Fabrication Of Cf/ZrC-SiC Composites Via Precursor Infiltration And Pyrolysis Process And Properties Investigation

The development of hypersonic vehicles is in great need of the application ofultra-high-temperature materials. Continuous fiber reinforced ultra high temperatureceramics matrix composites can not only get rid of the poor fracture toughness andthermal shock resistance of bulk ceramics, but also have the advantages of relativelylow density and the designment of materials, which makes it show a good prospect inthe fields such as thermal protection systems. In this paper, we set the target toinvestigate the fabrication of Cf/ZrC-SiC composites and its properties. Firstly, weinvestigated the preparation of ZrC precursors and the selection of ZrC precursor usedfor PIP process, and then Cf/ZrC-SiC was fabricated by PIP process. Finally, themechanical and anti-ablative properties of the composites were studied and theanti-ablative mechanism was discussed.Zirconium-containing complexes, which were obtained by chelation of organicligands to zirconium, were used to combine with phenolic resin to form precursors forZrC. The precursors using specific ligands including acetylacetone, ethyleneglycol, andsalicylic acid transformed into pure ZrC at a relatively low temperature (1550oC). Theprecursor using acetylacetone as the ligand had high stability in comparison with othersand was more suitable for the subsequent PIP process. Polyzirconoxane (PZO), obtainedby chelation of acetylacetone to zirconium, was used to combine with phenolic resin toprepare a ZrC precursor, namely PZO-ZrC. The pyrolysis transformation mechanismsof PZO and PZO-ZrC were studied. The results showed that the organics of PZO wasin-situ carbonized, resulting in a short diffusing path for carbothermal reduction.Therefore, the lower C/Zr molar ratio and pyrolyzed temperatures (1550oC) wereneeded, compared with that only using zirconium oxychloride and phenolic resin.Chitosan-zirconium complex, obtained by chelation of chitosan to zirconium, formeda novel precursor for ZrC, which also achieved the synthesis of ZrC at a relatively lowtemperature (1550oC). For the limitation of the low char yield of chitosan and its poorsolubility in water, the chitosan-zirconium complex was not very suitable for PIPprocess. On the other hand, PZO-ZrC, because of its superior chemical and physicalproperties, and the good infiltration effect, was more suitable for PIP process whencompared with zirconium-chitosan complex.Cf/ZrC-SiC, Cf/ZrC were prepared by PIP process with PZO-ZrC and PCS as theprecursors of ZrC, SiC, respectively. The Cf/ZrC-SiC composite with a CVD SiCinterface coating showed the improved mechanical properties, compared withCf/ZrC-SiC without any coating. The mechanical properties of Cf/ZrC, Cf/ZrC-SiCcomposites (with CVD SiC interface coatings) were poor for the long heat treatment athigh temperature (5times at1550oC,2h per time). The Cf/ZrC had a flexible strength of 52.1MPa, elastic modulus of7.1GPa, and fracture toughness of2.2MPa·m1/2; whileCf/ZrC-SiC had the superior mechanical properties: a flexible strength of69.7MPa,elastic modulus of18.5GPa, and fracture toughness of3.8MPa·m1/2, because of SiC,which can deliver the load effectively. After being ablated by oxyacetylene flame for80seconds, the Cf/ZrC composite, which had a mass loss rate of0.0024g/s and a linearrecession rate of0.0126mm/s, showed a superor anti-ablative property in comparisonwith Cf/ZrC-SiC and Cf/SiC composites. When SiC was introduced, the anti-ablativeproperty of Cf/ZrC-SiC decreased; however, it was still superior to Cf/SiC and had agood anti-oxidation property.Cf/ZrC-SiC composite was prepared through decreasing the times (2times) of thehigh temperature heat treatment, adjusting when to introduce SiC, and the content ofSiC in the matrix, as a result, its mechanical property was greatly improved. Theoptimized Cf/ZrC-SiC composite had a flexible strength of141.3MPa, elastic modulusof50.1GPa, and fracture toughness of11.6MPa·m1/2; however, it had a relatively pooranti-ablative property in comparison with Cf/ZrC, which was attributed to the finalintroduce of SiC and its high content in the matrix. After ablation, the Cf/ZrC-SiCcomposites had a strength retention rate of67%and showed a tough fracture behavior.The ablation mechanisms of the Cf/ZrC-SiC, Cf/ZrC composites in oxyacetyleneflame environment were discussed. As to Cf/ZrC, the ablation included chemical andmechanical process. In the ablation process, the oxidation product ZrO2had a highmelting point and a low vapour pressure, a little of which may be blown off by the gasflux, then the left ZrO2was sintered at high temperatures to form a dense layer, whichprotected the carbon fiber and the matrix. The ablation of Cf/ZrC-SiC involved inchemical, mechanical, and themal process. At the start-up of the ablation, the carbonfiber was oxidized and the SiC matrix in the surface of the composites was alsooxidized and its product SiO2was gasified because of its low vapour pressure. With thefurther oxidation of the matrix, the glass mixture of ZrO2and SiO2spreaded on theablated surface and protected the composites. Although SiO2may be gasified at hightemperature, the left ZrO2can be sintered to form a dense layer, which was regarded asa further protection barrier.Finally, C/ZrC-SiC composite was fibracated using chopped T-300felt as thereinforcement. It had a good mechanical property and its flexible strength and fracturetoughness were142.3MPa,8.0MPa·m1/2, respectively. On the other hand, it had arelatively high linear recession rate of0.0318mm/s, though which was still superior toCf/SiC.