Study On SiC Nanowire-toughened Silicon-based And Hafnium-based High-temperature Oxidation And Ablation Protective Coatings

C/C composites are prone to oxidize and ablate at elevated temperatures,which limits their applications as high-temperature structural materials.The anti-oxidation and anti-ablation ceramic coatings are considered to be the effective methods to solve this problem.At present,silicon-based and hafnium-based ceramic coatings have been attracted much attention as the most promising ceramic coating systems.However,the intrinsic brittleness of these coatings and the mismatch of the thermal expansion coefficients between these coatings and C/C substrate often give rise to the cracking of the coatings,which results in the failure of the coatings.To solve these problems,in this thesis,silicon-based and hafnium-based coatings toughened by the well-dipersion SiC nanowires with different morphology were proposed and prepared by chemical vapor deposition?CVD?,in-situ synthesis and pack cementation.The phase composition and microstructure of the as-prepared SiC nanowires and coatings were analyzed;the mechanical properties,thermal expansion and oxidation protective abilities of the as-obtained coatings were investigated;the growth of SiC nanowires,the oxidation protective failure mechanism of the coatings and the reinforcement mechanisms of SiC nanowires were researched.The main contents and conclusions are listed as follows:SiC nanowire-toughened SiC coating was prepared by a two-step technology of CVD and pressure-less pack cementation.Firstly,a porous SiC nanowire preform was deposited by CVD.Secondly,this material was subjected to pressure-less pack cementation to infiltrate the porous preform with the coating materials,which successfully achieved the well-dispersion of SiC nanowires in the coatings.Results showed that,after incorporating SiC nanowires,the fracture toughness of the coating increased by 50%,the microcrack density of the coating decreased by 70%,and the weight loss of the coated samples is only 2.68% after isothermal oxidation at 1500? in air for 44 h.This study reveals the toughening mechanisms of SiC nanowires including nanowire pull-out,nanowire bridging and crack deflection.Based on the relationship between the tougening effect of SiC nanowires and interfacial bonding state,a novel hot-pressure pack cementation method is put forward for preparing SiC nanowire-toughened silicon-based ceramic coatings,and SiC nanowire-toughened SiC-Si coating was prepared by a two-step technology of CVD and hot-pressure pack cementation,which successfully achieved the good interfacial bonding between the nanowires and the coatings,significantly improved the toughening effect of SiC nanowires,that is to say,the fracture toughness of the coating increased by 75%,the microcrack density of the coating decreased by 95%,and made the as-prepared coatings possess the excellent oxidation protective ability at 1500? in air.The weight loss of the coated samples is only 0.51% after isothermal oxidation at 1500? in air for 70 h.This study reveals the novel toughening mechanisms of SiC nanowires including the plasticity deformation of the nanowires and the plasticity fracture at the interface of the nanowire-matrix,which extends the toughening mechanisms of SiC nanowires in the composites.In addition,this study also reveals the oxidation protective failure mechanism of the coating during 1500? isothermal oxidation that the presence of Al³⁺ in the coating damaged the network structure of SiO₂ glass and resulted in the slight oxidation of C/C composites.To solve the problem that the oxidation protective temperature range of silicon-based ceramic coatings is too narrow,SiC nanowire-toughened SiC/SiC-Si multilayer coating was prepared by CVD and pressure-less pack cementation.The as-prepared coating possesses the excellent oxidation protective abilities between 1500? and room temperature,which can protect C/C composites from oxidation for more than 313 h at 900? in air and for more than 112 h at 1400? in air.The weight loss of the coated samples during isothermal oxidation at 900? is mainly attributed to the presence of the penetrating cracks in the coating during the whole oxidation process.The weight loss of the coated samples during isothermal oxidation at 1400? is mainly attributed to the presence of the penetrating cracks in the coating during thermal cycling from 1400? to room temperature.To solve the problem that SiC nanowire-toughened multilayer ceramic coatings prepared by CVD have some impurity and the poor interfacial connectivity with inner coatings,a novel in-situ synthesis method is put forward for preparing SiC nanowire-toughened multilayer ceramic coatings,and SiC nanowire-toughened SiC-Si/SiC-Si multilayer coating was prepared by in-situ synthesis and pressure-less pack cementation.The as-prepared coating could protect C/C composites from oxidation between 1500? and room temperature in that the oxidation behaviors of the coated samples were the continuous weight gain process,which achieved the oxidation protective of silicon-based ceramic coatings for C/C composites between 1500? and room temperature.This study discovered that the interfacial bonding strength of silicon-based ceramics with other materials was improved significantly by in-situ synthesizing SiC nanowires on their surface,and revealed the interface anchoring mechanisms of SiC nanowires involving the interfacial bonding anchoring and the mechanical interlocking anchoring.Motivated by the reinforcement mechanisms of the bone-shaped short fibers,a novel concept is put forward for the design of the special morphology SiC nanowire-reinforced ceramic coatings,and bamboo-shaped SiC nanowire-reinforced HfC coating was prepared by a two-step CVD technology,which successfully solved the problem of the poor interfacial bonding between the nanowires and the coatings in SiC nanowire-reinforced ceramic coatings prepared by the current CVD technology.Results showed that the incorporated bamboo-shaped SiC nanowires effectively suppressed the cracking and debonding of the coating.After incorporating bamboo-shaped SiC nanowires,the fracture toughness of the coating increased by 105%,and the interface bonding strength between the coating and C/C substrate increased from 0.35±0.12 MPa to 10.73±0.53 MPa,which was mainly attributed to the reinforcement and interface anchoring mechanisms of the bamboo-shaped SiC nanowires.This study reveals the novel reinforcement mechanisms of the bamboo-shaped SiC nanowires including the generated special mechanical interlocking between the nanowire nodes and the surrounding coating matrix and the generated microcrack toughening during the naowire pullout.