Densification,Microstructure Tailoring And Properties Of Ta_1-xHf_xC Based Ceramics

Rock-salt structured Ta_1-xHf_xC ceramics have attracted worldwide attention in recent decades due to their ultra-high melting point（>3700^oC）.Owing to the same crystal structure with Na Cl（B1,space group Fm3m）,continuous Ta_1-xHf_xC（0（27）x（27）1）solid solution could be obtained with Ta C and Hf C across the whole compositional ranges.They are promising materials to serve as thermal protection system（TPS）materials for various applications at temperatures above 2000^oC.However,densification of Ta_1-xHf_xC ceramics is difficult because of the strong covalent bonding and low self-diffusion coefficient.Started from the integrated processing of solid solutioning and densification using commercial Ta C,Hf C powders,this thesis firstly studied the pressurelessly consolidation of Ta_0.8Hf_0.2C ceramic.For purposes of grain refinement and properties optimization,Si C was introduced in Ta_0.8Hf_0.2C-based ceramics in both in-situ and ex-situ ways.Besides,two strategies（solid solutioning and carbon deficiency）were carried out to exploit ductility of Ta_1-xHf_xC_y.The following research work has been carried out in this study:（1）Pyrolysis carbon was chosen as sintering aid to densify Ta_0.8Hf_0.2C ceramic via pressure-less sintering.The process parameters were optimized including volume of binder（P.R.）addition,temperature and dwell time during sintering.Graphitized carbon,pyrolyzed from P.R.,can remove oxide impurities via carbon-thermal reaction and escort the solid solutioning process including the grain boundary migration and bulk diffusion.Solid solution formation proceeded in ranges of[1600^oC,2200^oC].The relative density of as-sintered Ta_0.8Hf_0.2C ceramic is 98.8%.（2）Si C was introduced in Ta_0.8Hf_0.2C-based ceramic in both in-situ and ex-situ ways. Ta_0.8Hf_0.2C-27vol%Si C was toughened and strengthened via pressurelessly in-situ reactive sintering.High aspect ratio,platelet-likeα-Si C grains formed and interconnected as interlocking structures.Toughness and flexural strength values of 5.4±1.2 MPa?m^1/2and 443±22MPa were measured respectively. The influence of Si C volume fraction on the densification,microstructure evolution and room-temperature properties of Ta_0.8Hf_0.2C-x vol%Si C（x=0,10,20,30）were examined.Ta_0.8Hf_0.2C-10vol%Si C exhibited significantly higher ablation resistance,with ablation rate of 0.7μm/sec in comparison to the monolithic system with 3.5μm/sec.（3）The relative density of ternary Ta_1-xHf_xC ceramics sintered using Cr₃C₂ additive reached>97.5%using spark plasma sintering（SPS）.The densification mechanism of Cr₃C₂ was to create liquid phase sintering when increasing temperature in Ta_1-xHf_xC matrix.Phase transition and decarburization,from orthogonal phase（Cr₃C₂,Cr₇C₃）to rock-salt structured(Cr₂₃C₆),would happen.The grain boundary phase could be turned to a multi-element rock-salt structured phase:Ta/Hf doped Cr₂₃C₆.Coefficient of thermal expansion（CTE）varied in the range of 7.17-7.51×10^-6/K.Three-point flexural strength and fracture toughness were in the range of 439-492 MPa and 4.0-5.8 MPa?m^1/2respectively for all Ta_1-xHf_xC ceramics.（4）A promising route to brittle-ductile transition was proposed and verified experimentally while retaining high hardness of the rock-salt structure Ta_1-xHf_xC_y by using density functional theory（DFT）in conjunction with special quasi-random（SQS）structures.The Pugh’s ratio（k）and Poisson’s ratio（v）values of Ta_0.8Hf_0.2C_0.8 were 0.58 and 0.26 implying a brittle to ductile transition point.A stronger peak appeared at the non-zero region（Fermi level）with carbon-deficiency that strengthened the metallic bonding characteristic by virtue of density of electronic states（DOS）and charge density analysis.Obvious indentation modulus dropped from 641.7±14.8 GPa to 555.8±9.9 GPa along with depleted carbon in Ta_0.8Hf_0.2C_y.The XPS characteristic peak of C 1s shifted to higher region near 282.9e V with more carbon vacancy in Ta_0.8Hf_0.2C_0.8 sample,which is close to the C-Ta binding energy.As more carbon atom depletion,the localized valence electron gradually deviated from the strong covalent bonding of C-Ta/Hf and contributed to a more‘metallic’bonding feather.