Trans. Nonferrous Met. Soc. China 28(2018) 2011-2016

In situ formation of Zr₂Al₃C₄/Al₂O₃ composites by combustion synthesis with PTFE and thermal activations

Chun-Liang YEH, Yi-Chang CHEN

Department of Aerospace and Systems Engineering, Feng Chia University, 100 Wenhwa Road, Seatwen, Taichung 40724, Taiwan, China

Received 20 December 2017; accepted 8 March 2018

Abstract: Preparation of Zr₂Al₃C₄-Al₂O₃ in situ composites was investigated by self-propagating high-temperature synthesis (SHS) involving both aluminothermic reduction of ZrO₂ and chemical activation of PTFE (Teflon). The starting materials included ZrO₂, Al, carbon black and PTFE. In addition to the conventional SHS method, the experiments were conducted by a chemical-oven SHS (COSHS) route to thermally assist the synthesis reaction. The threshold amount of 2% (mass fraction) PTFE was required to induce self-sustaining combustion. When the conventional SHS scheme was utilized, due to low combustion temperatures between 1152 and 1272 °C and insufficient reaction time, the dominant carbide forming in the composite was ZrC instead of Zr₂Al₃C₄. On the other hand, the COSHS technique increased the combustion temperature of the reactant compact to about 1576 °C, lengthened the high-temperature duration for the reaction, and prevented Al vapor from escaping away. As a consequence, Zr₂Al₃C₄-Al₂O₃ composites with a small amount of Zr₃Al₃C₅ were obtained. The microstructure of the COSHS-derived product showed that plate-like Zr₂Al₃C₄ grains were about 2 μm in thickness and 10-30 μm in length, and most of them were closely stacked into a laminated configuration.

Key words: Zr₂Al₃C₄; combustion synthesis; PTFE activation; chemical-oven SHS; aluminothermic reduction

1 Introduction

Ternary aluminum carbides, such as Ti₂AlC and Ti₃AlC₂, known as the MAX phase, have low density, high stiffness, high thermal conductivity, and good oxidation and corrosion resistance at high temperatures, as ceramics; while they are relatively soft, readily machinable, electrically conductive, resistant to thermal shock, and highly tolerable to damage, as metals [1-3]. In the Zr-Al-C ternary system, Zr₂Al₃C₄ has a hexagonal crystal structure [4] and possesses much better oxidation resistance than ZrC and comparable high-temperature mechanical properties to ultra-high temperature ceramics (UHTCs), such as ZrB₂-SiC and HfB₂-SiC composites [5]. As a conductive ceramic, Zr₂Al₃C₄ is a promising reinforcement for Cu without any detrimental effect on the electrical conductivity [6].

Zr₂Al₃C₄ was first synthesized by SCHUSTER and NOWOTNY [7] via arc melting of a mixture of Zr or ZrC, Al, and C powders at 1000 °C. Afterwards, FUKADA et al [8] produced Zr₂Al₃C₄ together with small amounts of Al₂O₃ and Zr₃Al₃C₅ by heating a compressed pellet made up of ZrC, Al, and graphite at 1380 °C for 1 h in Ar. HE et al [9] fabricated bulk Zr₂Al₃C₄ from Zr, Al, and graphite powders by in situ reactive hot pressing at 1900 °C under 30 MPa for 1 h in Ar and then annealing at 1600 °C for 0.5 h in low vacuum. Preparation of Zr₂Al₃C₄ thin films was recently conducted by pulsed cathodic arc deposition from elemental Zr, Al and carbon cathodes [10]. With the use of Zr, Al, graphite and B₄C as the starting materials, Zr₂Al₃C₄-ZrB₂ composites were produced by spark plasma sintering [11]. The addition of ZrB₂ was shown to improve hardness, elastic modulus, fracture toughness, thermal conductivity, and oxidation resistance of Zr₂Al₃C₄ [11].

