Research On Preparation And Properties Of C/C-TaC Composites

Carbon/carbon（C/C） composites have superior characteristics of low density,high strength,good ablation property,excellent thermal shock resistance and designable characteristics.These outstanding properties make them the fourth generation throat ablation materials, which is widely applied in Solid Rocket Motor（SRM） throats.With the further development of SRM,C/C composites can not satisfy the higher requirements.In this thesis,C/C-TaC composites were prepared by precursor infiltration pyrolysis（PIP） and adding TaC powder in preform （PP） for the first time.The microstructure,the degree of graphitization, the thermal conductivity coefficient and the mechanical and ablation properties of the C/C-TaC composites made by PIP and PP method were systematically investigated.The main work and conclusions presented in this thesis can be summarized as follows:（1） TaC precursor which is proper for infiltration was prepared by Ta powder,hydrofluoric acid and furan resin for the first time.And the best mass rate of different components was determined according to its characteristic of viscosity and solidification.The temperature for infiltration was decided by the chemorheology of the precursor,which was described by amended dual-Arrhenius equation.Its solidification technical paremeters were determined according to the solidification behavior,and solidified activation energy and reaction rate were accounted based on the DSC analysis.（2） The formation mechanism of TaC during heat treatment were investigated.It is found that H₂TaF₇ promote the solidification of furan-resin.After solidification,TaO₂F crystalline existed in precursor. With the increase of heat treatment temperature,Ta₂O₅ decomposed or hydrolyzed and.formed TaO₂F.When the temperature increased to 1200℃,Ta₂O₅ reacted with carbon then TaC formed.（3） C/C-TaC composites with different TaC volume contents were successfully prepared by PIP and PP methods,respectively.The distribution and microstructure of TaC,the degree of graphitization and thermal conductivities coefficient of these two composites were investigated.Compared with ones prepared by method of PP,C/C-TaC composites prepared by PIP method presented the less size and more uniform of TaC.The degrees of graphitization of these two composites are higher than that of C/C composite with the same technique.This was attributed to the stress graphitization at the interfaces between the fibers and TaC in C/C-TaC composite made by PP method and that between pyrolytic carbon and TaC in those made by PIP method.（4） The influence of the volume content of TaC and pyrolysis carbon and heat treatment temperature（HTT） of the above two composites were discussed.It is found that the mechanical properties of C/C-TaC composites made by method of PIP are better than those made by PP method.The relationships between mechanical properties and the volume content of TaC and pyrolysis carbon of above two composites are similar. That is,the lower content of pyrolytic carbon and the higher content of TaC,the lower the mechanical properties.The reason of reduce in mechanical properties of these two kinds of composites were analyzed and their fracture mechanism were discussed.The inter-laminar shear behavior of C/C-TaC composites made by PP method all show brittle fracture,while those made by PIP method changed from false plastic fracture into brittle fracture with the increasing volume content of TaC. The vertical and parallel compressive behaviors of C/C-TaC composites made by the two methods both show shearing or combination of shearing and delamination when the TaC volume content are low.They changed into delamination when the TaC volume content are high.（5） The oxyacetylene ablation properties of the C/C-TaC composites of the above two composites were investigated.The relationships between oxyacetylene ablation properties and the volume content of TaC and pyrolysis carbon of above two composites are similar.That is,the lower content of pyrolytic carbon and the higher content of TaC,the higher the oxyacetylene linear ablation rates.And the oxyacetylene ablation properties of C/C-TaC composites made by PIP method are better than those made by PP method.The oxyacetylene linear ablation rate of C/C-TaC composites with 47%pyrolytic carbon and 0.4%TaC made by PIP method is 0.0083mm/s,20%less than that of C/C composite with the same technique.The stagnation linear ablation rate of this material is 0.914 mm/s,35%less than that of C/C composite.The improvements of ablative resistance brought by proper content of TaC consist of restraining oxidation and offset the ablative defects.（6） TaC coatings are prepared by using TaO₂F for the first time.When the C/C-TaC composite made by PIP method with 47%pyrolytic carbon and 0.4%TaC was coated with TaC,its oxyacetylene linear ablation rate decrease to 0.0056 mm/s,49%less than that of C/C composite.