INCREASING THE RELIABILITY OF CERAMICS COMPONENTS
USING SELF-CRACK-HEALING
K. Ando 1, W. Nakao 1
1 Department of Energy and Chemical Engineering, Yokohama National University,
Tokiwadai Hodogayaku Yokohama-city 204, 8501 Japan – e-mail: andokoto@ynu.ac.jp
Keywords: structural ceramics, material design, self-crack-healing, high temperature strength, reliability of components
ABSTRACT
Structural ceramics are superior in high temperature strength and anticipated as leading materials for next generation energy machine. However, ceramics have disadvantage of brittleness, sensitive to flaw and low reliability. To overcome the disadvantages, the present authors proposed a new technology for the through life reliability guarantee concept. 1) Materials design with excellent self-crack-healing ability. 2) Through life reliability management process of ceramic components using self-crack-healing ability, proof test and compressive stress introduction. In this paper, each process is introduced.
1. INTRODUCTION
Structural ceramics are superior in high temperature strength and critical heat-proof temperature compared to that of metallic material. Structural ceramics are a leading candidate material for high-temperature apparatuses such as gas-turbines. However, the fracture toughness of ceramics is lower compared to metallic material, thus they are sensitive to flaw, and the following three problems occurred. (1) Cracks occur by the usual machining process, which lowers the component reliability considerably. In order to prevent this, precise polishing is required in the final stage, which is time-consuming, and costly. (2) Crack sizes of about 10~30 μm in depth affect the reliability. The nondestructive inspection technology for detecting the cracks is yet still undeveloped. (3) There is a possibility that a crack will initiate in the components while they are being used at higher temperatures.
To overcome these problems, authors developed four technologies. (a) Induce a self-crack-healing ability, so that all surface cracks can be healed. (b) Improve the fracture toughness of the material by fiber reinforcement. (c) Conduct a proof test to prevent use of a low reliability member containing inner flaw. (d) Introduce compressive residual stress to increase fracture resistance.
There are three advantages using a crack-healing technology. (A) If the self-healing of the surface crack which exists is carried out after an efficient machine operation is performed, then there is a great advantage in fabrication efficiency and fabrication cost. (B) Since all surface cracks are healed, reliability improves greatly. (C) It is great advantageous that a crack which occurs during service can be healed and completely recover the strength by the self-crack-healing ability. From the above ideas, the self-crack-healing behaviors of ceramics were investigated by Ando and co-worker. Silicon nitride, alumina, mullite, SiC, AlN and ZrO2 with the excellent
healing ability have been developed [1]. In this paper, outline of the self-crack-healing behavior of ceramics and through life reliability managements of ceramics components were introduced.
2. CONCEPT FOR MATERIALS DEVELOPMENT 2.1. Nano-composite material
Our nano-composite materials are defined as the following specificity [2]. (a) Nano-size SiC particles are added to the material, preventing grain growth of the matrix in the sintering process, and increasing the bending strength through grain refinement: For example, in the case of an alumina, bending strength can be increased from about 400 MPa to 700~1000 MPa [2]. (b) SiC particles of nano-size 15-30 vol% added to the material, and induce a self-crack-healing ability. (c) SiC particles of nano-size are distributed not only in the grain boundaries of an alumina, but in individual grains and increases the critical heat-proof temperature by about 300 K [2].
2.1. Multi-composite material
A multi-composite material is a material which combines nano-sized SiC particles and SiC whiskers in the proportion of 20-30 vol%, and has excellent self-crack-healing ability, fracture toughness, high temperature strength [3]. The nano-sized SiC particles mainly contribute to increase strength and self-crack-healing rate and the SiC whiskers mainly contribute to increase fracture toughness and high temperature strength. The proportion fraction of SiC particles to SiC whiskers is determined by taking into consideration the self-crack-healing ability and fracture toughness requirements [4]. For example, the room-temperature bending strength and fracture toughness of Alumina/SiC(p)15% and Alumina/SiC(p)10%/SiC(w)20% are 850 MPa, 980 MPa and 3.2 MPam1/2 , 5.0 MPam1/2, respectively.