As a promising alternative, combustion synthesis in the mode of self-propagating high-temperature synthesis (SHS) takes advantage of the self-sustaining merit from the highly exothermic reaction, and hence, has the benefits of low energy requirement, short reaction time, high-purity products, and simple facilities [12,13]. The SHS technique has been applied to preparing the MAX carbides, Ti₃AlC₂, Ti₂AlC and Ta₂AlC, from elemental powder compacts [14-16]. For the improvement in fracture toughness, flexural strength and hardness of ternary aluminum carbides, Al₂O₃ has been demonstrated as one of the effective reinforcing phases [17-19]. Previous studies showed that the SHS method involving aluminothermic reduction of metal oxides represented an in situ processing route capable of producing Al₂O₃- reinforced Ti₃AlC₂, Ti₂AlC, Nb₂AlC and NiAl [20-22]. Due mainly to weak exothermicity of reduction of ZrO₂ by Al, however, the Zr₂Al₃C₄-Al₂O₃ composite has not been investigated by the SHS process.

This work made the first attempt to prepare the Zr₂Al₃C₄-Al₂O₃ composite by combustion synthesis with a reduction stage. In order to chemically promote the SHS reaction, we adopt PTFE (polytetrafluoroethylene or Teflon) as an activator and carburizing agent. Besides, thermal activation is conducted by employing a modification of the conventional SHS method called chemical-oven SHS (COSHS), which provides extra heat flux to the sample and prolongs the high-temperature duration for the reaction [23,24]. In particular, the effects of chemical activation by PTFE and thermal boost by COSHS were investigated on combustion sustainability, combustion wave velocity, reaction temperature and product composition.

2 Experimental

The starting materials adopted in this work included ZrO₂ (Alfa Aesar Co., <45 μm, 99%), Al (Showa Chemical Co., <45 μm, 99.9%), carbon black (Showa Chemical Co., 20-40 nm, 99%), and PTFE (—(C₂F₄)_n—, Alfa Aesar Co., 6-10 μm). The mixture of reactant powders was formulated based upon the stoichiometry of Reaction (1):

                       (1)

where C_PTFE represents carbon supplied from complete decomposition of PTFE and is taken into account as a part in the total carbon balance. Reactant powders were well mixed and compressed into cylindrical samples with 7 mm in diameter, 12 mm in length, and a relative density of 55% for the test in the conventional SHS configuration. It was found that combustion ceased to propagate and quenched after ignition when no PTFE was added. The threshold amount of PTFE required for inducing self-sustaining combustion in Reaction (1) was experimentally determined to be 2% (mass fraction), which is correspondingly equivalent to x=0.54. As a reaction promoter, PTFE is able to peel off the Al₂O₃ shell on the surface of the Al particle and enhances the reactivity of Al powders [25]. As a carburizing agent, thermal decomposition of PTFE generates carbon (C_PTFE) that participates directly in the reaction [26,27]. The reactant compacts with 2%, 3%, and 4% (mass fraction) PTFE (i.e., x=0.54, 0.81, and 1.07) were conducted under the conventional SHS mode.

In the COSHS technique, composite test specimens were prepared that included a core reactant compact (7 mm in diameter and 10 mm in length) made up of the main mixture of Reaction (1) with 4% (mass fraction) PTFE and a shell composed of an equimolar Ti/C booster mixture. The resulting composite sample, as illustrated in Fig. 1, had a diameter of 12 mm and a height of 15 mm. Both SHS and COSHS experiments were performed in a windowed stainless-steel chamber under high purity argon (99.99%) of 0.15 MPa. A schematic diagram of the detailed experimental setup was presented elsewhere [28]. Ignition of either regular or composite samples was accomplished by a heated tungsten coil with a voltage of 60 V and a current of 1.5 A. The combustion process was recorded by a CCD video camera (Pulnix TMC-7). The combustion wave velocity was determined from the time sequence of recorded images. The combustion temperature was measured by a fine-wire (125 μm) Pt/Pt-13%Rh thermocouple attached on the sample surface. Phase constituents of the final product were identified by an X-ray diffractometer (Bruker D2) with Cu K_α radiation. The product microstructure was examined under a scanning electron microscope (Hitachi S3000N).

