Original ArticleOptimized mechanical properties and oxidation resistance of low carbon Al2O3-C refractories through Ti3AlC2 addition
Introduction
Carbon-containing refractories, including Al2O3-SiC-C, MgO-C, Al2O3-C, and Al2O3-ZrO2-C refractories are necessary applied materials for iron and steel making processes [1,2]. Generally, the high content of graphite (8−20 wt.%) is vital to ensure good slag corrosion resistance and thermal shock resistance [3,4]. However, low/ultra-low carbon-containing refractories (usually less than 3 wt.% of C) were highly demanded in recent years owing to the urgent requirements of clean steel smelting and low-carbon economy [5,6]. Therefore, the importance of oxidation resistance of low carbon-containing refractories was raised due to the reduction of carbon content [7]. Generally, the antioxidants such as commercial silicon (Si), aluminum (Al), and boron carbide (B4C) have drawn concentrated attention due to their balanced performance and price [8,9].
On the one hand, the antioxidants protect the carbon from oxidation; on the other, they can induce ceramic phases. Si can react with carbon in the coke bedded atmosphere to form chemically stable high-strength SiC whiskers that contribute to excellent high-temperature mechanical properties [10]. The Si content and proper oxygen partial pressure (p(O2)) dominate the formation of SiC whiskers [11,12], as too high p(O2) often lead to SiO(g) escape and abnormal growth of whisker, that is harmful to the mechanical properties [[13], [14], [15]]. Therefore, Al is usually added in combination with Si to decrease the p(O2). Meanwhile, Al can react with C, CO, and N2 to form ceramic reinforcements such as Al4C3, AlOC, and AlN at elevated temperatures [16,17]. Still, above aluminum carbides and nitrides are hydratable, causing the disintegration during processing and storage [18]. Thus, the use of Al needs an elaborate design. Again, B4C is another antioxidant that can protect the carbon from oxidation via the generation of byproducts such as borosilicate and aluminum borate [15]. Besides, B4C can also catalyze the formation of in-situ carbon nanotubes and nano onion carbon, improving mechanical properties and thermal shock resistance of Al2O3-C refractories [19]. Although the combination of the above antioxidants could satisfy the requirements, new additives are of great importance to guarantee good properties at elevated temperatures due to the massive escaping of vapors (SiO, Al, B2O3) would result in the structure degradation and strength loss [[13], [14], [15],18].
Researchers have been dedicatedly seeking new kinds of additives that can simultaneously improve oxidation resistance and mechanical properties at high temperatures. Recently, a layer compound Ti3AlC2, periodic planar stacking sheets of edge-sharing Ti6C octahedra and close-packed Al atoms along the c-axis, has been widely investigated due to its good mechanical properties, chemical stability, excellent oxidation and corrosion resistance [[20], [21], [22], [23]]. Moreover, the weak bonding between Ti6C octahedra layers and neighboring sheets of Al atoms enables the selective oxidation of Al [24,25], leading to a continuous protective Al2O3 layer formed on the surface of the Ti3AlC2 substrate [26,27]. Chen et al. adopted Ti3AlC2 to replace partially graphite in a relatively high carbon-containing system (4∼10 wt.%) and observed an improved slag resistance by the formation of a protective layer of CaAl2Si2O8–xAl2O3·yTiO2 [28,29]. Meanwhile, Ti3AlC2 showed no positive effects on improving the mechanical properties and thermal shock resistance in their system [30,31]. Although Ti3AlC2 has shown its application potential, the structure evolution of Ti3AlC2 and its specific influences on the microstructure evolution and the oxidation resistance of refractories at higher temperatures demand further investigation.
Inspired by the above works, the present work aimed to improve the mechanical properties and oxidation resistance by taking the advantages of lamellar Ti3AlC2 and volume expansion during oxidation. The phase, microstructure evolutions, mechanical properties, and oxidation resistance of the refractories were comparatively investigated. Furthermore, the in-situ structure evolution of Ti3AlC2 and related mechanisms were addressed in this work.
Section snippets
Materials and methods
The raw materials used for Al2O3-C refractories were tabular alumina (3.0∼0 mm, <75 μm, 98 wt.% Al2O3, Zhejiang Zili Holding Co. Ltd., China), reactive alumina powder (∼2 μm, 98 wt.% Al2O3, Henan Special Refractories Co. Ltd., China), silicon powder (45 μm, 98.47 wt.% Si, Anyang, China), flake graphite (<13 μm, 99 wt.% C, Qingdao Tianyuan graphite Co. Ltd., China), carbon black (N220, ∼25 nm, 99.5 wt.% C), aluminum powder (45 μm, 98.47 wt.% Al, Anyang, China), commercial Ti3AlC2 powder (<45 μm,
Phase compositions
The XRD patterns of Al2O3-C samples coked at different temperatures are shown in Fig. 1. After coking at 800 ℃, corundum (ICCD-01-088-0826), silicon (ICCD-01-089-5012), and graphite (ICCD-01-075-2078) were found in both cases, while intact Ti3AlC2 (ICCD-00-052-0875) and aluminum (ICCD-01-089-4037) were found in samples TAC and AL. At 1200 ℃, the peak intensities of Ti3AlC2 decreased in TAC, while aluminum disappeared in AL. Meanwhile, SiC (ICCD-01-074-2307) appeared at the cost of Si in both
Conclusions
The present work investigated the specific functions of Ti3AlC2 on the mechanical properties and oxidation resistance of low carbon Al2O3-C refractories. Based on the obtained results, the following conclusions can be drawn:
- (1)
Due to the selective oxidation of Al, Ti3AlC2 was gradually oxidized with increasing temperatures, leading to residual lamellar phases such as Ti3Al1-xC2 and TiC, and the formation of Al2TiO5. The controlled oxidation of Ti3AlC2 and its volume expansion contributed to the
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgments
The authors are grateful to the National Natural Science Foundation of China (51872211 and 51802230), the Special Project of Central Government for Local Science and Technology Development of Hubei Province (2019ZYYD003, 2019ZYYD076), and the Natural Science Funds of Hubei Province for the Distinguished Young Scholar, Wuhan, China, (2019CFA050).
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