Self-healing behavior of Ti2AlC at a low oxygen partial pressure

Ti2AlC, a MAX phase ceramic, has an attractive self-healing ability to restore performance via the oxidation-induced crack healing mechanism upon healing at high temperatures in air (high oxygen partial pressures). However, such healing ability to repair damages in vacuum or low oxygen partial pressure conditions remains unknown. Here, we report on the self-healing behavior of Ti2AlC at a low oxygen partial pressure of about 1 Pa. The experimental results showed that the strength recovery depends on both healing temperature and time. After healing at 1400 °C for 1–4 h, the healed samples exhibited the recovered strengths even exceeding the original strength of 375 MPa. The maximum recovered strength of ∼422 MPa was achieved in the healed Ti2AlC sample after healing at 1400 °C for 4 h, about 13% higher than the original strength. Damages were healed by the formed TiCx from the decomposition of Ti2AlC. The decomposition-induced crack healing as a new mechanism in the low oxygen partial pressure condition was disclosed for the MAX ceramics. The present study illustrates that key components made of Ti2AlC can prolong their service life and keep their reliability during use at high temperatures in low oxygen partial pressures.


Introduction 
Advanced ceramic materials are widely used in high-temperature environments due to their attractive stability, high strength, and oxidation resistance at high temperatures. However, the intrinsic brittleness of ceramics makes them sensitive to the presence of surface cracks, resulting in a loss in performance or even a sudden catastrophic failure during service [1].
to an element of the transition metal, A is an element of group IIIA or IVA, and X is C or N [14]) exhibit attractive crack healing performance at relatively low temperatures and short time [1,12,13,[15][16][17][18][19]. For example, Ti 3 AlC 2 can completely heal a crack with a length of 7 mm and a width of 5 μm after healing at 1100 ℃ in air for 2 h [15]. The main healing mechanism for Ti 3 AlC 2 is the filling of cracks with Al 2 O 3 and TiO 2 . Ti 2 AlC has an ability to repeatedly heal introduced cracks with a size of ~2 mm at a given location at 1200 ℃ for 2 h in air. Such cracks were filled by the formation of Al 2 O 3 and TiO 2 [1]. A fine-grained Cr 2 AlC (~2 μm) MAX ceramic also demonstrates an ability to restore the flexural strength through the filling of cracks with only Al 2 O 3 at 1100 ℃ in air for 4 h [16]. Moreover, Ti 2 SnC can heal thermal-shock-induced cracks at a relatively low temperature of 800 ℃ within 1 h in air, leading to the recovery of electrical conductivity and flexural strength [17]. Healing is accomplished by the formation of TiO 2 and SnO 2 oxides at the site of damage.
Based on the above discussions, it is known that the crack healing mechanism driven by oxidation is efficient in air. However, high-temperature structural components made of such ceramics applied in hypersonic vehicles during flight, gas turbines under combustion, and industrial vacuum furnaces during work are generally exposed to low oxygen partial pressures. Cracks in the components will not be completely healed by the formation of oxides due to the suppression of the formation of the oxides in Mo-Si-B alloys [20] or the evaporation of the oxides in SiC and its composites [21][22][23] under low oxygen partial pressure conditions. So far, most investigations have focused on the oxidation-induced crack healing behavior of the MAX ceramics in air. Work on the crack healing of MAX at low oxygen partial pressures has been much less focused. Only Ti 2 SnC was reported that it can heal quenching-induced cracks at 800 ℃ for 1 h at a low oxygen partial pressure of 5 Pa [24]. Elemental Sn precipitates from the structure of Ti 2 SnC and fills cracks. Therefore, Sn-precipitation-induced crack healing in Ti 2 SnC leads to the complete recovery of the electrical conductivity at low oxygen partial pressures [24]. In the MAX family, Ti 2 AlC is expected to be an attractive candidate for high-temperature applications in the gas turbines under combustion, heat elements and liners for industrial vacuum furnaces, and nozzles due to its good oxidation resistance [25][26][27], exceptional ablation resistance [28], and nonsusceptibility to thermal shock [29]. Such high-temperature components made of Ti 2 AlC generally work under low oxygen partial pressure conditions, while whether Ti 2 AlC can repair damages to prolong their service life is unknown. Therefore, it is necessary to investigate and understand the crack healing behavior of Ti 2 AlC under low oxygen partial pressure conditions.
In the present study, the self-healing behavior of Ti 2 AlC at a low oxygen partial pressure of about 1 Pa was investigated. The microstructure and strength of Ti 2 AlC before and after healing at temperatures ranging from 1200 to 1400 ℃ for 1-4 h at a low oxygen partial pressure of about 1 Pa were characterized.

