The Research On Matrix Modification Of C/C Composites And Their Oxidation Resistance

The precursor infiltration and pyrolysis (PIP) method was used to prepareC/C-SiC composites with hydrogen-containing polysiloxane as precursor, in order toimprove the ablation resistance and oxidation resistance of the C/C composites. Thecross-linking conditions of hydrogen-containing polysiloxane were studied. The bestratio of hydrogen-containing polysiloxane and crosslinker divinylbenzene is1/0.5; thefavorable cross-linking temperature is250℃, and the pyrolysis temperature shouldbe higher than1550℃. In the pyrolysis process, the break and rearrangement of themolecular chains mainly happen at the temperature range of300-800℃. The slowheating rate is preferred to improve the ceramic yield.C/C-SiC composites of1.24,1.36,1.69and1.84g/cm3were prepared through PIPmethod with hydrogen-containing polysiloxane as precursor, the density of the initialC/C composites was1.02,1.21,1.60and1.78g/cm3. After4times PIP process, thecontents of SiC of the above samples were15.78%,13.14%,5.28%and4.12%respectively.The ablation rates of C/C-SiC composites and C/C composites were determinedwith H2-O2flame ablation method. The results demonstrated that: the linear ablationrates of C/C-SiC composites with density of1.24,1.36,1.69and1.84g/cm3were3.11,2.22,1.67and1.44μm/s respectively, and the mass ablation rates were0.87,0.58,0.56and0.16mg/s. The ablation rates were proved that the introduction of SiCappearently improves the ablation resistance of C/C composites.The static air oxidation of C/C-SiC composites shows that the weight loss rates ofC/C-SiC composites with density of1.84g/cm3was76.3%that of C/C-SiCcomposites with density of1.85g/cm3. The fact proves that the addition of SiCsignificantly improves the oxidation resistance of C/C composites, Furthrmore, theanti-oxidation mechanism was analysed from the point of view of chemicalthermodynamics.Zr-containing polysilane was synthesized for the first time throughelectrochemical method, in order to introduce the refractory metal elements into C/Ccomposites, methyl trichlorosilane allyl chloride, cyclopentadiene, and zirconiumtetrachloride was used. The product contains48.8%Si (Wt%) and4.11%Zr.In the process, the crosslinking and pyrolysis condition of Zirconium-containingpolysilane was studied. the study of the crossling condition demonstrated that theZr-containing polysilane can cross-linked by itself, and the best cross-linkedtemperature is300℃, the highest ceramic yield is82.2%. Themogravimetrydifferential thermal curve analysis showed that the cleavage and rearrangement ofbond occur mainly between400~800℃.The C/C-ZrC-SiC composites of1.69g/cm3were produced after PIP for4cycles,using Zr-containing polysilanes as ceramic precursor, the density of the initial C/Ccomposites was1.28g/cm3. The density was increased by32.03%.The ablation property was measured with the H2-O2flame ablation method ofC/C-ZrC-SiC composites of1.69g/cm3, the linear ablation rate was2.11μm/s, andfor the mass ablation rate was0.518mg/s. The results showed that the addition of ZrCand SiC, to a great extent, can improve the anti-ablation properties of the C/Ccomposites. From the ablation morphology, the ablation surface can be divided into the central area, the transitional area and the rim area. The ablation mechanism indifferent regions was proposed.The static air oxidation experiment was done to C/C-ZrC-SiC composites from500to900℃, which showed that the oxidational weight loss rate of C/C-ZrC-SiCcomposites was80.7%that of C/C composites after oxidation for30min at900℃.The additions of ZrC and SiC greatly improve the oxidation resistance of C/Ccomposites. The anti-oxidation mechanism was analyzed.The anti-ablation and anti-oxidation mechanism of C/C-ZrC-SiC and C/C-SiCcomposites was analyzed from the perspective of chemical thermodynamics andchemical kinetics: Gibbs free energy(ΔG) of ZrC and SiC less than that of carbon, andthe oxidation of ZrC and SiC was prior to carbon, in addition, the apparent activationenergy of C/C-ZrC-SiC and C/C-SiC composites is greater than that of C/Ccomposites.