How Do Carbon Fibers Enhance Properties Of Alumina-Carbon Refractories For Continuous Casting?

Aluminium-carbon refractories are widely used in functional refractory products for long-lasting flow control and protection, such as continuous casting spouts, plugs, and sliding plates, due to their excellent mechanical properties, thermal shock resistance, and slag erosion resistance. In recent years, as the requirements for clean steel, low-carbon steel, ultra-low-carbon steel, and other special steel grades have continued to increase, the development of high-performance, multi-functional, and long-life carbon-containing refractory materials to meet the demands of new steel types and continuous casting technologies has become an urgent task.

To improve the service performance of refractories, researchers have enhanced and toughened these materials through the optimized design of the carbon chain structure and the generation of ceramic phases. Among these methods, the incorporation of carbon fibres has been considered a viable approach.

Carbon fibre not only offers excellent heat and corrosion resistance but also retains its strength without degradation in high-temperature inert environments above 2000 °C, thanks to its inherent flexibility. As a result, carbon fibre composites are widely used across various fields, including for reinforcing and toughening ceramic-based and metal-based composites.

For example:

Shao Binbin et al. introduced carbon fibres into C/SiC-based ceramic materials and investigated their dynamic mechanical properties.

Yang et al. incorporated carbon fibres into zirconium diboride-based ceramics and observed a significant improvement in fracture toughness.

Song Wentao applied carbon fibre-reinforced braided mesh composites to concrete beams, resulting in varying degrees of improvement in all stress-related properties.

Nasiri et al. prepared ZrB₂-SiC-Csf nanocomposites using pressureless sintering. The results showed that when 2.5% carbon fibre was added, both hardness and fracture toughness improved.

Li Chunhua used carbon fibre-reinforced nylon 6 to significantly enhance the mechanical properties of the composites.

Luo Sanfeng and Zhang Hong introduced carbon fibres into aluminium-carbon materials and found a clear reinforcing and toughening effect.

Gao Hua and Luo Ming added carbon fibres to low-carbon magnesium-carbon bricks, significantly improving oxidation resistance, strength at room and high temperatures, and thermal shock resistance.

However, it has been found that the uniform dispersion of carbon fibres significantly affects their ability to enhance material properties. Carbon fibres are highly susceptible to fracture under transverse shear stress and tend to agglomerate due to their high surface energy. As a result, achieving a homogeneous distribution of chopped carbon fibres within the matrix during composite preparation is quite challenging.

Currently, the dispersion methods for carbon fibres in functional carbon fibre composites include dry mixing, wet mixing, and alcohol pre-dispersion. However, these methods have several drawbacks, such as the tendency of carbon fibres to agglomerate, poor dispersion quality, complex dispersion procedures, and strict requirements regarding fibre length.

In response to these challenges, this paper introduces a new approach to dispersing carbon fibres during material preparation. In this method, carbon fibres are directly incorporated into a binder that has good affinity with the carbon surface. Ultrasound is then applied to achieve uniform dispersion, simplifying the pre-dispersion process and effectively separating individual filaments of chopped fibres. This ultrasonic dispersion method also minimizes the risk of damage to the carbon fibres.

In this study, we investigate the effect of different amounts of chopped carbon fibres on the properties of aluminium-carbon refractories. We compare and analyse changes in bulk density, apparent porosity, room-temperature flexural strength, compressive strength, and thermal shock resistance of the specimens after high-temperature treatment.

Experimental Part

1.1 Raw Materials and Specimen Preparation

The main raw materials used in this experiment were electrofused brown corundum (with particle sizes of 30, 70, and 200 mesh), flake graphite (C > 94%, 80 mesh), chopped carbon fibres, monolithic silica powder (Si > 98%), and silicon carbide (SiC > 98%). Thermosetting phenolic resin was used as the binder.

The properties of the chopped carbon fibres used are listed in Table 1, and their SEM (scanning electron microscope) morphology is shown in Fig. 1. The specimens were labeled CA to CE, corresponding to carbon fibre contents ranging from 0 to 0.4%. The detailed raw material compositions for each specimen are shown in Table 2.