1 Material preparation
Ti (average particle size = 48 μm, 99.5 wt% purity), Al (average particle size = 75 μm, 99.5 wt% purity), and TiC (average particle size = 48 μm, 99.5 wt% purity) powders were mixed in a plastic container for 10 h with a molar ratio of Ti : Al : TiC = 1 : 1.1 : 0.9. The mixture was hot-pressed in a graphite mold covered by BN at 1450 ℃ with 30 MPa for 1 h in the Ar atmosphere to synthesize Ti 2 AlC samples [30].
The synthesized dense Ti 2 AC samples were diamondcut into bars with dimensions of 4 mm in width, 3 mm in thickness, and 36 mm in length. The rectangular bars were ground with SiC papers, and their tensile surfaces were polished to 0.25 μm by a diamond paste, and then cleaned in an ultrasonic bath with ethanol. Some polished bars were used to perform a three-point bending test, and others were used to introduce damages and to perform the healing test.

2 Damaging and crack healing
Microdamages were generated by indentation method. On the polished surface, a row of four indentations along the width in the middle of the long bar are generated using a diamond indenter in a Vickers hardness tester (TH700, Beijing TIME High Technology Ltd., China). The load was 30 kg, and the dwell time was 15 s. The damaged bars were heat-treated in a vacuum furnace at temperatures of 1200-1400 ℃ for 1-4 h. The residual pressure in the furnace was ~5 Pa, corresponding to a low oxygen partial pressure of about 1 Pa. www.springer.com/journal/40145

3 Mechanical property measurement
To measure the initial strength, residual strength, and recovered strength, the three-point bending test was performed in a WDW-100E compression machine (Jinan Time Shijin Testing Machine Co., Ltd., China) by the initial polished bars, damaged bars, and healed bars. During the test, the polished surfaces and the damaged surfaces before and after healing were subjected to the tensile stress. The span size was 30 mm, and the crosshead speed was 0.5 mm/min. At least three bars were tested to achieve an average strength value.

4 Characterization
The microstructures of the initial, damaged, and healed bars were characterized by a scanning electron microscope (ZEISS EVO 18, Carl Zeiss SMT, Germany) equipped with an energy-dispersive spectrometer (X-Max50, Bruker, Germany). The polished surfaces of the initial bars were etched by a mixed acid solution of HF and HNO 3 , and then characterized with scanning electron microscopy (SEM)/energy-dispersive spectroscopy (EDS). The phase compositions of the samples before and after healing were identified by an X-ray diffractometer (XRD; D/Max 2200PC, Rigaku Co., Ltd., Japan) using Cu Kα radiation. The filling products in the healed zones were determined with XRD and EDS.