The material with a self-crack-healing ability described in this paper has all of the following attributes. (A) The material itself detects an occurrence of a crack and begins crack-healing activities. (B) If surface crack were introduced which reduce 50-90 % strength of a material, the material heals the crack completely, and the strength of the material is completely recovered. (C) The strength of a crack-healed zone is equivalent to or higher than that of a base material up to about 1673 K [1].
3. BASIC SELF-CRACK-HEALING BEHAVIORS 3.1 Mechanism of crack healing
The crack healing of ceramics developed by the authors is caused by the following oxidation reaction of SiC.
SiC + 3/2O2 = SiO2 + CO (ΔH = - 943 kJ/mol) (1) A schematic diagram of the self-crack-healing mechanism is shown in Figure 1. To achieve the complete strength recovery by self-crack-healing, following three conditions are necessary. (1) The healing substance has to fill the crack completely. This condition is achieved by the about 80% volume increase of SiO2 compared to
SiC. (2) The healing substance has to be welded to base material completely. This condition is attained by the huge exothermic heat of 943 kJ. The heat melts matrix and a healing substance and makes the mixture. (3) The strength of a crack-healed zone has to exhibit equivalent or higher strength than that of a base material up to about 1673 K. This condition is attained by forming the crystalline phase SiO2 in Equation (1).
Figure 1: A schematic diagram of the self-healing mechanism.
We used JIS three-point bending-test specimens except for the bending span of 16 mm to evaluate the bending strength (σB) of a crack-healed specimen. We introduced the crack into the central part of the test specimen. The crack is a semi-elliptical crack 100 μm in surface length and 45 μm in depth (hereafter called a standard crack). We used the shorter bending span than standard (40 mm) to evaluate the strength of crack-healed zone and to prevent the specimen to break into small pieces by reducing elastic energy at fracture.
3.2 Oxygen partial pressure and temperature dependency of crack-healing behavior
The most basic crack-healing behavior test results were shown in Figure 2 [5]. The bending strengths (σB) of an as-received test specimen are about 650 MPa. By the standard crack, the σB was reduced to 180 MPa. However, by the crack healing carried out in air at 1573 K for 1 h, the σB was improved up to about 800 MPa. The σB of a crack-healed specimen is larger than the σB of an as-received specimen. The reason is that even the smooth specimens have minute cracks on the surface and the cracks were healed completely. The crack-healed zone exhibit higher strength than base material, thus quite many samples fractured outside the crack-healed zone as shown in Figure 3. However, by the crack healing in a vacuum, N2 gas, and argon gas, the σB recovered to at most 350 MPa. The recovery of σB is insufficient. The slight increase of σB by this heat treatment occurred because the tensile residual-stress of the crack tip was removed. Similar crack healing and strength recovery behaviors were reported in mullite [5], Al2O3 [6] and Si3N4 [7-8].
O2 SiO
2
Figure 2: Effect of atmosphere on crack healing behavior.
Figure 3: Fracture initiation and crack path.
The crack-healing behavior is greatly dependent on healing temperature, material, oxygen partial pressure and crack size (mainly in depth). The influence of oxygen partial pressure on crack-healing behavior is shown in Figure 4, in Al2O3/SiC composite materials [9]. By the crack-healing carried out at 1673 K, in air, the crack healed completely in about 20 min. When the oxygen partial pressures were 5000 and 50 Pa, the crack healed completely in about 1 h and 70 h, respectively. Moreover, the test specimen which healed in thin oxygen showed a bending strength equivalent to a matrix division up to 1673 K. It is said that the oxygen partial pressure in the exhaust gas of a gas-turbine or a vehicle is about 8-10 kPa, which is approximately half in air atmosphere. So it is anticipated that the surface crack can be healed in oxygen partial pressure in the exhaust gas of a gas-turbine or a vehicle.
