Effects Of Electrophoretic Deposition Of MXene On High Modulus Carbon Fiber And Its Composites Interfacial Properties

The surface of high modulus carbon fiber（HMCF）prepared by ultra-high temperature heat treatment process is highly inert,and the interaction between HMCF and matrix is so poor that the load cannot be effectively transferred from the matrix to HMCF and tends to interlaminar delamination,resulting in poor interfacial properties of HMCF composites.Anodic oxidation and sizing agents are generally used to modify HMCF in industry,but the surface activity of HMCF with the above treatments is increased to a limited extent,and the interfacial bonding between the fiber and resin matrix is still difficult to meet the requirements of higher performance aerospace materials.By constructing a strong fiber-matrix interface to improve the load transfer efficiency is a pivotal factor to enhance the mechanical properties of HMCF composites,and it is conducive to the further development and application of HMCF composites.In this work,the interfacial properties of the composites were improved via electrophoretic deposition of Ti₃C₂T_x MXene nanosheets onto the HMCF surface,and we modified MXene by L-cysteine（LC-MXene）to address the problem of oxidation of MXene in aqueous solution during the experiment.This paper has investigated the effects of HMCF modified by electrophoretic deposition of MXene and LC-MXene nanosheets at different placement time at room temperature on the interfacial properties of composites.（1）The effects of electrophoretic deposition time on HMCF surface and the composites interfacial properties were investigated.The results showed that the deposition of MXene onto the surface of HMCF not only changed the surface morphology of HMCF,but also improved the surface roughness,surface O element content and surface energy.There was no significant effect on the monofilament tensile strength and modulus of HMCF before and after the modification,indicating that the electrophoretic deposition process of MXene would not damage the chemical structure or produce physical defects on the surface of HMCF.However,the interfacial shear strength（IFSS）and interlaminar shear strength（ILSS）of the modified HMCF composites reached93.4 MPa and 86.8 MPa,which showed an increase of 46.9%and 49.9%,respectively.The interfacial reinforcing mechanism was mainly attributed to the improved wettability,increased roughness（mechanical bonding）and interface modulus transition layer.（2）The effects of MXene modified with L-cysteine on its properties were investigated.The stability of MXene colloidal solution decreased significantly with increasing placement time,but LC-MXene still remained highly stable（-30.6 m V）after 4 weeks.The results showed that the MXene nanosheets were completely oxidized to Ti O₂ and disordered C structures after being placed for4 weeks in deionized water,while the two-dimensional layered nanosheets structures with uniform and neat surfaces and distinct edges were still evident for the LC-MXene nanosheets.It suggested that L-cysteine could effectively inhibit the oxidation of MXene nanosheets in deionized water and preserve the two-dimensional layered structure of MXene nanosheets.The mechanism is that the S-Ti and N-Ti bonds and hydrogen bonds on the surface of LC-MXene occupy the reaction sites where water and oxygen molecules attack MXene,thus inhibiting the oxidation of MXene.（3）The effects of electrophoretic deposition of MXene and LC-MXene at different placement time on the interfacial properties of HMCF composites were investigated.The surface of HMCF modified with MXene after 2 weeks of placement could only see a few small flakes of MXene and Ti O₂ particles formed by oxidation,and the surface roughness and surface energy progressively decreased with increasing placement time.In the contrast,the surface of LC-MXene modified HMCF could observe a great amount of MXene with large sheet size and showed a more closely arranged,and its surface roughness and surface energy were higher than those of MXene modified HMCF.The ILSS results showed that the ILSS of LC-MXene modified HMCF composites reached 88.5 MPa,which showed 52.8%improvement over that of the unmodified ones.After 2 weeks of placement,the ILSS of LC-MXene modified HMCF composites still reached 85.2 MPa,which was only a decrease of 3.7%,while the ILSS of MXene modified HMCF composites decreased to69.2 MPa,which was 20.3%lower,demonstrating that L-cysteine could effectively inhibit the oxidation of MXene nanosheets in deionized water,and further enhanced the interfacial bond strength of the composites.