Investigation On Preparation Of MAX Phase Coatings And Their Performance As The Interphase Of Composites

The interfacial properties play a crucial role in determining the final properties of composites.The present paper focuses on the design and optimization on the interfacial proterties of several composites,including continuous fiber reinforced silicon carbide matrix composites?C/SiC,SiC/SiC?,SiC whiskers reinforeced copper matrix composites?Cu/SiC?,and Ti₃C₂ MXene nanocomposites.Layred ternary compounds MAX phases combine some good attributes of ceramics and metals,such as good fracture energy absorption ability,oxidation resistance,and irradiation resistance.MAX phases are promising candidates for the interphase materials of composites owing to the folloing reasons:?a?MAX phases interphase can improve the oxidation resistance,irradiation resistance and damage tolerance of C/SiC,SiC/SiC composites;?b?MAX phases interphase can improve the wettability between SiC and Cu,thus improve the interfacial properties of Cu/SiC composites;?c?MAX phases are the precursor of MXene materials,designing MAX phases with special structure enables us to fabricating MXene-based composites for specified applications.The main research contents of the paper are as follows:?1?Uniform and continuous coating with the major phase of Ti₃SiC₂ was fabricated on SiC fiber and SiC whisker via the in-situ reaction between SiC and Ti in a molten salts bath.The influence of reaction temperature,reaction time on the constituent and structure of the coaiting was investigated.The formartion mechanisms of the coating in the molten salts bath were investigated.The thickness of the coating on SiC fiber was controlled by changing the reaction time.The thickness of the coating on SiC whisker was controlled by changing the Ti/SiC ratio.In order to evaluate the performance the coating,copper matrix composites reinforced with Ti₃SiC₂-coated SiC whiskers were fabricated.The influence of the Ti₃SiC₂ coating on the interfacial properties of the composites was investigated.The results showed that by forming a thin Ti₃SiC₂ coating on SiC whiskers,the interfacial bonding properties of the composites were effectively enhanced.Owing to the improved interfacial bonding properties,the load transfering efficiency from the matrix to whiskers was improved.The tensile strength of the composites reinfireced with uncoated SiC whiskers was only 275±7.5 MPa.In comparsion,when SiC whiskers were coated by a Ti₃SiC₂ layer with the thickness less than 100 nm,the tensile strength of the composites reached 504±8.5 MPa,and the fracture elongation was larger than 8%.?2?TiC/Ti₂AlC constituent gradient coating was fabricated on carbon fiber via an in situ reaction in a molten salts bath.The coating exhibited a multi-layered structure,in which the inner layer was TiC and the outer layer was Ti₂AlC.The influence of the reaction conditions on the morphology,composition,and thickness of the coating were investigated.The formation mechanism of the coating was discussed in term of thermodynamics,kinetics,and the coordination behavior and diffusion behavior of the reactants.The oxidation resistance properties of the as-prepared TiC/Ti₂AlC-coated C_f in static air and water vapor flow at 600°C,800°C,and 1000°C were investigated.The results indicated that the as-prepared TiC/Ti₂AlC coating could provide protection to the carbon fiber in both static air and water vapor flow up to 800°C.The oxidation resistance of the TiC/Ti₂AlC coating was not as good as bulk Ti₂AlC,which was attributed the insufficient thickness of the coating.The coating was not thick enough to form a continuous Al₂O₃ film,which could inhibit the further oxidation.?3?Graphite/TiC/Ti₃C₂?G/TiC/Ti₃C₂?hybrids with scale-like structure were proposed to achieve improved electromagnetic?EM?absorption properties.Graphite/TiC/Ti₃AlC₂?G/TiC/Ti₃AlC₂?hybrids were fabricated in a molten salts bath and G/TiC/Ti₃C₂ hybrids were obtained by an HF acid eacting process afterwards.In G/TiC/Ti₃C₂ hybrids,graphite and TiC layer formed a skin-like plane and Ti₃C₂ sheets stood vertically on this plane.The as-prepared hybrids were dispersed in paraffin matrix to evaluate their EM absorption properties.G/TiC/Ti₃C₂ hybrids showed excellent EM absorption at X-band?8-12 GZh?.The minimum RC of G/TiC/Ti₃C₂reached-63 dB and the effective absorption bandwidth was larger than 3.5 GHz.G/TiC/Ti₃C₂ showed much better EM absorption ability than the materials without microsture design,e.g.,Ti₃C₂ MXene.Owing to the special interfacial junction of G/TiC/Ti₃C₂,electron band bending occured at the interface,which result in the formation of a depletion layer.Therefore,under the EM field,space charges accumulated at the interface and interfacial polarization was enhanced.The interfacial polarization amplified the response to the incoming EM waves of the G/TiC/Ti₃C₂hybrids,and thus resulted in the improved microwave absorption performance.