Mechanical Properties, Thermal Shock And Ablation Resistance Of (C_f-SiC_f)/SiBCN Composites

SiBCN powers were prepared by mechanical alloying.(Cf-SiCf)/SiBCNcomposites were prepared by hot-press sintering process. The phase compositionsand microstructures of the composites were studied, as well as the affe cts of weakinterface layer and additives (ZrSiO4) on mechanical properties, thermal shock andablation resistance.The results show that amorphous SiBCN matrix can be prepared by mechanicalalloying method. It is formed by hard aggregates of nano-particles. C or BNcoatings on the surface of Cfand SiCfwere in preparation of dip and pyrolys ismethod. The thickness of C coatings were not uniform, and the adhesionphenomenon between fibers was very serious. The thickness of BN coatings was abit thin but fibers dispersed better.(Cf-SiCf)/SiBCN composite prepared by hot-press sintering were mainlycomposed of β-SiC,-SiC and BCN. BCN had a turbostratic and amorphousstructure. Fibers in composites without weak interface layers arranged in a certaindirection. The composites had a feature of brittle fracture. After introducing weakinterface layers, fibers lost its preferred orientation. The fracture form was pseudoplastic fracture. However, density and mechanical properties reduced whenintroducing weak interface layers. The introduction of additives could significantlyimprove the performances. Its density, flexural strength, elastic modulus andfracture toughness were2.58g/cm3,235.3±18.4MPa,127.4±1.7GPa and5.33±0.25MPa.m1/2, respectively. Debonding and pulling out were main tougheningmechanism of carbon fibers, and silicon carbide fibers mainly relied on crackdeflection and extend crack propagating path.Flexural strength sharply declined with the increased of thermal shocktemperature difference. CSS-Aids composites had the fastest declined speed. Theresidual strength of composites with weak interface layer declined more slowly. BNcoatings had the better effect on improving thermal shock resistance of the materials.The residual strength after thermal shock were71.4%,71.4%and61.8%of theoriginal strength, respectively. After thermal shock at800°C, a slight, denudationand fracture of fibers and small amounts of cracks in matrix could be found. Whenthe thermal shock temperature difference reached1000°C and1200°C, oxidationphenomenon occurred on the surface of materials. Damage o f internal fibers wasslight and fiber pulled out can be observed. After introduced weak interface layer, mass ablative rate of compositesincreased slightly, but line ablative rate did not change significantly. Additivescould significantly improved ablation resistance of the composites. The massablative rate and line ablative rate were0.00136mm/s and0.0273g/s, respective ly.After the ablative experiments under an oxyacelene flame, ablative pits appesred onthe surface of composites. CSS-Aids composites had the smallest ablation regionand shallowest ablation depth, followed by CSS composites. The ablative surfacewas divided into three areas, that was ablative zone, transition zone andheat-affected zone. Fiber denudation phenomenon occurred in ablative zone.Transition zone covered a oxidation layer. And sparkling phenomenon appeared inheat-affected zone.The ablative mechanis ms of the composites were mechanicaldenudation, high-temperature airflow scouring and thermal oxidation ablation.