INVESTIGATION OF THE THERMO-MECHANICAL AND ABLATIVE BEHAVIOUR OF SILICON CARBIDE BASED CONCRETES EXPOSED TO HYBRID PROPULSION ENVIRONMENTS

摘要

This research is part of the PERSEUS project, a space program concerning hybrid propulsion and supported by CNES. The main goal of this study is to characterize silicon carbide based micro-concrete with a maximum aggregates size of 800 μrn, in a hybrid propulsion environment. The nozzle throat has to resist to a highly oxidizing paraffin/LOX hybrid environment, under temperatures ranging up to 3000°C. The study is divided in two main parts: the first one deals with the thermo-mechanical characterization of the materials up to 1400°C and the second one with an investigation on the ablation behaviour in a standard atmosphere, up to 3000°C. The combustion time is of 15s. Young's modulus was determined by resonant frequency method: results show an increase with the stabilisation temperature. Four points bending tests have shown a rupture tensile strength increasing with stabilisation temperature, up to 1200°C. Sintering and densification processes are primary causes of this phenomenon. Visco-plastic behaviour appears at 1100°C, due to the formation of liquid phases in cement ternary system. High-temperature oxidation in ambient air was carried out at PROMES-CNRS laboratory, on a 2 kW solar furnace, with a concentration factor of 15,000. A maximum 15 MW/m2 incident solar flux and a 7 to 90 seconds exposure times have been chosen. Optical microscopy, SEM, EDS analyses were used to determine the microstructure evolution and the mass loss kinetics. During these tests, silicon carbide undergoes active oxidation with production of SiO and CO smokes and ablation. A linear relation between mass loss and time is found. Oxidation tests performed at 15 MW/m~2 solar flux have shown a mass loss of 20 mg/cm~2 after 15 seconds. After 90 seconds, the mass loss reaches 80 mg/cm~2. Surface temperature measurement is a main point in this study, because of necessity of a thermo-mechanical-ablative model for the material. Smokes appear at around 6.5 MW/m2, leading to the impossibility of useful temperature measurements by optical pyrometry. Micro-concrete is really interesting for the nozzle realisation, thanks to its workability, and its thermo-mechanical properties. After 30 seconds, mass loss in micro-concrete is one half of pure a-SiC. This result is really interesting to study SiC-based concretes in oxidizing environments, instead of pure SiC. Our goal is to improve the thermo-mechanical properties and to study micro-concrete in a hybrid propulsion environment, developing a phenomenological model.