Investigation on the Ablation Behaviors of C/SiC-based Composites in Plasma Wind Tunnel

Author:Luo Lei

Supervisor:wang yi guang


Degree Year:2018





Hypersonic vehicles which have high flight speed would sustain obvious aerodynamic heating.Thermal protection system are one of key technology for the hypersonic vehicles,and thermal protection system materials are guarantee of their success.C/SiC and modified C/SiC materials are one of the most candidate materials for thermal protection system,due to their high strength,high specific modulus,anti-ablation,high temperature resistance,and low density.C/SiC and modified C/SiC materials own the advantages of C/C and ceramics that may obtain comprehensive performance to meet the harsh demands.The design and application of the thermal protection system relies on the response characteristics and failure mechanism of thermal protection system materilas in service environment.The ground experiment evaluation is the main method for these studies.High-frequency plasma wind tunnel has been chosen as the main ablation equipment for simulating the service environment and ablation mechanism research among numerous simulation methods,due to its pure flow field,long running time,strong simulation ability,high test efficiency,and could provide high enthalpy and speed plasma environment.This article studies the response characteristics and ablation mechanism in simulation service environment of C/SiC and modified C/SiC composite materialsb by plasma wind tunnel.The main research contents and results are as follows:(1)The ablation behaviors of C/SiC composites prepared by CVI method in plasma wind tunnel were systematically studied.Under low heat flux and pressure,the main control reaction of C/SiC samples is the atomic oxygen oxidation of SiC,including atomic oxygen oxidation of SiC to form SiO2 and erosion of SiO2 by atomic oxygen.The synergies between two reactions lead to the slight depletion of SiC layer on the surface.Under high heat flux and pressure,the main reaction of the samples during ablation is the decomposition or volatilization of SiC,leading to the high erosion rate of SiC layer.When SiC layer on the surface of samples depleted,the combustion and catalytic reaction of carbon fiber would cause the surface temperature jump phenomenon.(2)The influences of different preparation methods(including PIP,RMI and CVI methods)on ablation of C/SiC composites were studied.The ablation mechanism of C/SiC composites prepared by different methods is consistent.The surface steady temperature reaching 1850 oC is the key point for C/SiC composites ablation mechanism which transform from atomic oxygen oxidation to decomposition or volatilization of SiC.Different preparation methods lead to different surface steady temperatures of C/SiC materials in same condition.The samples prepared by PIP method have the lowest surface steady temperature,followed by the samples fabricated by RMI method,and C/SiC samples fabricated by CVI method have the highest surface steady temperature.Different preparation methods also lead to different consumption rate of surface SiC layer.The consumption rate of SiC layer prepared by RMI and PIP method is 2-3 times higher than samples prepared by CVI method.(3)The ablation of UHTCs modified C/SiC composites in plasma wind tunnel was studied.A ZrO2 skeleton is formed with silica filling inside and covering surface by the atomic oxygen oxidation of SiC-ZrB2-ZrC in the C/SiC-ZrB2-ZrC surface ceramic layer during ablation.Under low heat flux and pressure,there have atomic oxygen oxidation of SiC like in C/SiC samples,which including atomic oxygen oxidation of SiC to form SiO2 and erosion of SiO2 by atomic oxygen.Along with the erosion of SiO2 in oxide layer by atomic oxygen,the SiC phase in the ceramic layer would oxidize to supply the erosion of SiO2,and leading to the increase of oxide layer thickness on the surface.When the thickness of ZrO2 skeleton rises to a certain thickness,the formation rate of SiO2 lower than the erosion rate,and the SiO2 could not effectively cover the surface ZrO2.Then,the surface temperature of the samples would rise in a short range,due to the high catalytic coefficient of ZrO2.The detachment of ZrO2 particles and oxidation of SiC could supply the erosion of SiO2,and reduce the catalytic coefficient of surface.In this way,the surface temperature would rises in a short range but not jump.Under high heat flux and pressure,the erosion mechanism of SiC in ceramic layer transfroms from atomic oxygen oxidation of SiC to decomposition or volatilization of SiC.The SiC phase in the surface ceramic layer would completely consume up in a short time.The SiO2 phase in the oxide layer could not get replenishment and erode by the atomic in high rate.But,before SiO2 phase in the oxide layer depleted,the sureface temperature would rise but not jump.After the SiO2 phase in the oxide layer consuming up,the ZrO2phase in the surface exposes to the high enthalpy airflow in a large area.The high catalytic efficiency of ZrO2 causes the surface temperature jump phenomenon.The ablation mechanism of C/SiC-HfC composites in plasma wind tunnel is similar to that of C/SiC-ZrB2-ZrC composites.However,due to the higher catalytic efficiency of HfO2which is higher than that of ZrO2,C/SiC-HfC composites showes surface temperature jump phenomenon in earily time at low condition.(4)The atomic oxygen oxidation behaviors of C/SiC and C/SiC-ZrB2-ZrC composites were studied.When the forming and erosion rate of SiO2 reach equilibrium during C/SiC composites ablation,the thickness of the oxide layer on the surface is the equilibrium thickness.The thickness of the equilibrium oxide layer would decreases with the increase of ablation condition.The atomic oxygen oxidation behaviors during ablation of C/SiC-ZrB2-ZrC composites not only include the atomic oxygen oxidation of SiC which involved the forming and erosion of SiO2,but also have the atomic oxygen oxidation of ZrB2-ZrC ceramics to form ZrO2 phase and detachment of ZrO2 particles by high speed airflow.When the rate of forming ZrO2 by atomic oxidation and peeling off rate of ZrO2 by airflow reaches the equilibrium,the surface atomic oxygen oxidation behaviors the C/SiC-ZrB2-ZrC composites also reach the equilibrium.