Performance Modification And Component Fabrication Of KD-? SiC Fiber-reinforced SiC Composites

Due to their low densit y,high specific strengt h,good toughness,excellent thermostability and oxidation resistance,et c,c ontinuous sil icon carbi de fiber reinforced si licon carbide(Si Cf/Si C)composites have been focused and applied in aeroengi nes and other areas.In this thes is,precurs or infiltration and pyro lysis(PIP)process has been adopted to prepa re SiC_f /SiC com posites,employi ng KD-II S iC fiber as reinfor cement,a low mol ecular polyc arbosil ane wi th –CH=C H2 groups(LPVCS)as precursor,P yC and BN coati ng as the interfac es.The single filament tensile strength of S iC fiber s with different coati ngs have been studi ed.Effects of(PyC /BN)n multilayer interfaces on the properties of SiC_f/S iC composit es have been discus sed.Performance s of SiC_f/S iC composi tes after heat and oxi dati on treatm ent have been investiga ted.A optimiz ed PIP process has been adopted to fabricate thin-walled SiC_f/SiC com posite components.Interface is a critical factor influencing the perform ance of ceramic m atrix composites.BN int erface coul d improve the performance of SiC_f /SiC compos ites as the previous researc hs.Two different processes(dip-coating and CVD)have been carried out to prepare BN coating on the s urface of SiC fiber.The raw materials ratio has significa nt effect s on the composit ion of BN coating when di p-coati ng process is adopted.As t he results of XR D and XPS,BN i s t he base of coat ing while the molar ratio of boric acid and urea is 1:1.The raw materials ratio and the deposi ting temperature have effects on the compos ition of BN coati ng when employi ng CVD process.The nitride content in the coating increases with the ratio of am monia and boron t richloride and the increasing trend becomes slower.The nitride content decrease s firstly and increa ses subsequently as the depositing tem perature increas es.The stabilit y of BN coating deposited at l ow tempera ture is very poor.While the morphology of the coating is complete,the S iC fibers are covered by the BN coating effectively.The thickness of the coati ng can be controlled by the deposit ing time.The single filament tens ile strengt hs of the dip-coated fibers decreases firstly and increa ses subs equently,then decreases at 6thcycle.The single fil ament tensile strength of t he SiC fi ber with a 5 dip-coating cyc les BN coating is 1.56 GPa,which is 4.7%higher than t he as-recieved fiber.The s ingle filament tensile strengt h of SiC fiber varies significantly after prepa ration of(DB /PyC)n multilayer coatings.The single filament tensile st rength of the PyC /DB/PyC coated fiber is 1.67GPa(the highest),while the fiber wi th DB/ PyC coating has the lowest single filament tensil e strength of0.98 GPa.After the prepar ation of(PyC / CB)n multilayer coatings,t he single filament tensile strengt h of SiC fi ber changes in a smaller range.The hi ghe st single filament tensile strength of P yC/CB/ PyC coated fi ber is 1.68 GPa,while that of CB/ PyC/ CB coated fiber was 1.30 GPa.The KL-N has the lowest flex ural strength of 144.1MPa.Among the composit es with(DB/PyC)n interfaces,KL-PyC posses ses the best mechanic al property,whose flex ural stre ngt h and fra ct ure toughness ar e 498.7MPa and 27.8MPa·m1 /2,respectively.While the flexural st rength and fracture toughness of KL-DB are 225.7MPa and9.0MPa·m1/2,res pectively.The KL-CB shows the wors t mechani cal perfor mance with a flexural strengt h of 298.8MP a and a fracture toughness of 13.1MPa ·m1/2in the composites em ployi ng(PyC/CB)n as interfaces.The flexural st rength and fracture toughness of KL-PyC/CB were 566.0MPa and 19.3M Pa·m1/2,respec tively,which are the best.The KL-PyC/D B has the highest int erfacia l shear stren gth of 138.6MP a,howev er,its flex ural s trength and fracture toughne ss are 275.4MPa and 3.5MPa·m1/2,respectively.The interfacial shear strength of KL-DB/PyC/DB is 18.9MPa,which is the lowest,while i ts flex ural strength is 302.4M Pa.The interfacial shear strength of composites with high flexural s trength such as KL-PyC(close to 500MPa)and KL-PyC/CB(566.0MPa)are 39.2M Pa and 38.6MPa,respect ivel y.The results indicate the machanic al properti es of compos ites degra de when the i nterfaci al shear strength of composit e is too high or too low,which vertify the interface theory of cer amic matrix composites well.The crystallization and grain size of S iC matrix increa ses with the heat treatment temperature.S iC fiber changes similar to that of the matrix.The weight loss rate of SiC_f/SiC composites after heat treatment is less than that of Si C matri x under the same condition.After heat treated at 1200? for 1 hour,t he flex ural strength and fracture toughne s s of KL-1200-1H decreas e to 452.1MPa and 15.5MP a·m1/2,respectively.The dat as decrease to 364.4MP a and 15.7MP a·m1/2when the heat treatment tempera ture increase s to 1400?.The mechanical properties of KL-1600-1H deteriora tes,with a flex ural st rength of 119.1MPa and a fracture toughness of 4.1MPa·m1/2.The results from nanoidentation showed that the hardness and modulus of fiber and matri x reduce as the heat treatment tempera ture increas es,which is the pri mary cause of the composi te performance degradation.The time preser vati on of heat treatment at 1200? has little effects on the mechanical propert iesof SiC_f/SiC compos ites.The flexural st rength and fracture toughness of KL-1100-1O decrease to297.8MPa and 10.1MPa·m1/2,respec tively,and the strength retention rat e is only 60%.The mechanical properti es of KL-1200-1O decreas es further wi th a fl exural strength of 246.1MPa and a fracture t oughne ss of 8.7MPa·m1 /2.The flexural strength of KL-1300-1O is nearly the same as t hat of KL-1200-1O,while the frac ture toughness increa ses to 10.3 MPa·m1/2.Extending the time of oxidation has si gni fica nt effects on the mechanic al properties of the com posites.The flex ural strength and fracture toughness of KL-1100-5O decreas e to 176.3MPa and 6.6MPa·m1 /2,respectively,while those of KL-1100-10 O decrease to 145.0MP a and 6.1MPa·m1/2,respectively,wit h a strength retention rate of only 29%.Thin-wall ed SiC_f/SiC composite com ponent has been prepared by a optimized PIP process.Firstly,weaving and PIP moul ds have been designed accor ding to the model of thin-walled component.P IP proce s s has been opti mized by learn ing matured PMC proce sses such as vacuum bag m olding and autocl ave as sisted crosslinking.The appropriate processing time has been chosen for the cone and supporting body i n the thin-walled component.Slurry and CVD SiC coating have been us ed to improve the appearance of the component products.The C AE module of Siemens PLM NX 8.0Software has been used to sim ulat e the local part of the component.The si mulation resul ts show that,under a load of 300 N on the local part of annulus,the outside of u-shaped hole on the annulus and t he support body have the largest displacement of7.924?m.The max imum of the units st ress and node stress are 8.782 MPa and 18.84 MPa,respectively,whi ch are l ocated at the ins ide of u-shaped hole on the annulus and the support body.