Healing performance of Ti2AlC ceramic studied with in situ microcantilever bending

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Journal of the European Ceramic Society 33 (2013) 383–391

Healing performance of Ti2AlC ceramic studied with in situ microcantilever bending H.J. Yang a , Y.T. Pei a,∗ , G.M. Song b , J.Th.M. De Hosson a a

Department of Applied Physics, Zernike Institute for Advanced Materials and Materials Innovation Institute M2i, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands b Department of Materials Science and Engineering, Delft University of Technology, Mekelweg 2, 2628 CD, Delft, The Netherlands Received 28 May 2012; received in revised form 4 September 2012; accepted 13 September 2012 Available online 3 October 2012

Abstract In situ microcantilever bending tests were carried out to evaluate the healing efficiency of pre-notched Ti2 AlC ceramic after annealing at 1200 ◦ C for 1.5 h. Microcantilevers of different orientations were fabricated with focused ion beam method at different locations, i.e. in a singular Ti2 AlC grain, at a grain boundary or at the Ti2 AlC–Al2 O3 interface after healing. Ti2 AlC microcantilever shows an anisotropic bending strength (ranging between 9.6 GPa and 4.6 GPa depending on the precise crystallographic orientations) that is closely related to the different atomic bonds in the layered structure. After healing, the Ti2 AlC–Al2 O3 microcantilevers exhibit almost the same strength of about 5.2 GPa, i.e. slightly higher than the cleavage strength (4.6 GPa) of the initial Ti2 AlC microcantilevers. It suggests that the orientation of the matrix grain has no significant influence on the strength of healed microcantilevers. Furthermore, it turns out that the strength of the microcantilever with a healed grain boundary is at least twice the strength of the initial Ti2 AlC cantilever with a grain boundary. It is concluded that the oxidation dominated self-healing mechanism of Ti2 AlC ceramics can result in a perfect recovery of mechanical performance. The paper shows that the in situ microcantilever bending test provides a quantitative method for the evaluation of the strength of self-healing ceramics. © 2012 Elsevier Ltd. All rights reserved. Keywords: Titanium aluminum carbides; Al2 O3 ; Interfaces; Mechanical properties; Self-healing

1. Introduction Recent years show a growing interest in the self-healing properties of Ti2 AlC and Ti3 AlC2 ceramics at elevated temperatures for applications as oxidation-resistant coating and as conducting ceramic in harsh environments.1–3 Previous investigations demonstrate an outward diffusion of Al atoms and preferential oxidation of Al atoms during annealing.4 These processes are responsible for self-healing of cracks or notches in Ti3 AlC2 5 or Ti2 AlC.6,7 The healing products are mainly an Al2 O3 scale of dense microstructure or a mixture of Al2 O3 with a small amount of TiO2 ,5–9 which provide the ceramic material excellent oxidation resistance and recovery at elevated temperature.

∗

Corresponding author. Tel.: +31 50 363 4344; fax: +31 50 363 4881. E-mail addresses: [email protected] (Y.T. Pei), [email protected] (J.Th.M. De Hosson). 0955-2219/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jeurceramsoc.2012.09.012

Although studies were focused on the oxidation and healing products,5,6,10–12 only scant effort was made to determine quantitatively the bonding strength of healed oxides to the matrix. Indeed these so-called MAX phases2 show considerable anisotropy in single crystalline form and the crystal orientation may have a strong influence on the recovery of strength. However, since the oxide layers/scales are very thin, classical methods, such as pull test13 and double-cantilever beam setup,14 have certain drawbacks for the evaluation of the bonding strength of oxide scales or healed zone. Lin et al.8 reported that the strength of Al2 O3 /Ti2 AlC interface was higher than 85 MPa, but failure occurred at the oxide scale/epoxy adhesive interface rather than at the oxide scale/substrate interface. Therefore a new methodology is necessary for determining the healing efficiency of self-healing ceramics. In this work the mechanical performance of self-healed Ti2 AlC ceramic is examined through micro-cantilever bending test (MCBT) in conjunction with focused ion beam (FIB) technique. In the past MCBT had been executed on thin films15,16 and

