# Wetting behaviors of the Ti2AlC ceramics by nickel and aluminum fillers

Chengjie Lu, Liangbo Sun, Qin Qi, Jie Zhang, Gilles Hug

► To cite this version:

Chengjie Lu, Liangbo Sun, Qin Qi, Jie Zhang, Gilles Hug. Wetting behaviors of the Ti2AlC ceramics by nickel and aluminum fillers. Materials Letters, 2018, 219, pp.240-242. 10.1016/j.matlet.2018.02.077. hal-02319668

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<figure><img src="image_1.png" /></figure>
# Wetting behaviors of the Ti₂AlC ceramics by nickel and aluminum fillers

Chengjie Lu¹,², Liangbo Sun¹, Qin Qi¹, Jie Zhang¹,*, Gilles Hug²

¹School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China

²Laboratoire d'Études des Microstructures, CNRS-ONERA, Châtillon 92322, France

## Abstract

In the present work, the behaviors of nickel and aluminum based fillers on the Ti₂AlC ceramics are studied by wetting experiments and characterization of the interface cross-section. The results show that the two kinds of fillers possess different wettability on the Ti₂AlC ceramics: the contact angle of the nickel fillers decreases continuously with rising temperature and reaches less than 20° finally, whereas that of the aluminum fillers remains larger than 135° in the full temperature range investigated. The characterization of the interface from cross-section observations reveals the presence of an interaction layer in the Ti₂AlC ceramic substrate, which promotes the wetting process.

**Keywords:** MAX phases; wetting; interface; reaction

## 1. Introduction

In recent years, a group of nanolaminate ternary carbide or nitride ceramics have attracted considerable attention from researchers because of their excellent combination of metal and ceramic properties [1, 2]. They have been systematically

E-mail address: hitzhangjie@hit.edu.cn (J. Zhang).

<footnote>* Corresponding author: Tel/Fax: +86 451 86414234</footnote>
introduced by Barsoum *et al.* [3] with the family name ‘MAX’ and the general formula
‘*M*<sub>*n*+1</sub>*A*X*<sub>*n*</sub>’, where *M* is an early transition metal, *A* is an A-group element and *X* is
either carbon or nitrogen. The *n* parameters may vary from 1 to 3 and the different
structure are often named 211, 312 or 413 after their composition. The Ti<sub>2</sub>AlC
ceramics, a typical member of the family with *n*=1 (known as the ‘211’ structure), have
gained extensive interests since the compound possesses the lowest density [4] and the
best oxidation resistance [5] of the MAX family. However, as for other ceramics, the
synthesis of bulk Ti<sub>2</sub>AlC ceramics has proven to be difficult [6, 7]. Therefore, studies
devoted to the joining of Ti<sub>2</sub>AlC ceramics are of great significance for applications [8].

Only a few efforts have been carried out to study the joining of Ti₂AlC phases,
among which the diffusion technique is the mostly adopted one. These studies revealed
that the formation of Ti₂AlC-MAX diffusion couples requires high temperature, along
with a long diffusion time [9]. Such conditions are often unfavorable for the thermal
stability of the Ti₂AlC ceramic substrates. In addition, mechanical tests of
Ti₂AlC-metal diffusion couples [10] exhibit low mechanical strength in general, which
is mainly attributed to the large mismatch of the coefficients of thermal expansion of
the two materials. Among the various existing joining techniques, brazing is one of the
most promising for joining MAX phases, especially concerning the ceramic-metal
joints [11]. In the brazing of some traditional ceramic materials, the poor wettability of
the molten metallic alloys on the ceramic surface is always a critical problem, which
requires to be overcome. Usually, it is done by introducing some reactive elements in
the brazing filler or by including a surface modification of the ceramic substrates.

However, there is still no literature reporting on the fundamental wetting behaviors of the liquid brazing filler onto the Ti₂AlC ceramics at this moment. The present work aims at illustrating the wetting behaviors of some fillers on the Ti₂AlC substrate, thereby disclosing the wetting mechanisms of this MAX phase.

## 2. Materials and methods

Since the elimination of the *A* element in a MAX compound is considered to be the major factor accounting for its decomposition behavior [12], two kinds of fillers are chosen in this work: Nickel based fillers (including BNi-2 alloy and NiPd eutectic alloy) which are reactive to the *A* element, and aluminum based fillers (including the pure Al metal filler and AlSi eutectic alloy) which are not. All these fillers are provided in powder state with an average particle size of 50 µm, by Shanghai Simike Welding Materials Limited Company, China. The wetting experiments were carried out in a high vacuum furnace (VAF-30, working at around 10⁻³ Pa). The contact angle during the experiment was recorded with the camera (DV-HV 1303UM) equipped on the furnace, and the cooling speed was 5 °C/minute after wetting experiments. Finally, the cross-section morphology was investigated using a scanning electron microscope (SEM, Sigma 500, Zeiss).

