| Continuous silicon carbide fiber-reinforced silicon carbide matrix composites?Si Cf/Si C?are regarded as"star"candidate material for the hot section components in high performance aero-engines,due to their advantages,such as high thermal stability,low density,high specific strength,oxidation resistance,ablative resistance,high reliability,etc.The researches show that the service life of ceramic matrix composites?CMCs?presents a mechanism related to stress level?crack?,but the cracking stress of the matrix determined by the traditional stress-strain curve is often higher than the real stress,which makes the service life of the composites fluctuate greatly.In addition,different loads in service can cause damage like matrix microcracks,large cracks and interfacial debondings in the composites,which will provide channels for the oxidizing atmosphere?such as O2 and H2O?in the service environment to invade the interior of the material,thus leading to the oxidation of the constitutes of the composites and reducing the service efficiency.Therefore,it is urgent to improve the evaluation and understanding of the damage behavior?especially crack evolution?under specific stresses and their influence on oxidation resistance of Si Cf/Si C composites.The experimental platform including acoustic emission?AE?,X-ray tomography imaging?X-CT?to explore the damage behavior of CMCs,and platform to test the mechanical properties of composites at both ambient and high temperature in air have been built in this project.And first,the damage characteristics of Cf/Si C composites during loading/unloading processes corresponded to the specific stress levels are analyzed.Results show that the damage development of Cf/Si C composites can be closely related to their acoustic emission characteristics,and the irreversible damage is produced from the initial loading.Four types of matrix cracks are initiated during the whole loading/unloading process:cracks initiated in the matrix near the edge of the sample,some of the which extended through several fiber layers;cracks existing in the internal layer of pure matrix without fiber bundles;cracks occurred in the transverse fiber bundles;and cracks formed and propagated in the longitudinal fiber bundles.The crack distribution is almost unchanged under the stress less than 23 MPa.The development of new and old cracks can be observed after unloading from 63 MPa,while the stress above 120 MPa can greatly accelerate the damage development and failure.Based on the successful analysis of the damage behavior of Cf/Si C composites,the damage development of Si Cf/Si C composites during loading/unloading,fatigue and dwelling at specific stresses is studied,in particular.And it is found that the?mini?minimum damage formation stress?and?onset?large damage formation stress?are generally higher than that of Cf/Si C composites,and the Si Cf/Si C composites have higher resistance to damage.And their native crack density and size are significantly lower than that of Cf/Si C composites.Apart from the above four kinds of crack forms in Cf/Si C composites,cracks are easily initiated near the hole of Si Cf/Si C composites during the loading/unloading process,the multi-step fatigue can produce another two matrix cracks,which are cracks that can be easily formed along the boundaries of various matrix components?such as the Si C and Si?,and cracks generated in the axial fiber bundles,but blocked by certain axial fibers and propagate along the interfaces.Fatigue stress can cause more interfacial damage and reduce effectiveness of materials in application.In the unloading process of fatigue cycle,low energy damage such as interfacial debonding and fiber sliding will occur.The dwelling stress can widen the crack formed in the early stage and make the bridged longitudinal fiber bear higher stress for a long time,thus increasing the probability of its fracture.The damage of materials under specific stress levels will almost reach saturation during the dwelling period,while?onset is increased after each dwelling process.No through-thickness crack or crack network was formed during the 120 h dwelling process at 80 MPa,which has little influence on its service performance.Based on the comprehensive exploration of damage development of Si Cf/Si C composites,this project focuses on the oxidation behavior of the material at 1200°C in air atmosphere after pre-damaged by 3 mechanical treatment above,as well as the high-temperature fatigue and creep behavior in the same environment.Studies have shown that,for the composites with no loading before oxidation,the tensile strength retention rate after oxidation is over 80%,the anti-oxidation performance is closely related to the mechanical processes before oxidation,as for the samples pre-damaged by the dwelling process at 100 MPa,the tensile strength retention after oxidation is only 60%,and strain to failure is only 23%of the original composites.The open pores inherent in Si Cf/Si C composites and the various cracks connected with the sample surface or the open pores can be channels for oxygen to diffused into the materials.Oxidation occurs often near the interface of the composites,and the matrix can be oxidized before the fiber.The decrease of?miniand?onset indicates the increase of oxidation severity.When the high-temperature fatigue test with the given peak stress was carried out,the specimens failed without reaching the fatigue life of 106 cycles,and the number of cycles to failure significantly decreased with the increase of peak stress.The average fatigue life with peak stress of 120 MPa was only 6207 cycles.In the creep test with the same conditions,except the fracture at 0.75 h under 120 MPa,the creep test at 100 MPa and 80 MPa lasted for 12h without failure.The repeated opening and closing of matrix cracks in the composites under fatigue stress will introduce oxidizing atmosphere such as oxygen,while expelling gaseous products,and further promote the oxidation reaction,in addition,the detritus of solid oxidizing products or the original constitutes can be introduced into the interface,leading to the failure of fibers due to the more serious friction and wear under the rapid loading/unloding cycles.However,under the application of low creep stress?<100 MPa?,the crack scale is relatively low and the propagation capacity is limited,and the minor cracking and other damage inside the composites will be closed by solid oxidation products,which hindering the further oxidation reaction,so the composites has a relatively high service life. |
PDF Full Text Request