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bulk materials17,18 for measuring strength, modulus and fracture toughness. In order to study the recovery efficiency of mechanical performance after healing, notches were manufactured at selected locations. Ti2 AlC and Ti2 AlC–Al2 O3 microcantilevers of different orientations were prepared for in situ bending tests: i.e. cantilevers along different orientations, with a grain boundary or Ti2 AlC–Al2 O3 interface located at the fixed end. In addition, the work of adhesion of Ti2 AlC–Al2 O3 interface is calculated and the mechanisms responsible for the different mechanical behavior of the various microcantilevers will be discussed. It will be shown that the MCBT combined with FIB slicing technique contributes to a quantitative and direct evaluation of the strength recovery of self-healing materials. 2. Experiments Ti2 AlC ceramic was fabricated using an in situ solid–liquid reaction of Ti, Al and graphite powders under hot pressure sintering conditions. The sintered bulk material was confirmed as Ti2 AlC by X-ray diffraction (XRD). A detailed description of the preparation can be found in Ref. 19. Rectangular bars with dimensions of 8 mm × 2 mm × 1 mm were cut from the center of the sintered bulk disk. These bars were carefully ground with abrasive SiC paper till 2400#. They were further polished down to 1 ␮m diamond paste and finished with polishing in SiO2 suspension. All the microcantilevers for MCBT were fabricated with FIB in a dual-beam FIB/SEM microscope (Lyra, Tescan, Czech Republic) (FIB, Ga+ ions, 30 kV). To minimize the Ga+ implantation effect, ion milling with current of 50 pA was used for the final polishing. Fig. 1 shows the microstructure of initial Ti2 AlC ceramic and microcantilevers without (Fig. 1b) and with (Fig. 1d) Al2 O3 healed zone. Fig. 1a is a scanning electron microscopy (SEM) image of Ti2 AlC surface close to the sample edge, in which a relatively coarse grain (marked as Grain-A) was chosen to be machined into a microcantilever. The small unit cell in Fig. 1a represents the crystal orientation of the Ti2 AlC grain, which was determined by electron backscatter diffraction (EBSD) (XL30-TSL OIM, 20 kV). As indicated by the unit cell, the long direction of platelet Grain-A is perpendicular to [0 0 0 1]. Fig. 1b shows a well polished microcantilever which was fabricated from Grain-A. The microcantilever in Fig. 1b with its long axis (LA) parallel to [0 0 0 1] is coded as MC-T. Similarly, the microcantilevers with LA perpendicular to [0 0 0 1] coded as MC-L and the microcantilevers with grain boundaries coded as MC-GB were also prepared. Fig. 1c shows an SEM image of Ti2 AlC surface where Grain B was chosen for preparing a Ti2 AlC–Al2 O3 microcantilever. First, the crystal orientation of Grain-B in Fig. 1c was determined by EBSD. As indicated by the unit cell, the long direction of the grain is perpendicular to [0 0 0 1]. A rectangular notch with dimensions of 1.8 ␮m in width, 5 ␮m in length and 6 ␮m in depth was cut by FIB parallel to the surface normal and basal plane of the grain, as show in Fig. 1c. Then the sample with notches

was healed at 1200 ◦ C for 1.5 h in air. After healing, the notch was filled up with fine Al2 O3 grains.6 Next, a microcantilever was cut out of the healed grain with its fixed end located at the Ti2 AlC–Al2 O3 interface, as shown in Fig. 1d. The backscattered electron (BSE) image (Fig. 1d) reveals elemental contrast where Ti2 AlC grain and Al2 O3 healed zone can be readily distinguished. The microcantilever in Fig. 1d with LA parallel to [0 0 0 1] of the matrix grain is denoted as MC-T/HN, where HN refers to healed notch. Similarly, the microcantilevers with LA perpendicular to [0 0 0 1] and with the Ti2 AlC–Al2 O3 interface located at the fixed end are denoted as MC-L/HN. As for the MCGB, the rectangle notch was cut along the GB and extended to the neighbor grain, see Fig. 1e. After healing, these microcantilevers with Ti2 AlC–Al2 O3 interface located at the fixed ends are denoted as MC-GB/HN (Fig. 1f). The MCBTs were carried out in situ in SEM (Lyra) by using a UNAT-SEM 2 nanotester (ASMEC, Germany), with a maximum load as high as 200 mN, force resolution <0.5 ␮N and displacement resolution <0.5 nm, which are outstanding for precise dynamic measurements. The load and displacements were recorded simultaneously at a data acquisition rate of 32 Hz (max. 64 Hz). Bending was performed at a deflection rate of 4 ␮m/min. The bending length lb , which is the distance between the loading position and the fixed end of a cantilever, was measured in SEM. With SEM, the dimensions of the cross section of a cantilever were measured prior to the bending test and the fracture surfaces were characterized after fracture of each specimen. The Ti2 AlC–Al2 O3 interface microstructure of Ti2 AlC after annealing at 1200 ◦ C for 3 min was also investigated in the transmission electron microscopy (TEM). Cross sectional laminae for TEM observation were sliced with FIB. A protective tungsten layer (2 ␮m thick) was deposited before making a lamina. A JEM 2010F TEM operating at 200 kV was used for selected area electron diffraction (SAED) analysis and microstructural observations.

