Study On The Fabrication And Mechanisms Of ZrC Ceramics And Composites

Zirconium carbide（ZrC）has drawn great attentions due to its promising characteristics such as high melting temperature,low density,good corrosion-resistance,remarkable thermal and electrical conductivity,great neutron transparency and excellent thermal shock resistance as well as high temperature mechanical performance.Those combined mechanical and physical properties make ZrC a good candidate for a variety of ultra-high temperature applications including the future hypersonic vehicles and the fuel shielding of nuclear reactors.Unfortunately,the application of ZrC are restricted by its poor sinterability.The densification of ZrC needs high temperature beyond 2100,which coarsens the ZrC grains,and consequently leads to a decrease in mechanical performance.The present doctoral dissertation is forcing on improving the densification,microstructure and mechanical performance of ZrC ceramics and ZrC-ZrB₂-SiC ceramic composites.The dissertation is composed of four parts:（1）The densification and grain growth process of commercial micron-scale ZrC powder during spark plasma sintering（SPS）was studied.Rising the applied pressure is an effective way to enhance the densification of ZrC.The temperature for achieving dense ZrC ceramics dropped from over 2000 to 1900,and mean grain size also decreased from 15.7μm to 4.1μm while the applied pressure increased from 20 MPa to 100 MPa.Based on the densification equation for pressure assisted sintering and pore-dragged creep model,it was found that when the relative density was below 90%,the densification of ZrC was controlled by grain boundary sliding mechanism under 20MPa and by plastic deformation mechanism under 100 MPa;when the relative density was above 90%,the densification was controlled by lattice diffusion.The grain growth of ZrC was sensitive to temperature.When the temperature was above 1900,the grain growth was observed,and controlled by the grain boundary migration mechanism.（2）Nano-scale ZrC powders with different stoichiometry were synthesized in order to improve the sinterability from the bottom up.The sintering behavior of the nano-scale ZrC powders was also studied.The synthesized nano-scale ZrC powder have performed improved sinterability.The improvement should attribute to the enlarged surface energy,the dislocation area in the crystalline of the ZrC particles,and the effects of stoichiometry on the mass transfer.It was found that the increase of synthesis temperature of ZrC powders would lower down the sinterability,and the increase of oxygen content in powders would lead to enhanced densification and grain growth,As the composition of the powder got close to stoichiometric ZrC,it became hard for densification as well as grain growth.For example,the dense ZrC_0.79O_0.18 with mean grain size of 2.4±0.5μm was achieved at 1750/5 min/100 MPa,while dense ZrC_0.93O_0.06 with mean grain size of 1.2±0.2μm was obtained at 1850/5 min/100MPa.（3）ZrH₂ was selected as sintering additive to achieve the low temperature consolidation for fine-grained ZrC,and remove the carbon impurities.The addition of ZrH₂ have showed great enhancement on the densification and grain growth of ZrC.During the heat treatment,the ZrH₂ would depose into Zr and H₂,then the Zr would react with ZrC and introduce carbon vacancy into ZrC lattice.Sequentially,the yield strength of ZrC crystal decreased,which enhanced the control of plastic deformation to the densification,promoted the mass transfer through solid-state diffusion during sintering,and lowered the active energy for grain growth.As a result,the densification and grain growth became easier in the reacted ZrC.For the nano-scale ZrC,fully dense ZrC ceramics with fine grain size of 1.3±0.2μm were achieved at 1650/5 min/100MPa with 6 wt.%ZrH₂.The final product exhibited the Vicker’s hardness of 21.2±1.0GPa and fracture toughness of 2.2±0.3 MPa·m^1/2.Compared with the ZrC without addition,the temperature for densification and grain size were 200 and 13%lower,respectively.Additional,the ZrH₂ were effective for the removal of carbon impurity,which would improve the mechanical properties.（4）B₄C and Si were added into ZrC to fabricate ZrC-ZrB₂-SiC ceramic composites via reactive sintering.The results shows that the reaction process in this system involved two steps.It was found that the formation of ZrB2 was earlier than that of SiC,and the intermediate product carbon might contribute to the removal of oxygen impurities.ZrC-ZrB₂-SiC composites without impurity phases was obtained at temperature above 1700.The further rising of temperature led to densification and grain growth,and the highest strength was achieved at 1800.On the other hand,the increasing on（ZrB₂+SiC）content in the composites led to refined microstructure and consequently the mechanical properties was improved.The composite ZrC-47.4ZrB₂-47.9SiC prepared at 1800/5 min/30 MPa exhibited the most satisfying combination of properties.The flexural strength,fracture toughness and Vickers hardness were 760±19 MPa,6.3±0.3 MPa·m^1/2 and 22.7±1.4 GPa,respectively.