Influence Of Boron Compound On Microstructure And Properties Of C_f/Lithium Aluminosilicate Composites

In this paper, carbon fiber(Cf)reinforced boron compound(H3BO3, Si B6, BN)doped lithium aluminosilicate(LAS) composites Cf / LAS(B) were prepared by sol-gel method combined with hot pressing process using carbon fiber(Cf),boron compound powder,lithium nitrate, aluminum nitrate, silica sol and ammonia as the main raw materials.The influence of boron compound content and pressing temperature on the microstructure, mechanical properties and oxidation resistance of Cf/LAS(B) composites were studied. Composites with excellent comprehensiveproperties were fabricated by adjusting the process parameters. Studieshave shown that the addition of boron compounds produced a significant effect on the structure and properties of the composites.The main results of this study are as follows:With the increase of the boron compound(H3BO3, Si B6, BN) content, the mechanical properties of Cf/LAS(B) composite increased first then decreased.The composites prepared at 1350 oC with 6wt.%H3BO3 showed the best mechanical properties. The flexural strength and fracture toughness were 773±42MPa and 22.72MPa·m1/2. When the H3BO3 content continues to rise, thedecreasing of β-spodumene phase content of the composite material leads to the decrease ofthe mechanical properties of the composites, even lower than that of the free H3BO3 composites; The composites prepared at 1300 oC with 2wt.%Si B6 showed the best mechanical properties.The flexural strength and fracture toughness were 520±28MPa and 19.73MPa·m1/2. When Si B6 content continues to rise, silicon hexaboride reacts with oxygen to generate Si O2 leads tochanging the content of Si O2 of the material, so that the main phase changes result in material properties change significantly; The composites prepared at 1300 oC with 3wt.% BN showed the best mechanical properties. The flexural strength and fracture toughness are up to 909 ± 36 MPa and 20.68MPa· m1 / 2, which reached highest flexural strength of the boron compound(H3BO3, Si B6, BN) doped Cf / LAS composite.With the increase of the boron compound content, from the load-displacement curve, fracture mode of the material by the quasi-ductile fracture transform to brittle fracture; from the fracture morphology, the length of the fiber pull-out progressively shorter until it is flush with the fracture; from the interface layer morphology, the interface layer thickness decreases, the bonding strength increases. The bonding strength of the fiber and the matrix increases with the increase of the boron compound content, increased binding strength led to a sharp decline in the flexural strength and fracture toughness.The addition of boron compounds significantly improved the oxidation resistance of Cf /LAS composites. The mass loss rate of Cf/LAS composites was significantly decreased after high temperature oxidation, and the retention rate of bending strength was significantly increased after high temperature oxidation. Two enhancement mechanisms of the oxidation resistance of boron compound doped Cf/LAS composites are summarized: 1.The graphitization degree of carbon fibers in the boron doped Cf/LAS composites is enhanced due to the catalytic effect ofboron. 2. Under high temperature Boron compounds generated boron oxide which can heal cracks and pores as well asprevent oxidative gas from entering the interior of the fiber, so the inner fiber can be effectively protected from oxidation.