3. Results 3.1. In situ bending tests Fig. 2a and b shows the in situ bending configuration of microcantilever without and with a healed zone, respectively. The interface between Ti2 AlC and Al2 O3 is indicated by a dotted line in Fig. 2b. During the bending test, the indenter comes from the top of the image and moves downwards to bend the microcantilever. Fig. 2c and d shows two representative load–deflection curves of MCBTs. For the single crystalline microcantilevers without and with Al2 O3 healed zone, the load increases linearly with the deflection as shown in Fig. 2c. These curves demonstrate that the microcantilevers of single grain Ti2 AlC do not show any obvious plastic deformation. The microcantilevers ended at grain boundary (MC-GB) undergo a little amount of plastic deformation before fracture occurs, as shown in Fig. 2d. Apparently the grain boundary in Ti2 AlC ceramic can accommodate some plastic deformation.

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Fig. 1. SEM micrographs revealing the microstructure of Ti2 AlC ceramic (a, c, e) where the orientation of selected grains is indicated with the unit cells determined by OIM. FIB cut notches and the locations of microcantilevers are marked. A microcantilever MC-T (b) was cut with FIB from the grain A in (a), a microcantilever MC-T/HN (d) was prepared from the grain B in (c) after healing the notch, and a microcantilever MC-GB/HN (f) was made from the healed grain boundary notch on the right side of (e), respectively.

The bending strength of the microcantilevers is derived from linear-elastic bending theory20 : 6Fl σ= 2 wt

ε= (1)

and the elastic modulus (E) of the microcantilevers according to E=

4Fl3 δwt 3

Furthermore, the fracture strain ε is obtained via Hooke’s law

(2)

3δt σ = 2 E 2l

(3)

where w and t are the width and thickness of a microcantilever, respectively; F is the maximum load, l is the bending length and δ is the vertical deflection at the loading point. The fracture strength and fracture strain of Ti2 AlC and Ti2 AlC–Al2 O3 microcantilevers are listed in Table 1.

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Fig. 2. SEM image showing the in situ bending configuration of a single crystal Ti2 AlC microcantilever MC-L (a) and microcantilever MC-T/HN with the Al2 O3 healed notch at the fixed end (b) where the interface of Ti2 AlC–Al2 O3 is indicated by the dotted line. Representative load–deflection curve of the single crystal Ti2 AlC microcantilevers MC-L and MC-T (c) and of the microcantilevers MC-GB ended at a grain boundary (d).

Fig. 3 presents an overview of the bending fracture strength and fracture strain for the single grain Ti2 AlC and Ti2 AlC–Al2 O3 microcantilevers, and those microcantilevers ended at a grain boundary before and after self-healing. It is seen that MC-L shows the highest strength, i.e. over 9.6 GPa. The strength of its corresponding cantilever with a healed zone, MC-L/HN, is 5.1 GPa, which is about 53% of that of MC-L. The average fracture strength of MC-T and MCT/HN microcantilevers is close to each other, 4.6 GPa and 5.2 GPa, respectively. The MC-GB ended at a grain boundary shows the lowest strength, 1.4 GPa. It is noteworthy that

the strength of grain boundary is about one third of the cleavage strength of Ti2 AlC grains themselves. Since the cleavage strength is a measure of the surface energy the ratio of 1/3 is all very reasonable since measured and calculated (high-angle) grain boundary energies are often found roughly 1/3 of the surface energies (in metals the grain-boundary energy is a function of temperature decreasing with increasing temperature).21 After healing the notch that ran through a grain boundary and extended to the opposite neighbor grain (Fig. 1e), the strength of MC-GB/HN improved significantly to 3.3 GPa.

