Self-Crack-Healing Behavior of Mullite/SiC Particle/SiC Whisker Multi-Composite

Wataru Nakao, Koji Takahashi and Kotoji Ando

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Posted: September 2005

Topics Covered

Abstract

Keywords

Introduction

Experimental

Results and Discussion

Conclusion

References

Contact Details

Abstract

Self-crack-healing is proposed as a new method for improvement in reliability of structural ceramics by the present authors.  Self-crack-healing is that surface cracks and flaws are healed by oxidation of SiC admixing in the matrix.  In this study, mullite/ SiC particle/ SiC whisker multi-composites were developed for improvement in fracture toughness as well as endowing with excellent self-crack-healing ability.  The fracture toughness and the bending strength after crack-healing treatment of the composites were investigated.  Admixing with SiC whisker was effective for improvement in fracture toughness.  It was, however, found that the crack-healed part welded by oxidation of only SiC whiskers became mechanically weaker than the matrix at high temperature.  In contrast, the crack-healed part welded by oxidation of SiC whiskers and particles was superior to the matrix in fracture strength at each examined temperature.  Moreover, fracture toughness was not reduced by admixing with SiC particles.  

Keywords

Crack-healing, Multi-composite, Synergy Ceramics, Structural Ceramics, High-temperature Oxidation

Introduction

Structural ceramics such as alumina and mullite are excellent in resistance to heat, corrosion, wear and oxidation.  However, the fracture toughness is low.  This low reliability has, thus, limited their applications.  Many investigators tried to improve the fracture toughness of structural ceramics by admixing with whisker and fiber.  Guo et al. [1] improved the fracture toughness of silicon nitride by admixing with carbon fiber.

To improve reliability of structural ceramics, the present authors [2-14] have progressed in development of structural ceramics endowed with crack-healing ability by using oxidation of SiC.  When the ceramics are kept in air at high temperature, SiC located on the crack surface reacts with O₂.  Then, the crack is completely restored by the products and the heat of the reaction.  Moreover, the crack-healed part is mechanically stronger than original bulk parts.  If the crack-healing is used on structural components in engineering use, great benefits can be anticipated to an improvement in reliability as well as a decrease in machining and polishing costs.

In the previous study [15], the fracture toughness and crack-healing ability of mullite admixed with 15 vol% SiC whiskers were investigated.  The fracture toughness of the composites was 3.8 MPa·m^1/2.  However, the welded part by crack-healing is not covered over whole of the crack surface in order that SiC is not well dispersed.  As a result, the fracture strength of the specimens crack-healed was not completely recovered to that of the as-received specimen.

This study aims a development of new mullite composites having high fracture toughness and excellent crack-healing ability.  For this purpose, mullite/SiC whiskers /SiC particles multi-composite and mullite/ SiC whiskers composites increasing the volume ratio of SiC whiskers were prepared.  Crack-healing ability of these specimens are estimated by comparison in fracture strength between crack-healed and as-received composites.

Experimental

Mullite powder (KM 101, Kioritzz Co. Ltd., Japan) used is an average particle size of 0.2 µm and Al₂O₃ content of 71.8%.  SiC whisker (SCW, Tateho Chemical Industry Co. Ltd., Japan) used has a diameter of 0.8 µm to 1.0 µm and a length of 30 µm to 100 µm.  SiC powder (Ultrafine grade, Ibiden Co. Ltd., Japan) used has a mean particle size of 0.27 µm.  In case of the mixing the raw powder of mullite/ 15, 20 and 25 vol% SiC whiskers, abbreviated as MS15W, MS20W and MS25W, respectively, the mixture of mullite powder and SiC whiskers was mixed well in isopropyl alcohol for  12 h using alumina balls and an alumina pot mill.

In case of the mixing the raw powder of mullite/ 15 vol% SiC whiskers/ 10 vol% SiC particles, abbreviated as MS15W10P, SiC powder admixed to mullite powder and the mixture was blended well in alcohol for 12 h.  Additionally, SiC whiskers admixed to the mixture and the mixture of mullite- SiC whisker- SiC powder was also blended for 12 h.  Rectangular plates of 50 mm ×50 mm × 9 mm were sintered in argon for 1 h via hot press under 40 MPa at 1700^oC.  The sintered plates were cut into the 3 mm × 4 mm × 22 mm rectangular bar specimens.  The specimens were polished to mirror finish on one face and the edges of specimens were beveled 45°, as shown in Figure 1, to reduce the likelihood of edge initiated failures.

A semi-elliptical surface crack of 100 µm in surface length was made at the center of the tensile surface of specimens with a Vickers indenter, using a load of 19.6 N.  The ratio of depth (a) to half the surface length (c) of the crack (aspect ratio a/c) was 0.9.  The specimens cracked and as-received were subjected to crack-healing treatment at 1300^oC for 2 h in air, where the crack-healing condition is referred to the pervious study [15]. 

