Study On Preparation And Properties Of ZrB₂-SiC Composites

As a kind of important ultra-high-temperature ceramics(UHTCs),zirconium diboride(ZrB2)with excellent properties,such as high melting point(32450C),high hardness(23 GPa),high conductivity and excellent thermal stability,has been widely used in the field of thermal protection systems for supersonic aircraft,parts of scramjet engines,high temperature electrodes and molten metal sealing systems.The ceramic coating prepared by using ZrB2-SiC composite powders has excellent thermal shock resistance,corrosion resistance and ablation resistance,and is one of the ideal candidates for C/C composite ultra-high temperature oxidation resistant coatings.The addition of SiC to the ZrB2 matrix can also improve its sintering properties and expand its application fields.In this study,high-purity ZrB2 powders and ZrB2-SiC composite powders were synthesized by optimized carbothermal reduction process.ZrB2-SiC composite ceramics were prepared by hot pressing using the obtained powders as raw materials.Ultrafine ZrB2-SiC powders were synthesized through one-step reduction process.In this paper,ZrO2,H3BO3 and carbon black were used as starting materials to optimize the carbothermal reduction process.The effects of boron source contents,calcination temperatures and holding time on the phase composition and microstructure of ZrB2 powders were studied.The growth process of ZrB2 grains was preliminarily discussed.The research showed that the optimal synthesis conditions of ZrB2 powders were:80 wt%H3BO3 in excess,calcination temperature 1600℃,and holding time 90 min.Due to the presence of molten B2O3 during calcination,the liquid conditions were provided for the carbothermal reduction process.ZrB2 grains growed along a certain direction and finally showed a regular columnar structure with an average particle size of 6-8 μm.After adding NaCl,the directional growth of ZrB2 grains was more obvious due to the liquid molten salt environment at high temperature,eventually forming a long strip structure.ZrB2-SiC composite powders with uniform composition and tight microstructure were prepared through introducing different proportions of SiC into the raw materials to form ZrB2 grains in situ.The influence of SiC contents and calcination temperatures was studied.The results showed that SiC could limit the grain boundary mobility during calcination,acting as the grain growth inhibitor.When the content of SiC was 30 vol%,the particle size of ZrB2 was about 1-2μm after thermally treated at 1600℃ for 90 min,and the columnar ZrB2 and the granular SiC were uniformly distributed.When the volume fraction of SiC increased to 40 vol%,the coarsening of ZrB2 grains was strongly suppressed due to a significant increase in diffusion distance.The presence of layer-like hexagonal SiC was attributed to Ostwald ripening and the mechanism of two-dimension nucleation and growth.In addition,the introduction of SiC could hinder the recrystallization process of ZrB2,and ZrB2 grains which were farther away would not continue to grow,reducing the required energy,thereby reducing the calcination temperature of the powders.In this study,ZrB2-SiC composite ceramics were prepared by hot pressing ZrB2 and commercial SiC powders,and the effects of additive(B4C)and SiC contents on the density of ceramics were studied.It was found that the optimal addition amount of B4C and SiC is 1 wt%and 30 vol%,respectively.Then,the obtained ZrB2-SiC composite powders were hot pressed to prepare ceramics,and the influence of different ceramic powders on the properties of ceramics was studied.The results showed that the density and mechanical property of ceramics could be improved by using ZrB2-SiC powders as the raw material.In this paper,granular ZrB2 with the particle size of 0.5-2μm was synthesized at 1500℃ for 60 min,using ZrO2,B4C and carbon black as raw materials.After introducing silicon source(Si or SiO2),ZrB2-SiC composite powders were obtained by ZrO2-B4C-C-Si and ZrO2-B4C-C-SiO2 systems through one-step reduction process.Results showed that the optimal conditions for the synthesis of composite powders were as follows:20 wt%B4C in excess,calcination temperature 1500℃,holding time 60 min.When Si was used as the silicon source,ultrafine ZrB2-SiC powders could be prepared.ZrB2 and SiC particles were uniformly distributed together,and the particle size was about 100-300 nm.In addition,the CO generated during calcination reacted with silicon to produce SiC whiskers.The relative mass content of SiC was calculated to be about 14.4%(close to the theoretical fraction)via the Rietveld refinement method.