Machanical Properties Of Unidirectional SiC Fiber Reinforced Potassium-based Geopolymer Composites

Potassium-based geopolymer （KGP） and unid irectio nal S iC fiber reinforcedpotassium-based geopolymer composites （SiC_f/KGP） were prepared by a simp leultrasonic-assisted slurry infiltration method us ing metakaolin, potassiumsilicate solution and unidirectiona l SiC fibers. Variety of techniques inc ludingXRD, SEM, TEM, therma l analyzer, dilatometry and three point bending ectwere used to study the effects of the mechanica l properties and fracture behaviorof the composites with different fiber contents at room te mperature. Theproperties of the composites and interface between fiber and matrix were alsoinvestigated after exposure to air fro m600oC to1000oC. Additio nally, the hightemperature mechanical properties and fracture behavior were discussed.The results show that the obtained S iC_f/KGP presented a full infiltrationand homoge nous distributio n of unid irectional SiC fibers in the matrix.Compared with the potassium-based geopolymer, the composites exhib iteddramatically improved mecha nical properties due to the incorporation of SiCfibers. Especia lly to the composites reinforced by20vol.%SiC fibers, theflexural strength, Young’s modulus, fracture toughness and work of fractureincreased14.4times,3.4times,14.4times and81.5times, respective ly. Theflexural stre ngth alo ng with the direction o f load parallel to fiber sheet （xdirection） was higher than that of the directio n of load perpendicular to fibersheet （z direction）, and in contrast, the work of fracture was lower. Comparedwith the typica l bending shear fracture mode in the x d irection, the fracture modein the z direction was interla minar shear fracture.Leucite and a little of kaliophilite were observed in the K-based geopolymerafter exposured to air at1000℃, while only a little of kaliophilite could be foundin the composite in the same cond ition. So, the addition o f SiC fiber tends tohinder crysta llization of the aluminum silicate geopolymer. The compositesexpanded alo ng with SiC fiber axial direction. The average linear thermalexpansion coefficie nt fro m RT¹300oC was3.6×10^-6/oC, which s ma ller than thatof SiC fiber. The co mposite samp le shrinked significant ly in thickness and widthat1000oC and reaches the maximum va lue of11.98%and13.82%, respectively.Meanwhile, mechanica l properties of materia ls with increasing exposuretemperature gradua lly lower relative to their original state and the compositestend to fracture in a brittle way. The flexura l strength retention o f samp leexposed at900oC is80.0%. However, after exposure at1000oC, the flexural strength o f composites sharply deteriorated, with only25.8%strength retentionThe strength decrease is ma inly attributed to the microcracks formedperpend icular to the axia l direction of fiber due to the therma l expansioncoefficie nt mis match and interface reaction between fiber and matrix.The high te mperature mecha nical test results show that the mecha nicalproperties of geopolymer increased due to the sintering densification, andimproved with the temperature ris ing of the test. The high tempreture flexuralstrength and work of fracture increased171.0%and325.6%, respective lycompared with the room te mperature properties. After tested at high temperature,the hardness of geopolymer increased83.7%than before, and the compositesfracture in a typ ical brittle way. However, the mecha nical properties of theSiC_f/KGP decreased with the test te mpeture increased, and the fracture modechanges fro m a pseudo-ductile way to a brittle way at900℃. The flexuralstrength reached110.1MPa at1100℃, with only69.3%strength retention. Thereasons for the strength decrease of the composites are the therma l mis match andpoor interface between fiber and matrix.