Table 1 Fracture strength and fracture strain of Ti2 AlC and Ti2 AlC–Al2 O3 microcantilevers. Cantilever type/codea

No. of specimens

Max. strength (GPa)

Max. strain (%)

Ave. strength (GPa)

Ave. strain (%)

Measured modulus (GPa)

Ti2 AlC cantilevers

MC-L MC-T MC-GB

3 3 3

9.9 5.4 1.9

3.9 3.4 2.3

9.6 ± 0.4 4.6 ± 0.5 1.4 ± 0.5

3.7 ± 0.3 2.4 ± 1.1 1.5 ± 0.9

284 ± 19 234 ± 16 152 ± 21

Ti2 AlC–Al2 O3 cantilevers

MC-L/HN MC-T/HN MC-GB/HN

3 3 3

5.7 5.9 4.8

3.6 2.8 2.5

5.1 ± 0.7 5.2 ± 0.9 3.3 ± 1.4

2.1 ± 1.3 2.6 ± 0.6 1.7 ± 0.8

274 ± 20 251 ± 8.3 238 ± 49

a “L” means cantilever axis perpendicular to [0 0 0 1], “T” means cantilever axis parallel to [0 0 0 1], “GB” means cantilever with the fixed end located at a grain boundary and “HN” means healed notch.

H.J. Yang et al. / Journal of the European Ceramic Society 33 (2013) 383–391

a

Strength (GPa)

10 8 6 4 2 0

MC-L MC-T MC-GB

N HN HN B/H -L/ -T/ G C C M M MC

b

Fracture strain (%)

4 3

387

isotropic fracture strength of healed microcantilevers is determined by the bonding strength of the interface between the Ti2 AlC matrix grain and Al2 O3 phase and by the polycrystalline microstructure of Al2 O3 healing product (see Section 4.3). According to the result of bending experiments, the interfacial strength between Ti2 AlC and Al2 O3 phases is close to but still beyond the cleavage strength of Ti2 AlC matrix. In this sense, the efficiency of strength recovery after crack healing is about 100%. Thirdly, although MC-GB shows the lowest strength (1.4 GPa) among all the cantilevers, it is much higher than the bulk strength of polycrystalline Ti2 AlC ceramic,2 indicating that the weakest point for crack initiation is not the grain boundary of Ti2 AlC but rather other type of defects such as surface microcracks and voids. After healing a grain boundary notch, the strength of MC-GB/HN (3.3 GPa) is more than doubled the original fracture strength of Ti2 AlC grain boundaries. All these results obviously indicate that the oxidation-derived self-healing of Ti2 AlC ceramic can not only recover its strength fully, but also eliminate the intrinsic anisotropic properties of Ti2 AlC, which might be of significance for the application of ceramics in harsh atmosphere.

2

3.2. Fracture morphologies of Ti2 AlC and Ti2 AlC–Al2 O3 microcantilevers

1 0

MC-L

MC-T MC-GB

N HN HN B/H -L/ -T/ -G MC C MC M

Fig. 3. Summary of bending strength (a) and fracture strain (b) of different types of Ti2 AlC microcantilevers coded with the letter “L” indicating the cantilever axis perpendicular to [0 0 0 1], “T” indicating the cantilever axis parallel to [0 0 0 1], “GB” meaning the fixed end of cantilever located at a grain boundary and “HN” implying healed notch.

The differences in the fracture strain of microcantilevers follow similar trends as those of the strength, as shown in Fig. 3b. The fracture strain decreased from MC-L to MC-T, and the MCGB has the lowest fracture strain. The fracture strains of the Ti2 AlC–Al2 O3 microcantilevers are roughly at the same level. It should be noticed that the error bars of the fracture strain are sometimes considerable even after testing at least 3 specimens for each type of microcantilevers. The measured modulus of single crystalline Ti2 AlC microcantilevers perpendicular to and parallel to [0 0 0 1] is 284 GPa and 234 GPa (Table 1), respectively, which are very close to the calculated values (290 GPa and 250 GPa).22 The MCBT result points to at least three important conclusions. First, the single crystalline Ti2 AlC microcantilevers show strong anisotropy in strength. The strength of Ti2 AlC microcantilevers perpendicular to [0 0 0 1] direction is almost two times the strength of the microcantilevers parallel to [0 0 0 1], which is well consistent with the expectation that the cleavage strength of MAX compounds delaminating basal planes is the lowest. Second, in contrast the Ti2 AlC–Al2 O3 microcantilevers show nearly isotropic fracture strength. It is understandable that the

