Spontaneous pulverisation of Al 4 C 3 containing MAX bulk ceramics at room temperature

Abstract

The deleterious effect of Al₄C₃ on the performance of MAX ceramics has not attracted much attention. In the present study, Al₄C₃ containing Cr₂Al(Si)C and SiC/Cr₂AlC ceramics have been prepared. It was found that spontaneous pulverisation of the as synthesised samples took place at room temperature after exposure in air and water for various durations. Volume expansion induced by the interaction of Al₄C₃ with moisture is held responsible for the disintegration of Al₄C₃ containing MAX ceramics. Preventive measures to avoid the formation of Al₄C₃ upon preparing MAX ceramics were herein proposed.

Keywords

MAX ceramics Pulverisation Microstructure Mechanism

Introduction

MAX phases (M denotes an early transition metal, A is a mostly IIIA or IVA group element and X is either C or N), including the layered ternary carbides and nitrides, have attracted much attention owing to their attractive properties at room and high temperatures. Some Al containing MAX phases such as Ti₂AlC, Cr₂AlC, Ti₃AlC₂ and Ti₃Al(Si)C₂ exhibit good resistance to thermal shock, oxidation and corrosion.^1–12 In addition, the MAX phases can be easily prepared by different processing methods from various starting mixtures. The attractive properties, together with the ease of fabrication of the MAX materials, make them promising candidates for high temperature applications such as gas turbines, heat exchangers, solid oxide fuel cells and nuclear reactors.

To prepare Al containing MAX materials, Al elemental powder is generally used in starting mixtures. However, the easy loss of Al (melting point: 630°C) at high temperatures induced unwanted impurities in the MAX products. These impurities are generally binary carbides such as TiC, Al₄C₃ and Cr₇C₃, and they impair some properties of the MAX materials. To improve the purity of MAX, Al₄C₃ has been used to replace Al in starting mixtures because, at sintering temperatures, Al₄C₃ can provide both Al and C atoms in the system of M–Al–C, and also Al₄C₃ cannot be easily lost due to its higher melting point (2200°C) than Al. The Al containing MAX phases and their solid solutions such as (Ti_1−xNb_x)₂AlC,¹³ Ti₃Si_1−xAl_xC₂,¹⁴ Cr₂AlC,^15,16 Ti₂AlC,^17,18 Ti₃AlC₂,¹⁹ ZrAl₄C₄,²⁰ Zr₂Al₄C₅ and Zr₃Al₄C₆,²¹ Nb₂AlC and (Ti,Nb)₂AlC²² have been consequently synthesised.

Although Al₄C₃ is a desirable component in starting mixtures to prepare the abovementioned MAX materials, yet it is hygroscopic, unavoidably introducing another impurity of Al₂O₃ during mixing.¹⁹ In addition, if Al₄C₃ appears as an unwanted impurity in the MAX bulk ceramics, it will impair the electrical conductivity, self-lubricating ability and oxidation resistance of the MAX materials. Furthermore, the moisture sensitive Al₄C₃ may result in the surface degradation or even complete failure of the MAX materials. It has been reported that the appearance of Al₄C₃ is detrimental to the mechanical integrity of metal matrix composites.^23–26 However, work on the potentially detrimental influence of Al₄C₃ on the structural integrity of MAX ceramics has been much less focused.

Therefore, in the present study, the main purpose was to investigate the harmful effect of Al₄C₃ on MAX materials.

Experimental

In the present study, a Cr₂Al(Si)C ceramic containing Al₄C₃ was deliberately prepared. Powders of Cr (particle size: <75 μm; 99·50% purity), Al (particle size: ∼70 μm; 99·5% purity), Si (particle size: <45 μm; 99·9% purity) and graphite (C; particle size, <45 μm; 99·7% purity) were used as a starting mixture. The powders of Cr, Al, C and Si with a molar ratio of 2∶1·1∶1∶0·3 were mixed for 10 h. The mixed powders were put into a graphite die coated with boron nitride and then hot pressed at 1450°C for 1 h under 30 MPa in Ar atmosphere. A 20 vol.-%SiC/Cr₂AlC composite containing Al₄C₃ was prepared by hot pressing a mixture of Cr₂Al, C and SiC powders at 1400°C under 20 MPa for 1 h in Ar atmosphere.

