Fabrication Of ZrO₂ Modified C/SiC Composites Via Precursor Infiltration Pyrolysis Method And Properties Investigation

Carbon fibre reinforced silicon carbide(C/SiC) composites is a new type of high temperature structural material with the potential application in the aeronautic, astronautic and military fields.However,C/SiC cannot be long-term utilized above 1650℃and properties are obviously declined above 1700℃due to the shortage of high temperature endurance and oxidation resistance. In this dissertation, ZrO₂ modified C/SiC composites were designed and fabricated by precursor infiltration pyrolysis (PIP) method. Systematic investigation were studied of the induction route of ZrO₂, oxidation and ablation behavior of matrix modified by ZrO₂,and the effect of content of ZrO₂ on the properties of C/SiC composites.Firstly, this paper focused on the choice of precursor on candidation of zirconium-2,4-pentanedionate(Zr(AcAc)4), zirconium n-butoxide(Zr(OBu)4) and zirconium n-propoxide(Zr(OPr)4) as organical precursor, ZrOCl2?8H2O as inorganical precursor. The result revealed that the organical precursor can dissolve into PCS and divinylbenzene(DVB) mixture, and Zr(OBu)4 is the most suitable precursor with the proportion of PCS:DVB:Zr(OBu)4=2:1:2 of which the yield is 43.93%. ZrOCl2?8H2O can be utilized as precursor due to high solubility in methanol and good infiltration in SiC ceramics and C/SiC composites and the yield is 12.3%.For the purpose of investigating the effect of ZrO₂ additional to properties of matrix, ZrO₂ modified SiC ceramics were prepared. ZrO₂ existed in the form of thin grains among SiC blocks.With increasing content of ZrO₂, the porosity increased consistently, the porosity of ceremics with 29wt% ZrO₂ was 23.6% and density was 2.16g/cm3. Infiltration with pressure can decrease the porosity. Ceramics prepared by this method of which ZrO₂ content was 14.3wt% had porosity 23.6%, and density 2.31g/cm3. After oxidation at 1700℃, with the content of ZrO₂ increasing, weight loss increased and retention of flexural strength decreased. Compared to ceremics with 7wt% ZrO₂, weight retention of ceremics with 29wt% ZrO₂ decreased from 59.16% to 36.84%. Oxidation behavior of ZrO₂ modified SiCceramics may be divided into three steps according to the temperature gradient. Firstly, oxidation of free C and passive oxidation of SiC forming SiO₂ happened at 800℃to 1500℃. Secondly, according to the temperature rising and pressure of O₂ decreasing, active oxidation of SiC gradually became dominant which formed SiO gas at 1500℃to 1600℃. Then, above 1600℃, carbo-thermal reaction began and pyrolysis of PCS continued with gas forming, which caused the protective SiO₂ destroyed and the inner region of the ceramics was oxided. The ablation rate decreased with content of ZrO₂. when ZrO₂ increased from 7wt% to 29wt%, the mass loss rate changed from 0.0367g/s to 0.0195g/s, and the linear recession rate changed from cracked to 0361mm/s. After ablation, proportion of ZrO₂ decreased from the ablation center to edge, ZrO₂ existed mostly at the ablation center and SiO₂ at the edge.ZrO₂ modified C/SiC composites were fabricated on base of the ceramics, and the structure and properties were investigated, the mechanisms of oxidation and ablation were also researched. The result showed that, C fibers were efficiently protected by ways of infiltrating PCS/Xylene at the former cycles.Flexual strength was obviously improved, flexural strength of composites with 25wt% ZrO₂ was 216MPa and the one without protection was only 103MPa. Infiltration with pressure can make up this limitation. Composites fabricated with pressure had ZrO₂ content 9.0wt%, porosity 4.78% and flexural strength 292MPa. The intrinsically resistant to oxidation of ZrO₂ emerged at 1700℃, which improved the oxidation resistance of whole material. Composites with ZrO₂ 61wt%,25wt% and 16wt% had weight loss of 29.98%, 34.71% and 35.07%. Compared to C/SiC composites(0.0223g/s and 0.0602mm/s), the mass loss rate and linear recession rate of 61wt% ZrO₂ composites was 0.0161g/s and 0.0185mm/s.