Self-propagating high-temperature synthesis of (Zr,W)C/WC/Al2O3 composite powders from WO3-ZrO2-Al-C system

https://doi.org/10.1016/j.ijrmhm.2019.05.006Get rights and content

Highlights

  • (Zr,W)C/WC/Al2O3 composite was produced by SHS reaction of 2(1-x)WO3-3xZrO2-Al-2(1 + 0.5x)C system.

  • (Zr,W)C powder was synthesized by the SHS aluminothermic reaction of ZrO2 for the first time.

  • The (Zr,W)C/WC ratio varied in the composite by altering the WO3/ZrO2 ratio of the reactants.

Abstract

In the present study, the self-propagating high-temperature synthesis of 2(1-x)WO3-3xZrO2-Al-2(1 + 0.5x)C system was considered to prepare WC-ZrC-Al2O3 composite powders. The effects of an increase in the x-value between 0 and 1 as well as the addition of 50 wt% extra carbon to the initial reactant mixture on the adiabatic temperature, reaction front velocity, structure and microstructure of the combustion products were investigated. Results showed that the adiabatic temperature and the reaction front velocity decreased with increasing the x-value until it reached zero for x = 0.5. For x = 0 sample, WC, W2C and W phases were formed after the synthesis process. The addition of extra carbon led to an increase in the (WC + W2C)/W ratio. Furthermore, (Zr,W)C/WC/W2C/Al2O3 composite powders were obtained for x > 0. Moreover, the (Zr,W)C concentration with near-stoichiometric composition and blocky morphology increased with the x-value. Differential thermal analysis results illustrated that the SHS reaction in the WO3-ZrO2-Al-C system is initiated by the aluminothermic reaction of WO3. Moreover, it was postulated that the presence of W facilitated the formation of zirconium carbide.

Introduction

Zirconium carbide (ZrC) is a typical ultra-high temperature ceramic with considerable hardness (25.5 GPa) that is used for making cutting tools [1,2]. In addition, owing to the resistance to irradiation damage, this carbide is implemented as field emitters and coating of nuclear particles [3,4]. Unfortunately, the strong covalent bond and low self-diffusion coefficient deteriorate its sinterability which can diminish its application [1]. The sinterability can be improved through the fabrication of ZrC containing composites [5,6].

Zirconium carbide is usually produced by carbothermal reduction of zirconium oxide due to the affordable cost of the primary materials and simplicity of the process [1]. However, the high temperature required for the process leads to the unwanted grain growth of the final powder which in turn, degrades its mechanical properties [1,7]. During the last two decades, there are some attempts to synthesize ZrC by low-temperature methods such as sol-gel [8] or fast high-temperature methods like the self-propagating high-temperature synthesis (SHS) to inhibit the grain growth during the synthesis process [[9], [10], [11], [12], [13], [14], [15]].

The self-propagating high-temperature synthesis is a facile route to synthesis various ceramic compounds like carbides [[16], [17], [18], [19], [20]], nitrides [21,22], borides [[23], [24], [25]], etc. Lower cost, simplicity, energy-saving property and short reaction time are the main advantages of this process [24].

Therefore, SHS is a promising route to obtain ZrC containing composites. For example, Tsuchida and Yamamoto [10] fabricated ZrC-ZrB2 composite by mechanically activated SHS reaction of Zr-B-C system. In another research, Song and co-workers [9,14] synthesized ZrC by the SHS reaction of Zr-Al-C system. They investigated the effect of Al concentration on the particle size of zirconium carbide. It is noteworthy that the cost of the SHS product will decrease if pure Zr is replaced with the inexpensive zirconium containing precursors such as zirconium oxide. In this case, Al or Mg is usually used to reduce zirconium oxide to metallic Zr. Then, ZrC is obtained by the reaction of Zr with C. Depending on whether Al or Mg is used the reaction is called aluminothermic reaction or magnesiothermic reaction, respectively. A few articles have been published on the magnesiothermic reduction of ZrO2 for the synthesize of ZrC. Li et al. [11] produced ZrC particles through SHS reaction of ZrO2-Mg-C system. They investigated the effect of Mg on the purity of the final product. In another research, Fe-ZrC composite was produced by the SHS reaction of ZrO2-Mg-C-Fe system by Ebrahimi-Kahrizsangi et al. [12]. Wang et al. [13] fabricated nanocrystalline ZrC by the low-temperature reaction between Mg powder with ZrO2 and lithium carbonate. They showed that the product had a good oxidation resistance below 250 °C in the air. To the best of our knowledge, there is no report on the SHS synthesis of ZrC by the aluminothermic reaction of ZrO2 in the open literature. It may be due to this fact that the liberated heat from this reaction is not high enough and therefore, it cannot propagate in the self-sustaining mode. On the other hand, some SHS systems like WO3-Al-C are extremely exothermic and liberate considerable amount of heat [17]. Thus, it seems that if these two systems are merged, the extra heat delivered by the WO3-Al-C system will help the ZrO2-Al-C system to propagate in the self-sustaining mode. Therefore, simultaneous synthesis of ZrC and WC powders using aluminothermic reaction of ZrO2 and WO3 is studied in the present work. In this regard, the SHS reaction of the WO3-ZrO2-Al-C system is investigated for the fabrication of WC-ZrC-Al2O3 composite powders with different WC/ZrC ratios.

Section snippets

Experimental procedure

Pure aluminum (Rankem A2265), tungsten trioxide (Alfa Aesar 11,828), zirconium oxide (Aldrich 230,693) and activated carbon (Merck 102,184) powders were used as the starting materials. The powders were weighed and mixed according to the following reaction:21xWO3+3xZrO2+4Al+21+x2C=21xWC+3xZrC+2Al2O3

Two sets of samples were prepared based on reaction 1. In the first set, the x-value varied between 0 and 1 (0 ≤ x ≤ 1) with 0.1 intervals. In the second set, 50 wt% extra carbon was added to the

Adiabatic temperature and combustion front velocity

In the SHS systems, the reaction is usually initiated in a small area of the compacted mixture. Subsequently, it propagates along the compact which leads to the conversion of the reactants to the products. Merzhanov [18] proposed an experimental criterion which is based on the thermodynamic data and usually is used to predict the stability of the reaction front in the SHS systems. According to this criterion, an exothermic reaction can propagate in the self-sustaining mode if its adiabatic

Conclusion

The aim of the present study was to synthesize WC/ZrC/Al2O3 composite by self-propagating high-temperature synthesis of 2(1-x)WO3-3xZrO2-Al-2(1 + 0.5x)C system. The x-value and the addition of 50 wt% extra carbon to the stoichiometric compositions were considered as the influencing parameters to increase the WC and ZrC concentrations in the final composites. The main results can be summarized as follows:

  • 1.

    The adiabatic temperature and the SHS reaction front velocity declined with increasing the x

Acknowledgment

The authors acknowledge Mr. A.R. Gardeshzadeh for proofreading of the manuscript.

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