Air Vacuum Ar N2
Crack-healing atmosphere
Crack-healed (1573 K, 1h) * Center-lined symbols indicate the specimens fractured from the crack-healed zone 0 200 400 600 1000 800 Be ndi ng st re ngt h (M P a ) As-cracked As-received As-Cracked As-Healed
Figure 4: Effect of oxygen partial pressure on crack healing behavior.
The influence of temperature and time on crack-healing behavior in air was conducted on various materials, systematically. By similar experiments as shown in Figure 4, we found the shortest time tHM which can heal a crack completely from a certain temperature THL. The relationship between 1/ tHM and 1/ THL can be shown by the Arrhenius equation as follows [10].
(
) (2)
The activation-energy (QH) and proportional moduli (Q0) of each material were calculated and the result is shown in Table 1. Equation (2) is applicable for standard crack in tHM = 1~300 h and in air condition. If crack were deeper than the standard crack, and oxygen partial pressure were lower than that of air, the necessary time for the complete crack-healing became longer.
Table 1. Activation energy & proportional constant.
3.3 Crack-healing behavior of heavily machined ceramics
In general, crack can be introduced by machining processes such as polishing and lapping. The crack-healing behavior of machined specimens was investigated systematically by the authors.
Figure 5 shows the strength recovery behavior of heavily machined Al2O3/20 vol.% SiC whiskers [11]. A semicircular groove was made at the center of the smooth specimens by using a diamond-coated ball-drill as shown in Figure 5. The cut depth
102 0 200 400 600 800 1000 Crack-healing time, tH(s) Be n d in g s tre n g th (M P a)
Bending strength of cracked sample
Air Atmosphere N2/O2Atmosphere
(PO2= 5000 Pa)
N2/O2 Atmosphere
(PO2= 50 Pa)
103 104 105 106 Solid Symbol: Crack initiated outside the
crack healed zone
Activation Energy Proportional Coefficient QH(Kj/mol) Q0(1/hour) Si3N4/SiC 277 4.2×1011 Si3N4 150 5.3×104 Al2O3/SiC 334 1.7×1011 Mullite/SiC 413 4.7×1013 Material
by one pass (dc) during machining of the semicircular groove was dc = 5~15 μm. The cut depth is 0.5 mm. The local fracture stresses (σLF) considering the stress concentration factor of the semicircular groove (α = 1.2) were evaluated.
Figure 5: Strength recovery behavior of heavily-machined components.
The open triangle symbols show the local fracture stress (σLF) of the as-machined specimen. The σLF of the as-machined specimens decreased with increasing dc. It was found by SEM images of the fracture surface that as-machined specimens always fractured from the surface long cracks of 20~50 μm in depth caused by machining. The open square symbols in Figure 5 show the σLF of the machined specimen healed at 1673 K for 10 h in air. The σLF of these specimens increased significantly by crack healing. The σLF of the machined specimens was almost equal to the fracture stress of the healed smooth specimens (solid circles). Thus, the surface cracks introduced by the machining process were healed. Therefore, it was concluded that crack healing could be an effective method for improving the structural integrity of heavily machined alumina and reducing machining costs.
4. HIGH TEMPERATURE STRENGTH OF CRACK-HEALED SPECIMENS
The temperature dependency of the σB of a crack-healed specimen is shown in Figure 6 [1]. Each test specimen was crack healed by an optimum condition after a standard crack was introduced. As shown in the figure, a newly developed SiC and Si3N4 exhibit excellent critical heat-proof temperature of a crack-healed zone of about 1673 K, and we examined its usage [12]. The large improvement in such a critical heat-proof temperature was attained by the crystallization of a grain boundary and a crack-healing substance. In addition, most test specimens fractured from the base material below to the critical heat-proof temperature among the materials of Figure 6, except the commercial SiC. This implied that the crack-healing zone had sufficient bending strength up to1673 K.
