Microstructure Controling And Properties Of MoSi₂/RSiC Composites With Three-dimensional Network Structure

Molybdenum disilicide?MoSi₂?has been widely used as electrical-heating material for high temperature,due to its high melting point?2030 ??,low volume resistivity?2.2×10-2m?·cm?,good thermal conductivity?28.5W/?m·K??,and excellent oxidation resistance at high-temperatures.Unfortunately,there are some shortages,including the brittleness at ambient temperature,inadequate creep resistance at high temperature,accelerated oxidation between 400 and 800 ?,restricting the further applications of MoSi₂,which become the critical problems of MoSi₂ materials to be solved.Recrystallized silicon carbide?RSiC?maintains high strength at high temperatures,shows excellent corrosion and thermal shock resistance and so on.However,the sintering mechanism of RSiC is dominated by evaporation and condensation,which leads to no shrinkage during the sintering process,the porous microstructure of RSiC leads to the negative influence on mechanical properties and oxidation resistance.Morever,RSiC has high hardness and electrical resistance,resulting in hard machining.Since the molten MoSi₂ exhibits good wettability with SiC,a novel MoSi₂/RSiC composite with the microstructure of three-dimensionally?3D?continuous and interpenetrating each other both MoSi₂ and RSiC could prepared by impregnating MoSi₂ melt into the porous RSiC.The MoSi₂/RSiC composite can solve the brittleness at room temperature and poor creep resistance at high temperature of MoSi₂,and improve the mechanical properties,oxidation resistance and electrical conductivity of RSiC,which is possible to combine the advantages of the two materials,and to overcome their shortcomings.MoSi₂/RSiC composite is expected as a new type of electric heating elements or structural materials applied at high temperatures.Direct melt infiltration and alloy-activated melt infiltration processes were employed to prepare MoSi₂/RSiC composites with 3D interpenetrated network structure,and the microstructure was further controlled by the organic precursor impregnation pyrolysis?PIP?technique.The effects of microstructure and composition on mechanical properties,electrical conductivity and oxidation resistance of the composites were investigated.The main work and the results are summaried as follows:1.The effects of infiltration temperature and time on the microstructure,composition,mechanical properties,electrical conductivity and oxidation resistance of the composites were systematically investigated.The results show that the MoSi₂/RSiC composites exhibit 3D network structure.Both the RSiC matrix and the infiltrated MoSi₂ are continuous.The XRD patterns confirmed that the composites are mainly composed of ?-SiC,MoSi₂ and Mo_4.8Si₃C_0.6.The mechanical properties,electrical conductivity and oxidation resistance of the composites are improved compared to RSiC matrix.A high performance MoSi₂/RSiC composite with high density and low electric resistivity?165.3m?·cm?was obtained after melting infiltration at 2050? for1 h.When oxidized at 1500 ? for 100 h,the weight gain of the composite was only0.29mg/cm2,and the bending strength were increased by 2.2%.2.A Mo_4.8Si₃C_0.6 powder with a purity up to 98.5% was synthesized using Mo,Si and C powders.The thermal expansion coefficient,volume resistivity,mechanical properties and oxidation resistance of Mo_4.8Si₃C_0.6 bulk sintered using the synthesized powders were characterized.The results show that Mo_4.8Si₃C_0.6 possesses good electrical conductivity,but poor oxidation resistance and higher coefficient of thermal expansion than those of both SiC and MoSi₂.Therefore,the Mo_4.8Si₃C_0.6 phase should be avoided for maintaining the mechanical properties and oxidation resistance of the composites.The increase of Si vapor pressure in the melt infiltration environment can effectively reduce the reaction between MoSi₂ and SiC at high temperature,and inhibit the generation of Mo_4.8Si₃C_0.6.3.The infiltration process was carried out in the environment of increased Si vapor pressure after a thin SiC or C layer has been coated on the inner surface of the pores of RSiC matrix by PIP process using polycarbosilane?PCS?or phenolic resin?PF?as precursor.A thin amorphous SiC?Py SiC?or C layer was formed between the RSiC matrix and the infiltrated MoSi₂,which effectively reduced the micro cracks in the melt infiltration phase.Comparing to the Py SiC layer,the C layer seems to be more effective to improve the interface of matrix and infiltrated phase,so that the mechanical properties,electrical conductivity and oxidation resistance of the composites were further improved.4.MoSi₂/RSiC composites were prepared by a combination of PIP?PF?and alloy-activated melt infiltration processes.The Si and Ti with low melting point were added into MoSi₂,and the composite with high density was prepared by infiltration of1800? and 1900?.The composites obtained at 1800? containing some Si and Ti Si₂.when the infiltration temperature increased to 1900?,the composite shows excellent high temperature performance,which is mainly composed of MoSi₂,Ti5Si4,SiC,and asmall amount of Ti C.Compared to the direct infiltration,the infiltration temperature of alloy-activated melt infiltration decreased at least 200 ?.The lower infiltration temperatures were beneficial to decrease the damage of matrix during thermal shock at high temperatures,and improve the properties of the composites.The bending strength and elastic modulus of the composites obtained by alloy-activated melt infiltration increased 35.7% and 46.3% compared with those of RSiC,and the volume resistivity of the composites decreased to 57.6m?·cm,which decreased by nearly 5 orders of magnitude compared with that of RSiC matrix.