LIU Haichang ^{1, 2}, WU Songsong ^{1, 2}, WEN Guangwu ^{1, 2}, LI Zhuoyang ^{1, 2},

ZHANG Chunpeng ^{1, 2}, WANG Bo ¹

(1. School of Materials Science and Engineering, Shandong University of Science and Technology, Zibo 255000, Shandong, China; 2. Engineering Ceramics Research Institute, Shandong University of Science and Technology, Zibo 255000, Shandong, China)

Extended abstract:[Background and purposes] Due to their ultra-low/negative thermal expansion coefficient, excellent heat resistance and thermal shock resistance, Li₂O-Al₂O₃-4SiO₂ glass-ceramics have important applications in the field of precision instruments, high-temperature structures and microelectronics packaging. However, their intrinsic brittleness and low bending strength seriously restrict their engineering application as structural materials. Although the mechanical properties of C_f/LAS composites can be significantly improved through fiber toughening mechanism, high temperature application is still limited. Currently, surface coating is usually used to protect the fiber, but the cost of the coating process is always high and the manufacturing process is relatively complex. Therefore, ZrB₂ particles were selected as the reinforcement phase to improve the mechanical properties and oxidation resistance. On one hand, ZrB₂ has high melting point, excellent oxidation resistance and matching thermal expansion characteristics, which can be used as a strengthening phase to improve the densification of the matrix and hence binding strength of the final composites. On the other hand, the formation of ZrO₂-B₂O₃ composite oxide layer due to the high temperature oxidation can self-repair the matrix and inhibit oxygen diffusion. In order to meet the target requirements, C_f/LAS composites with different contents of ZrB₂ were prepared by slurry impregnation and hot pressing. The main purpose of this study is to improve the mechanical properties and oxidation resistance of C_f/LAS composites by using a simple and effective method, aiming to establish the relationships between microstructure, oxidation resistance and mechanical properties.[Methods] Li₂O-Al₂O₃-4SiO₂ glass-ceramic precursor was prepared by using sol-gel method. The aluminum nitrate solution was hydrolyzed to form Boehmite sol (γ-AlOOH), followed by silica sol and lithium nitrate solution. After mechanical stirring for 4 h, aluminosilicate sol with stoichiometric ratio (n_Li:n_Al:n_Si=1:1:2) was formed. The precursor powder was obtained after drying at 120 ℃ and calcination at 550 ℃. The impregnated paste was prepared with 0–7 wt.% ZrB₂ additive, methyl cellulose (binder), polyethylene glycol (plasticizer) and precursor, through ball milling. The carbon fiber was evenly loaded with the paste by winding process. After drying for 48 h, it was cut into a 40×60 mm prefabricated sheet (the axial direction of the fiber was parallel to 40 mm). At 1300 ℃ and 10 MPa, the C_f/LAS composites with 35–40% carbon fiber were prepared by laminating the prefabricated sheets combined with heat preservation and pressure for 40 min. Three point bending method (sample size 3×4×40 mm, span 30 mm, rate 0.5 mm·min⁻¹) and single notch beam method (sample size 2×4×25 mm, notch depth 0.5 mm, span 16 mm, rate 0.05 mm·min⁻¹) were used to measure the bending strength and fracture toughness of the samples, respectively. Density and porosity were characterized by using Archimedean method. Phase evolution was analyzed by using XRD [Cu Kα, 10°–90°, 8 (°)·min⁻¹]. Fracture morphology and element distribution were analyzed by using SEM-EDS and XPS. A synchronous thermal analyzer (air atmosphere, temperature rise from RT to 1000 ℃ at a heating rate of 10 ℃·min⁻¹) was used to evaluate high-temperature oxidation behavior of the materials. The regulatory mechanism of ZrB₂ on thermal stability was discussed.[Results] The effects of ZrB₂ addition on the phase composition, fracture behavior and oxidation resistance of C_f/LAS composites were studied systematically. ZrB₂ doping (0–7 wt.%) increased bulk density of the composite to 2.09–2.24 g·cm⁻³ and reduced the porosity to 0.25%–0.68%, which proved that the densification during hot pressing sintering was improved. There was a double effect on mechanical properties. When the content of ZrB₂ increased to 5 wt.%, the bending strength reached a peak of (921±32) MPa, which was 110% higher than that of the undoped sample, but the fracture toughness decreased by 15% to (15.2±1.0) MPa·m^1/2. The undoped sample (0ZB) showed the character of long fiber pulling out, indicating the weak binding of matrix and fiber. 5 wt.% ZrB2 doping shortened the fiber pulling length and strengthened the bonding between matrix and fiber, but excessive doping (>5 wt.%) led to interfacial embrittlement and decreased flexural strength. No residual ZrB₂ phase was detected after 1–3 wt.% ZrB₂ was added. However, the undoped sample (0ZB) showed typical brittle fracture characteristics, while the doped sample showed significantly reduced fiber desticking. Fiber pull-out length was shortened and no macroscopic cracks were found in the matrix. The high temperature oxidation experiment showed that molten B₂O₃ was formed on the surface of ZrB₂-doped samples can effectively fill the pores of the ZrO₂ skeleton and penetrate into the gap between the fiber and the matrix to form a dense anti-oxidation layer, which significantly inhibits oxygen diffusion. 5 wt.% ZrB₂-doped samples still maintain 77% mass retention after oxidation at 900 ℃, while 0ZB samples only maintain 66% mass retention. The is attributed to the self-healing effect of the oxide layer in the ZrO₂-B₂O₃ composite. As the 5ZB sample was oxidized at 900 ℃ for 30 min, the strength retention was 74.4%.[Conclusions] ZrB₂ was oxidized to ZrO₂ and B₂O₃ during hot pressing sintering at 1300 ℃ and 10 MPa, which significantly improves the densification of the matrix and enhances the fiber-matrix bonding strength. The bending strength of the 5 wt. % ZrB₂ doped sample is (921±32) MPa, which is 110% higher than that of undoped sample. However, excessive densification leads to the blockage of crack deflection and the strong bonding strength inhibits the fiber bridging effect, resulting in a 15% reduction in fracture toughness to (15.2±1.0) MPa·m^1/2. As the content of ZrB₂ is ≥4 wt.%, the residual ZrB₂ crystal phase is positively correlated with the diffraction intensity. The ZrO₂-B₂O₃ oxide layer shows self-healing effect at 600–900 ℃. Molten B₂O₃ can dynamically fill the pores of ZrO₂ skeleton and infiltrate into the gap between fiber and matrix, so that the mass retention rate of the 5 wt.% ZrB₂ doped sample is 77% after oxidation at 900 ℃, while that of the undoped sample is only 66%. Meanwhile, the strength retention rate was 74.4% after oxidation at 900 ℃ for 30 min, while that of the undoped sample is 60%. It can be concluded that co-optimization of mechanical properties and oxidation resistance of C_f/LAS composites can be achieved by adjusting the doping content of ZrB₂, thus providing a new idea for the design of high-performance ceramic matrix composites

Key words: C_f/LAS glass-ceramic; ZrB₂; mechanical properties; oxidation resistance