Fig. 1 Schematic diagram of composite sample for COSHS test

3 Results and discussion

3.1 Self-propagating combustion characteristics

Figure 2 presents a combustion sequence recorded from a 2% (mass fraction) PTFE-added powder compact ignited under the conventional SHS mode. As illustrated in Fig. 2, shortly after initiation, a localized reaction zone formed at the combustion front traveling along a spiral trajectory over the sample surface. It took about t=3.27 s for the combustion wave to propagate from the ignition plane to the bottom of the sample. The combustion front velocity (V_f) of about 3.18 mm/s was deduced from the time-sequence images. Moreover, the spinning combustion wave left visible corkscrew marks on the sample surface. The increase of PTFE increased the combustion velocity to V_f=3.68 mm/s at 3% PTFE and 4.31 mm/s at 4% PTFE, but did not change the combustion behavior. Figure 2 also reveals that the burned sample was subjected to axial elongation. This could be caused by the ejection of gaseous species from the powder compact, which cracked the burned sample. Consequently, due to fracture formation over the sample, the height of the sample was significantly increased. To be discussed below, gaseous species possibly formed from Reaction (1) are CO, F₂, and Al vapor. For the COSHS experiment, a burst of incandescent burning glow emitted almost volumetrically from the composite sample was detected. This suggests a highly energetic and fast combustion reaction occurring in the booster shell.

Fig. 2 Time sequence of self-propagating combustion images recorded from powder compact of Reaction (1) with addition of 2% PTFE

Propagation of self-sustaining combustion in a spinning manner is considered as an unstable SHS process. There are thermodynamic and kinetic reasons which provide the basis for departure from the steady condition [29]. Thermodynamic considerations arise from the degree of reaction exothermicity. Once the heat flux released from combustion is not enough to maintain longitudinal propagation of a planar reaction front, the combustion zone is confined to one or several reaction spots moving along a spiral path. Kinetic reasons are largely attributed to insufficient reactivity on account of the presence of diffusion barriers.

In Reaction (1), aluminothermic reduction of ZrO₂ with H_r=-24.8 kJ/mol of Al₂O₃ is weakly exothermic [30]. It is believed that for the PTFE-added sample, fluorine (F₂) generated from thermal decomposition of PTFE reacts with the reduced Zr and produces ZrF₄ along with a large formation enthalpy of H_f =-1911.3 kJ/mol [30], which has a great benefit to the SHS process. The highly reactive ZrF₄ subsequently reacts with carbon to yield ZrC and F₂. ZrC is an important intermediate for the evolution of Zr₂Al₃C₄ [9]. Moreover, the active interaction between PTFE and Al₂O₃ produces AlF₃ and CO, which removes the oxide shell on the Al particle surface [25]. As a result, the reactivity of Al and the degree of reduction of ZrO₂ were enhanced. The above PTFE-involving reactions confirm the role played by PTFE in the chemical activation.

3.2 Combustion temperatures of SHS and COSHS reactions

Combustion temperature profiles depicted in Fig. 3 were associated with PTFE-activated powder compacts conducted by the conventional SHS scheme. The rise in temperature signifies arrival of the combustion wave and the peak value corresponds to the combustion front temperature. After the passage of the combustion front, an appreciable decrease in temperature is a result of heat loss to the surroundings. The maximum temperature arrived at 1152 °C for the sample with addition of 2% PTFE and was further up to 1272 °C for the 4% PTFE-added sample. The increase of combustion front temperature with PTFE was attributed to a larger amount of ZrF₄ formed in the course of combustion synthesis. According to MUSA et al [27], the reaction exothermicity is enhanced by PTFE through formation of metal fluorides as intermediate phases. As mentioned above, fluorine (F₂) generated from PTFE reacts with the reduced Zr to produce ZrF₄ and a large reaction enthalpy [30]. However, the dwell time at the temperatures close to its peak value for the sample compact was very short and approximately 0.2-0.4 s.

