Characterization Of SiC Fibers And Study On Their Resistance To High Temperature And Oxidation

SiC fiber is an ideal reinforced phase material for ceramic matrix composites?CMC?,and its performance directly determines the overall performance of the CMC.In order to meet the steady development of domestic SiC fibers,a set of systematic,standardized,and complete structural properties characterization methods are needed to provide basis for the evaluation and improvement of domestic SiC fibers.Moreover,research on the structure and performance changes of domestic SiC fiber under typical service environment?high temperature,oxidation?can provide technical support for the application and development of domestic SiC fiber.In this paper,four kinds of domestic SiC fibers were heat-treated?1200°C,1350°C,1600°C?in Ar atmosphere for 2 hours in a vacuum hot-pressing sintering furnace.The fibers were subjected to oxidation treatment?1050°C,1200°C,1350°C?in static dry air for 1 h in a muffle furnace.Then the Weibull distribution was used to characterize the mechanical properties of the SiC fibers.The AES,XPS,SEM,XRD,RMS and other methods were used to study the changes of fiber microstructure,elemental composition,grain size and other structural changes.Research indicates:?1?The strengths of fiber are in accordance with the two-parameter Weibull distribution.At room temperature,the strength of A-II is the highest,and A-I and B-I are similar,B-II fiber has the lowest strength due to defects.The strength of the SiC fiber depends on the grain size and C/Si ratio of the fiber,excessive grain size and too large C/Si ratio result in a decrease in strength.When the grain size is 4.6nm,it can be represented bys=4.54-1.58×I_c/si;?2?The elastic modulus E of the SiC fiber depends on the fiber density?and can be expressed by E=?455+480??/?₀-480;?3?In the high-temperature treatment process,the strength gradually decreased with the increase of temperature,mainly due to?-SiC grain growth and free carbon increase and regularity.At 1600°C,the SiC_xO_y phase decomposes strongly,resulting in a rapid increase in grain growth rate and free carbon regularity,which ultimately leads to a rapid decrease in strength of fiber.The high temperature resistance of A-II and B-II fiber is superior to that of A-I and B-I fiber.The modulus of the fiber is less affected by the high temperature treatment;?4?During the oxidation process,the strength of fiber gradually decreases with increasing temperature and a dense Si O₂ oxide layer is formed on the surface of the fiber.The faster the reaction,the thicker the Si O₂ layer and the faster the strength of fiber decreases.B-I fibers have the worst antioxidant properties,while A-I is slightly better than A-II fibers.Because the oxidation time is short and the temperature is low,the grain size increases less,so the grain size has less effect on the fiber strength.At the same time,the modulus of the fiber is less affected by oxidation.The method proposed in this paper to evaluate young's modulus with density and evaluate strength of fiber with grain size and C/Si ratio can quickly evaluate the performance of SiC fibers.