Ceramization mechanism of ZrB₂ - boron phenolic resin modified silicone rubber thermal protection system materials and the influence of oxygen in service environment

The ceramization reaction of silicone rubber thermal protection system (TPS) materials, along with the resultant ceramic phase structure, is critical for enabling these materials to endure high-temperature environments and withstand erosion from high-velocity flows. Under service conditions, in addition to intrinsic physical and chemical transformations, the reactions between the materials and environmental constituents significantly influence the ceramization process. To investigate the influence of oxygen on ZrB₂ - boron phenolic resin modified silicone rubber TPS materials, thermal analysis experiments were conducted under varying oxygen concentrations (0 %, 5 %, 10 %, and 21 %) across a temperature range from room temperature to 1800 K. These experiments utilized a thermal analyzer, fourier-transform infrared spectrometer, X-ray diffractometer, and other instruments. The results indicate that the influence of oxygen in the service environment on the ceramization reaction can be attributed to two primary aspects. Firstly, oxygen affects the oxidation behavior of the silicone rubber matrix. In a nitrogen atmosphere, cyclic siloxanes formed during pyrolysis undergo structural reconstruction at temperatures above 1300 K. In an air atmosphere, these cyclic siloxanes are oxidized to form new C = O organic groups before undergoing pyrolysis and subsequent structural reconstruction at elevated temperatures. Secondly, in an air atmosphere, zirconium boride and boron phenolic resin fillers undergo oxidation to produce boron oxide. This boron oxide reacts with silica to form borosilicate B-O-Si structures. Additionally, the molten boron oxide acts as a pore-filling agent within the ceramic phase structure, leading to the formation denser ceramic phase structure than the network structure in a nitrogen atmosphere.