SELF-HEALING BEHAVIOR OF FIBER-REINFORCED SELF-HEALING
CERAMICS
S. Sugiyama 1, W. Nakao 1
1 Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama, 240-8501 Japan
– e-mail: sugiyama-saho-yp@ynu.ac.jp; wnakao@ynu.ac.jp
Keywords: Structural ceramics, Fiber reinforced ceramics, Mechanical reliability, oxidation induced self-healing, autonomic self-healing
ABSTRACT
The alumina fiber reinforced alumina containing the interlayer of SiC as healing agent was found to have self-healing ability for the delimitation of the fiber/matrix interface. Main crack in the composite is branched to fiber/matrix interface when the main crack reached the interface, because the interlayer has lower strength than those of the fiber and matrix. The surface of the branched crack consists of the healing agent so that self-healing reaction which is the high temperature oxidation of the healing agent effectively acted to the re-bonding of the branched crack. Therefore, it was found that the developed composite can completely recover the degraded strength during the shorter time than the ordinary SiC particles dispersed self-healing ceramics.
1. INTRODUCTION
Fiber-reinforced self-healing ceramic (shFRC) is most candidates of the advanced turbine blade materials. Turbine blades need to stand high temperature environment, high temperature oxidation, and crush of foreign object with high speed. In spite of its excellent refractoriness and oxidation resistance, ceramic turbine blades have not been realized yet, because ordinary ceramics cannot stand overloading, including to the foreign objective damage (FOD). The proposed shFRC has a high resistance to foreign object impact damage, as shown in Figure 1. As the shFRC has weak interlayer at the fiber/ matrix interface, cracks can be branched along the interlayer when cracks propagate to the interlayer. Further crack propagation leads to the friction at the fiber/matrix interface. Therefore, the shFRC can stand excessive loads by high fracture energy due to the firicion at the branched crack surfaces. Moreover, the high temperature oxidation of the interlayer acting as healing agent can re-bond the branched crack surfaces, because the crack allows the surrounding atmosphere flow into the crack surface and react with the healing agent. The branched crack is no longer playing as most severe defect, and thereby, the shFRC can survive the repeated overload, such as FOD.
In the present study, the above materials concept was demonstrated by using continuous alumina fiber reinforced alumina with SiC interlayer.
Figure 1: Schematics of the developed continuous fiber reinforced ceramics having self-healing ability and the mechanism of the function combined fiber reinforcement
and self-healing 2. MATERIAL AND SAMPLE PREPARATION
For typical sample of shFRC, alumina fiber/ alumina composite with SiC interlayer was prepared in this study. The used alumina fiber (NITIVY CO.,LTD. S-640D) is filaments bundle with diameter of 0.3 mm. Fine alumina (Sumitomo Chemical Co., Ltd. AKP-50, 0.23 m) and SiC ( IBIDEN CO., LTD. Ultrafine, 0.35 m) powders were used as matrix and interlayer, which plays role in healing agent.
Figure 2(a) shows the procedure of sample preparation of shFRC. The whole of the coating and figuration were subjected by using the fiber winding apparatus as shown in Figure 2(b). Prior to the interlayer coating, the alumina fine particles were filled with the voids of alumina fiber bundle by letting the fiber bundle pass through alumina slurry. By using similar slurry coating, the SiC interlayer was formed on the alumina fiber bundle. After letting the SiC coated fiber bundle pass through alumina slurry, the fiber bundle was figurated by winding on 50 mm x 50 mm x 6 mm stainless plate. The figurated fiber bundle lamination was sintered at 1300 oC for 1h in Ar.