Fig. 1 SEM morphology of carbon fibre

Table 1 Performance indicators of carbon fibres

Table 2 Raw material composition of the specimens

The dry powder materials for each component were weighed according to the ingredient ratios listed in Table 2 and then pre-mixed. The chopped carbon fibres and thermosetting phenolic resin were also weighed according to the specified ratios and mixed using ultrasonic stirring.

The pre-mixed powders were then combined with the binder and mixed at high speed. The resulting mixture was dried to obtain the test slurry. This slurry was pressed into 200 mm × 150 mm × 50 mm blocks using isostatic pressing and dried at 200 °C for 24 hours.

The dried blocks were then fired at 1100 °C and 1500 °C under a reducing atmosphere, and removed after cooling.

1.2 Testing and Characterisation Methods

The Archimedes method was used to determine the bulk density and apparent porosity of the specimens, according to the standard Test Method for Bulk Density, Apparent Porosity and True Porosity of Dense Defined Refractory Products (GB/T 2997-2015). The room temperature compressive strength was measured using a pressure testing machine, following the standard Test Method for Room Temperature Compressive Strength of Refractory Materials (GB/T 5072-2008). The room temperature flexural strength was determined by the three-point bending method according to Test Method for Room Temperature Flexural Strength of Refractory Materials (GB/T 3001-2017), and high-temperature flexural strength was tested following Test Method for High Temperature Flexural Strength of Refractory Materials (GB/T 3002-2004).

An X-ray diffractometer (DX-2700BH) was used to identify the phase composition of the samples after firing. The microstructure of the specimens was observed using a scanning electron microscope (PhenomProX).

Thermal shock resistance was evaluated according to Test Method for Thermal Shock Resistance of Refractory Products Part 3: Water Rapid Cold-Crack Determination Method (YB/T 376.3-2004). After subjecting the specimens to thermal shock by burying in charcoal at 1100 °C followed by water cooling, the room temperature flexural strength was measured after one, two, and three cycles, respectively.

2.1 Dispersion Effect of Carbon Fibre after Introduction of Binding Agent

The microscopic morphology of carbon fibres after being introduced into the resin binder and cured at 1000 °C is shown in Fig. 2. The chopped carbon fibres were uniformly dispersed without agglomeration when directly dispersed in the binder, and a strong bond was observed between the carbon fibres and the resin.

Fig. 2 SEM morphology of carbon fibre after introduction of resin bonding agent and curing at 1000°C

2.2 Effects of Different Carbon Fibre Contents on Material Properties, Physical Phase, and Morphology after Heat Treatment at 1100°C

The flexural and compressive strengths of the specimens after heat treatment at 1100°C are shown in Figure 3. The room temperature flexural and compressive strengths of specimens with added carbon fibres improved compared to those without carbon fibres. However, when the carbon fibre content reached or exceeded 0.1%, both strengths initially increased and then began to decrease.

Fig. 3 Flexural and compressive strengths of specimens with different carbon fibre contents after heat treatment at 1100°C

Figure 4 shows the variation curves of high-temperature flexural strength (at 1500°C) for specimens with different carbon fibre contents. The high-temperature flexural strength of specimens containing carbon fibres was improved compared to specimens without carbon fibres. However, when the carbon fibre content was ≥ 0.1%, the strength initially increased and then decreased.

Fig. 4 High temperature flexural strength of specimens with different carbon fibre contents (1500°C)

The room temperature flexural strength of each specimen after thermal shock is shown in Fig. 5. It can be seen that the residual flexural strength of the specimens after one to three thermal shock cycles follows the same trend as that of the original samples. The residual flexural strength of specimens containing carbon fibres is higher than that of specimens without carbon fibres, with the optimal strength observed at a carbon fibre content of 0.1%.

Fig. 5 Flexural strength of specimens after thermal shock for different carbon fibre contents

The bulk density and apparent porosity of each specimen after heat treatment at 1100°C are shown in Fig. 6. Compared with specimens without carbon fibres, those containing 0.1% carbon fibres showed an increase in bulk density and a decrease in apparent porosity. However, specimens with carbon fibre content of ≥0.2% exhibited a decrease in bulk density and an increase in apparent porosity.

