were measured at room temperature in air. These tests were carried out using a universal monotonic testing machine.
4. RESULTS
Figure 2 shows the residual stress distributions for SP and SP + crack-healed specimens. A maximum compressive surface residual stress of 710 MPa was observed in the SP specimen. However, the compressive residual stress decreased in the depth direction. Moreover, a surface residual stress of 370 MPa was observed
Figure 1: Schematic diagram of in-situ observation device
Figure. 2: Relationship between residual stress and depth from the surface
SP
SP + Crack-healed (1100oC, 5h, air)
Polished
Depth from the surface[m]
0 10 20 30 40 50 0 -400 -200 -600 -800 -1000 Resid ua l st re ss [MP a ] CCD camera Zoom microscope Specimen Infrared lamp Load cell Pre-crack 26mm Zoomed-in view of the
loaded part in 3-point bending test
in the SP + crack-healed specimen. Thus, some of the compressive residual stress induced by SP remained after the crack-healing process.
Figure 3 shows the relationship between the stress applied during crack-healing and bending strength at room temperature. The average value of the bending strength of smooth specimens was 780 MPa. The pre-cracks largely reduced the bending strength to 340 MPa. After SP, the bending strength of the pre-cracked specimens increased up to 580 MPa. The increase in the bending strength was attributed to compressive residual stress. The critical stress for crack-healing (C
app) was defined as the maximum stress below which a crack-healed specimen recovered its bending strength and below which most specimens fractured outside the crack-healed zone. The Capp values for pre-crack + crack-healed and pre-crack + SP + crack-healed specimens were determined to be 200 and 450 MPa, respectively. These results indicated that SP increased the C
app value by 2.3 times.
Figure 4 shows C
app as a function of the bending strength of pre-cracked specimens (c). Nakao et al. have conducted similar tests on Al2O3/SiC particles, Al2O3/SiC whiskers, mullite/SiC particles [4]. They reported that C
app is proportional to c. These results indicated that the increase in C
app by SP was caused by an increase in c. The proportional constant for the relationship between Capp and c (Capp/c) was approximately 64% for non-SP specimens in spite of the different material and crack-healing conditions. However, the value of Capp/c for the SP specimens was 78%. Figure 5 shows the specimen surface observed using an optical microscope and pre-cracked area observed using a scanning electron microscope. The crack opening displacement (COD) in the non-SP zone was 0.2 m, whereas the COD in the SP zone was less than 0.1 m. Moreover, the volume between crack surfaces in SP specimens was smaller than that in the non-SP specimens. Further, the volume of the healing substance, which is necessary to fill and bond surface cracks, in the SP specimens was smaller than that in the non-SP specimens. Thus, the crack-healing rate in the SP specimens was higher than that in the non-SP specimens. Therefore, the value of C
app/c was increased by subjecting the specimens to SP.
∗
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∗ ∗
∗
∗
∗
∗
∗
∗
Applied stress during crack-healing, app[MPa] Non-healing 0 100 200 300 400 500 B endin g Str eng th a t R. T ., c [MP a ] 200 0 400 600 800 1000 1200
* indicates the specimen fractured outside the crack-healed zone
Pre-crack + crack-healed Pre-crack + SP + crack-healed Smooth Pre-crack Pre-crack + SP Crack-healed at 1100oC for 5h
Figure 3: Bending strength of crack-healed specimens as a function of applied stress during crack-healing
5. CONCLUSIONS
The effects of shot peening on the critical stress for crack-healing (C
app) in Si3N4/SiC composite ceramics was investigated. Shot peening increased the C
app value of Si3N4/SiC by 2.3 times. The improved Capp value of the peened specimen was due to the compressive residual stress. Thus, crack-healing combined with shot peening is a useful technique for increasing the structural integrity of ceramic components.
REFERENCES
[1] W. Nakao, K. Takahashi and K. Ando, Self-Healing of Surface Cracks in Structural Ceramics, in S.K. Ghosh Ed.,: Self-healing Materials, Chapter 6, 183-217, WILEY-VCH, Weinheim, Germany, (2009).
[2] K. Takahashi, K. Ando, H. Murase, S. Nakayama and S. Saito, Threshold Stress for Crack-Healing of Si3N4/SiC and Resultant Cyclic Fatigue Strength at the Healing Temperature, Journal of the American Ceramic Society, 88 (2005) 645-651.
[3] K. Takahashi, Y. Nishio, Y. Kimura and K. Ando, Improvement of Strength and Reliability of Ceramics by Shot Peening and Crack-healing, Journal of European Ceramic Society, 30 (2010) 3047-3052.
Figure 4: Relationship between critical stresses for crack-healing and bending strength of pre-cracked specimens
100 m
Non-SP zone SP zone
Pre-crack Pre-crack Pre-crack
SP
Pre-crack
Indentation
5m
Non-SP
Figure 5: Crack closure by shot peening
Al2O3/SiC whisker 4)(1200℃, 8h)
Al2O3/SiC particle 4)(1200℃, 24h)
Mullite/SiC particle4)(1100℃, 15h)
Si3N4/SiC SP (1100℃, 5h)
Si3N4/SiC Non-SP (1100℃, 5h)
Bending strength of pre-cracked specimen
at R.T., C[MPa] Cr it ic al s tr e ss f or c ra ck -he al ing , Cap p [MP a] 0 200 400 600 800 200 400 600 800 0 Ref: Nakao et al4) Peened Si3N4/SiC: Fracture:
[4] W. Nakao, K. Takahashi and K. Ando, Threshold Stress during Crack-healing Treatment of Structural Ceramics having the Crack-healing Ability, Materials Letters, 61 (2007) 2711-2713.
