Study On Oxidation Behavior Of Silicon-based Ceramic Coatings

Carbon/carbon?C/C?composites are prone to oxidize at elevated temperatures,which limits their applications as high-temperature structural materials.The anti-oxidation ceramic coatings are considered to be effective methods to solve this problem.At present,Si C ceramic coatings have been attracted much attention as the promising ceramic coating.The oxidation behavior of Si C coating is greatly affected by additives,so it is essential to do a systematic research on them and then fully realize the application potential of Si C-based coatings.To improve the oxidation resistance of Si C coatings for C/C composites,it was studied that the effects of additives?Al₂O₃,Zr B₂,and Si C nanowire/pyrocarbon?Si Cnw/Py C?structures?on the microstructures and oxidation behavior of Si C coatings by experiment and first-principle calculation.Moreover,the initial oxidation mechanisms of Si C and Zr B₂ ceramics were studied;the influence mechanisms of Al and Zr atoms on the Si O₂ glass layer formed after oxidation were analyzed;the transformation mechanism of Si C nanowires and the toughening mechanism of Si Cnw/Py C structure were also investigated.The main contents and conclusions are listed as follows.Si C coating was prepared on C/C composites by pack cementation using Si,C and Al₂O₃powers as raw materials.Si C ceramic sheets were prepared to simulate the oxidation behavior of the coatings themself.The prepared Si C coating with Al₂O₃ was dense due to the liquid Al₂O₃ filling the gaps among Si C particles.during the preparing process.The introduction of Al₂O₃ had a negative effect on the oxidation resistance of Si C ceramic.Small amounts of Al atoms substituted Si atoms in the Si O₂ lattice to form Al-doped Si O₂ lattice.With CO molecule entering the lattice,the Si O₂ network structure was destroyed by releasing CO₂molecule.The remaining O vacancy increased the permeability of oxygen.As an O₂ or CO molecule reached near the vacancy,a new network structure was formed to reduce the permeability of oxygen and protect the internal materials.Higher Al₂O₃ contents expanded the volume of the Si O₂ lattice and breaked some Si-O rings.When the gaseous-byproduct CO was close to Al-O bond,it inserted into Si-O ring and resulted in the expanding of Si O₂ lattice and the increase of the permeability of oxygen,causing internal materials to be oxidized.The morphology stability of 3C-Si C nanowire with or without a Py C layer prepared by chemical vapor deposition?CVD?have been studied at different temperatures?1800-2100°C?.For the 3C-Si C nanowire without a Py C layer,the coarsening was observed after heat treatment at 1800°C.With the increase in temperature,the 3C-Si C nanowire transformed to the structure with decorated hexagonal prisms and new nuclei were synthesized on it.As the growth of the new nuclei,the 3C-Si C materials were transformed to stable?-Si C crystals.For the Si Cnw/Py C structure with the diameter of 3?m,it possessed the stably filament-shaped morphology and the Si C nanowire was cubic structure after heat treatment at 1800-2100°C.This was due to that Py C layer changed the free surface of the 3C-Si C nanowires by nucleating and growth on them during the CVD process,and Py C layer prevented the atomic glide of stacking faults by the strong binding force during the heattreatment stage.Atomistic mechanisms of the initial oxidation on the 6H-Si C?0001?and Zr terminated-Zr B₂?0001??Zr-Zr B₂?0001??and B terminated-Zr B₂?0001??B-Zr B₂?0001??surfaces at different temperatures were investigated by first-principle calculation.The oxygen coverage rate was lower at 1500°C.As O₂ molecule adsorption on the surface of 3×3supercell,the achieved 6H-Si C?0001?adsorption structure with the oxygen molecule along all channels were same because the adsorption space of O₂ molecule was large enough,and the dissociation of the O₂ molecule occurred.The achieved Zr-Zr B₂?0001?adsorption structures with the O₂ molecule along the vertical channel were different from these along the parallel channel due to the change of the dissociation process of the O₂ molecule.Four B-Zr B₂?0001?adsorption adsorption structures were obtained.Only part of the O₂ molecule was dissociated and had a stable adsorption state.The oxygen coverage rate increased at 800°C.With O₂molecule adsorption on the surface of 2×2 supercell,the adsorption space of O₂ molecule decreased,which caused the different binding states of O₂ molecule on 6H-Si C?0001?,Zr-Zr B₂?0001?and B-Zr B₂?0001?surfaces.Moreover,by comparing the energy barriers of the O₂ molecule adsorption on 6H-Si C?0001?,Zr-Zr B₂?0001?and B-Zr B₂?0001?surface,the initial oxidation occurred first on the 6H-Si C?0001?surface,followed by Zr-Zr B₂?0001?surface,and finally B-Zr B₂?0001?surface.However,both B-and Zr-Zr B₂?0001?belong to Zr B₂?0001?,so the oxidation barriers of Zr B₂?0001?were higher than these of 6H-Si C?0001?.Thus,once Si C-Zr B₂ multiphase ceramics were oxidized,Si O₂ was firstly formed and B₂O₃generated.Si Cnw/Py C structures reinforced Si C-Zr B₂ coating was prepared on Si C coated C/C composites by a three-step technique consisting of CVD,CVD and pack cementation.The as-prepared coating was dense because that the gaseous C-containing materials infiltrated into the porous Si C nanowire layer and formed a new porous layer with large open pores during the CVD process.The large open pores were beneficial to the permeation of the raw materials used to prepare Si C-Zr B₂ coating.In addition,Si in raw materials surrounded and reacted with the Py C to generate dense Si C layer,which combined with the Si C and Zr B₂ particles in the coating during pack cementation.After 25 thermal cycles of“room temperature?1500°C”,the coated samples with Si C nanowires lost mass of 4.92%.After 40thermal cycles of“room temperature?1500°C”,the coated samples modified by Si Cnw/Py C structures lost mass of 0.30%?Si O₂-Zr O₂ glasses with different Zr O₂content were prepared by cold pressing and sintering,and the influence of Zr on the high-temperature stability of Si O₂ was studied.The results showed that the volatilization of Si O₂ decreased with the introduction of Zr.The Zr atom in the topmost surface of the generated Zr Si O₄?001?preferred to diffuse into Si O₂ lattice and occupied its interstitial region or substitutes a host Si atom,and the interstitial Zr atom increased the adhesion energy of the Si O₂ interface,which contributed to improving the stability of Si O₂ glass at high temperature.