Investigation Of The Ablative Performance Of The Modified ZrC Ceramic Coatings

The development of ultra-high temperature materials that can be used at temperature over2000℃is the technical bottleneck of the research and development of the thermal protection system and power system of new aerospace aircraft.Carbon/carbon composites own lots of distinct properties,for instance,high melting point,low density,low coefficient of thermal expansion（CTE）and exceptional ablation and thermal shock resistance.In addition,the strength of the C/C composites increase with the temperature.However,C/C composites are easily oxidized in an environment containing oxygen at temperature above 400℃,which makes C/C composites cannot resist the erosion of high temperature and high velocity airflow.With the development of technology,C/C composites cannot meet the requirements for thermal protection system and power system of space and space vehicles.In order to improve the ablative performance of the C/C composites,the commonly methods are to introduce ultra-high temperature ceramic particles into the C/C composites or to prepare ultra-high temperature ceramic coatings（ZrC,Hf C,etc.）on the C/C composites.However,the ultra-high temperature ceramic coatings or particles are easily pulverized and oxidized under the erosion of high temperature and high-speed airflow,forming Zr O₂ and Hf O₂ porous structures.These porous structures provide diffusion channels for oxygen.Porous Zr O₂ will peel off under the erosion of high-speed airflow,resulting in the failure of component.Crystal structure of Zr O₂ transit from t-Zr O₂ to m-Zr O₂ during cooling,which results in the expansion of volume and the formation of cracks in the coating.To solove the above problems,Cr,Al₂O₃,SiC and Zr O₂,as the additives phase,are successfully introduced into ZrC ceramic coating by the method of in-situ reaction and supersonic atmospheric plasma spraying in this paper.The influence of additives content on the crystal phase composition and microstructure of ZrC coating are systematically studied.The effects of the modified ZrC coatings on the anti-ablation properties of the C/C composites are investigated.The main contents and results of this study are as follows:SiC coating and Cr modified ZrC composite coatings are fabricated on C/C composites by in-situ reaction and supersonic atmosphere plasma spraying.Firstly,SiC coating is fabricated on C/C composites by in-situ reaction.Then ZrC coatings with different Cr content are fabricated on SiC coated C/C composites by supersonic atmosphere plasma spraying.Results show that ZrC coatings with different Cr content show different anti-ablation property.After introducing15 wt.%Cr,Cr-ZrC composite coatings show denser structure and better ablation resistance property than other prepared coatings.Chemical ablation and mechanical denudation of oxyacetylene torch determine on central region of the coating during ablation.On the surface of the specimen,it forms dense Zr-O-Cr composite structure during ablation.And this dense dendritic structure can seal the pinholes during ablation test,which can reduce permeability path for oxygen and act as a thermal barrier.Besides,the dense Zr-O-Cr oxide film can prevent metal cation to diffuse outward and oxygen ions to permeate into the coating,which can effectively alleviate or inhibit the oxidation of the specimen.SiC and Al₂O₃ are introduced into ZrC coating by in-situ reaction.Firstly,inner SiC coating is fabricated on C/C composites by in-situ reaction method.Then SiC-ZrC-Al₂O₃ coating is prepared on SiC-coated C/C composites by in-situ reaction method.The microstructure,phase composition and anti-ablation property of the SiC-ZrC-Al₂O₃ coating are investigated.Results show that the SiC-ZrC-Al₂O₃ coating is dense and no obvious crack can be seen on the surface.Ablation test results demonstrate that SiC/SiC-ZrC-Al₂O₃ coating can protect C/C composites under heat flux of 2.4 MW/m² over 60 s.The mass and linear ablation rates of the SiC/SiC-ZrC-Al₂O₃ coated C/C composites are-0.235 mg/s and-0.24μm/s,respectively.After ablation,Zr O₂-Si O₂,Si O₂-Al₂O₃ and Zr O₂-Al₂O₃ composite oxides formed on the surface of the specimen.Coating forms a layered structure:Si O₂ glass layer with some hemi-spherical particles（Zr-O-Al）,dense dendrites structure（Zr-O-Al-Si,Zr-O-Al,Zr-O-Si）,less affected SiC-ZrC-Al₂O₃ and SiC layer.As the ablation time increases,the ablation depth increases and Si O₂ evaporates,leading to the increase of the hemispherical particles（Zr-O-Al,dense dendrites structure）.SiC and SiC-ZrC coatings with different Al₂O₃ content are prepared on C/C composites by in-situ reaction and supersonic atmosphere plasma spraying,respectively.Firstly,inner SiC coating is fabricated on C/C composites by in-situ reaction method.Then SiC-ZrC coatings with different Al₂O₃ content are prepared on SiC-coated C/C composites by in-situ reaction.The sprayed SiC-ZrC-Al₂O₃ coatings are composed of ZrC,SiC,Zr O₂ and Si O₂ based on the XRD,EDS and XPS analysis.Ablation test results show that ZSA10 coating can protect C/C composites under heat flux of 2.4 MW/m² over 90 s.The grain size of Zr O₂ in the SiC-ZrC coating is decreased with the introduction of Al₂O₃.Al₂O₃ can help to increase the sintering ability of the coating,promote the transfer of the materials during the ablation test and form a compact glassy layer.The SiC-ZrC coating with 10 wt.%Al₂O₃ owns the lowest mass and linear ablation rate.The scale could effectively inhibit the infiltration of oxygen.ZrC-Zr O₂ coatings with different Al₂O₃ content are fabricated on C/C-SiC composites by supersonic atmosphere plasma spraying,respectively.Firstly,inner SiC coating is fabricated on C/C composites by in-situ reaction method.Then different content Al₂O₃ modified ZrC-Zr O₂coatings are fabricated on SiC-coated C/C composites by supersonic atmosphere plasma spraying.The influence of the Al₂O₃ additive on high temperature ablation behavior of the ZrC-Zr O₂ coating is studied.Results show that the addition of Al₂O₃ can help to form compact composite coating during supersonic atmosphere plasma spraying.After ablation test for 60 s,ZrC-Zr O₂-10 vol.%Al₂O₃ and ZrC-Zr O₂-20 vol.%Al₂O₃ coatings can keep the surface integrity and compact.The amount of the micropores and microholes in the Al₂O₃ modifed ZrC-Zr O₂coating decreases.And on the ablated coating form Al-Zr-O compact grains,which contributes to decrease the defects on the surface of the coating and reduce the area exposed to oxygen.Therefore,the compact coating can resist the scouring of the oxyacetylene flame with the pinning effect of the Al-Zr-O grains.