| Silicon carbide (SiC) ceramics has many excellent properties, such as highstrength, high hardness, corrosion resistance and high temperature resistance, however,the intrinsic brittleness of the ceramics results in poor device reliability, and restrictsits development and application as structural ceramics. Therefore, strengthening/toughening of SiC ceramics is always a hotspot in materials science research.Laminated SiC ceramics has good strength/toughness, and can overcome thebrittleness of bulk ceramics and also can improve nonlinear mechanical behavior ofcontinuous fiber reinforced silicon carbide ceramic matrix composites (CMC-SiC)which is one of the great potential structural ceramics. At present, hot pressingsintering method (HP) is always used to fabricate laminated SiC ceramics, but theproblems of this approach are obvious: firstly, SiC laminated ceramics has isotropyonly in the layer, and HP is very difficult to satisfy the requirements of fabricatingcomplex shape components as considered direction; secondly, laminated SiC ceramicsmust be increased volume fraction of whiskers to overcome the influence of weakinterface binding on strength, but high temperature and hot pressing have seriousdamage of whisker; finally, laminated SiC ceramics has two strengtheningmechanisms with and within the layers, but the interface is difficult to control duringsintering process, resulting in difficult to play the synergy role of these twomechanisms. Above three problems prevents engineering application of laminated SiCceramics, developing new fabricating method of the ceramics has an importantapplication value, which can also enrich and develop strengthening and tougheningtheory of ceramic materials.Isothermal chemical vapor infiltration (ICVI) and tape casting (TC) were proposedto fabricate laminated SiC whisker (SiCw)/SiC structural ceramics. To optimize thefabrication process for SiCwslurry and laminated SiCw/SiC structural ceramics; usingheat treatment method to respectively modify SiCwand laminated SiCw/SiC structuralceramics; using polymer infltration and pyrolysis (PIP) and liquid silicon infiltration(LSI) method to densify the ceramics; by addition of micron SiC particles (SiCp) andnano SiC particles (SiCnp) to modify the ceramics, and respectively investigatingeffect of heat treatment, densification and particles modification onstrength/toughness of laminated SiCw/SiC structural ceramics. The main researchcontents and results are as follows:(1) The effects of dispersant content, binder content, plasticizer content,ball-milling time and the volume fraction of SiCwon SiCwslurry viscosity werediscussed, and investigated the rheological properties of SiCwslurry. The results showed that: when the dispersant content was2wt.%, the viscosity of the slurry had aoptimal value of1.21Pa·s, which was suitable for tape casting; the viscosity of SiCwslurry increased with increasing binder content, the large-adhesive viscosity of theslurry was not convenient for post-bubble process and was not suitable for tapecasting, resulting in reduction of the quality of sheet preform, so binder content chose3wt.%. The viscosity of SiCwslurry increased with increasing plasticizer content, inorder to reduce the organic content in the slurry, plasticizer content was3wt.%. Withincreasing ball-milling time, the viscosity of SiCwslurry firstly decreased and thenincreased. The ball-milling time can change the whisker morphology, so ball-millingtime was controlled in12h. With increasing solid content of SiCw, the viscosity ofSiCwslurry increased, due to the shorter distance between the whisker and largerattraction, resulting in increase of internal friction. SiCwslurry had a shear thinningcharacteristics and was suitable for tape casting.(2) The effects of tape casting process, ICVI process and content of SiCwon thetensile properties of laminated SiCw/SiC structural ceramics were investigated, theprocess parameters of fabricating laminated SiCw/SiC structural ceramics wereoptimized and the advantages of TC-ICVI process and properties of the ceramics wereelaborated. The results showed that: the ceramics had a gradient thichness, resulting inthe best density and tensile strength, owing to more crack deflection in the interlayerand longer and more whisker pull-out. The thickness of SiC layer is deposited on thesurface of single-layer SiCw/SiC composite, which can change the interface bondingstrength. When the thickness of SiC layer was thin, interface bonding strength wasmoderate, leading to high tensile strength of the ceramics; with increasing content ofSiCw, large-sized holes of30~200m significantly reduced in internal of laminatedSiCw/SiC structural ceramics, and small-sized holes of0.1~1m increased anddensity and tensile strength of the ceramics were improved significantly. TC-ICVIprocess can increase content of SiCw, reduce damage of whiskers and fabricatecomplex shape compents. The tensile strength, flexural strength and fracturetoughness of the ceramics with SiCwof40vol.%and thichkness of3mm wererespectively158MPa,315MPa and8.02MPa·m1/2。(3) The effects of heat treatment modification, densification modification andparticle modification on mechanical properties of laminated SiCw/SiC structuralceramics were investigated. The results showed that: whiskers after heat treatment candecrease stress concentration of surface to improve the load-bearing capacity ofwhiskers, reduce oxygen content on the surface and improve interface bondingstrength of whisker/matrix, leading to improvement of mechanical properties of theceramics. High temperature strength of the ceramics increased with increasingtemperature due to the effect of temperature on the strength of the porous matrix. When the temperature increased from room temperature to1500oC, interficialbonding strength increased to be helpful for toughening mechanisms of crackdeflection, bridging and whisker pull-out to improve mechanical properties of theceramics. When the temperature increased to1700oC, interface of whisker/matrixoccurred solid phase sintering action, resulting in the failure of interface and brittlefracture of the ceramics. Vacuum heat treatment had little effect on the strength of theceramics. After five PIP process, density increased linearly and porosity decreasedgreatly as parabolic type. Flexural strength and fracture toughness of the ceramicswere respectively356MPa and8.66MPa·m1/2. As the appropriate time of siliconinfiltration selected0.5h, density and mechanical properties of the ceramics can beeffectively improved in a short time. When the content of SiCwkept25vol.%, adding6vol.%SiCpcan improve density of the ceramics, tensile strength, flexural strengthand fracture toughness were respectively152MPa,333MPa and7.49MPa·m1/2.When the total volume fraction of SiCwand SiCnpwas fixed30vol.%, the density andmechanical properties of laminated (SiCw+SiCnp)/SiC structural ceramics increasedwith increase of SiCnpproportion.(4) Taking into account mechanical properties and microstructure of laminatedSiCw/SiC structural ceramics and the ceramics modified by heat treatment,densification and particles, strengthening/toughening mechanisms of the ceramicswere investigated. The results showed that: the main toughening mechanisms oflaminated SiCw/SiC structural ceramics were interlaminar crack deflection, interficialcrack deflection, bridging and whisker pull-out. The strengthening mechanism of theceramics was the high volume fraction of whiskers. Whiskers after heat treatment canchange interficial interface bonding strength, leading to improvement ofstrength/toughness of the ceramics. Laminated structural ceramics after heat treatmentcan change interficial and interlaminar interface bonding strength, resulting inimproving the strength/toughness of the ceramics. Densification modification canimprove density of the ceramics, in order to improve load-bearing capacity of theceramics, extending crack propagation path in the layer and between the layers,leading to improvement of strength/toughness of the ceramics. SiCp+SiCworSiCnp+SiCwwere together introduced to improvement of strength/toughness of theceramics by synergistic effect, involving toughening mechanisms of whiskers,particles and laminated structure. |
PDF Full Text Request