Crack healing induced electrical and mechanical properties recovery in a Ti2SnC ceramic

https://doi.org/10.1016/j.jeurceramsoc.2015.09.019Get rights and content

Abstract

Ceramics are able to heal cracks and flaws by triggering local sintering (internal cracks) or oxidation reaction (surface cracks) which may induce the recovery of mechanical strength. Crack healing in ceramics, however, is limited to micro-sized cracks and requires elevated temperatures (above 1000 °C) and long time periods even up to 100 h. In this work we report on accelerated crack healing of a Ti2SnC ceramic which is capable to repair thermal shock induced cracks at a relatively low temperature of 800 °C within only 1 h. After healing treatment both the low flexural strength and electrical conductivity measured on the damaged material after quenching were almost fully recovered. Furthermore, Ti2SnC exhibits repeatable healing capability which offers a high potential for developing durable ceramics with prolonged lifetime under harsh thermal conditions.

Introduction

Research on ceramics able to restore mechanical damage by crack healing was inspired by biological healing processes [1], [2], [3], [4], [5]. Though in contrast to biological healing processes ceramic materials require an external stimulus (usually high temperature) to cause closure or filling of the crack space either by mass transport (sintering) or phase formation (oxidation reaction). Crack healing or self-healing is often used to characterize this behavior. Crack healing in ceramics has been studied on single oxide crystals [6], [7], [8], amorphous silicate glasses [9], [10] and polycrystalline oxide as well as non-oxide materials [11], [12], [13], [14], [15], [16], [17]. Recently, some ternary carbides such as Ti3AlC2, Ti2AlC, and Cr2AlC (also called MAX phases where M denotes an early transition metal, A is a mostly IIIA or IVA group element, and X is either C or N) were demonstrated to exhibit efficient crack healing capability at 1100–1200 °C in air atmosphere even for rather long cracks up to 1–7 mm [18], [19], [20], [21]. Filling of the cracks by the formation of solid oxide reaction products such as Al2O3, TiO2 and Cr2O3 was identified as the main healing mechanism for surface cracks accessible for air to penetrate. Although some MAX phase ceramics can heal millimeter-sized cracks in a short period of time, yet oxidation-induced crack healing still requires high temperatures exceeding 1000 °C. In order to lower the healing temperature of MAX phases, Ti2AC with the element on the A-site being a metal element of low melting temperature (A = Ga, Cd, Sn, In and Pb) might be of interest since a low cohesion as well as migration energy of the A-element [22] may favor mobilization and oxidation reaction at lower temperatures. Ti2SnC is one of the most attractive materials because it is damage tolerant, corrosion resistant, easily machinable, and it has high thermal and electrical conductivity (4–14 × 106 Ω−1 m−1) [23], [24], [25]. Ti2SnC was demonstrated to start to oxidize in air at 300 °C [26]. Accordingly healing of the Sn-containing Ti2Al(1−x)SnxC solid solutions was reported to be accomplished below 1000 °C [27]. Exploration of crack healing is of particular interest for extending the fields of engineering and functional applications of the group of Ti2AC ceramic materials.

In this work we report on the recovery of the mechanical and electrical properties of Ti2SnC by healing in air atmosphere at relatively low temperature and within a short period. The crack healing was studied on materials containing patterns of surface cracks which were generated by thermal shock damage. The microstructural evolution of the crack filling with solid oxidation products was analyzed by scanning and transmission electron microscopy techniques.