Fig. 6 Bulk density and apparent porosity of specimens with different carbon fibre contents after heat treatment at 1100°C

From the above, it can be seen that the flexural and compressive strengths of the material are significantly improved when the carbon fibre content is 0.1%, compared to specimens without carbon fibres, at a heat treatment temperature of 1100°C. When the carbon fibre content is 0.2%, the room temperature flexural and compressive strengths are slightly higher than those of specimens without carbon fibres, but significantly lower than those with 0.1% carbon fibre addition. Among the specimens tested, thermal shock resistance is also optimal at a carbon fibre content of 0.1%. This indicates that a moderate carbon fibre content can effectively toughen the material.

When the carbon fibre content is ≥ 0.2%, the interfacial area between the fibres and the matrix increases, resulting in a decrease in bulk density and an increase in apparent porosity, which in turn reduces the flexural and compressive strengths. Additionally, the pressing of carbon fibres and sample blanks generates substantial internal stress. Due to the complex distribution and direction of plastic deformation caused by these internal stresses, elastic rebound and stress relaxation occur during pressure relief and demoulding. These effects reduce the bulk density and increase porosity. This explanation is consistent with the observed trends in bulk density and apparent porosity after heat treatment at 1100°C.

The XRD patterns of each specimen after heat treatment at 1100°C are shown in Figure 7. At this temperature, the phases present in all specimens are essentially the same, with the main phases being carbon (C) and alumina (Al₂O₃), along with small amounts of silicon (Si) and silicon carbide (SiC).

Fig. 7 XRD patterns of specimens with different carbon fibre contents after heat treatment at 1100°C

Figure 8 shows scanning electron micrographs of carbon fibres in the cross section of a specimen (heat-treated at 1100°C) containing 0.1% carbon fibres. On the fracture surface of the material, carbon fibres were observed to be dispersed in strips. Some of the carbon fibres were encapsulated in the matrix [Fig. 8(a)], some interacted with the matrix by bridging cracks [Fig. 8(a), Fig. 8(b)], and others showed debonding and pull-out [Fig. 8(b)].

When the material fractures, cracks may be impeded as crack propagation in the matrix reaches the carbon fibres during loading. Moreover, the deflection and twisting of cracks along the carbon fibre/matrix interface consume more energy than direct crack extension, thereby increasing the fracture surface energy and enhancing strength and toughness. Crack bridging by carbon fibres also contributes significantly to toughening, as the fibres can prevent the matrix from peeling off by bridging cracks when the applied stress exceeds the fracture strength of the matrix but remains below that of the carbon fibres. Additionally, the energy required for debonding, fibre pull-out, and friction also consumes a certain amount of energy.

Through the combined effect of these mechanisms, aluminium-carbon refractories with carbon fibre additions can achieve improved strength and fracture toughness at service temperatures of 1100°C, thereby achieving toughening and reinforcement. Furthermore, Fig. 8 shows that the short carbon fibres introduced into the resin by direct dispersion at 0.1% content are uniformly dispersed in the sintered samples without agglomeration.

Fig. 8 SEM morphology of wrapped bridged carbon fibres and debonded and pulled out carbon fibres in the cross-section of the specimen with 0.1% carbon fibre content (heat-treated at 1100°C)

2.3 Effects on the structural properties, physical phase and morphology of carbon fibre-containing specimens after heat treatment at 1500°C

Figure 9 shows the variation curves of bulk density and apparent porosity of the specimens with different carbon fibre additions at 1500 °C. Similarly, it can be seen that with the increase of carbon fibre addition from 0 to 0.4%, the bulk density firstly increases and then decreases, while the apparent porosity firstly decreases and then increases.

