Al₂O₃-C refractories are one of the important categories of refractory materials in steel smelting. Improving their service life has a great application value. The service life of Al₂O₃-C refractories is closely related to their mechanical properties. Therefore, an effective approach to improve the mechanical properties of Al₂O₃-C refractories and further enhance their service life is to introduce a structural reinforcement such as carbon fibers. However, using commercial carbon fibers directly as a structural reinforcement cannot bind with other components of the material tightly at various temperatures due to its smooth surface and low reactivity, thus restricting the reinforcing effect of carbon fibers. Therefore, surface treatment such as thermal oxidation on carbon fibers is an effective way to improve the reinforcement effect of carbon fibers and improve the mechanical properties of carbon fibers reinforced Al₂O₃-C refractories. In this paper, short carbon fibers were treated by a thermal oxidation method, and Al₂O₃-C refractories were then prepared using the treated carbon fibers as a reinforcement. The structural evolution of short carbon fibers with different degrees of oxidation in Al₂O₃-C refractories, as well as the effect of adding thermal oxidation short carbon fibers on the mechanical and thermal shock resistance properties of Al₂O₃-C refractories were investigated.

White corundum, active alumina powder, silicon powder, boron carbide, flake graphite, nano carbon black, and short carbon fiber were used as raw materials, and thermosetting phenolic resin was used as a binder. In addition, some commercial carbon fibers were heated in air at 400, 500 ℃, and 600 ℃ for 0.5 h to prepare thermal oxidation carbon fibers in order to investigate the effect of thermal oxidation treatment of carbon fibers on their structural evolution and service performance in Al₂O₃-C refractories.

The Al₂O₃-C samples without carbon fibers and with untreated carbon fibers, as well as thermal oxidation carbon fibers at different temperatures (i.e., 400, 500 ℃, and 600 ℃) were numbered as sample BK, sample F, sample FT4, sample FT5, and sample FT6, respectively. The raw materials were mixed evenly and then shaped into 25 mm×25 mm×140 mm at 120 MPa. Each sample was cured at 200 ℃ for 24 h, and then heated in a saggar filled with coke grit at 1 200, 1 300 ℃, and 1 400 ℃ for 5 h, respectively.

The micromorphology of samples was determined by a field emission scanning electron microscope (TESEM). The apparent porosity and bulk density of samples were measured based on the Archimedes principle. The CMOR and HMOR of samples were measured by a three-point bending method. The CCS of samples were measured by a uniaxial compression method. The thermal shock resistance of the samples was evaluated according to the residual strength ratio of the CMOR (CMORrsr). The elastic modulus (E) of samples was analyzed by a resonance frequency and damping analyzer. The fracture-related parameters of samples were detected by a single-edge notched beam (SENB) method.

Based on the TESEM images, the surface of the untreated carbon fibers are relatively smooth without obvious defects. After thermal oxidation at 400 ℃, the morphology of carbon fibers does not change much and still have a relatively smooth and intact surface. After thermal oxidation at 500 ℃ and 600 ℃, however, visible structural changes occur on the surface of carbon fibers, becoming rough in some areas. Also, the reaction between carbon fibers and Si heat-treated at 1200 ℃ occurs slightly, leading to that the surface morphology of carbon fibers is similar to their initial state. After heat treatment at ≥1300 ℃, some structures in the form of whiskers and particles appear on the surface of carbon fibers. In addition, for carbon fibers after thermal oxidation treatment, in addition to growing the structures outside the surface of fiber like the original carbon fibers, it is also possible to further react in fiber surface defects to generate new structures inside the fibers. This reaction process will continuously increase the defect size of the fibers and damage the structural integrity of the fibers.

Although the addition of short carbon fibers reduces the compactness of Al₂O₃-C refractories, their introduction can still play a positive role in improving the mechanical properties. For instance, for Al₂O₃-C refractories containing carbon fibers treated at 500 ℃, after being heat treated at 1 400 ℃, their CMOR, CCS HMOR are increased by 28.2%, 22.4%, and 87.2%, respectively, compared with the control material. This can be due to the high-temperature chemical reaction between carbon fibers and other components of refractories, as well as the additional energy dissipation mechanism introduced by fibers, which greatly increases the energy required for crack propagation, thereby hindering crack propagation and improving the mechanical properties of refractories.

Thermal oxidation treatment could create defects on the surface of carbon fibers, thereby affecting their reactivity and mechanical properties. Adding them to Al₂O₃-C refractories could promote the formation of in-situ ceramic phases due to the surface defects, having positive impacts on the improvement of mechanical properties. However, the temperature of thermal oxidation treatment should be appropriate. If the temperature was too low or too high, it could not fully exert the reinforcing effect of carbon fibers. In addition, although the addition of thermal oxidation carbon fibers had a positive impact on the performance improvement of refractories in some cases, there were still weaknesses in the performance.