All the microcantilevers fractured at the fixed end. Fig. 4 shows the characteristic morphology of the fracture surfaces, which is rough and step-like for the microcantilevers MC-L (Fig. 4a) resulted from fracture of thin lamina, smooth for the microcantilevers MC-T (Fig. 4b), and serrated for the microcantilevers MC-GB (Fig. 4c), respectively. The typical fracture morphologies of the Ti2 AlC–Al2 O3 microcantilevers are shown in Fig. 5. The Ti2 AlC matrix and Al2 O3 healed zone exhibit a large contrast in the BSE images due to atomic weight difference of the elements. In order to record the crack propagation path, the bending tests of the MC-L/HN and MC-T/HN were terminated manually, as shown in Fig. 5a. During bending, the crack initiated at the top surface of the microcantilever and propagated along the interface of Al2 O3 /Ti2 AlC in MC-L/HN. In contrast, the microcantilevers MC-T/HN often fractured next to the Al2 O3 /Ti2 AlC interface and the crack propagated inside the Ti2 AlC grain as shown in Fig. 5b, suggesting that the strength of the interface exceeds the cleavage strength of Ti2 AlC single crystal (ignoring differences in fracture mode mixture). Fig. 5c shows the cracking behavior of MC-GB/HN, in which the crack definitely propagated along the Al2 O3 /Ti2 AlC interface. Comparing the fracture morphologies of all the microcantilevers investigated, the following conclusions can be drawn: first, all the microcantilevers fractured at the fixed ends where the maximum tensile stresses occur. It should be mentioned that the cantilever bending test is extremely sensitive to surface defects, and all the fractures at the fixed end of microcantilevers indicate that the surface smoothness of FIB cut microcantilevers avoids most of surface defects. Second, the different fracture morphologies of the microcantilevers MC-L, MC-T and MC-GB suggest different cracking mechanisms as to be discussed.

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Fig. 4. SEM micrograph showing the fracture surface of single crystalline Ti2 AlC microcantilevers MC-L (a), MC-T (b) and MC-GB (c).

Fig. 5. SEM micrographs showing the crack and fracture morphologies of microcantilever MC-L/HN (a), MC-T/HN (b) and GB-N (c), respectively.

4. Discussion

strength of Ti2 AlC under uniaxial tension along [0 0 0 1] is about 25 GPa, which is two times larger than the ideal shear  ¯ (0 0 0 1) shear deformation, indistrength (11 GPa) under 1210 cating strong anisotropy in the strength of Ti2 AlC. However, the theoretical strength is generally much higher than our experimental results (shown in Fig. 3), most likely due to defects.24 Here in situ MCBT of microcantilevers with different orientations fabricated by FIB technique unambiguously demonstrates anisotropic strength of Ti2 AlC ceramics (Fig. 3). Ti2 AlC can be treated as an elastic material according to the load–deflection curves (see Fig. 2c), the elastic modulus E

4.1. Crystal structure inducing anisotropic bending behavior of Ti2 AlC microcantilevers In the layered structure of Ti2 AlC, the atomic bonds between Ti Ti and Ti C are much stronger than that of Ti Al, implying that failure of Ti Al bonds occurs at a lower stress, as having been revealed by the first-principle calculations.22 Liao et al. reported the deformation modes and theoretical strengths of Ti2 AlC via first-principle calculations.23 The ideal tensile

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cleavage planes as seen in Fig. 6b. This should be the reason why the fracture surface of MC-T is clean and flat (Fig. 4b). In addition to the normal stress (σ), the shear stress (τ) would affect the crack initiation and propagation behavior during bending. As illustrated in Fig. 6, the maximum shear stress can be calculated from τ=