All fracture tests of the specimens crack-healed were performed on a three-point loading system with a span of 16 mm, as shown in Figure 1, at room temperature and temperatures from 600 to 1300^oC.  The cross-head speed in the monotonic test was 0.5 mm/min.  Moreover, the crack-healed specimens without pre-crack in order to heal the crack introduced the mechanical process were also carried out the above bending test.  The specimens called below shortly heat-treated specimen.  Fracture toughness was obtained by indentation fracture method, where Young’s modulus used to calculation is 220 GPa [16]. 

Results and Discussion

Table 1 shows the bending strength of MS15W, MS20W, MS25W and MS15W10P at room temperature.  The bending strength of all specimens is reduced to less than that of as-received one by cracking, and recovered even more than as-received specimen by crack-healing.  The crack-healed MS20W, MS25W and MS15W10P have same or higher bending strength than the heat-treated specimens and are always fractured from the position other than the pre-crack.  In contrast, the crack-healed MS15W is always fractured from the pre-crack and has the lower bending strength than the heat-treated MS15W.  At the view point of the strength recovery at room temperature, it is confirmed that MS20W, MS25W and MS15W10P can completely heal the pre-crack.  Thus, admixing above 20 vol% SiC whiskers is necessary for endowing adequate crack-healing ability by only SiC whiskers.  From the previous study [2], it was found that fracture strength is completely crack-healed by admixing with 15 vol% SiC particles.  Therefore, it is confirmed that crack-healing ability of SiC whisker is lower than that of SiC particle.

Table 1. The bending strength of MS15W, MS20W, MS25W and MS15W10P at room temperature

* Fractured from pre-crack

Figure 2 shows the temperature dependence of the bending strength of the crack-healed and heat-treated (a) MS20W, (b) MS25W and (c) MS15W10P.  The closed and open triangles indicate the bending strength of the crack-healed specimens fractured from the position other than the pre-crack and from the pre-crack, respectively.  Also the closed circles indicate the bending strength of the heat-treated specimens.  The bending strength of the all crack-healed and heat-treated specimens is slightly decreased by increasing temperature below 1200°C, but the bending strength is reduced greatly at 1300°C.  Moreover, the bending strength of the crack-healed specimen is almost equal to the values of the heat-treated specimens at every temperature.  However, the crack-healed MS20W and MS25W were fractured from the pre-crack above 800°C and 1000°C, respectively.  These results indicated that the crack-healed part became weaker than the matrix in order that the pre-crack was not completely welded by the products of the crack-healing reaction.  However, MS15W10P was never fractured from the pre-crack at temperatures from room temperature to 1300^oC.  It is, therefore, confirmed that the welded part of MS15W10P by crack-healing can be covered over whole of the crack surface.

Table 2 shows the fracture toughness of the specimens.  The fracture toughness of MS15W10P is improved 1.5 times high compared to the value of the monolithic mullite (K_IC = 2.5 MPam^1/2) in spite of the lowest value of the specimens sintered in this study.  Fracture toughness is increased with increasing SiC whisker contents.  Therefore, an improvement in fracture toughness is not affected by admixing with SiC particles but affected greatly by admixing with SiC whiskers.

Table 2. Fracture toughness of the specimens

Conclusion

For improving fracture toughness as well as endowing with excellent self-crack-healing ability, mullite/ SiC whiskers/ SiC particles multi- composites were sintered.  The fracture toughness and the bending strength after crack-healing treatment were investigated. 

Admixing above 20 vol% SiC whiskers is necessary for endowing adequate crack-healing ability in the strength recovery at room temperature by only SiC whiskers.  The crack-healed part of MS20W and MS25W became weaker than the matrix above 800^oC and 1000^oC, respectively.  In contrast, the crack-healed part of MS15W10P was superior to the matrix in fracture strength at every experimental temperature.

Admixing with SiC whisker was effective for improvement in fracture toughness, and the fracture toughness was not reduced by admixing with SiC particles.  Therefore, SiC whiskers- SiC particles multi-compositing is a valuable method for improving fracture toughness and endowing with excellent self-crack-healing ability.

References

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5.       K. Ando, K. Tuji, M. Nakatani, M.C. Chu, S. Sato and Y. Kobayashi, “Effects of Y₂O₃ on Crack Healing Temperature Strength of Structural Mullite”, J. Soc. Mat. Sci. Jpn., 51 (2002) 458-464.

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Contact Details

Wataru Nakao

Department of Energy & Safety Engineering

Yokohama National University

79-5 Tokiwadai

Hodogaya-ku, Yokohama 240-8501

Japan

Email: [email protected]

Koji Takahashi

Department of Energy & Safety Engineering

Yokohama National University

79-5 Tokiwadai

Hodogaya-ku, Yokohama 240-8501

Japan

Kotoji Ando

Department of Energy & Safety Engineering

Yokohama National University

79-5 Tokiwadai

Hodogaya-ku, Yokohama 240-8501

Japan

This paper was also published in print form in “Advances in Technology of Materials and Materials Processing”, 6[2] (2004) 236-239.