Abstract: Due to the high temperature required for ceramization of ceramicable composites, the ceramic layer formed in the early stage of ablation is not dense enough, which will affect the ablation performance of the composites. While the glass frits with low melting point can decline ceramization temperature of the composites. In this work, two low melting point glass frits G1 and G2 were introduced into the MoSi₂ modified boron phenolic resin (BPR). Two vitreous silica fabric reinforced composites were prepared by prepreg-molding process. The effects of the glass frits on the thermal properties of the pyrolysis residues of MoSi₂ modified BPR and the flexural properties of vitrified structure and ablative properties of its high-silica-fiber reinforced composites were studied. Test results show that the ceramifiable temperature of the composites declines by about 600 ℃,and room temperature bending strength of vitrified structure at 800 ℃~1200 ℃ of the composites with glass frit G2 increases by 6.5 MPa~7.6 MPa. In addition, with the introduction of glass frit, a dense melt structure is formed on the surface of the composites in the process of oxyacetylene ablation, which inhibits the diffusion of hot oxygen to the inner carbonized matrix. Furthermore, glass frit G2 with more SiO₂ content is beneficial to promote the interaction between MoSi₂ particles and BPR pyrolysis products to form high-melting ceramic phases of Mo_4.8Si₃C_0.6 and MoB, which reduce the mass ablation rate of the composites by 10.7%.

Key words: thermal protection, boron phenolic resin, MoSi₂, glass frit, ceramization mechanism, composites