The as synthesised Al₄C₃ containing Cr₂Al(Si)C sample was machined into bars. The bars were polished and exposed in air and water for different times. The 20 vol.-%SiC/Cr₂AlC sample with dimension of 36×25×6 mm was stored without any treatment in a plastic bag. The phase composition and microstructure of samples were characterised using X-ray diffraction (XRD) analysis with a D/Max 2200PC diffractometer (Tokyo, Japan) using Cu K_α radiation and a ZEISS EVO 18 scanning electron microscope (SEM, Carl Zeiss SMT, Germany) equipped with an energy dispersive spectroscopic system respectively.

Results

Figure 1 shows the diffractogram of the sample sintered at 1450°C. The synthesised sample consisted mainly of Cr₂Al(Si)C plus small amounts of Cr₅Si₃C_x and Al₄C₃. Previous work²⁷ showed that a Cr₂Al(Si)C ceramic with Cr₅Si₃, but no Al₄C₃, was obtained by hot pressing a mixture of 2Cr/1·1Al/0·2Si/C at the same conditions as for 2Cr/1·1Al/0·3Si/C. Hence, the appearance of Al₄C₃ should be induced by extra Si in the mixture of 2Cr/1·1Al/0·3Si/C during hot pressing.

X-ray diffraction pattern of synthesised Cr₂Al(Si)C sample

Figure 2 shows a typical microstructure of the Cr₂Al(Si)C sample. Small particles (black) were Al₄C₃ distributing in the matrix. Most Al₄C₃ grains are needle-like. Some relatively larger black particles were Al₂O₃, which was not detected by XRD due to its low content below the detection limit of XRD. Al₂O₃ should be formed at high temperatures from a reaction between Al and oxygen absorbed at the original powders.

Image (SEM) of polished surface of Cr₂Al(Si)C sample

After exposure of the polished specimens in air for 3 days, small pits on the polished surfaces was observed by naked eyes. A typical backscattered SEM image clearly shows the detail of the pits (Fig. 3). In the pits, there are fine particles (black), which should be induced by the hydrolysis of Al₄C₃. The fine particles were identified by energy dispersive spectroscopy, with only appearance of Al and O. However, it is reasonable to speculate that these fine particles may be an aluminum hydroxide, Al(OH)₃, because Al₄C₃ is hygroscopic and easily reacts with moisture in atmosphere to form Al(OH)₃. This phase has been further confirmed by XRD as shown in Fig. 5a. The above feature gives a clear evidence of the material degradation.

Backscattered SEM image of polished surface of Cr₂Al(Si)C sample after exposure in air for 3 days

Photographs of Cr₂Al(Si)C samples after exposure in air for a 1 month and b 2 months and c in water in beaker for 1 week

a X-ray diffraction pattern and b backscattered SEM image of pulverised powder; inset in b showing delamination and formation of kink bands in broken grain

To further study the detrimental influence of Al₄C₃ on the MAX materials, the polished specimens were deliberately exposed in air and water respectively at ambient temperature for various durations. It is worth noting that the polished specimen got loose and became part of the powder after exposure in air for 1 month (Fig. 4a) and completely transformed to powder after exposure in air for 2 months (Fig. 4b). More interestingly, as shown in Fig. 4c, the specimen was quickly pulverised as immersed in water for only 1 week, indicating that direct exposure in water promotes the hydrolysis rate of Al₄C₃.

The pulverised powder is composed of Cr₂Al(Si)C, Cr₅Si₃C_x and Al(OH)₃, with no evidence of Al₄C₃ (Fig. 5a). The disappearance of Al₄C₃ indicates the complete hydrolysis of Al₄C₃ to Al(OH)₃. Figure 5b shows the microstructure of the pulverised powder without any grinding. It should be noted that some Cr₂Al(Si)C grains have been broken into small pieces. Delamination and kink bands were found in some broken grains as shown in the inset in Fig. 5b. The above feature indicates that the Cr₂Al(Si)C grains suffered large compressive stresses induced by the hydrolysis of Al₄C₃. The Cr₂Al(Si)C solid solution has a hexagonal structure with weak bonding between nanolaminates and a high axis ratio of c/a = 4·5.²⁷ As the direction of compressive stresses is parallel to the basal planes of Cr₂Al(Si)C, the easy slip along the basal planes and the formation of kink bands occurs. Deformation of the MAX phases with delamination and kink band formation is always observed at room temperature under the applied stresses, and the main mechanism has been discussed in detail.^28,29