## 3. Results and discussion

Fig. 1 shows the variations of the contact angle θ as a function of temperature T. It suggests that the two kinds of fillers possess different wetting behaviors on the
Ti₂AlC ceramics. The contact angle of the two nickel based filler alloys, as displayed in Fig. 1(a), both stay lower than 90° during the whole wetting process, and can even reach as small as 20° finally. Although the presence of the additive Cr and B elements in the BNi-2 filler alloy is believed to be favorable for the wetting process, it is still reasonable to deduce that the nickel melts possess excellent wettability on the Ti₂AlC ceramics. On the contrary, the two aluminum fillers, shown in Fig. 1(b), are not wettable on the ceramic surface, whose contact angle keeps larger than 135°. It is worthwhile pointing out that the presence of the Si element in the AlSi eutectic filler contributes to a smaller contact angle at the initial wetting stage, however, it does not alter the non-wetting behavior of the filler on the Ti₂AlC ceramics. Therefore, it can be safely assumed that the aluminum melts possess poor wettability on the Ti₂AlC ceramics.

<figure><img src="image_2.png" /><figcaption>Fig. 1. Variations in the contact angle $\theta$ as a function of temperature $T$ for the Ti₂AlC ceramics wetted by (a). Nickel based fillers; (b). Aluminum based fillers.</figcaption></figure>

The cross-section of the BNi-2/Ti₂AlC wetting sample is displayed in Fig. 2. In the left side of the figure the original separation surface is clearly apparent.
Accordingly, the reaction layer can be divided into two regions: beneath the original surface, there exists a diffusion area, which is supposed to be a solid state reaction during the wetting experiments since a highly ordered microstructure can be observed; above the original interface, there exists the solidified filler melt with the typical isothermal solidification microstructure of BNi-2 filler alloy [13], which is considered to be liquid state during the wetting experiments. Fig. 2(b) shows the magnified morphology of the cross-section area. Beneath the original surface, the formation of the interaction area is likely to be related to the diffusion behavior of the Ni element in the Ti₂AlC substrate. In addition, some modified Ti₂AlC (with lighter contrast in the figure when compared to the unmodified Ti₂AlC substrate) can be observed, which is assumed to be the original grain boundary according to the distribution morphology. Therefore, it is suggested that the nickel element from the brazing filler diffuses into the Ti₂AlC substrate mainly along the grain boundary, and then contributes to the formation of the interaction layer with the increasing nickel content. The average chemical composition of the interlayer is determined from EDS analysis to be 28.1 at.%, 11.8 at.%, 31.7 at.% and 28.4 at.% for Ti, Al, C and Ni elements, respectively.

Thus it is assumed to be a mixture of Ni₃(Al, Ti) and TiCₓ. It can be concluded from the above analysis that the diffusion of Ni element from the filler in the Ti₂AlC substrate accounts for the formation of a thick interaction layer. In addition, the presence of the additive elements (The B element for example) in the BNi-2 filler contributes to a strong interaction between the melt and the substrate, thereby leading
to a quick reduction in the contact angle. However, for the NiPd filler, without the
assistance of the reactive elements, the formation of the interaction layer by diffusion
of Ni and Pd elements in the Ti₂AlC substrate requires some time, which accounts for
the slow reduction rate in the contact angle at the initial stage. Afterwards, the
following wetting process is promoted with the formation of interaction layer, then the
contact angle decreases rapidly. In conclusion, the formation of the interaction layer is
assumed to be the major wetting mechanism of the nickel fillers on the Ti₂AlC
ceramics.

<figure><img src="image_3.png" /><figcaption>Fig. 2. Cross-section morphology of the BNi-2/Ti₂AlC wetting sample: (a). morphology of the edge region; (b). morphology of the middle region.</figcaption></figure>