Fig. 6. Schematic illustration of fracture mechanisms of Ti2 AlC microcantilever MC-L (a) and MC-T (b) of different orientations indicated. The red and the green open-arrows indicate the tensile stress and the shear stress under bending, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

and the fracture strain ε can be thus used to estimate the fracture strength of macrocantilevers following Hooke’s law, σf = Eεf . The calculated E(L) and E(T) are 290 GPa and 250 GPa,22 respectively. The fracture strains observed are ∼3.7% for MCL and ∼2.4% for MC-T, as shown in Fig. 3b. Accordingly, the “theoretical” fracture strength of MC-L and MC-T are 10.73 GPa and 6 GPa, respectively, which are in a good agreement with the measured fracture strength in bending tests (Fig. 3). Hence, both the experimental and “theoretical” fracture strength of microcantilevers with different orientations exhibit clear anisotropic behavior, which is related to the intrinsic crystal structure of Ti2 AlC ceramic. It should be noted that the fracture strengths obtained from the MCBTs are significantly higher than that of polycrystalline Ti2 AlC materials, which are found usually below 1 GPa under compression.1,2 Since the diameter of the microcantilevers is around 1 ␮m, defects inside the microcantilevers are assumed to be minimal. Also very low ion beam current (∼50 pA) was utilized for the final FIB polishing to avoid surface damages. Therefore the microcantilevers can be considered as ideal specimens. The C44 (shear modulus) is quite high (118 GPa)22 also contributing to a high fracture strength. Fig. 6 illustrates a distinct cracking behavior of the Ti2 AlC cantilevers with two typical orientations. For MC-L, the basal planes are horizontally oriented. During bending, the crack initiates at the top surface of a microcantilever when the tensile stress reaches the ultimate strength, and then deflects along basal planes during propagation owing to the weakest Ti Al bonds that makes it running more easily along than across. A zigzag path is thus formed as shown in Fig. 6a. It also explains why the fracture surface of MC-L is rough and step-like (Fig. 4a). In contrast, since the basal planes in an MC-T microcantilever are transverse (vertical), once a crack initiates it will propagate quickly along the basal plane, leaving behind a pair of smooth

3F 2wt

(4)

The shear stress of MC-L (τL ) and MC-T (τT ) is ∼300 MPa and 200 MPa, respectively. τT is large enough to move dislocations in the basal plane since the critical resolved shear stress of dislocations gliding along the basal planes is only 24 MPa.25 Although τL is larger than τT , τL is not expected being large enough for moving c-type dislocations on a prismatic plane, i.e. with line directions perpendicular to the basal planes. c-Type dislocations have been discovered in ceramic oxides only when deformed under high strain rates.26,27 Therefore it is expected that the crack propagation of MC-L will be deflected by the basal planes, as shown in Fig. 6a; the crack propagation of MC-T will be confined within a basal plane. This also explains the differences in the fracture morphology of MC-L and MC-T. Moreover, the MC-T is much more sensitive to the surface defects than MC-L due to the straight cracking path along the basal planes, no need to break further Ti C bonds. It also implies that the toughness of MC-L is higher than MC-T. The cracking mechanism for MC-GB is much complicated since the grain boundary microstructure of Ti2 AlC ceramic is less clear yet. The bending strength of MC-GB is 5–6 times lower than that of MC-L and 3–4 times lower than that of MC-T, which indicates the grain boundary in the Ti2 AlC ceramic is a weak path. Thus, the crack will preferentially initiate at GB and propagate along GB. 4.2. Microstructure of Ti2 AlC–Al2 O3 interface determining the isotropic bending behavior The fracture strength of the Ti2 AlC–Al2 O3 cantilevers with different orientations is very close, indicating that the microstructure of Ti2 AlC–Al2 O3 interface is not strongly correlated to the orientation of Ti2 AlC matrix grains. The microstructures of Ti2 AlC–Al2 O3 interface have been reported recently.28,29 Although several orientation relationships have been observed, none of them dominates the growth of Al2 O3 during annealing. This is supported by the fact that polycrystalline Al2 O3 layers are formed in the notch or on the surface of single crystalline Ti2 AlC grains. Fig. 7 is a typical TEM bright field micrograph of the cross-section of Ti2 AlC–Al2 O3 interface, which was formed after annealing at 1200 ◦ C for 3 min. The darkest outer layer is tungsten (W) that was deposited to protect the oxidation surface before milling with FIB. The healed surface layer about 500 nm thick consists of a large number of fine Al2 O3 grains (about 200 nm in size) as confirmed by the selected area diffraction pattern shown in Fig. 7c. The underneath Ti2 AlC matrix orientates with its surface roughly parallel to (0 0 0 1), as confirmed by the diffraction pattern in Fig. 7b. It is obvious that the Al2 O3 layer is polycrystalline without a dominant orientation relationship with the Ti2 AlC matrix grain,

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H.J. Yang et al. / Journal of the European Ceramic Society 33 (2013) 383–391 Table 2 The work of adhesion of Ti2 AlC/Al2 O3 interfaces.

