High Temperature Self Dual Friction And Wear Characteristics And Their Regulation Of C_f/SiC Composites Via Precursor Infiltration Pyrolysis

Continuous carbon fiber reinforced silicon carbide(Cf/SiC)combines many advantages of carbon fiber and silicon carbide,and is a heat-resistant and load-bearing integrated material that can meet high temperature applications.The precursor infiltration and pyrolysis(PIP)process has the advantage of near-net-size forming.Owing to the low fabrication temperature(～1000℃)of the PIP process,the composition and microstructure of Cf/SiC composites obtained by this process could be different in a hightemperature environment.The elevator,aileron and rudder of space vehicles are mostly made of Cf/SiC composite.They are hinged with the same material external hanging points fixed on the stabilizer surface through the sliding friction pair structure.The friction and wear characteristics of the hinged surface under high temperature bearing conditions are an important indicator of their performance,which seriously affects the attitude control of the space vehicle.Studying of the composition and microstructure evolution of PIP-Cf/SiC at high temperature and its influence on the mechanical properties and self-dual friction and wear characteristics of composites can provide technical support for the application of Cf/SiC as thermal structural materials in the aerospace field.In this paper,PIP-Cf/SiC composites containing pyrolytic carbon(PyC)interphase were prepared.In order to obtain the performance of the self-dual friction pair of PIPCf/SiC composites under the relevant working conditions of a space vehicle,the mechanical and wear tests were carried out by using high temperature direct test at RT800℃ and first heat treatment and then room temperature test at 800℃-1800℃.The microstructure evolutions of matrix,fiber,PyC interphase and fiber-PyC and PyC-matrix interfaces were systematically investigated,as well as the influence of these evolutions on the mechanical properties and friction and wear characteristics.The relationship between the mechanical properties and friction and wear characteristics was deeply discussed for the practical engineering application.According to the requirements of actual working conditions,the regulation technology of friction and wear characteristics of PIP-Cf/SiC composite at high temperature was explored.The conclusions are concluded as follows.(1)In air environment below 800℃,the mechanical properties of PIP-Cf/SiC composite are basically unchanged under the test condition,but at 400℃,the mechanical strength of carbon fiber and PyC layer on the surface of the composite decreases due to oxidation,and no friction film is formed on the surface,the tribological properties deteriorate.At 800℃,the surface of the SiC matrix oxidizes to form SiO2,and friction coefficient decreases because SiO2 melts to a certain extent.(2)Under the anaerobic condition of 800-1800℃,the matrix crystallizes and defects increase.The free carbon dissolved in the SiC continuum deposits,and the free carbon phase becomes turbostratic from amorphous.The carbothermal reduction reaction changes the PyC-SiC interface from a porous interface to a highly interlocking one,and causes chemical damage to the carbon fiber above 1600℃.The mechanical properties change little below 1600℃,and it decreases significantly above 1600℃;In the friction test,due to the stronger interface which can reduce the fibers stripping,the carbon content in the surfaces increases and the cutting effect of the large-size carbon fiber wear debris on the material surface is reduced,thereby friction coefficient reduce significantly.(3)In the process of friction,the friction film would not be formed,or will be suddenly damaged once the stress on the surface exceeds the failure limit of carbon fibre or the matrix,resulting in a huge rise of friction and wear.So good mechanical properties and high carbon content are the necessary conditions to ensure the application of PIP-Cf/SiC composite under high pressure and low speed reciprocating conditions.By depositing PyC layer on the surface of carbon fiber,appropriately increasing the pyrolyzed temperature and increasing pitch impregnation and pyrolysis for 1-2 times,PIP-Cf/SiC composite can have good mechanical properties as well as tribological properties.In the application of self dual friction pair on the aerospace thermal structure system,PIP-Cf/SiC composite can have good mechanical and friction and wear characteristics by the applicable tailor of carbon fiber,SiC matrix and interphase in the main working temperature range of 800℃-1800℃.In air environment below 800℃,due to the high oxygen content,the mechanical strength of carbon fiber decreases,which would lead to its poor tribological performance at about 400℃.In the future,it is necessary to improve the oxidation resistance of the composite or take some structural changes to reduce or avoid the oxidation of the friction surfaces at this stage.