Fatigue Properties Of SiC_f/SiBCN Composites Prepared By Precursor Impregnation Pyrolysis Method

With the increasing demand of ultra-high speed aircraft,the development of component materials which has high temperature has been paid attention by many countries in the world.Fiber reinforced ceramic matrix composites have attracted much attention of researchers because of their low density,high temperature resistance,oxidation resistance,high temperature creep resistance and high specific strength.SiC/SiBCN composites are the hotspot of this kind of materials today.However,the fatigue properties of this material are rarely reported,which limits its design,application and further development.Therefore,the tensile-tensile fatigue behavior of SiC/SiBCN composites at room and high temperatures has been studied in this paper.The composite material studied is made of KD-II SiC fiber preform and PBSZ by precursor infiltration and pyrolysis method.The effects of residual stress,oxidation damage,frequency and stress ratio on the fatigue properties of the material are discussed by fatigue test at room temperature.The results showed that:(1)In high cycle fatigue,the residual tensile stress in the material will be released during the fatigue process,which will make the material have higher apparent fatigue limit stress.In low cycle fatigue,the crack propagation in the material is too fast,and the residual tensile stress in the material will break before it can release.(2)The fatigue properties of SiBCN matrix remain excellent because of the selfhealing and self-healing properties of SiBCN matrix at 1200 C.At 1350 C,due to the rapid consumption of BNC phase at high temperature,the fatigue properties of SiBCN matrix and fibers are oxidized apprantly,and the fatigue properties of SiBCN matrix decrease a lot.(3)The fatigue life of materials decreases with the increase of frequency and increases with the increase of stress ratio.This is because the fatigue damage mechanism of long-term fatigue is mainly caused by the friction damage of fiber/matrix interface.The frequency and stress ratio of tensile fracture have little influence on the tensile fracture,and the fracture behavior is similar.(4)The fatigue limit of materials is predicted by S-N curve and Goodman curve.The error is less than 5%,and the error of fatigue life prediction is also within the scope of engineering.The effects of temperature,frequency and stress ratio on fatigue properties of materials are studied in high temperature fatigue test.The results showed that:(1)The fatigue life of materials decreases with the increase of temperature.The higher the temperature,the more serious the oxidation erosion of the interface,the greater the damage caused by the friction of the fiber/matrix interface,and the higher the temperature,the more serious the creep damage of the material.(2)The lower the frequency and the greater the stress ratio,the lower the high temperature fatigue life,and the creep damage has the main influence on the fatigue fracture of materials.The higher the frequency,the smaller the stress ratio and the lower the high temperature fatigue life,the greater the role of oxidation damage.In general,the interfacial wear of fiber/matrix is the main cause of material fatigue failure and fracture in normal temperature fatigue test,and the creep of fiber is the main factor of material fracture failure at high temperature.The material has excellent fatigue properties at room temperature and good fatigue properties at high temperature.