Investigation Of The Oxygen Barrier Properties Of Binary Transition Group Metal Silicide Modified ZrB₂ Coatings Based On Self-Propagation Combustion Synthesis

Carbon structural materials are widely used in aerospace,deep space and deep earth,and other extreme environments with alternating high and low temperatures due to their excellent physical and chemical properties.However,due to the low oxidation resistance and oxygen embrittlement at high temperatures,the application of carbon structural materials is constrained.In coatings applied to the surface of carbon structural materials,ZrB₂-based coatings exhibit excellent protection against oxidation in a wide temperature domain.However,the vapourisation of the low melting point oxidation product B₂O₃ not only allows for catastrophic evolution and structural splitting of the oxygen barrier structure,but also weakens the ability of the sealing glass film to inhibit oxygen penetration.Transition group metal silicide modification can not only be used as a silicon source to rapidly generate higher melting point glass films,but also can utilize the synergistic effect among multiple transition group metal oxides to strengthen the ability of inerting oxygen diffusion and permeation in sealed complex phase glass layers,further improving the protective performance of the coatings.In addition,given the strong ionic complexation ability of the high-valence,small-radius transition group metal Ta⁵⁺,it is expected to achieve the creation of hermetic complex glass films with low oxygen permeability on the coating surface.In order to quantitatively explain the role of tantalum modifiers,there is an urgent need to deepen the study of the effect of tantalum content variation on the oxygen barrier ability of composite coatings.Therefore,this thesis first investigates the oxidation resistance of coatings with different TaB₂ content and the effect of film formation temperature on the oxygen barrier capacity of Ta-B-Si-O composite glass films.Furthermore,in order to suppress the destructive dynamic evolution of ZrB₂-based coatings,this thesis used self-propagating high-temperature synthesis technique to prepare highly reactive composite powders such as ZrB₂-MoSi₂-TaSi₂ and ZrB₂-HfSi₂-TaSi₂ to enhance the sintering properties of the coatings,and then combined with the low-temperature hot-pressure sintering technique of discharge plasma sintering to prepare oxygen-barrier-enhanced ZrB₂-MoSi₂-TaSi₂ and ZrB₂-HfSi₂-TaSi₂ composite coatings were prepared on the carbon substrate surface,respectively,and the effects of the Ta-based compound content and the synergistic reinforcement of the double transition group metal silicides on the oxygen barrier properties of the ZrB₂-based coatings were investigated.The main research contents and experimental results are as follows:The results of tantalum content variation on the oxidation of the composite coating at 1500℃ show that the lower tantalum content can effectively inhibit the generation of pores in the glass layer and improve the structural oxygen barrier capacity of the coating and the inert oxygen barrier capacity of the glass film,resulting in a 62.16%reduction in the oxygen permeability of the 40TaB₂-60SiC coating.During the oxidation process,the high-valent metal cation Ta⁵⁺has a strong ionic complexation ability,which not only improves the overall viscosity and skeleton strength of the complex glass film,but also enhances the inert response of the coating to the dynamic evolution of the oxygen barrier structure.However,too high Ta content will,on the contrary,weaken the self-healing efficiency of the sealed glass layer and increase the oxygen permeation rate.In addition,the film-forming treatment study in the wide temperature range of 1100-1600℃ showed that higher film-forming treatment temperature could improve the fluidity of the self-generated glass film and promote the uniform dispersion of Ta nanocrystal particles in the Ta-B-Si-O complex glass layer,which in turn enhanced its self-healing and sealing effects.With the increase of film-forming temperature,the values of structure factor and inertia factor of the coating gradually decreased,and the coating had the best oxygen barrier effect after the film-forming treatment at 1500℃,and the values decreased by 43.12%and17.33%,respectively,compared with the untreated specimens.By studying the formation of double silicide modified ZrB₂ coating by alloying Ta with transition metal Mo and Hf silicide,it was found that the maximum weight gain and oxygen permeability of 50ZrB₂-（MoSi₂-10TaSi₂）coating decreased by 48.21%and 28.87%respectively compared with 50ZrB₂-MoSi₂ coating.Compared with the50ZrB₂-（HfSi₂-30TaSi₂）coating,the maximum weight change rate of the 50ZrB₂-（HfSi₂-30TaSi₂）coating decreased from 5.27 g·cm^-2·s^-1 to 1.09 g·cm^-2·s^-1,a decrease of 79.32%,and the oxygen permeability decreased by 93.75%.This shows that the composite coating formed by alloying of Ta with transition metal Mo and Hf silicide has excellent antioxidant protection effect.During the high-temperature oxidation process,appropriate volume expansion caused by TaSi₂ oxidation can promote the sealing of the moltenSiO₂ glass phase,repair the structural damage inside the coating,and suppress the structural looseness caused by ZrB₂ oxidation,thereby consolidating the structural oxygen barrier of the ZrB₂-MeSi₂（Me=Mo,Hf）coating itself.In addition,the self-separation effect and complexation film-forming effect of TaSi₂ help to exfoliate and refine the agglomerated double transition metal Zr/Hf oxide particles and promote their flow diffusion in the amorphousSiO₂ glass film,thus forming a Zr-B-Me-Ta-Si-O sealed complex phase glass film.When the glass layer is in a softened state by high temperature oxidation,the high melting point transition metal oxides are pinned in the glass film in the form of hard particles,which increases the unmelted area of the glass layer,which not only hinders the linear diffusion process of oxygen,but also prolongs their diffusion paths and thus reduces the diffusion rate of oxygen,and the antioxidant protection efficiency of the coating is increased to 99.71%（50ZrB₂-（MoSi₂-10TaSi₂））and 99.84%（50ZrB₂-（HfSi₂-30TaSi₂））,which indicates that the appropriate amount of TaSi₂ in the process of Ta synergistic transition metal Mo and Hf silicide alloying modified ZrB₂ coating has better oxygen barrier protection performance.When the TaSi₂ content in TaSi₂-MeSi₂ is excessive,the large amount of dissolved Ta⁵⁺intensifies the local aggregation of oxides and the formation of loose dendritic structures or slab-like structures,which not only makes the complete structure of the sealed glass film destroyed,but also accelerates the oxidation by providing oxygen diffusion channels.In addition,the formation of more macromolecular TaZr_2.75O₈ or Hf₆Ta₂O₁₇ phases made it difficult for the transition group metal oxides to diffuse and dissolve into the SiO₂ glass,resulting in a dramatic increase in the glass film viscosity,which not only weakened the self-healing ability of the glass layer,but also destroyed the structural stability of the ZrB₂-MeSi₂ coating.The oxygen permeability of the coating increased to 0.73%（50ZrB₂-（MoSi₂-30TaSi₂））and 0.53%（50ZrB₂-（HfSi₂-40TaSi₂））,while the protection efficiency decreased to99.18%（50ZrB₂-（MoSi₂-30TaSi₂））and 99.47%（50ZrB₂-（HfSi₂-40TaSi₂））,indicating that the TaSi₂ modifier exceeded the threshold addition amount and weakened the oxygen barrier performance of the composite coatings instead.This thesis includes 47 figures,2 tables and 126 references.