Fabrication And Properties Of Aligned SiCNWs Reinforced SiC Nanocomposite

Improving toughness while maintaining high strength is always research focus for ceramics,especially for silicon carbide ceramics.Introduction of reinforcing agent is a widely adopted method to improve strength and toughness.Silicon carbide nanowires?Si CNWs?,one-dimensional nanomaterial with outstanding mechanical properties and high aspect ratio morphologies,are promising candidates as reinforcing agents.Strengthening and toughening performance of Si CNWs is related to its orientation and interfacial bonding strength.Currently major reports focus on random Si CNWs,and rarely on interface tailoring between Si CNWs and matrix.Hot pressing,slip casting and extrusion molding are traditional technologies to induce Si CNWs alignment.But dimension of the alignment,orientation angle and uniformity need more consideration,including its complex manufacturing process.Direct ink writing,as an extrusion type additive manufacturing,which equipped with small size nozzle,is a desired method to obtain one-dimensional orientation of Si CNWs while enabling its homogeneity.3D printing technology can realize intelligent manufacturing of aligned Si CNWs networks with preferred orientation.Based on the above background,it is proposed to achieve strengthening and toughening through aligned Si CNWs by DIW.Preparation and performance study of aligned Si CNWs/Si C nanocomposite was carried out.And the following contents are obtained.First,aligned Si CNWs network is fabricated through DIW technology.The Lotgering index of Si CNWs in printed lines is?0.956 which indicates pefect unidirectional alignment.High temperature sintering and chemical vapor infiltration were adopted as densification method.The structure revolutions of aligned Si CNWs during the two processes were studied.High aspect ratio morphologies of Si CNWs gradually transformed to rod-like morphologies and eventually to particles in Si CNWs and Si C powder mixed samples through high temperature sintering with the aids of Si C powder and sintering additives.Si CNWs doesn't play the part in strengthening and toughening due to the loss of its aligned structure.For the Si CNWs networks densified through CVI process,the high aspect ratio morphologies of Si CNWs were kept as also the well-aligned structure of Si CNWs.The compressive strength of the porous Si C ceramics,which is assembled by aligned Si CNWs with high aspect ratio,was significantly improved.According to the above results,aligned Si CNWs/Si C nanocomposite,with integral structure features of orthogonal overlapping between adjacent layers,was fabricated through densification of aligned Si CNWs skeleton by CVI process.The effects of printing distance and Si CNWs content on microstructures and properties were studied.For samples with compacted struts,only outer zones of samples were filled leaving inner zones as loosen structure due to the limits by CVI process.And flexural strength was only 10.2±2.5MPa.Adopting aligned Si CNWs skeleton with minimum strut distance is a way to solve the problem.The optimal properties were achieved by sample with 13vol%Si CNWs content.The density,flexural strength and fractural toughness is2.62±0.02g/cm³,198.4±24.8MPa and 3.5±1.0 MPa·m^1/2respectively.Two typical microstructures existed in sample,aligned Si CNWs/Si C region and CVI-Si C matrix.Si CNWs fracture and pull-out were obvious in Si CNWs/Si C region.The left pore size is 10?200?m.And based on calculation,the pore size was larger than critical flaw size,which would lower the material mechanical properties.In order to diminish the disadvantage aroused by large pores,in-situ grown Si C nanowires were introduced on aligned Si CNWs skeleton to optimize structure of aligned Si CNWs/Si C nanocomposite.Through parameters tailoring,desired morphologies of in-situ Si C nanowires were realized.And the 10?200?m pore size was successfully shifted to<3?m.But due to limits of CVI process,the density descended to 2.30g/cm³.The flexural strength is 226.8±17.6MPa and the fractural toughness is3.9±0.1 MPa·m^1/2.Mechanical properties can be further improved by reducing defect density.On the basis of structure optimizing by in-situ Si C nanowires,precursor impreganation and pyroplyia?PIP?and liquid silicon infiltration?LSI?were adopted to reduce porosity of Si CNWs/Si C nanocomposites.The microstructures and mechanical properties were studied.The pores which left by CVI process were successfully fulfilled by liquid polycarbosilane during PIP process.But Si C matrix pyrolysed from polycarbosilane were discontinued with defects left in the structure such as cracks.Therefore cracks propagated through defects in the matrix or interface between PIP matrix and CVI-Si C matrix.PIP-Si C matrix possessed low load-bearing capacity including load transmitting capacity to Si CNWs.Hence there was no obvious improving of flexural strength.During LSI process,Si matrix fullfilled the space among in-situ grown Si C nanowires which resulted in dense structure.But thermal damages to the commercial Si CNWs raw material at relatively high temperature during LSI process were obvious.It deteriorated mechanical properties of aligned Si CNWs/Si C zone.Heat treatment of in-situ grown Si C nanowires improved bonding strength between in-situ grown Si C nanowires and matrix,hence the mechanical properties of this micro-zone were improved.The key to the performance reduction of Si CNWs/Si C nanocomposites densified by LSI process is the poor high-temperature stability of the purchased commercial Si CNWs raw material.Py C interphase with varied thickness were deposited on aligned Si CNWs through chemical vapor deposition process in order to tailoring bonding strength between Si CNWs and Si C matrix for better toughening effects.With?5nm Py C interphase,the load transmitting between Si CNWs and matrix were effective.The flectural strength,elastic modulus and fracture toughness were enhaced significantly.When the thickness of Py C interphase was increased to?15nm,load-bearing capacity of Si CNWs are poor.Thick interphase resulted in low bonding strength,and crack propagated along the interface between Si CNWs and matrix.Si CNWs can not carry the load effectively,which resulted in property decrease.Thickness increase of Py C interphase results in lowering of flexural strength.But compared to micro-zone of Si CNWs/Si C without Py C interface,micro-zones with Py C interphase possed potential of higher toughness due to 53%incresement in plastic energy.