Design,Preparation,and Characterization Of Ta/Ta_0.5Hf_0.5C Laminated Composites

The development of next-generation hypersonic vehicles puts forward an urgent demand for materials that can withstand extreme temperatures above 2000oC in oxidative atmospheres.Ta_0.5Hf_0.5C ceramic,which owns a high melting point,high hardness,and excellent ablation resistance,shows great potential to be used as wing leading edges and nose caps though its applications never succeed in reality due to intrinsic brittleness.In this context,improving its toughness while not harming thermal endurance has attracted extensive interest over the world.Herein,we fabricate a novel Ta/Ta_0.5Hf_0.5C laminated composite with superior strength,toughness,and ablation resistance,and then investigate the effects caused by the introduction of Ta foil systematically.Detailed results and discussions are shown below.The method to fabricate strong and tough Ta/Ta_0.5Hf_0.5C laminated composite was explored through experiments and calculations on optimizing the sintering parameters and lamination structure.Results reveal that the composite can obtain a multi-scale laminated structure among 2000～2200oC,10～20 min,and 30～50 MPa but its mechanical properties achieved the maximum values when the sintering condition is 2100oC,15 min,and 40 MPa.In terms of lamination structure,the theoretical strength of the composite is positively correlated with the thickness of Ta foil as well as the thickness ratio of Ta_0.5Hf_0.5C and Ta layers.Based on the above findings,two composites containing Ta foil with different thicknesses（100μm V.S.200μm）were fabricated by sintering the green body that was alternately stacked with Ta foil and Ta_0.5Hf_0.5C powder.Three layers were observed at the interfacial region of Ta/Ta_0.5Hf_0.5C laminated composite.The one close to Ta phases,which is Ta₂C MXene,possesses the lowest hardness;the one close to Ta_0.5Hf_0.5C phases,which is composed of hcp-Ta₂C phase and twin c-Ta_0.16⁰.2Hf_0.8⁰.84C phases,possesses the highest hardness;while the one located between them,which is homogeneous hcp-Ta₂C phases,possesses the moderate hardness.Moreover,the interfacial reaction was confirmed only involving Ta phases and the Ta C part in Ta_0.5Hf_0.5C phases.The mechanical properties and toughening mechanisms of Ta/Ta_0.5Hf_0.5C laminated composite were investigated.It was found that the introduction of Ta foil improves the toughness of Ta_0.5Hf_0.5C significantly.Among 100～200μm,decreasing the thickness of Ta foil is good for strength,while increasing the thickness of Ta foil is good for toughness.The strengthening mechanisms of Ta foil consist of self-strength addition of Ta foil,the interfacial layer ensures the smooth transfer of load,and laminated structure increases the crack tolerance limit of Ta_0.5Hf_0.5C.The toughening mechanisms of Ta foil mainly involve the passivation and deflection effects contributed by twin c-Ta_0.16⁰.2Hf_0.8⁰.84C phase,hcp-Ta₂C phase,Ta₂C MXene,Ta layer,and residual stress at atomic scale,micro scale,and macro scale,respectively.The thermophysical properties of Ta/Ta_0.5Hf_0.5C laminated composite were studied.Results show that the introduction of Ta foil improves the thermal conductivity of Ta_0.5Hf_0.5C ceramic but rarely changes its coefficient of thermal expansion.The composite exhibits little difference in appearance after being treated at 2200oC for 10 min,and it owns a strength retention of 86.0%after being treated at 2400oC for 10 min.The evolution behaviors of the composite during high temperatures include grain coarsening,porosity increasing,and interfacial reaction continuing.The thermal shock resistance of Ta/Ta_0.5Hf_0.5C laminated composite was evaluated by employing plasma flame for pulse heating.Based on the measured temperatures at the ablated surface and back,the internal temperature and thermal stress fields of the assessed sample were established using finite element software.Results show that no macro and micro cracks were observed at whether ablated surface or cross-section of all samples,indicating Ta/Ta_0.5Hf_0.5C laminated composite possesses a good thermal shock resistance.The maximum thermal stress obtained by simulation as well as the mechanical properties obtained in the cyclic loading test indicates the excellent thermal shock resistance of the composite is the result of its outstanding ability to resist crack initiation and propagation.The thermoablative resistance and evolution behavior of Ta/Ta_0.5Hf_0.5C laminated composite under plasma flame was studied.Combining with the established volatile phase diagram,a probable ablation mechanism of the composite was proposed.Due to the introduction of Ta foil decreasing the ablative surface temperature and retard the diffusion of oxygen from the atmosphere to the inside of ablating sample,the composite exhibits an equivalent linear ablation rate（-5.40 V.S.-4.08μm/s）in comparison to Ta_0.5Hf_0.5C ceramic though it increases with extending the ablation time.The composite mainly undergoes oxidation of Ta and Ta_0.5Hf_0.5C phases,a solid solution of Ta₂O₅ with Hf₆Ta₂O₁₇ phases,melt stripping of Ta₂O₅ phases,and dissolved oxygen of Ta and hcp-Ta₂C during the ablation.With extending ablation time,the density of the ablated surface of the composite increases,while its oxidation behavior changes from passive oxidation to active oxidation.As a result,its linear ablation rate continues to increase but mass ablation increases first and then decreases.