Research On The Near-Net-Shape Forming And Performances Of Oxide Fibers/SiO₂ Ceramic Composites

Silica ceramics and oxide fibers have been widely used in many fields such as aerospace and metallurgy,but the low strength of ceramics has seriously restricted their further application while the strength of ceramic composites is relatively high.However,there are some problems in the existing forming methods to prepare ceramic composites,such as inhomogeneous composition,long preparation period,and difficult to prepare complex-shaped parts.Direct coagulation casting via high valence counter ions?DCC-HVCI?and selective laser sintering-vacuum impregnation(?SLS-VI?forming methods have great application potential in forming complex-shaped ceramics with high mechanical performance,however,up to now,it has been rarely reported to prepare oxide fibers/ceramic composites by these two methods.Based on the above situation,in this paper,SiO₂ and the oxide fibers?Al₂O₃ and mullite fibers?were used as the research objects to prepare ceramic composites by DCC-HVCI and SLS-VI method.The microstructure,phase composition,and mechanical properties were systematically studied by means of SEM,X-ray diffraction and electronic universal material testing machine.The main results were summarized as follows:?1?Al₂O_3?f?/SiO₂ composites were successfully prepared by the DCC-HVCI method.the Al₂O_3?f?/SiO₂ suspension could be rapidly in-situ coagulated with the increase of temperature.On the one hand,the ions strength would be increased via controlled release of Ca²⁺from calcium iodate,which would compress the double-layer of the ceramic particles.On the other hand,the hydrolysis of glycerol diacetate would generate acetic acid and adjust the pH of suspension to the isoelectric point.When pH was 11 and the contents of dispersant TMAOH was 2.5 wt%,Stable suspension with high solid loading?50 vol%?and low viscosity?<1 Pa·s?could be prepared at room temperature.The addition of 0-15wt%alumina fibers had little effect on the viscosity of the suspension.The suspension could be coagulated by adding 6.5 g L^-1 calcium iodate and 1.0 wt%GDA after heating at 70? for 30 min.And green bodies with complete shapes and high strength could be obtained after demolding.Due to the fiber pull-out and fracture behaviors,the mechanical properties of Al₂O_3?f?/SiO₂ composites with certain contents of Al₂O₃ fibers could be improved remarkably.When the fiber contents were 10 wt%and the sintering temperature was 1350?,the flexural strength of the composites reached the highest value of 30.3±2.7 MPa.?2?Mullite_?f?/SiO₂ composites were successfully prepared by the SLS-VI method.The good-quality mullite fiber green body could be prepared with 10 wt%epoxy resin E12,laser power of 7 W,scanning speed of 2000 mm/s,scanning space of 0.13 mm,layer thickness of 0.13 mm.The sample pre-sintered in 1300? could successfully remove the binder and possess some strength.The pre-sintered mullite green body could be impregnated with SiO₂particles when placed in vacuum condition for 15 min because of the low bulk density of SLS forming method,the pores left by the binder removal process and the gaps between mullite fibers.SiO₂ particles would form a liquid phase when sintered at 1600?,which would fill the pores and the joints between mullite fibers and thus improve the flexural strength of mullite_?f?/SiO₂ composites.The flexural strength of the Mullite_?f?/SiO₂composites increased first and then decreased as increasing the volume fraction of SiO₂suspension.When the volume fraction of SiO₂ suspension was 20 vol%,the flexural strength of the Mullite_?f?/SiO₂ composites reached the highest value of 12.2±0.14 MPa.The near-net-shape forming methods of DCC-HVCI and SLS-VI could be used to prepare complex-shaped oxide fibers/SiO₂ composites with high performance,which could largely shorten the forming period and cut down the cost.Thus,these two forming methods have a great application potential in aerospace and other fields and provide a process reference for the preparation of other complex-shaped ceramic composites with high mechanical properties.