Results
The as-prepared sample mainly consists of Ti 2 AlC, with a small amount of TiC as an impurity, evidenced by the XRD pattern ( Fig. 1(a)). The etched surface demonstrates that the Ti 2 AlC grains are plate-like, with sizes of ~45 m in length and ~6 m in thickness ( Fig. 1(b)). Figure 2 shows the flexural strengths for the original, damaged, and healed Ti 2 AlC bars. The original Ti 2 AlC material has a flexural strength of about 375 MPa. It should be noted that the healing of the original samples at 1400 ℃ for 2 h leads to an improved strength up to 421 MPa ( Fig. 2(a)). The main reason will be explained in Section 4. Introducing damages by the indentation method causes a great decline of 50.4% in strength, from the initial strength of 375 to 186 MPa. The fractured samples after the bending test showed that the fracture propagation was through the four indentations in the damaged samples (the upper image in Fig. 2(b)).   Our previous work [1] demonstrated that the oxidation-induced crack healing of Ti 2 AlC leads to the flexural strength returning from the initial strength (211 MPa) to 224 MPa after healing at 1200 ℃ for 2 h in air. It should be noted that the fracture path in the 1400 ℃-2 h-healed samples after the bending test was away from the healed zone, suggesting that the healed zone becomes stronger (the lower image in Fig.  2(b)). Such feature has also been observed in the Ti 2 AlC and Cr 2 AlC MAX materials after healing in the air [1,16]. Figure 2(c) presents another feature, in which the damaged zones around the indentations are completely covered, and the indentations are almost filled by some products. To identify the formed products and clarify the crack healing behavior of Ti 2 AlC at a low oxygen partial pressure of about 1 Pa, the phase compositions and the microstructures of the healed samples were characterized with XRD and SEM/EDS.
After healing at temperatures of 1200-1400 ℃ for different times, the surfaces of the healed Ti 2 AlC samples were characterized with XRD (Fig. 3). After healing at 1200 ℃ for 2 h, some Ti 2 AlC peaks disappeared, and the main peak of (103) locating at 39.54 decreased greatly in intensity (Fig. 3(b)) as compared with that of the initial Ti 2 AlC sample ( Fig. 3(a)). In addition, new phases of Ti 3 AlC 2 , TiC, and Al 2 O 3 were detected ( Fig. 3(b)). After healing at 1300 ℃ for 2 h, the peak intensities of TiC became stronger, indicating that TiC is the predominant phase. However, Ti 2 AlC, Ti 3 AlC 2 , and Al 2 O 3 peaks still appeared. The above results indicate that the decomposition of Ti 2 AlC into TiC x (the value of x is theoretically 0.5) occurs at above 1200 ℃, and the appearance of Ti 3 AlC 2 may be induced by a reaction between Ti 2 AlC and TiC x ; the detailed information was analyzed in Section 4. After healing at 1400 ℃ for 2 h, only TiC peaks appeared, without Ti 3 AlC 2 and Al 2 O 3 peaks, indicating that the Ti 2 AlC sample might have been completely transformed into TiC x , or the surface of the healed sample may be covered by a TiC x www.springer.com/journal/40145 layer with a thickness of large than 10 m. The above supposition will be confirmed by the following cross-sectional SEM observations. Cross-sectional SEM images of the damaged and healed Ti 2 AlC samples are presented in Fig. 4. The fracture surface through the indentation shows a semielliptical damaged zone, where the larger Ti 2 AlC grains are broken into small ones under a compressive load of 30 kg (Fig. 4(a)). Many micro cracks with transgranular and intergranular propagation paths were found under the indentation, as shown in the areas marked by "1" and "2" (Fig. 4(a)). These damages cause a drop of 50.4% in strength, as depicted in Fig. 2.
A low-magnification cross-sectional SEM micrograph of the 1200 ℃-2 h-healed Ti 2 AlC reveals that the indentation was not completely filled by the products (the inset in Fig. 4(b)). A high-magnification SEM micrograph taken from the marked area in the inset clearly shows a dense layer covering the surface. The dense layer is mainly composed of fine TiC x grains, identified by XRD and EDS. The TiC x layer is about 1.4 m in thickness (Fig. 4(b)). It should be noted that the micro cracks under the indentation completely disappeared, as compared with Fig. 4(a). With the temperature increasing, the TiC layer becomes thicker. The thickness of the TiC x layer is about 5 m in the 1300 ℃-2 h-healed Ti 2 AlC (Fig. 4(c)), and about 20 m in the 1400 ℃-2 h-healed Ti 2 AlC (Fig. 4(d)). In addition, the thickness of the TiC x layer is also influenced by the healing time. Healing at 1400 ℃ for 1-4 h leads to the layer thickness changing from 10 to 45 μm. The elemental distribution results showed that the surface layer mainly consists of Ti and C elements (59 at% Ti and 41 at% C), corresponding to TiC x ; the area below the TiC layer is comprised of Ti, Al, and C, indicating Ti 2 AlC or Ti 3 AlC 2 (Fig. 4(e)). Based on the XRD results, Ti 3 AlC 2 appeared in the samples after healing at above 1200 ℃. Therefore, a mixture of Ti 2 AlC and Ti 3 AlC 2 may exist under the TiC x layer.
Based on the above discussions, it can be concluded that Ti 2 AlC can heal damages even at a low oxygen partial pressure of about 1 Pa; the main mechanism is that the formed TiC from the decomposition of Ti 2 AlC fills the damages, leading to the strength recovery. For the Ti 2 AlC MAX phase, it exhibits a new crack healing mechanism, i.e., decomposition-induced crack healing in the low oxygen partial pressure environments.

