Preparation Of ZrB₂/HfB₂-SiC Composite Powders By Microwave Molten Salt Method And Their Spark Plasma Sintering

Due to its high melting point and hardness,good thermal/electrical conductivities,excellent erosion resistance,and outstanding oxidation resistance,ZrB₂-SiC ceramic were widely applied in versatile ultra-high-temperature fields,including rocket propulsion and hypersonic flight.Nowadays,sintering of ZrB₂-SiC composite powders at high temperature and high pressure was the commonly-used method to prepare ZrB₂-SiC ceramics.Microwave-heating and molten-salt modified boro/carbothermal reduction method had the advantages of lowering the firing temperature,shortening the dwelling time,obtaining high-purity composite powders with anisotropic morphology.At the same time,ZrB₂-SiC ceramics with high density,and excellent mechanical properties could be prepared by using spark plasma sintering.Therefore,we prepared ZrB₂-SiC composite powders by microwave-heating and molten-salt modified boro/carbothermal reduction method and studied the effect of firing temperature,dwelling time and the usage of raw materials.Finally,the composite powders were sintered by spark plasma sintering.The results demonstrated that:?1?ZrB₂-SiC composite powders were synthesized by microwave-heating and molten-salt modified boro/carbothermal reduction method by using ZrO2,SiO2,activated C and B₄C as raw materials,and NaCl-KCl as reaction medium.The optimal processing conditions for synthesizing ZrB₂-SiC powder via the present method was n?ZrO₂/SiO₂/C?=1.00:0.37:2.61,60 mol%excess B₄C,a salt/reactant weight ratio of 2.0 and 1200°C/20min.It is worth noting that the firing temperature was 200°C lower than conventional solid reaction method and the dwelling time reduced from several hours to 20 min.ZrB₂had well-defined rod-like structure with lengths of up to dozens of micrometers and the diameters of several micrometers,and the SiC sheets generally had hexagonal-sheet-like morphology with submicron-scale lengths and nano-scale thicknesses.The great enhancement to synthesize rod-like ZrB₂ and plate-like SiC composite powders at such low temperature should be attributed to the combined effects of microwave heating and molten-salt medium,which accelerated the epitaxial growth of the single-crystal with anisotropic structure.?2?ZrB₂-SiC composite powders were synthesized by microwave-heating and molten-salt modified boro/carbothermal reduction method by using ZrSiO₄,activated C and B₄C as raw materials,and NaCl-KCl as reaction medium.The optimal processing conditions for synthesizing ZrB₂-SiC powder via the present method was n?ZrSiO₄/C?=1.0/4.5,60 mol%excess B₄C,a salt/reactant weight ratio of 2.0 and1200°C/20 min.Hexagonal plates of ZrB₂ were fabricated through a“self-weaving process”accelerated by microwave heating and molten-salt medium.The self-weaving processes could be described as follow:Firstly,the rod-like ZrB₂ appeared,then they weaved into a network,finally,the gaps were filled and the nano-plates were obtained.?3?HfB₂-SiC composite powders were synthesized by microwave-heating and molten-salt modified boro/carbothermal reduction method by using HfO2,SiO2,activated C and B₄C as raw materials,and NaCl-KCl as reaction medium.The optimal processing conditions for synthesizing HfB₂-SiC powder via the present method was n?HfO₂/SiO₂/C?=1.00:0.37:2.61,60 mol%excess B₄C and a salt/reactant weight ratio of 2.0 and1250°C/20 min.HfB₂ had well-crystallized rod-like structure with lengths of up to dozens of micrometers and the diameters of several micrometers,the granular SiC homogeneously dispersed around HfB₂ rods.?4?The as-prepared ZrB₂-SiC powders were consolidated by spark plasma sintering under the conditions of 2000^oC,15 min and 100 MPa.The relative density of as-obtained ZrB₂-SiC composite ceramics reached 99.2%,respectively,with Vickers hardness and fracture toughness rising to 24.5 GPa and 4.8 MPa·m^1/2,SEM images indicate that the rod-like structured ZrB₂ in starting mateials still existed in the ZrB₂-SiC ceramics after spark plasma sintering,and thus significantly improved the mechanical properties of the sintered products,finally promoted the fracture toughness of as-prepared composite ceramics via crack deflection and bridging.After polishing and chemical etching,SEM images showed that there existed a large number of randomly arranged ZrB₂ rod in ZrB₂-SiC ceramics with an average aspect ratio of up to 5.3.