Fig. 7. (a) Bright field TEM micrograph showing a cross-section of Ti2 AlC–Al2 O3 interface formed by annealing Ti2 AlC at 1200 ◦ C for 3 min, with the dotted lines highlighting the grain boundaries of Al2 O3 oxide; (b) and (c) diffraction patterns of Ti2 AlC and Al2 O3 , respectively.

so the orientation effect of the Ti2 AlC matrix is diminished. As has been reported recently,6 the influence of the crystal orientation of Ti2 AlC is of secondary importance on the morphology of Al2 O3 scale that formed during annealing. The study in Ref. 6 has demonstrated that the two-layered microstructure of Al2 O3 grains can be formed on Ti2 AlC grains with various surface orientations. In this sense, the property of Ti2 AlC–Al2 O3 interfaces in MC-L/HN and MC-T/HN should be quite similar. As far as MC-GB/HN is concerned, its fracture strength is lower than the strength of MC-L/HN and MC-T/HN. It is likely related to the initial Al2 O3 layer formed on a grain boundary less compact in comparison with that formed in the notches inside singular grains of Ti2 AlC, which is confirmed in our recent paper.9 Once the initial Ti2 AlC grain boundary was oxidized during annealing (healing), however, the growth and the final microstructure of Al2 O3 healed zone are similar to those in MC-L/HN and MC-T/HN. 4.3. Theoretical consideration of the fracture energies The main conclusion of the ab initio study on the surface energies of the MAX phases30,31 is that the Ti X bonds are roughly 50% stronger than the Ti Al bonds (in the latter paper the absolute values seem too high but the ratios sound valid). Based on this conclusion one would expect the MC-L microcantilevers to be stronger by about 50% as experimentally observed in this work. The “macroscopic atom” (MA) approach developed by Miedema and collaborators32 is used to calculate the surface

Microcantilevers

MC-L/HN

MC-T/HN

Interfaces Work of adhesion (J/m2 )

Ti2 AlC/Al2 O3 4.23

Ti2 C/Al2 O3 4.55

energies of Ti2 AlC and Al2 O3 , Ti2 C, and their work of adhesion. The MA approach is a semi-empirical method that connects the enthalpy effects of alloys with the interface between two (Wigner–Seitz) cells. It has been successfully used to predict the enthalpy of alloys, amorphous aluminum oxides33 and the work of adhesion of metallic coating on metallic substrate (e.g. Fe–Zn systems34 ) and oxide scale on metallic substrate.34,35 According to the MA approach, we built a suitable model for the system of Ti2 AlC and Al2 O3 . Using a series of equations which has been reported in,33,35 the surface energy of Ti2 AlC (isotropic) is calculated 1.89 J/m2 and its fracture energy or work of adhesion in a Griffith approximation is thus 3.78 J/m2 . This value is lower than the work of adhesion of Ti2 AlC/Al2 O3 interface (4.23 J/m2 ). Ti2 AlC could exhibit three different kinds of end planes: Ti2 C, Al and a mixture. However, the Al atoms at the surface will be oxidized during annealing to form the Al2 O3 scale. As a result, the interface between the matrix and the oxides for MC-L/HN and MCT/HN should be considered as Ti2 AlC/Al2 O3 or a combination of Ti2 AlC/Al2 O3 and Ti2 C/Al2 O3 , respectively. As shown in Fig. 7, Al2 O3 scale formed on the Ti2 AlC matrix has a polycrystalline microstructure, and no predominating orientation relationship was found between Ti2 AlC and Al2 O3 . Thus, the work of adhesion for MC-L/HN is calculated by averaging the work  of adhesion of Ti2 AlC/Al  2 O3 (0 0 0 1), Ti2 AlC/Al2 O3 ¯ and Ti2 AlC/Al2 O3 1100 ¯ 1120 , while that of MC-T/HN is calculated by averaging the work   of adhesion of Ti2 C/Al  2 O3 ¯ , and Ti2 C/Al2 O3 1100 ¯ . The (0 0 0 1) and Ti2 C/Al2 O3 1120 results for MC-L/HN and MC-T/HN are 4.23 J/m2 and 4.55 J/m2 , respectively (see also Table 2). Therefore, the fracture behaviors of these two microcantilevers show similar strength, which is consistent with our experimental observations (Fig. 3). According to the calculations, the microcantilever fracture will occur in Ti2 AlC, rather than along the interface. Now, in this experiment, the crack occurs partly along the interface and partly on the side of Ti2 AlC matrix close to the interface. To a first approximation we would expect that rupture of the interface Ti2 AlC/Al2 O3 will take place because Al2 O3 is more brittle than Ti2 AlC. Failure will occur without a substantial dislocation nucleation and propagation whereas plastic energy involved in fracture of Ti2 AlC and the crack will be a bit blunted by plasticity and the work of adhesion of Ti2 AlC/Ti2 AlC will be affected by the plasticity contribution. A reservation of this concept is that the issue whether brittle fracture occurs may depend intricately and subtly on preexisting defects rather than on the crack tip nucleated and emitted dislocations. It seems that failure is determined by the “theoretical” work of adhesion being lower along Ti2 AlC/Ti2 AlC than along Ti2 AlC/Al2 O3 . The experimental result of oxide bonded Ti3 AlC2 and Ti2 AlC plates36 and repeatedly crack healing in Ti2 AlC7 revealed the same phenomenon that fracture occurs

