The effect of SiC particle size on the healing efficiency of alumina at high temperatures

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THE EFFECT OF SIC PARTICLE SIZE ON THE HEALING

EFFICIENCY OF ALUMINA AT HIGH TEMPERATURES

L. Boatemaa1, J. Zhou2, S. van der Zwaag3 and W. G. Sloof1

1

Department of Materials Science and Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands – e-mail: L.Boatemaa@tudelft.nl; w.g.sloof@tudelft.nl

2 Department of Biomechanical Engineering, Delft University of Technology, Mekelweg 2,

2628 CD Delft, The Netherlands – e-mail: J.Zhou@tudelft.nl

3

Novel Aerospace Materials, Delft University of Technology, Kluyverweg 1, 2629 HS Delft, The Netherlands – e-mail: s.vanderzwaag@tudelft.nl

Keywords: Self-Healing, Al2O3, SiC, High Temperature, Oxidation Kinetics

ABSTRACT

Structural ceramics like alumina are usually brittle and sensitive to flaws leading to a reduction in their mechanical properties. This situation is worsened by machining prior to service and during service by contact, which introduces micro cracks and hidden damages. Crack damage can easily be developed in such ceramics when loaded, which often leads to catastrophic failure. However, if sacrificial SiC healing particles are embedded into the matrix of alumina, then the cracks can be healed by exposure at high temperatures in an oxidizing environment. Then, the cracks are filled with SiO2 restoring the strength of the composite and thereby prolonging its

lifetime. Reducing the size of the SiC healing particles while keeping the volume fraction the same, will increase the efficiency and lowers the onset temperature of the oxidation induced crack healing.

1. INTRODUCTION

Structural ceramics such as alumina, mullite and zirconia are anticipated to be used as structural components operating at high temperatures because of their excellent heat, corrosion, chemical and wear resistance [1]. However, their application is restricted due to their brittleness, which makes them very susceptible to crack damage leading to a considerable decrease in strength. The reliability of these materials could greatly improve if inevitable cracks can be healed autonomously. One way to make alumina a viable option as a structural ceramic is to embed SiC intermetallic particles into the matrix, so that when cracking occurs these sacrificial particles would react at high temperatures with oxygen. SiO2 is expected to form,

which fills the crack. To obtain a strong healed zone, the products of oxidation (i.e. SiO2) must be mechanically strong in comparison with the base material, the volume

between the two crack surfaces should be completely filled with the oxidation product and there should be a strong bond between oxidation product and matrix. Experiments have shown that it is possible to fully recover the fracture toughness of a cracked sample [2].

It has also been demonstrated that submicron sizing of these sacrificial healing agents enables faster healing at lower temperatures [3], but comprehensive information is lacking. In this work the crack healing behaviour as a function of the size of the sacrificial SiC particles is investigated.

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2. MATERIALS AND METHODS

The crack healing in a ceramic with SiC as healing agent is schematically illustrated in Figure 1. Cracking allows SiC particles located on the crack walls to react with the oxygen in the atmosphere at high temperatures resulting in the formation of SiO2,

which fills the crack and recovers the structural integrity.

Figure 1: Schematic of crack healing with SiC particles, taken from [3].

SiC as healing agent is a viable option to regaining the strength of damaged ceramics because the reaction oxygen produces large exothermic heat, which bonds the product strongly to the matrix. In addition, SiO2 has a higher volume expansion of

80% in comparison to SiC and this gives a good probability of completely fill a crack. Before embedding the SiC healing agent into the matrix a thorough understanding of the healing behaviour is necessary. To this end, the oxidation kinetics of SiC particles with different sizes (1, 2, 5 and 10 µm) was investigated with Thermogravimetric and Differential Thermal Analysis (TG-DTA) analysis.

The rate of transformation of SiC particles into SiO2 by oxidation in Ar with 20 vol.%

O2 was studied at different but constant heating rates 𝛽, of 2, 5, 10 and 20 ºC/min up

to 1400 ºC. The size effect of SiC particles to heal crack damage is assessed through TG-DTA signal analysis.

3. RESULTS AND DISCUSSION

The oxidation rate of the SiC healing particles is obtained from the DTA signal whiles the TG signal shows mass gained during the reaction as shown in Figure 2. The activation energy Ea of the reactions can be evaluated using the

Kissinger-Sunase-Akahira equation, which reads [3]: ln 𝛽

𝑇 + 𝐸

𝑅𝑇 = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡      (1), where, 𝛽 is the heating rate, Tp is the peak temperature for the reaction, and R is the

gas constant.

The peak temperature (Tp) for each reaction is determined by finding the region on

the TG signal with the highest slope, which indicates the rate of oxidation. The temperature, at which the highest mass change occurs, usually correlates to a peak in the DTA signal. This temperature is taken as the peak temperature Tp.

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Figure 2: TG-DTA curves of 2 µm SiC oxidized in Ar with 20 vol,% O2

recorded with a heating rate of 10 °C/min.

The effect of the size of the SiC healing particles on the kinetics of oxidation induced crack healing is determined with TG-DTA. Evaluating the TG-DTA curves of the oxidation of SiC particles with different sizes recorded with different but constant heating rates and adopting Equation (1) gives the activation energy of the oxidation reaction. This activation energy increase with the size of the SiC particles. Moreover, the temperature decreases with decreasing SiC healing particle size considering the same oxidation induced crack healing kinetics.

4. CONCLUSIONS

The size of the SiC healing particles influences the kinetics of oxidation induced crack healing. The temperature at which these particles are capable to heal cracks decreases when smaller particles are applied.

AKNOWLEDGEMENTS

Financial support of the European Commissions 'Marie Curie International Training Network for Self-Healing   Materials:   from   Concept   to   Market’   (www.SHeMat.eu), project number 209308, is gratefully acknowledge.

REFERENCES

[1] Nakao, W., K. Takahashi, and K. Ando, Threshold stress during crack-healing treatment of structural ceramics having the crack-healing ability. Materials Letters, 2007. 61(13): p. 2711-2713.

[2] Ando K., et al., Self-crack-heling behavior and fracture strength of Al2O3/SiC

composites at the high temperature. European Conference on Fracture Series 15. Sweden, Aug. 11-13, 2004.

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[3] Nakao, W. and S. Abe, Enhancement of the self-healing ability in oxidation induced self-healing ceramic by modifying the healing agent. Smart Materials and Structures, 2012. 21(2): p. 025002.

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