Additionally, it was found that the 20 vol.-%SiC/Cr₂AlC sample containing rod-like Al₄C₃ grains (Fig. 6a) was also completely pulverised into powder after exposure in air for 2 months (Fig. 6b). This further proves the severe detrimental effect of Al₄C₃ on the mechanical integrity of the MAX materials.

a backscattered SEM image of polished surface of 20 vol.-%SiC/Cr₂AlC and b photograph of 20 vol.-%SiC/Cr₂AlC after exposure in air for 2 months

Discussion

Hydrolysis of Al₄C₃ is unavoidable when exposed to moisture. A hydrolysed product, Al(OH)₃, maybe formed according to equation (1)²⁶ 1

In the above reaction, 1 mol Al₄C₃ produces 4 mol Al(OH)₃. Based on their theoretical densities of 2·4 g cm⁻³ for Al₄C₃ and 2·36 g cm⁻³ for Al(OH)₃, 1 V (unit volume) of Al₄C₃ introduces ∼2·2 V of Al(OH)₃, leading to a 120% volume expansion. In addition, larger volume expansion would be achieved if Al(OH)₃ completely transformed to AlO(OH).αH₂O because AlO(OH).αH₂O has a porosity of 90% and a low density of about 0·23–0·25 g cm⁻³.^23,30 Therefore, interaction of Al₄C₃ with moisture causes the volume expansion, which correspondingly leads to internal stresses in the bulk materials. The internal stresses make the pulverisation of MAX bulk samples.

Based on the above discussion, it is clear that the hydrolysis of Al₄C₃ leads to the spontaneous pulverisation of MAX materials. To guarantee the structural integrity and service life of the MAX materials used as key ceramic components, formation of Al₄C₃ must be prevented upon preparing the Al containing MAX materials. Hence, it is necessary to consider the following conditions.

First is the adoption of small C particles in starting mixtures upon preparing MAX materials. Al₄C₃ was found around larger C particles that were not completely consumed during sintering a Cr/Al/C mixture for 1 h to obtain Cr₂AlC (Fig. 7). Small C particles in the starting mixtures can enhance the reactivity and improve purity of MAX materials. Second is the careful selection of second phases as reinforcing agents in the MAX composites. If the second phases react with MAX matrixes, unwanted impurities will be introduced. For example, the presence of Al₄C₃ around SiC as shown in Fig. 6a is due to reaction of SiC with Cr₂Al or Cr₂AlC. Third, optimisation of sintering temperatures and dwelling times is also an effective way to avoid the formation of Al₄C₃ in MAX materials.

Image (SEM) of Al₄C₃ grains around unconsumed C particle after hot pressing mixture of Cr, Al and C powders

In spite of the deleterious effect of Al₄C₃ stressed here, the hydrolysis of Al₄C₃ may be useful in practical applications. For example, it may be an effective mould release agent to separate ceramic or metal samples from graphite moulds after exposure in atmosphere or in water for certain durations. In addition, the hydrolysed product of Al₄C₃ is a desirable precursor for making a nanosized Al₂O₃ film³¹ and also a good flame retardant.

Conclusions

Al₄C₃ containing Cr₂Al(Si)C and SiC/Cr₂AlC samples have been spontaneously pulverised after exposure in air for 2 months, whereas pulverisation has been promoted as exposure in water for only 1 week. The driving force for spontaneous pulverisation of Al₄C₃ containing MAX phases is the internal stresses caused by volume expansion due to the fact that 1 V Al₄C₃ reacts with moisture to produce 2·2 V Al(OH)₃. This mechanism can be also used to explain the degradation of other materials containing Al₄C₃. To prevent the formation of Al₄C₃ in MAX materials, some preventive measures have been proposed. Furthermore, potential uses of Al₄C₃ as a mould release agent, a precursor to make nanosized film and a flame retardant in practical applications have been expected.

Footnotes

Acknowledgements

This work was supported by the National Natural Science Foundation of China under grant nos. 51172015 and 51372015, and SRF for ROCS, SEM and ‘Hongguoyuan’ Talent Foundation of Beijing Jiaotong University.

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