Effect Of Mg On The Structure And Ablation Properties Of C/C-ZrC-SiC Composites

C/C-ZrC-SiC composites have gained significant attention in the aerospace materials field due to their low density,low thermal expansion coefficient,and high specific strength of C/C composites,as well as the oxidation resistance,ablative resistance,and wear resistance of ZrC-SiC composites.However,the large melting point gap between ceramic oxides Si O₂ and Zr O₂ makes the oxide film formed in service environments above 2000°C prone to breakage.Therefore,the high-temperature stability and ablative resistance of C/C-ZrC-SiC composites still require improvement.In this paper,Mg-modified ZrC-SiC ceramic-based C/C composites were prepared using the sol-gel method to address the limitations of C/C-ZrC-SiC composites.Specifically,Mg-modified ZrC-SiC and ZrC-SiC-Ti C ceramic precursors were prepared via the sol-gel process,and then obtained Mg-modified ZrC-SiC and ZrC-SiC-Ti C multiphase ceramics through carbothermal reduction reaction and spark plasma sintering（SPS）process.The ablation behaviour of these multiphase ceramics was also investigated.Moreover,Mg-modified C/C-ZrC-SiC and C/C-ZrC-SiC-Ti C composites were prepared via the sol-gel process and the effects of Mg/（Zr+Si）and Zr/Ti ratio on the mechanical and ablative properties of these composites were also systematically studied.（1）The Mg-modified ZrC-SiC ceramic precursor was successfully synthesized using zirconium oxychloride as the zirconium source,tetraethyl orthosilicate as the silicon source,furfuryl alcohol as the carbon source,magnesium chloride as the modifier.During precursor pyrolysis,the formation of the Mg-Si-O oxide promoted mesophase Zr Si O₄formation,thereby reducing the temperature required for ZrC-SiC synthesis.The modified precursor exhibited a carbothermal reduction temperature nearly 200°C lower than the unmodified precursor,with decreasing residual carbon content as Mg Cl₂ content increased.Building on these findings,the Mg-modified ZrC-SiC-Ti C ceramic precursor was synthesized by adding tetrabutyl titanate as the titanium source.The amorphous Mg-Si-O oxides also played a catalytic role during pyrolysis,further reducing the temperature needed for SiC and（Zr,Ti）C solid solution formation.Moreover,the（Zr,Ti）C solid solutions in the product transitioned from Ti-rich to Zr-rich with an increasing Zr/Ti ratio.（2）High-density Mg-modified ZrC-SiC multiphase ceramics were successfully synthesized by SPS.During plasma flame ablation,the Mg-Si-O phase reacted with Si O₂ to produce Mg Si O₃ and Mg₂Si O₄,which melted to form a high-viscosity Mg-Si-O liquid phase.This phase combined with Zr O₂ particles to create a dense oxide film that prevented further ablation of the ceramic matrix.Furthermore,the addition of Mg atoms inhibited the transformation of t-Zr O₂ to m-Zr O₂,improving the stability of the oxide film.As the Zr/Ti ratio increased during the ablation of Mg-modified ZrC-SiC-Ti C multiphase ceramics,the Zr O₂ particles in the oxide film transitioned from dispersed to dense skeleton structures.The viscosity of the Ti-Mg-Si-O liquid phase also increased with the Zr/Ti ratio,which prevented the erosion of ablation airflow and oxidizing gas on the matrix ceramics,thus improving ablative resistance.Multiphase ceramics with a Zr/Ti molar ratio of 3:1 exhibited the best ablative properties,with a linear ablative rate of 1.26μm/s and mass ablative rate of-2.56 mg/s after90 s of plasma flame ablation.（3）The Mg-modified C/C-ZrC-SiC composites were prepared by incorporating the Mg-modified ZrC-SiC ceramic precursor into the C/C porous body using the sol-gel process.The ceramic particles gradually sintered and agglomerated with an increasing Mg/（Zr+Si）ratio,leading to a decrease in composite density.Additionally,the formation of the Mg-Si-O oxide inhibited the ceramization of carbon fibres,thereby enhancing the composite’s mechanical and anti-ablation properties.The composite with an Mg/（Zr+Si）molar ratio of 0.040:0.18 exhibited the best mechanical properties and ablative resistance,with a bending strength and elastic modulus of 331.22 MPa and 23.09 GPa,respectively.After 120 s of plasma flame ablation,the linear and mass ablation rates of the composites were3.91μm/s and-0.23 mg/s,respectively.During the ablation process,the viscous mixed liquid phase of Mg-Si-O and Zr-Si-O combined with the coarse-Zr O₂ solid phase skeleton to form a continuous dense oxide layer that covered the surface of the material and inhibited the further ablation of the composite matrix.（4）The formation of the Mg-Si-O oxide also inhibited the ceramization of carbon fibres in Mg-modified C/C-ZrC-SiC-Ti C composites.However,this inhibition effect decreased gradually as the Zr/Ti ratio increased,leading to a decrease in the density and mechanical properties of the composites.Among the composites tested,those with a Zr/Ti molar ratio of 3:1 exhibited the best ablative properties.After 180 s of ablation,the composites showed a mass ablative rate of-0.2351 mg/s and a linear ablative rate of 2.66μm/s.During the ablation process,the multi-component Ti-Mg-Si-O liquid phase filled the gaps among the Zr O₂particles,forming a continuous dense solid-liquid oxygenated film that covered the ablative surface of the composites.This film effectively prevented erosion from both the plasma ablative airflow and the oxidizing gas,enhancing the ablation resistance of the composites.