Lo ca l F ra ctu re S tr es s a t R T , s LF (M P a ) 0 1200 1000 800 600 400 200 5 10 15 20 0
Cut Depth by One Pass (mm) As-machined specimen (semicircular groove)
:
Machined specimen healed. (1673 K, 10 h)
:
Healed smooth specimen. (1573 K, 1 h)
:
Figure 6: Temperature dependency of strength of a healed sample.
5. CRACK-HEALING BEHAVIOR DURING SERVICE
If the crack could be healed under even service conditions, and the components recover the strength completely, the reliability and lifetime of ceramic components could be increased. The crack-healing behaviors under constant or cyclic stress have been systematically studied for Si3N4/SiC [13-14], Mullite/SiC [15-16], Al2O3/SiC [17], and SiC [18].
To take advantage of the crack-healing ability during service, it is essential to determine the threshold stress for crack-healing under the stress a crack can be completely healed.
The standard crack was introduced on the bending test specimens. After introducing the pre-cracks, crack-healing tests under cyclic or constant stress were carried out. Figure 7 shows the crack-healing process [13]. Crack-healing was carried out under cyclic (σmax,ap) or constant (σap) bending stresses at a healing temperature (Th) for a prescribed healing time (th). After the crack-healing process, the bending strengths of the specimens were measured at room temperature or at healing temperature in air.
Figure 7: Crack healing method under stress and crack behavior.
200 600 1000 1400 1800 Test temperature [K] Be ndi ng st re ngt h [M P a ] 0 200 400 600 800 1000 Si3N4/20 vol.% SiC composite (8 wt% Y2O3)
Si3N4/20 vol.% SiC composite
(5 wt% Y2O3 + 3 wt% Al2O3) Commercial SiC Al2O3/15 vol.% SiC composite Mullite/15 vol.% SiC composite
Sintered SiC composite (Sc2O3 + AlN) th Th Cyclic stress Constant stress sap smax,ap Temperature time C ra c k le n g th (II) (III) T e m p e ra tu re , S tr e ss (I) Crack-healing
Figure 8 shows the results of the bending tests on specimens crack-healed at 1473 K for 5 h under constant stress in PO2 = 500 Pa [19]. The material studied was a Si3N4 which contained 20 wt% SiC particles and 8 wt% Y2O3 as a sintering additive.
Figure 8: Crack healing and strength recovery behavior under stress.
The asterisks indicate specimens that fractured outside the crack-healed zone, suggesting that the pre-cracks were healed completely. The threshold stresses for crack-healing were defined as the maximum stresses below which specimens recovered their strengths completely and most specimens fractured outside the crack-healed zone even at healing temperature. From Figure 8, the threshold stress was determined to be 200 MPa. Similar crack-healing test under cyclic stress under various PO2 was carried out. if PO2 is higher than 800 Pa, the threshold stress is independent on the PO2, the threshold stress was found to be 250 MPa. In all materials developed, the threshold stress under cyclic test exhibited higher one than that of under constant stress. From above test results, the Si3N4 tested is able to recover the strength completely even under service conditions, i.e., with an applied stress below σapp. = 200 MPa and an oxygen partial pressure over PO2 = 500 Pa. The threshold stress for crack healing under constant or cyclic stress as a function of the bending strength of pre-cracked specimens were investigated systematically [2]. The threshold stress is almost proportional to the bending strength of pre-cracked specimens, and the proportional constants for the relationship between the threshold stress and bending strength of pre-cracked specimens is 64% for constant stress and 76% for cyclic stress.
