Study On Preparation And Properties Of Ta₄HfC₅-MoSi₂-ZrB₂ Ceramic Composites

Ta₄HfC₅ ceramic powders were acquired through solvothermal method combined with carbothermal reduction using commercial TaCl₅ and HfCl₄ powder,phenolic resin,acetylacetone and n-butanol as raw materials.Ta₄HfC₅-MoSi₂-ZrB₂（TMZ）ceramic material was prepared by Spark Plasma Sintering（SPS）technology using obtained Ta₄HfC₅ ceramic powder,commercial MoSi₂ and Zr B₂.The sintering process,the sintering mechanism,the microstructure feature,mechanical properties together with the oxidation behavior of the subject were well investigated.Fine and uniform Ta₄Hf C₅ ceramic powder with mean particle size of 1μm were obtained by solvothermal method and carbothermal reduction process in Ar atmosphere at 1500°C for 2 h.The precursors were respectively subjected to different temperature pyrolysis treatments,their phase composition and microstructure were characterized in order to analyze the carbothermal reduction process which includes:carbonization of aldehyde resins,cracking of organics,and reduction of metal oxides,in which the oxide of Tantalum is more easily reduced than the oxide of Hafnium,accompanied with the oxide crystal structure transformation.SPS technology was used to fabricate TMZ ceramics with different contents of ZrB₂ at 2100°C,45 MPa and 20 min.It was found that the main stage of sintering happened above 1600°C.At 2000°C,the sintering displacement started to change slowly,and sintering was basically completed when the temperature reached 2100°C.The effect of different ZrB₂ content on the mechanical properties of TMZ ceramics was studied to find that with the increase of Zr B₂ content,the relative density of TMZ ceramics increased,the hardness increased from 6.14±0.86 GPa to 8.53±0.67 GPa,and the bending strength also increased from 376.45±15.62 MPa to 435.17±37.07 MPa,while the fracture toughness decreased firstly and then rose up.The fracture toughness value of TMZ30 was the highest,reaching 4.46±0.02 Mpa·m1/2 thanks to the formation of the disordered graphite phase.In addition,the fracture mode of the material was a mix of both transgranular and intergranular fracture.The TG-DTA comprehensive thermal analysis and static oxidation test of TMZ ceramics were carried out to investigate the oxidation behavior of TMZ ceramics.By analyzing the TG-DTA curve,it was found that the there is an obvious oxidative weight gain at 865°C,indicating that the oxidation reaction started to occur at this very temperrature.The microstructures of TMZ ceramics after static oxidation in air atmosphere at 1100°C-1400°C was characterized.The results showed that the surface morphologies of TMZ samples with different Zr B₂ content was different at different temperatures for different time.Surface defects such as“edge warping”,“bulging”and holes appeared on the surface because the thermal expansion coefficient of the oxide and the matrix did not match and the gas product failed to escape from the material surface.The main phase components in the oxide layer structure were Ta₂O₅ and Ta（Zr,Hf）_2.75O₈,based on which it formed a multilayered structure of dense surface layer→porous oxide layer→dense transition layer→matrix structure.According to the oxidation kinetics analysis of TMZ ceramics,it was learned that TMZ30 could always exhibit better oxidation resistance among others under the same conditions.However,the oxidation behavior of the three components was different at different temperatures.At lower temperatures,the anti-oxidation performance of TMZ with medium or high ZrB₂ content was better,but with the increase of temperature,the anti-oxidation performance of TMZ with lowest or highest ZrB₂ content started to be superior.This was caused by the main factors improving the oxidation resistance in the oxidation process which are:liquid B₂O₃ and borosilicate glass phase that slow down oxygen diffusion,dense Ta（Zr,Hf）_2.75O₈ oxide layer who cuts off oxygen in the air,and the carbon containing oxides that obstructs oxygen infiltration.For TMZ ceramic materials,the former two components played a key role at relatively lower temperatures.At higher temperatures,the volatilization of B₂O₃ or the destruction of the integrity of the glass phase would accelerate the oxidation when the carbon containing oxides and dense Ta（Zr,Hf）_2.75O₈ oxide start to take over.