High Temperature Properties Of C_f-SiC_f-Al₂O₃Preinforced Potassium-based Geopolymer Composites

Short carbon/silicon carbide hybrid fibers reinforced geopolymer matrixcomposites(Cf-SiCf/KGP composites) and the compostites adding withaluminosilicate particle(Cf-SiCf-Al2O3p/KGP composites) with different siliconecarbide fiber lengths and fixed carbon fiber lengths were prepared by the methodthat mixing a kind of hybrid fiber preform pre-developed and geopolymer resin.High temperature properties of the composites were focused and studied bycomparing the microstructure and mechanical properties of the two compositesunder different conditions including high-temperature exposure, high-temperatureceramic treatment and high-temperature mechanical properties testing.The results show that the composites exhibited dramatically improvedmechanical properties owing to the reinforcement of short carbon/silicon carbidehybrid fibers when compared with the potassium-based geopolymer. Especially, theCf-SiCf/KGP composites with1mm length silicone carbide fibers, the flexuralstrength, Young s modulus, fracture toughness, work of fracture are5.7times,1.9times,5.1times and69.7times, respectively. The mechanical properties wereenhanced further with the incorporation of Al2O3p. Take the Cf-SiCf-Al2O3p/KGPcomposites with1mm length silicone carbide fibers as an example, the flexuralstrength, Young s modulus, fracture toughness, work of fracture are6.4times,2.9times,4.2times and59.6times, respectively.For the Cf-SiCf-Al2O3p/KGP composites, the incorporation of Al2O3pincreasesthe apparent density, reduces the high-temperature shrinkage and improves theYoung s modulus, which indicates the synergistic toughness effect of particle andfibers. However, the toughness effect of Al2O3pon the composites flexural strengthis not always positive. The experiment shows that with the incorporation of Al2O3p,the composites flexural strength is improved at800°C and lowered at1000°C ormore.After high-temperature exposure, carbon fibers are oxidized and the mechanicalproperties of the composites drop sharply. While comparing with the compostieswith carbon fibers alone, the composites reinforced by silicone carbide fibers andcarbon fibers diplay a certain advantage that silicone carbide fibers play a role intoughening and maintain the integrity of the composites at high temperature.Comparing the phase and morphology of the two composites under differentconditions including high-temperature exposure, high-temperature ceramictreatment and high-temperature mechanical properties testing, it is found that the incorporation of Al2O3phave positive effects on delaying the phase transformationof matrix and slowing down the interface reaction between fiber and matrix. Oxygenhas a strong role in promoting the matrix phase transformation and the interfacereaction.The fracture behavior of the initial composites is typical noncatastrophicpseudo-plastic fractue and the sample shows the phenomenon of apparentlysevered but actually connected. Afer high-temperature exposure, the fracture modehas changed into brittle one. As for high-temperature ceramic treatment andhigh-temperature mechanical properties testing, the samples s fracture mode changeinto brittle one until the temperature reach1200°C.Above all, mechanical properties and fracture behavior of particles and fibershybrid reinforced geopolymer matrix composites are not only related to theperformance of the reinforcement itself, but also related to the matrix strength andthe the interface bonding strength between reinforcement and matrix. Theintroduction of Al2O3pimproves the strength of the matrix and achieves thesynergistic toughening between the particle and fibers reinforcement. The incrementof bonding strength between fiber and matrix can improve the strength of thecomposites within a certain range. But when the bonding strength is over high, itwill make fiber debonding and pulled-out toughening mechanism weakened, andthen reduce work of fracture of the composites, even worse cause catastrophicrupture. Under the condition of high temperature oxidation, carbon fiber areoxidized completely, but silicon carbide fibers play a role in ensuring the integrityof composites, which greatly improves the application field of the composites.