Discussion
To further investigate the microstructure of the healed zones under the indentations in Ti 2 AlC at a low oxygen partial pressure of about 1 Pa, the polished fracture surface of the 1400 ℃-2 h-healed Ti 2 AlC sample was characterized by SEM/EDS (Figs. 5 and 6). No exfoliation and crack in the area between the TiC layer and the damaged zones in the matrix were detected, indicating that the formed TiC layer bonds the matrix strongly (Fig. 5). For the original Ti 2 AlC samples after healing at 1400 ℃ for 2 h, the surface defects such as scratches and holes should be filled by the formed TiC x . For the damaged samples, the formed TiC x fills all the damaged zones induced by indentation, contributing to the strength recovery. It can be found that small Al 2 O 3 grains (black particles) fill the pores and micro cracks, explaining why the TiC layer is dense. The backscattered SEM micrograph taken from the polished surface by removing the TiC layer about 15 μm in thickness from the surface clearly presents some small black particles in the pores or small cracks (Fig. 6(a)). The EDS mapping analysis shows the elemental distributions of Ti, Al, O, and C ( Fig. 6(b)). The black particles are mainly composed of Al and O, being Upon healing, the high temperature induces the decomposition reaction of Ti 2 AlC. The decomposition preferentially occurs on the sample surface, especially  in the damaged zones caused by the indentation due to the deformed Ti 2 AlC grains with high strain energy. According to Reaction (1), Ti 2 AlC decomposes into TiC 0.5 and Al. However, the content of C may be changed if Reaction (2) occurs [31]. Herein, TiC x is denoted as the decomposed product.
The TiC x grains from the Ti 2 AlC decomposition cover www.springer.com/journal/40145 the surface and form a TiC x layer, which becomes thicker with increasing temperature. The decomposition reaction of Ti 2 AlC to form TiC x results in a volume shrink. Therefore, many pores were detected in the TiC x layer (Fig. 5), which act as diffusion channels for the evaporation of Al at a low oxygen partial pressure of about 1 Pa. When gaseous Al passes through the porous TiC x layer, some Al atoms react with residual oxygen atoms in the system to form Al 2 O 3 , filling the pores and microcracks in the TiC x layer. This explains the reason why small Al 2 O 3 particles distribute in the TiC x layer, as shown in Figs. 5 and 6. According to the XRD results, the Ti 3 AlC 2 peaks were detected after healing Ti 2 AlC at 1200 and 1300 ℃ for 2 h (Fig. 3). The appearance of Ti 3 AlC 2 may be due to a reaction between Ti 2 AlC and TiC, as described by Reaction (3) [32]. However, the Ti 3 AlC 2 phase finally decomposes into TiC 0.67 and Al as a gaseous phase at above 1200 ℃ in vacuum, as described by Reaction (4) [33]. It has been reported that the decomposition of Ti 3 AlC 2 occurs at 1250 ℃ in an Ar or Ar-10 vol% CO atmosphere, resulting in the resultant products of TiC 0.67 and Al 2 O 3 [34]. Wang and Zhou [35] found that Ti 3 AlC 2 powder even decomposes at a temperature of as low as 600 ℃, and its decomposition rate is faster than that of the bulk counterpart in Ar [35].
As the temperature was 1400 ℃, the decomposition of both Ti 2 AlC and Ti 3 AlC 2 became severe, and the formed TiC x as the main phase completely filled the micro-damages both under indentations and on the original surface. The Al 2 O 3 content may be lower than the detection limit of the XRD. This explains why only TiC was detected in the 1400 ℃-2 h-healed sample by the XRD (Fig. 3).