Figure 2: (a) procedure of sample preparation of shFRC
(b)Photograph of the experimental apparatus of wire winding process
Interlayer Matrix
Fiber Formed Oxide
Fiber Reinforcement Self-healing
Interlayer Fracture Interface Slip =Friction Fracture Energy Increase Interface Rebonding + Crack Filtration Strength Recovery High Temperature Oxidation O2 slurry heater (b) (a)
filled with the voids with Al2O3slurry
SiC slurry coating
forming
sintering(1300 oC,1 h, Ar)
Al2O3slurry coating drying
drying the fiber bundle
Figure 3 shows cross-sectional microscopy of the prepared shFRC. It is found that the shFRC had the SiC interlayer thickness of ~60 m and the means fiber distance of ** mm. Thus, the fiber content was evaluated to be 58 %. Furthermore, the relative density of the shFRC was found to have 78 % from the Archimedes measurement.
Figure 3: microscopic photograph of cross-section surface of specimen 3. MECHANICAL PROPERTIES
Mechanical properties and self-healing ability of shFRC were investigated form three-point bending using 3 mm x 4 mm x 50 mm rectangular specimen with a V-shaped notch whose depth is 1 mm.
Figure 4 shows stress-strain curves of ”virgin” V-notched shFRC and the V-notched shFRC healed at 1000 oC for several time in the air after pre-loading. Black solid line indicates the stress-strain curve of “virgin” shFRC. The bending stress increases linearly as the bending strain increases below the strain which the stress shows a maximum. Above that, the stress drops but the shFRC did not fracture. This behavior implies that the crack introduced and propagated from the tip of the V-shaped notch and then branching along the interlayer when the crack reached to fiber bundle. The branching crack was observed in Figure 5(a). Therefore, the whole self-healed shFRCs had the branching crack before self-healing. Although the shFRC healed for 1 h had lower maximum stress and gradient of primary part of stress-strain curve than those of “virgin” shFRC, the shFRCs healed above 5 h had same or more than maximum stress, and the gradient for the shFRC healed for 50 h reached the same value of “virgin” specimen. In other words, the self-healing of the shFRC can recover the degraded strength and stuffiness for 5 h and 50 h, respectively. Figure 5(b) shows the fracture initiation of the shFRC healed at 1000 oC for 50 h. Since the fracture initiation change from the pre-introduced branching crack, the self-healing was found to enable to re-bond the crack surface completely. Moreover, all stress-stain curves revealed that the shFRC had large deformation ability after self-healing.
Similar self-healing behavior was investigated for 1200 oC healing. In 1200 oC healing,
the complete strength and stuffiness recoveries can be attained within only 1 h. From the obtained self-healing behaviors, it was found that the shFRC has superior self-healing ability to the ordinary SiC particles dispersed self-healing ceramic. From the previous research [1], SiC dispersed alumina composite was reported to enable to heal semi-elliptical surface crack having surface length of 100m for 100h @ 1000 oC
and for 10h @1200 oC. Therefore, it is confirmed that the shFRC can effectually heal
fiber bundle
SiC interlayer
200mm
the crack, because the shFRC entices cracks into propagating to the interlayer constituting of the healing agent.
Figure 4: Load-displacement curves of the developed fiber reinforced self-healing ceramics applied by three-point bending with span of 30 mm
Figure 5: Microscopic photograph of the clack near the notch in specimen 4. SUMMARY
In this study, the alumina fiber reinforced alumina containing the interlayer of SiC as healing agent was found to have self-healing ability for the delimitation of the fiber/matrix interface. it was also found that the developed composite can completely recover the degraded strength during the shorter time than the ordinary SiC particles dispersed self-healing ceramics.
REFERENCES
[1] K. Ando, B.S. Kim, S. Kodama, K. Takahashi, S. Saito, Development of the alumina ceramics which have the excellent crack-healing ability and a heat-resistant limit, Materials and processing conference 115 (2003-11) pp. 75-76.
0 10 20 30 40 50 60 70 0 0.002 0.004 0.006 0.008 0.01 bending strain[-] b en d in g stre ss [M P a] 1h 50h 10h Healing time As-machined Specimen 5h specimen self-healed at 1000oC 500µm 100mm
(a) As-clacked specimen (b) failure specimen