6. EFFECT OF SHOT PEENING ON THE RELIABILITY OF CERAMICS COMPONENT
By shot peening, compressive residual stress can be introduced. The value of the compressive residual stress introduced is dependent on peening method and material. The maximum compressive residual stress introduced in Si3N4 and ZrO2 were about 900 MPa and 1200 MPa, respectively. These compressive residual stresses increase apparent fracture toughness and contact strength and fatigue
［MPa］
**
Po
2=500Pa
*
*
＊：Fracture outside healed zone : As -cracked specimen (2C=100 μm)
:Crack -healed under constant stress at 1200℃ for 5h in Po2=500Pa ［M P a］
200
0
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0
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Applied Stress［MPa］**
Po
2=500Pa
*
*
＊：Fracture outside healed zone : As -cracked specimen (2C=100 μm)
:Crack -healed under constant stress at 1200℃ for 5h in Po2=500Pa ［M P a］
200
0
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300
0
200
400
600
800
1000
**
Po
2=500Pa
*
*
＊：Fracture outside healed zone : As -cracked specimen (2C=100 μm)
:Crack -healed under constant stress at 1200℃ for 5h in Po2=500Pa B en d in g S tr en g th ［M P a］
200
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0
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400
600
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0
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strength considerably. In conclusion, the compressive residual stress increases reliability of ceramics components, considerably [20].
7. A PROOF TEST THEORY
Oxygen is necessary for crack-healing, and thus embedded flaws such as pores and abnormally large grains cannot be healed. These facts suggest the importance of a proof test for higher reliability. Ando et al. have proposed a theory to evaluate the temperature dependence of a guaranteed (minimum) fracture stress(σG) of a proof-tested sample based on nonlinear fracture mechanics [21]. If proof test were carried out at room temperature, the σG can be expressed as Eq. (3) with regard to room temperature and high temperature materials constant:
σ σ0 T arccos{(K T K ) (σ0 σ0T) {sec ( σp σ0) 1} 1} 1 (3 where KIC, σ0 and σpR are the plane strain fracture toughness, the strength of plain specimen (intrinsic bending strength) and the proof test stress at room temperature, respectively, and the superscript R and T indicate the value at room temperature and elevated temperature T, respectively. By obtaining the temperature dependence of KIC and σ0, one can estimate σG.
To evaluate the validation of the proof test theory, fracture tests were made using Si3N4/SiC[21], coil spring made of Si3N4[22] and Al2O3/SiC[23]. Al2O3/SiC was sintered at relatively low temperature to introduce many embedded flaws such as pores and cracks. The results obtained was shown in Figure 9, where the measured σFmin is plotted as a function of the evaluated σG [23]. N in the figure denotes the number of samples used to obtain σFmin. Four solid diamonds indicate the data on the ceramic coil spring. All σFmin showed good agreement with σG. From this figure, it can be concluded that Equation (3) can be applied to Al2O3/SiC, Si3N4, and even to a coil spring.
Figure 9: Correlation between σG and σFmin.
Evaluated Minimum Fracture Stress, sG(MPa)
200 300 400 500 600 700 800 200 300 400 500 600 700 800 M ea su re d M in im u m F ra ct u re S tre ss , s Fm in (M P a)
●Al2O3/SiC: Proof test (spR= 435 MPa) after crack-healing
(N=15, this paper)
■Al2O3/SiC: Proof test (spR= 530 MPa) after crack-healing
(N=9, this paper)
○Si3N4: Proof test (spR= 598 MPa) no crack-healing (N=15)
□Si3N4: Proof test (spR= 598 MPa) before crack-healing (N=15)
△Si3N4: Proof test (spR= 700 MPa) before crack-healing (N=14)
8. FLOW CHART OF THROUGH LIFE RELIABILITY MANAGEMENTS
All processes were explained above sentence, thus in this sentence, only flow chart was shown in Figure 10.
Figure 10: Flow chart for the through-life reliability increase using self-healing ability.
9. CONCLUSION
In this paper, basic self-crack-healing behavior of ceramics and application to increasing reliability were shown. In conclusion, the crack-healing ability of structural ceramics is a very useful technology for higher structural integrity and for reducing the machining and non-destructive inspection costs.
ACKNOWLEDGEMENT
The author shows sincerely thanks to Prof. K. Takahashi, Associ. Prof. W. Nakao and Assist. Prof. T. Osada of Yokohama National Univ. for their creative and active joint researches, and Miss K. Iwanaka for making revised Figures.
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