It has been reported that the Ti 2 AlC and Ti 3 AlC 2 phases may decompose into TiC x and two gaseous phases of Al and Ti at high temperatures [31]. In the present study, the residual pressure in the furnace was ~5 Pa, the vapor pressures of Al rise from 1.4 Pa at 1200 to 26 Pa at 1400 , but the vapor pressures of ℃ ℃ Ti are over 3 orders of magnitude lower than those of Al at the same temperature (  [36]. Therefore, Once the relatively dense TiC x layer forms on the surface of the Ti 2 AlC sample, it acts as a barrier layer to retard the Al atomic migration from Ti 2 AlC, and thus slows down the decomposition process. The volume shrink caused by the decomposition of Ti 2 AlC will generate a compressive stress in the TiC x layer. In addition, the thermal expansion mismatch between the TiC x layer and the Ti 2 AlC matrix upon cooling down from the healing temperature also leads to the generation of the compressive stress on the healed surface. The coefficients of the thermal expansion for Ti 2 AlC and TiC x are 8.810 −6 [14] and (7.4-8.3)10 −6 K −1 [14,37], respectively. The compressive stress generated in the TiC x layer acts as a surface prestress to counteract the tensile stress applied to the surface of the Ti 2 AlC material. In addition, it will be influenced by the thickness and microstructure of the TiC x layer. This is the reason why the original and damaged Ti 2 AlC samples after healing at 1400 ℃ have the recovered strength higher than the initial strength.
Based on the above discussions, it can be concluded that Ti 2 AlC as a representative of the MAX phases can repair damages in vacuum or low oxygen partial pressure environments. The main mechanism is the decomposition-induced crack healing.
Up to now, two kinds of mechanisms have been confirmed for the self-healing of MAX phases at low oxygen partial pressures. Sn-precipitation-induced crack healing in Ti 2 SnC is the main mechanism at a low oxygen partial pressure of 5 Pa [24]. The precipitation-induced crack healing mechanism is predominant in the MAX phases containing lowmelting-point "A" elements such as Ga, Sn, In, and Pb, which have low migration energy. However, these MAX phases still keep the stable structural integrity after healing [24]. Decomposition-induced crack healing is the other mechanism confirmed in Ti 2 AlC at temperatures above 1200 ℃ at low oxygen partial pressures. This mechanism may be effective in the Alor Si-containing MAX phases (Ti 2 AlC, Cr 2 AlC, Ti 3 AlC 2 , and Ti 3 SiC 2 ), which are triggered by the decomposition of the MAX on the surface into carbides such as TiC x or Cr 23 C 6 [38] at high temperatures.

Conclusions
The crack healing behavior of Ti 2 AlC was firstly investigated in the temperature range of 1200-1400 ℃ for 1-4 h at a low oxygen partial pressure of about 1 Pa. The healing tests demonstrated that Ti 2 AlC has the ability to heal damages even under the low oxygen partial pressure condition. Healing at 1300 ℃ for 2 h leads the strength recovery to 371 MPa, close to the initial strength of 375 MPa. After healing at 1400 ℃, all the healed samples have the strength recovery exceeding the initial strength. A high recovered strength of 422 MPa was achieved after healing at 1400 ℃ for 4 h, increased by 13% of the initial strength. Decomposition-induced crack healing as a new healing mechanism in the Ti 2 AlC MAX phase was confirmed upon healing in the low oxygen partial pressure environment. The damages were healed by the formed TiC x from the decomposition of Ti 2 AlC on the surface. In addition, some Al atoms from Ti 2 AlC reacted with residual O in the system to form Al 2 O 3 . The small Al 2 O 3 grains fill the pores in the TiC x layer, thereby making the healed zones relatively dense. The healing ability of Ti 2 AlC in low oxygen partial pressure environments widens the potential applications of MAX phases in different environments.