Fig. 3 Combustion temperature profiles of PTFE-activated samples under conventional SHS mode

Figure 4 shows the combustion temperature profiles measured from the booster shell and core reactant compact of a COSHS composite sample. For the Ti/C booster mixture, a nearly vertical temperature gradient signified an extremely fast reaction rate and the peak temperature reached as high as 1722 °C. The temperature curve of the core reactant compact exhibited an increase to a peak value of 1576 °C followed by a decrease to a plateau region, where the sample temperature was retained at about 1455 °C for 2.0-2.5 s. After that, the sample temperature decreased at a slower rate compared to that experienced by the sample in the conventional SHS process. It is evident that with the aid of COSHS the synthesis reaction was thermally boosted.

Fig. 4 Combustion temperature profiles of booster shell and core reactant compact of composite sample under COSHS mode

3.3 Phase constituents and microstructures of synthesized composites

Figures 5(a) and (b) display the XRD patterns of final products obtained from PTFE-added powder compacts conducted in a conventional SHS process. Figure 5(a) indicates that for the 2% PTFE-added sample, the resulting composite consisted of ZrC and Al₂O₃ as the major constituents. The Zr-Al-C ternary phase Zr₂Al₃C₄ was poorly formed and some ZrO₂ was found as the remnant. This suggested that not only the phase transformation from ZrC to Zr₂Al₃C₄ was incomplete, but also aluminothermic reduction of ZrO₂ was unfinished. Formation of Zr₂Al₃C₄ is considered to involve the reaction of ZrC, Al, and C. It is believed that weak exothermicity of the reaction (T_c=1152 °C) and evaporation loss of Al during the SHS process are responsible for the poor formation of Zr₂Al₃C₄.

Fig. 5 XRD patterns of final products synthesized by conventional SHS from Reaction (1)

Figure 5(b) shows that for the sample containing 4% PTFE, the yield of Zr₂Al₃C₄ was slightly improved and the signature of ZrO₂ disappeared. This could be attributed to a higher combustion temperature of about 1272 °C. Other ternary carbide Zr₃Al₃C₅ was found in Fig. 5(b). According to HE et al [9], Zr₃Al₃C₅ was produced at high temperatures through the interaction of Zr₂Al₃C₄ with ZrC. Even though the formation of Zr₂Al₃C₄ was improved by increasing PTFE, ZrC was still the dominant carbide in the end product. This implied that evaporation loss of Al from the fissure-filled sample remained.

When the COSHS method was utilized, as revealed in Fig. 6, a Zr₂Al₃C₄-Al₂O₃ composite with a small amount of Zr₃Al₃C₅ was the end product. Compared with the conventional SHS process, the COSHS scheme substantially increased the reaction temperature (T_c=1576 °C) and prolonged the reaction time. Moreover, the booster shell could prevent Al vapor from escaping away. Therefore, phase evolution was practically completed.

Fig. 6 XRD pattern of final product synthesized by COSHS from Reaction (1) with 4% PTFE

Fig. 7 SEM micrographs illustrating fracture surface of SHS-derived products synthesized

The typical microstructures of fracture surface of the final products synthesized by the conventional SHS and COSHS processes are shown in Figs. 7(a) and (b), respectively. Figure 7(a) is associated with the product of Reaction (1) with 2% PTFE and exhibits granular ZrC crystals with a size of about 10-20 μm and fine bright Al₂O₃ particles of 2-5 μm. A COSHS product is displayed in Fig. 7(b), showing that the ternary carbide Zr₂Al₃C₄ has a different morphology from ZrC. The columnar or plate-like grains in Fig. 7(b) are Zr₂Al₃C₄ and fine bright particles of 1-2 μm are Al₂O₃. Zr₂Al₃C₄ grains are about 2 μm in thickness and 10-30 μm in length, and some of them are closely stacked into a laminated configuration. The morphology of Zr₂Al₃C₄ grains shown in Fig. 7(b) is similar to that observed by GUO et al [11].