The cross-section morphology of the Al/Ti₂AlC wetting sample is displayed in
Fig. 3(a) and (b). No interaction area in the Ti₂AlC substrate can be observed around
the wetting surface: the flat morphology remains unchanged, and the chemical
composition ratio of Ti: Al: C stays 2: 1: 1 in the Ti₂AlC grains nearby according to the
EDS results. In order to reveal the functionalization mechanisms of the pure Al filler
metal on the Ti₂AlC ceramics, a supplementary wetting experiment has been carried
out, using an Al foil with a thickness of 20 µm. Instead of gathering together to form a
liquid sphere when the Al powders are heated, the molten Al foil spills on the surface,
and then interacts with the Ti₂AlC ceramics during the wetting process. It finally
contributes to the formation of the surface with the morphology displayed in Fig. 3 (c)
and (d). It can be observed from the Fig. 3(c) that a mass of grey phase with sheet
morphology forms on the surface. It is identified as the intermetallic compound TiAl₃
according to the EDS result. Fig. 3(d) shows the magnified morphology of the fine
particles, and the corresponding chemical composition is found to be 4.9 at.% and 85.1
at.% for Ti and Al elements, respectively, suggesting to be a solidified Al melt on the
Ti₂AlC substrate. The formation of the TiAl₃ compound is most probably considered to
be due to the decomposition of the Ti₂AlC ceramics thereby providing a source of Ti
element. However, this reaction does not improve the wettability of the Al filler on the
Ti₂AlC ceramics, because the reaction product, the TiAl₃ compound, comes into during
the solidification period. It is worthwhile pointing out that the additive Si element is
able to diffuse into the Ti₂AlC substrate during the wetting experiment. The diffusion
behavior is likely to be favorable for promoting the interaction between the melt and
the Ti₂AlC substrate, which accounts for the smaller contact angle at the initial wetting
stage. However, the formation of the Ti₂(Al, Si)C promotes little to the wetting
behavior of the AlSi filler on the Ti₂AlC ceramics, and fails to alter the non-wetting
phenomenon of the Al melt on the ceramic surface, whose contact angle remains
around 135° during the whole wetting experiment.
<figure><img src="image_4.png" /><figcaption>Fig. 3. Morphology of the Al/Ti₂AlC sample: (a). cross-section; (b). triple line; (c). surface morphology of the reacted area; (d). magnified morphology of the particles.</figcaption></figure>

4. Conclusions

In summary, the two kinds of brazing fillers examined in this present work possess different wettability on the Ti₂AlC ceramics. The nickel based filler alloys are wettable, with the contact angle reaches as low as 20° finally, which is mainly ascribed to the interaction between Ni filler and Ti₂AlC substrate. On the contrary, the aluminum based filler alloys are non-wettable on the ceramic surface, whose contact angle remains higher than 135° during the whole wetting experiments. The weak interaction between Al filler and Ti₂AlC substrate should be blamed for the poor wettability. Therefore, the active Al atoms in the Ti₂AlC structure are considered to be
the major factor promoting the wetting process, as a result, neither the active brazing
elements nor the surface pretreatment are required for the wetting and brazing of
Ti₂AlC ceramics in most cases, and those fillers which are reactive to the Ti₂AlC
substrate, for example, the nickel based fillers in this work, are more suitable for the
brazing of Ti₂AlC ceramics.

Acknowledgements

This work was financially supported by the National Natural Science Foundation
of China (NSFC) under Grant Nos. 51372049, 51621091 and U1537206. The author
gratefully acknowledges the funding from China Scholarship Council (CSC).

References

[1] M.W. Barsoum, D. Brodkin, T. El-Raghy, Scripta Mater. 36(5) (1997) 535-541.

[2] M. Radovic, M.W. Barsoum, Am. Ceram. Soc. Bull. 92(3) (2013) 20-27.

[3] M.W. Barsoum, Prog. Solid State Chem. 28(1-4) (2000) 201-281.

[4] Z.M. Sun, Int. Mater. Rev. 56(3) (2011) 143-166.

[5] J.L. Smialek, Oxid. Met. 83(3-4) (2015) 351-366.

[6] S.R. Kulkarni, A. Wu, J. Alloy. Compd. 490(1-2) (2010) 155-159.

[7] Y.L. Bai, H.X. Zhang, X.D. He, et al, Int. J. Refract. Met. Hard Mat. 45 (2014)
58-63.

[8] R.E. Loehman, A.P. Tomsia, Am. Ceram. Soc. Bull. 67(2) (1988) 375-380.

[9] A. Ganguly, M.W. Barsoum, R.D. Doherty, J. Am. Ceram. Soc. 90(7) (2007) 2200-2204.
[10] D.J. Tallman, J. Yang, L. Pan, et al, J. Nucl. Mater. 460 (2015) 122-129.

[11] N. Eustathopoulos, M.G. Nicholas, B. Drevet, Wettability at high temperatures,
Elsevier1999.

[12] W.K. Pang, I.M. Low, B.H. O'Connor, et al, J. Alloy. Compd. 509(1) (2011) 172-176.

[13] S.K. Tung, L.C. Lim, M.O. Lai, Scripta Mater. 34(5) (1996) 763-769.