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near the interface rather than along the interface, which also supports the present calculation. 5. Conclusions We have studied by in situ bending experiments the fracture behaviors of Ti2 AlC microcantilevers before and after selfhealing. The in situ microcantilever bending test provides a convenient and quantitative method for evaluating the strength of Ti2 AlC–Al2 O3 interface after healing. The following conclusions can be drawn: • Anisotropic strength of Ti2 AlC microcantilevers has been determined and is related to the crystallographic structure. • Oxidation induced self-healing of Ti2 AlC ceramic may reach a full recovery of strength: the bonding strength of Ti2 AlC–Al2 O3 interface is beyond the cleavage strength of Ti2 AlC grains and the strength of healed grain boundary is two times the strength of original grain boundary. • Healing notches does not only recover strength but also eliminate the anisotropic behavior of Ti2 AlC ceramics. • Pre-notching and successive healing MAX ceramics in conjunction with MCBT provides an advanced approach for quantitative characterization of interface strength and healing efficiency of self-healing materials. Acknowledgment This work was supported by the Netherlands Innovation Oriented Program on Self Healing Materials under Grant No. IOP-SHM 0871. References 1. Barsoum MW, Radovic M. Mechanical properties of the MAX phases. In: Buschow KHJ, Robert WC, Merton CF, Bernard I, Edward JK, Subhash M, editors. Encyclopedia of materials: science and technology. Oxford: Elsevier; 2004. p. 1–16. 2. Barsoum MW. The MN+1 AXN phases: a new class of solids. Prog Solid State Chem 2000;28:201–81. 3. Wang XH, Zhou YC. Layered machinable and electrically conductive Ti2 AlC and Ti3 AlC2 ceramics: a review. J Mater Sci Technol 2010;26:385–416. 4. Wang XH, Zhou YC. High-temperature oxidation behavior of Ti2 AlC in air. Oxid Met 2003;59:303–20. 5. Song GM, Pei YT, Sloof WG, Li SB, De Hosson JThM, Van der Zwaag S. Oxidation-induced crack healing in Ti3 AlC2 ceramics. Scripta Mater 2008;58:13–6. 6. Yang HJ, Pei YT, Rao JC, De Hosson JThM. Self healing performance of Ti2 AlC. J Mater Chem 2012;22:8304–13. 7. Li SB, Song GM, Kwakernaak KZ, Van der Zwaag S, Sloof WG. Multiple crack healing of a Ti2 AlC ceramic. J Eur Ceram Soc 2012;32:1813–20. 8. Lin ZJ, Zhuo MJ, Zhou YC, Li MS, Wang JY. Microstructures and adhesion of the oxide scale formed on titanium aluminum carbide substrates. J Am Ceram Soc 2006;89:2964–6. 9. Yang HJ, Pei YT, Rao JC, De Hosson JThM, Li SB, Song GM. High temperature healing of Ti2 AlC: on the origin of inhomogeneous oxide scale. Scripta Mater 2011;65:135–8. 10. Ando K, Shirai Y, Nakatani M, Kobayashi Y, Sato S. (Crack-healing + proof test): a new methodology to guarantee the structural integrity of a ceramics component. J Eur Ceram Soc 2002;22:121–8.

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