Section snippets

Material preparation

Ti2SnC was prepared by hot pressing a mixture of Ti (<45 μm, >99.2% purity, General Research Institute for Nonferrous Metals, China), Sn (<75 μm, >99.5% purity, Beijing Reagent Company (BRC), China) and C (graphite, <45 μm, >99.5% purity, BRC, China) powders with a molar ratio of 2:1:1 at 1250 °C with 30 MPa for 1 h in vacuum. The dense Ti2SnC samples were diamond cut into rectangular bars with a dimension of 4 mm × 3 mm × 36 mm. The bars were ground with 1200-grit SiC paper and then polished to 0.25 μm by

Oxidation of Ti2SnC

The Ti2SnC hot-pressed at 1250 °C is characterized by an average particle size of 1.5 μm and a relative density of 98%. Ti2SnC is the dominating phase with only minor fractions of Sn and Ti6Sn5 detected by XRD, Fig. 1(a). Thermally activated decomposition of Ti2SnC is likely to yield intermetallic Ti–Sn phases as well as elemental Sn during sintering or hot-pressing at elevated temperatures [26], [28], [29], [30]. Annealing of bulk Ti2SnC samples in air at 600 °C revealed SnO2 with a little amount

Discussion

Penetration of oxygen from the environmental atmosphere into the crack causes Ti2SnC at the crack surface to undergo an oxidation reaction which was reported to follow two stages [26], [27], [32].

At temperatures below 800 °C destabilization of the MAX phase crystal lattice tends to release Sn leaving a distorted Ti2Sn1−xC phase (Reaction (1)).Ti2SnC  Ti2Sn1−xC + xSn

At higher temperatures TiO2 formation tends to be dominative (Reaction (2)) due to the lower standard Gibbs free energy of TiO2

Conclusions

Healing of thermal shock induced crack patterns in the Ti2SnC ceramic was investigated. During annealing in air atmosphere oxygen may penetrate into cracks and react with Ti2SnC on the crack surface. In large cracks near the surface a high oxygen potential triggers the formation of TiO2 and SnO2 filling the crack space. Limitation of oxygen transport in small secondary cracks gives rise to a low oxygen potential and metallic Sn remains. Due to restoration of solid contact in the crack, flexural

Acknowledgements

This work was supported by the National Natural Science Foundation of China under Grant Nos. 51372015 and 51172015, and SRF for ROCS, SEM, and Beijing Government Funds for the Constructive Project of Central Universities. Financial support from DFG-grant no. GR 961/33-1 is gratefully acknowledged. Financial support by the DFG through the Cluster of Excellence EXC 315 “Engineering of Advanced Materials” (EAM) and the research training group GRK1896 “In situ microscopy with electrons, X-rays and

References (40)

  • S. van der Zwaag et al.

    Self-healing behaviour in man-made engineering materials: bioinspired but taking into account their intrinsic character

    Philos. Trans. R. Soc. A

    (2009)
  • S. van der Zwaag

    Self Healing Materials an Alternative Approach to 20 Centuries of Materials Science

    (2007)
  • S.K. Ghosh

    Self-Healing Materials: Fundamentals, Design Strategies and Applications

    (2009)
  • B.J. Blaiszik et al.

    Self-healing polymers and composites

    Annu. Rev. Mater. Res.

    (2010)
  • P. Greil

    Generic principles of crack-healing ceramics

    J. Adv. Ceram.

    (2012)
  • A.H. Heuer et al.

    The influence of annealing on the strength of corundum crystals

    Proc. Br. Ceram. Soc.

    (1966)
  • C.F. Yen et al.

    Spheroidization of tubular voids in Al2O3 crystals at high temperatures

    J. Am. Ceram. Soc.

    (1972)
  • J. Rodel et al.

    High-temperature healing of lithographically introduced cracks in sapphire

    J. Am. Ceram. Soc.

    (1990)
  • S.M. Wiederhorn et al.

    Crack healing in glass

    J. Am. Ceram. Soc.

    (1970)
  • H.D. Ackler

    Healing of lithographically introduced cracks in glass and glass-containing ceramics

    J. Am. Ceram. Soc.

    (1998)
  • Cited by (43)

    • MAX phases, structure, processing, and properties

      2021, Encyclopedia of Materials: Technical Ceramics and Glasses
    View all citing articles on Scopus
    View full text