| Zirconium diboride(ZrB2)is a typical ultra-high temperature ceramic(UHTC).It has a unique combination of high melting point(3245 ?),high hardness,excellent corrosion resistance,good chemical stability,excellent thermal and electrical conductivity.Those excellent properties make ZrB2 material became a popular candidate for thermal protection materials of hypersonic aircraft,and cause extensive attention from researchers in recent years.Researches about ZrB2 material are focused on improving the poor sinterability,poor toughness and poor oxidation resistance.The main idea of this work is to prepare self-toughening multiphase ceramics.Firstly,a simple way to synthesis one-dimensional(1D)ZrB2 crystals with large aspect ratio has been determined.Secondly,using 1D-ZrB2 crystals as raw materials,?-SiC as additive,Al,B4C and C powders as sintering aids to prepare ZrB2-based ceramic materials.The effects of process parameters on the 1D-ZrB2 crystals morphology and the effects of raw materials ratio on the properties of ZrB2-SiC-Al3BC3 ceramic composites materials have been studied.The mian results are as follows:(1)1D-ZrB2 grains can synthesized by boro/carbothermal reduction using ZrOCl2·8H2O,B4C and C powders as raw materials at 1600 0C for 60 min in Ar atmosphere.1D-ZrB2 crystals prepared with good morphology showed a diameter about 0.5?3 ?m and an aspect ratio about 8.The growth mechanism of 1D-ZrB2 crystals was studied.The interproduct B2O3 played a key role in growth of ZrB2 crystals into elongated shape.Small B2O3 particles preferentially adsorbed to specific crystal planes would reduce the growth rate of some crystal planes and affect the growth pattern of ZrB2 crystals.Adding flux NaCl to raw materials was an effective way to improve the morphology of 1D-ZrB2 crystals.1D-ZrB2 crystals with a diameter of about 1?m and an aspect ratio of about 30 were prepared by adding 50 wt.%NaCl.(2)The prepared 1D-ZrB2 crystals with an aspect ratio about 8 were used as raw materials,adding 5-25 wt%SiC and 5-15 wt.%Al-B4C-C(ABC)for sintering ZrB2-SiC-Al3BC3 ceramic matrix composites.Dense ZrB2-SiC-Al3BC3 ceramic composites materials were prepared by hot pressing at 1950? for 60 min under a pressure of 30 MPa in Ar atmosphere.ZrB2-SiC-Al3BC3 ceramic matrix composite with 25 wt.%SiC and 10 wt.%Al-B4C-C(ABC)sintering aid composites showed best sinterability and mechanical properties.This sample has a density of 96.18%and a Vickers hardness of 19.18 GPa.The flexural strength and fracture toughness of this sample was 545.67 ± 14.36 MPa and 6.12± 0.18 MPa·m1/2,respectively.(3)ZrB2-25 wt.%SiC-10 wt.%Al3BC3 ceramic matrix composite use prepared 1D-ZrB2 crystals with an asprct ratio about 30 as raw material showed a better mechical properties.The flexural strength and fracture toughness of this sample was 545.67 ±14.36 MPa and 6.12± 0.18 MPa·m1/2.Sintering aids can effectively improve the sinterability of ceramic.Liquid phases provided by in-situ reaction between sintering aids(ABC system)could promote the movement and rearrangement of ZrB2 and SiC grains.Some ?-SiC crystals transform to a-SiC crystals and grew into elongated shapes.These elongated SiC grains and 1D-ZrB2 crystals with large aspect ratio can reduce the diffusion energy by causing the deflection of cracks and acted as bridges at cracks reduced the width of cracks.(4)The thermal shock resistance of ZrB2-SiC-Al3BC3 ceramic matrix composites was studied by water quenching method.It was found that ZrB2-SiC-Al3BC3 ceramic matrix composite with 10 wt.%Al-B4C-C(ABC)sintering aid composites showed the best thermal shock resistance.The critical temperature difference is 521.5?.The microstructure of ZrB2-SiC-Al3BC3 ceramic matrix composites after thermal shock at different temperature indicated the thermal shock failure mechanism was thermal shock damage.The ceramic didn't break instantly when subjected to thermal shock.Some internal grains became loose at first,and then some of them peeled from the ceramic.After that,the ceramic broke. |
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