Preparation Of SiC Nanowires And Their Application In Anti-oxidation Coating Of HfB₂-WB₂-Si/SiC-SiC_NW For C/C Composites

Carbon/Carbon（C/C）composites are provided with excellent properties at high temperature,and are extensively used in the fields of aviation and space flight.However,C/C composites are prone to oxidize at high temperature,which leads to a sharp decline in its mechanical properties.It can be inhibited at a certain extent through the preparing of oxidation coating on C/C composites.But,the mismatch of thermal expansion coefficients between the ultra-high temperature ceramic coating and the C/C composites matrix couldn’t be solved absolutly,which will lead to the cracks generating inevitably inside the matrix and coating during the course of work due to the thermal stress.In order to solve the above-mentioned problems,a creative method of preparing SiC nanowires(SiC_NW)was proposed in this parper.And it was introduced into the C/C composites and ceramic coating to reinforce the coating and modify the interface inside matrix and coating,which will improve the antioxidatant properties and extent the work life of C/C composites.And the main contents and results are as follows:1.SiC nanowires were successfully synthesized by pyrolysis of silicon doped coal-tar pitch.The effects of pyrolysis temperature and the doping ratio of PCS/TSCTP on the growth of nanowires were investigated.And the possible growth mechanism of the SiC nanowires was also discussed.The results are as follows:（1）Increasing the pyrolysis temperature results in an increase in the average diameter and in the quantity of nanowires produced.The morphology of the nanowires also changes with the variation of pyrolysis temperature.It could be well explained that the pyrolysis of SiOC and the thermal reduction of SiO₂ and C are intensified with the increasing the pyrolysis temperature,resulting in the increase of reaction components of gaseous SiO and CO.（2）With the increase of the ratio of PCS/TSCTP,the quantities of nanowires on samples increase firstly and reduce subsequently.When the ratio of PCS/TSCTP is 40%,the largest SiC_NW content is got.When the ratio of PCS/TSCTP is lower,the less SiC microcrystals from the pyrolysis of PCS will result in the reduction of the quantities of SiC_NW nucleation.And when the ratio of PCS/TSCTP is higher,the lack of active C will lead to the block of thermal reduction of SiO₂ and active C and the decrease of reaction components.The above-mentioned reasons have a disadvantage of the quantities of SiCNW.（3）The synthesized SiC nanowires areβ-SiC and grow along the[111]direction,and the formation of the SiC nanowires may be dominated by the vapor-solid（VS）mechanism,where SiC nanocrystals act as nuclei and gaseous SiO and CO act as active substance.Both the nuclei and active substance originate from the separation of amorphous SiOC glass and/or the reduction of SiO₂ by free carbon during pyrolysis.2.The SiC nanowires(SiC_NW)network was firstly synthesized on the surface of C/C composites with low density through precursor infiltration and pyrolysis（PIP）method.Then,the SiC-SiC_NW coating was prepared via reactive melt infiltration.The oxidation resistance of SiC-SiC_NW coated C/C composites at 800℃,1100℃,1300℃ and 1500℃ and the thermal shock resistance between 1500℃ and room temperature were investigated.In order to analyze the influence of SiC_NW in the coating,the oxidation resistance and the thermal shock resistance of SiC coated C/C composites were investigated at same condition.The results are as follows:（1）The preparation of SiC_NW layer can improve the oxidation resistance and thermal shock resistance of SiC-SiC_NW coated C/C composites.And benefit from the formation of SiO₂ glassy layer and the reinforced toughening of SiC_NW,SiC-SiC_NW coated C/C composites has a better oxidation resistance.（2）Compared with the oxidation resistance at 1300℃ and 1500℃,the antioxidant property of SiC-SiC_NWW coated C/C composites at 800℃ and 1100℃ is worse,which results from that the SiC ceramic couldn’t form the continuous SiO₂ glassy layer at lower temperature.3.The HfB₂-WB₂-Si outer coating on SiC-SiC_NW coated C/C composites was prepared by slurry brushing and the HfB₂-WB₂-Si/SiC-SiC_NW coated C/C composites toughed by SiC_NW was obtained finally.The oxidation resistance of the HfB₂-WB₂-Si/SiC coated C/C composites and HfB₂-WB₂-Si/SiC-SiC_NW coated C/C composites with different ratio of W/Hf were tested at 1500℃.And oxidation resistance of HfB₂-WB₂-Si/SiC-SiC_NW coated C/C composites at 1300℃,1100℃ and 800℃ was also tested.What’s more,the HfB₂-WB₂-Si/SiC-SiC_NW coated C/C composites with worse antioxidant property at 800℃ was improved by pre-oxidation technique.The results are as follows:（1）The anti-oxidation property of HfB₂-WB₂-Si/SiC-SiC_NW coated C/C composites is better than that of HfB₂-WB₂-Si/SiC coated C/C composites.The excellent oxidation resistance of HfB₂-WB₂-Si/SiC-SiC_NW coating is attributed to the formation of continuous HfSiO₄-SiO₂-WO₃ multiphase glass layer with good fluidity and its dense structure.（2）The weight loss of the HfB₂-WB₂-Si/SiC-SiC_NW coated C/C composites decreases firstly and then grows with the increase of W content.When the molar ratio of W/Hf=2:3,the anti-coating composites has a better anti-oxidation effect.The oxidation resistance of the coatings is influenced by the pinning effect of HfSiO₄ formed by the oxidation of the coating and the phase separation of WO₃.（3）The poor oxidation resistance of the samples at 800℃ is due to the failed formation of a continuous glassy oxygen barrier.But,the pre-oxidation process can form SiO₂-B₂O₃-WO₃ glass bar oxygen barrier layer on the surface of coating and improve the anti-oxidation property.