4 Conclusions

1) Formation of the Zr₂Al₃C₄-Al₂O₃ composite was investigated by PTFE- and chemical oven-activated combustion synthesis. The starting materials included ZrO₂, Al, and carbon black powders, and 2%-4% (mass fraction) PTFE that was employed as a reaction promoter and a carburizing agent. Experiments were conducted under the conventional SHS and COSHS configurations. By means of the conventional SHS scheme, self-sustaining combustion was characterized by a localized reaction zone propagating in a spinning mode. With the increase of the PTFE content from 2% to 4%, the combustion front velocity increased from 3.18 to 4.31 mm/s and combustion temperature from 1152 to 1272 °C. However, the as-synthesized product was a ZrC-Al₂O₃ composite with a trivial amount of Zr₂Al₃C₄. This was attributed to the low combustion temperature, insufficient reaction time, and evaporation loss of Al.

2) The COSHS method employed a composite sample consisting of a Ti/C booster shell and a core reactant compact. Combustion of the Ti/C mixture provided the core sample with a large amount of heat to raise its reaction temperature up to 1576 °C. In addition, the outer shell prolonged the dwell time at high temperatures for the core sample and prevented Al vapor from escaping away. Formation of Zr₂Al₃C₄ was greatly improved by COSHS and the as-synthesized product was a Zr₂Al₃C₄-Al₂O₃ composite with Zr₃Al₃C₅ as the minor phase. Zr₂Al₃C₄ grains had a plate-like morphology with a thickness of about 2 μm and length of 10-30 μm. Most of the long Zr₂Al₃C₄ laminas were tightly stacked and some were closely interlocked.

Acknowledgements

This research was sponsored by the Ministry of Science and Technology of Taiwan, China, under the grant of MOST 105-2221-E-035-039-MY2. Authors are grateful for the Precision Instrument Support Center of Feng Chia University in providing materials characterization facilities.

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结合PTFE与热激化的燃烧合成原位生成Zr₂Al₃C₄/Al₂O₃复材

Chun-liang YEH, Yi-Chang CHEN

Department of Aerospace and Systems Engineering, Feng Chia University, 100 Wenhwa Road, Seatwen, Taichung 40724, Taiwan, China

摘  要：采用自蔓延燃烧合成(SHS)结合ZrO₂铝热还原与PTFE化学激化制备Zr₂Al₃C₄–Al₂O₃复材。原料包括氧化锆、铝、碳黑和PTFE粉末。除了传统的SHS方法外，合成实验以化学炉辅助的燃烧合成法(COSHS)进行。实验发现，达成自蔓延燃烧的最低PTFE含量为2%(质量分数)。当使用传统SHS方法时，由于燃烧温度低(1152~1272 °C)且反应时间不足，合成复材中主要的碳化物为ZrC，而不是Zr₂Al₃C₄。但是，当应用COSHS技术时，燃烧温度可提高至1576 °C，而且反应在高温下的时间增长，同时可避免铝蒸气的流失；因此，可以成功制备内含少许Zr₃Al₃C₅的Zr₂Al₃C₄-Al₂O₃复材。Zr₂Al₃C₄晶粒为平板状，厚度约2 μm，长度约10~30 μm，多数晶粒紧密堆栈成层状微结构。

关键词：Zr₂Al₃C₄；燃烧合成；PTFE激化；化学炉燃烧合成；铝热还原

(Edited by Bing YANG)

Corresponding author: Chun-liang YEH; Tel: +886-4-24517250; E-mail: clyeh@fcu.edu.tw

DOI: 10.1016/S1003-6326(18)64845-9