Fig. 9 Bulk density and apparent porosity of specimens with different carbon fibre contents after heat treatment at 1500°C

The XRD patterns of each specimen after heat treatment at 1500°C are shown in Fig. 10. At this temperature, the phases present in all specimens are essentially the same, with the main phases being carbon (C) and alumina (Al₂O₃), along with a small amount of silicon carbide (SiC). However, compared to the specimens heat-treated at 1100°C (Fig. 6), the diffraction peaks of silicon (Si) disappeared after heat treatment at 1500°C under a carbon burial atmosphere, and new diffraction peaks of SiC appeared at 2θ = 33.4° and 2θ = 38°, corresponding to the diffraction peaks of α-SiC (72-0018).

A comparison of the XRD patterns of specimens with different carbon fibre contents after carbon burial heat treatment at 1500°C shows a general increase in the intensity of the SiC diffraction peaks with increasing carbon fibre content. This suggests that higher carbon fibre content at 1500°C promotes the formation of new silicon carbide phases. The microscopic morphology of these new phases was characterized by SEM.

Fig. 10 XRD patterns of specimens with different carbon fibre contents after heat treatment at 1500°C

Figure 11 shows the SEM images of the surface of a single carbon fibre in the section of the specimen with 0.1% carbon fibre content after heat treatment at 1100°C [Fig. 11(a)] and 1500°C [Fig. 11(b)], respectively. As seen in the figure, the surface of the carbon fibre after heat treatment at 1100°C is smooth and unetched, showing no difference compared to the morphology of the short-cut carbon fibre before treatment (Fig. 1). In contrast, compared with the smooth surface after heat treatment at 1100°C [Fig. 11(a)], the surface and interior of the carbon fibres in the specimen after heat treatment at 1500°C have become rough, and well-developed SiC whiskers have formed on the surface.

Fig. 11 SEM morphology of specimens with 0.1% carbon fibre content after heat treatment at 1100°C and 1500°C

The reaction process for the generation of silicon carbide whiskers on the surface of carbon fibres is as follows: during the sintering of the material, the corresponding SiO and CO are first generated according to reactions (1) and (2), and when the reaction occurs to the point where the SiO and CO pressures are comparable, according to reaction (3), SiC whiskers are formed.

2C+O₂→2CO (1)

Si+CO→SiO+C (2)

SiO+3CO→SiC+2CO₂ (3)

Therefore, after heat treatment at 1500°C, the silicon carbide whiskers generated in the carbon fibre-containing specimens act as bridges, connecting the matrix and the carbon fibres to form a weak interfacial bond that induces crack propagation. This mechanism enhances the mechanical properties and thermal shock resistance of the material. Thanks to the dual reinforcement from carbon fibres and silicon carbide whiskers, the service performance of aluminium-carbon refractory materials for continuous casting can be effectively improved under high-temperature conditions.

Conclusion

(1) The short carbon fibres added by direct dispersion in the binding agent were evenly dispersed without agglomeration. This shows that directly introducing carbon fibres into the binding agent for ultrasonic dispersion is effective, and it successfully achieves the separation of short-cut fibre monofilaments while avoiding damage to the carbon fibres.

(2) Under heat treatment conditions of 1100°C and 1500°C, the room temperature flexural strength, compressive strength, residual flexural strength, and high-temperature flexural strength of specimens with added carbon fibres were significantly improved compared to those without carbon fibres. The best performance was achieved with a carbon fibre content of 0.1%. Meanwhile, as the carbon fibre content increased from 0 to 0.4%, the bulk density first increased and then decreased, while the apparent porosity first decreased and then increased. This indicates that an appropriate amount of carbon fibre addition can not only toughen the material but also optimize bulk density and apparent porosity, thereby improving flexural strength, compressive strength, and thermal shock resistance. This effectively achieves the toughening of aluminium-carbon refractory materials for use in medium- and high-temperature environments.

(3) After heat treatment at 1500°C, the surface and interior of carbon fibres-which were originally smooth after heat treatment at 1100°C-became etched, with a reduction in silicon content and the formation of well-developed silicon carbide whiskers on the surface. These silicon carbide whiskers bridge the gap between the matrix and the carbon fibres, enhancing the mechanical properties and thermal shock resistance of the material. Thanks to the dual reinforcement from carbon fibres and silicon carbide whiskers, the service performance of aluminium-carbon refractory materials for continuous casting can be effectively improved under the high-temperature infiltration environment at 1500°C.