Research On Low Temperature Preparation And Performance Of Ti₃（Si,Al）C₂/Al₂O₃ Composites

Ti₃Si C₂ is a MAX-phase ceramic with high strength,high toughness,and good machinability,but it exhibits poor oxidation resistance at high temperatures.Ti₃（Si,Al）C₂ can be produced by doping Ti₃Si C₂ with a small amount of Al.Ti₃（Si,Al）C₂ shows comparable mechanical properties to Ti₃Si C₂,but it has better high-temperature oxidation resistance than Ti₃Si C₂.Al₂O₃ is a hard and brittle high-temperature resistant ceramic,and the mechanical properties of Ti₃（Si,Al）C₂ and those of Al₂O₃ are complementary.Ti₃（Si,Al）C₂/Al₂O₃ composite is considered to be a perfect structural material.However,due to the weak Ti-Si bonds and Ti-Al bonds in Ti₃（Si,Al）C₂,the high temperature required for the densification of Al₂O₃ could lead to the decomposition of Ti₃（Si,Al）C₂.To address this issue,Ti₃（Si,Al）C₂/Al₂O₃ composites were prepared by current-assisted sintering at 1100～1300℃using Ti₃Al C₂/Si O and Ti₃Al C₂/Si O₂ as raw materials.The molar ratios of Ti₃Al C₂:Si O and Ti₃Al C₂:Si O₂ were both 1:1.This study found that Si in Ti₃（Si,Al）C₂accounted for more than 85 at%of the A-layer atoms,indicating that the majority of the Al atoms in Ti₃Al C₂ were substituted by Si atoms during the reaction.The relative densities of the composites prepared by sintering Ti₃Al C₂/Si O and Ti₃Al C₂/Si O₂ at 1300℃can exceed 95%.The composites prepared by Ti₃Al C₂/Si O were easier to densify during low temperature sintering,and their mechanical properties and high temperature oxidation resistance were less affected by the decrease in sintering temperature.When the sintering temperature decreased from 1300℃to 1100℃,the flexural strength,hardness,and fracture toughness of Ti₃（Si,Al）C₂/Al₂O₃ composites prepared by Ti₃Al C₂/Si O decreased by 35.9%,27.4%,and33.4%,respectively.Under the same circumstances,the flexural strength,hardness,and fracture toughness of the composites prepared by Ti₃Al C₂/Si O₂ dropped by 81.2%,85.4%,and 68.6%,respectively.After oxidation at 1200℃for 100 hours,compared with the composites prepared by sintering Ti₃Al C₂/Si O at 1100℃and 1200℃,the composites prepared by sintering Ti₃Al C₂/Si O₂ at same temperatures exhibited larger oxidation weight gains per unit area.Therefore,Ti₃Al C₂/Si O is more suitable for low-temperature preparation of Ti₃（Si,Al）C₂/Al₂O₃composites.In this paper,the reaction mechanism of Ti₃Al C₂/Si O and the low-temperature densification mechanism of the prepared Ti₃（Si,Al）C₂/Al₂O₃ composites were studied.Due to the weak Ti-Al bonds,the Al atoms in Ti₃Al C₂ diffused first during sintering,forming Ti₃Al_1-xC₂ and Al-Si-O phases.As the reaction temperature increased,Si gradually entered Ti₃Al_1-xC₂,forming Ti₃（Si,Al）C₂,and the Al-Si-O phases gradually transformed into Al₂O₃ and a liquid phase.Al and Si atoms could diffuse through the A-layer of the MAX phase,and with the assistance of the liquid phase,they could also across the Al₂O₃ layer that had not been densified,allowing the Al-Si interdiffusion to proceed during the reaction.Al-Si substitution and interdiffusion caused that the proportion of Al₂O₃ in the composites continuously rose,and Al atoms in the Ti₃Al C₂ grains were gradually substituted by Si atoms,and in the end,Ti₃（Si,Al）C₂/Al₂O₃ composites were formed.Compared with the spherical Al₂O₃ formed by the reaction between Si O₂ and Ti₃Al C₂,the irregular and strip-shaped Al₂O₃ produced by the reaction between Si O and Ti₃Al C₂ might have larger surface energy,which was beneficial to the low-temperature densification of Ti₃（Si,Al）C₂/Al₂O₃ composites.Current-assisted sintering provided a greater heating rate than hot-press sintering,which could prevent the loss of surface energy brought on by grain coarsening during heating and contribute to the low-temperature densification of Al₂O₃.In addition,the residual Al metal produced by the reaction between Ti₃Al C₂and Si O might also fill the pores in the composites.