Study On Toughening Of Ultra High Temperature Ceramics With The Rod Like ZrB₂ Powders

As one of the ultra-high temperature ceramics,ZrB₂ based ceramics have high melting point,low density,high conductivity and high thermal conductivity.However,the intrinsic brittleness of ceramics has been restricting its wide application.In this paper,the rod like ZrO₂ powders were prepared by water quenching method using ZrCl₄ as raw material.And the rod like ZrO₂ powders were used as raw material to prepare rod like ZrB₂ powders.Then ZrB₂-SiC composites toughened by the rod like ZrB₂ powders which used the rod like ZrB₂ powders and SiC powder as raw materials were prepared by SPS sintering and hot pressing sintering.The microstructure,physical properties,mechanical properties and thermal shock resistance of the ZrB₂-SiC composites were analyzed and studied.The rod like ZrO₂ powders were prepared by heated ZrCl₄ to 1000 ? for water quenching after 1hour.The rod like ZrO₂ powders are about 10?m in diameter and 200?m in length.The rod like ZrB₂ powders which using the self-made rod like ZrO₂ powders were prepared under different temperature and atmosphere with four systems which are ZrO₂-B₄C-C?ZrO₂-B₂O₃-C?ZrO₂-H₃BO₃-C and Zr O₂-B₂O₃-Al.The results show that the size of the rod like ZrB₂ particles increases with the increase of the reaction temperature and the holding time.And ZrB₂ powders prepared by 4 differents systems have different purity.The relatively pure ZrB₂ powders are synthesized by ZrO₂-B₄C-C?ZrO₂-B₂O₃-C?ZrO₂-H₃BO₃-C,and the content of ZrB₂ powder is above 98% while only a small amount of ZrC and C are remained.Meanwhile,ZrB₂ powders synthesized by Zr O₂-B₂O₃-Al have a large number of Al2O₃ byproducts,besides a small amount of ZrC and C residues.It is worth noting that the rod like ZrB₂ particles with a diameter of 2?m and length of 10?m were synthesized by the ZrO₂-B₂O₃-C system and ZrO₂-H₃BO₃-C system at 1700? for 1 hour in the argon atmosphere.In this paper,ZrB₂-SiC composites toughened by rod like ZrB₂ powders which used the rod like ZrB₂ powders which prepared by ZrO₂-H₃BO₃-C system and SiC powder as raw materials were prepared by SPS sintering and hot pressing sintering.The results show that the density is 4.51g/cm³,the relative density is 81.9%,the flexural strength is 159.28 MPa,and the fracture toughness is 3.81MPa·m^1/2 of ZrB₂-SiC composites ZS-4.It is mainly attributed to the three main aspects that the fracture toughness of the rod like ZrB₂-SiC composites can reach the general level which is 3.9 MPa·m^1/2 in the case of other performance with low level.On the one hand,the path of crack propagation is extended due to deflection occurs when the crack expands.On the other hand,a large amount of fracture work can be consumed due to the extraction of the rod like ZrB₂ particles.In addition,there are a lot of pores in the material which can make the stress relaxation at the crack tip when the crack extends to the pores.It is attributed to the large number of micro cracks formed on the inside and surface of the material after thermal shock,the strength of the rod like ZrB₂-SiC composites decreased significantly after thermal shock test.And with the increase of thermal shock temperature the number of micro cracks increase.In addition,the fracture mode of the ZrB₂ particles in the material are changed from transgranular fracture to intergranular fracture due to the binding force between the particles is greatly reduced after thermal shock test.It is shown that the critical temperatures of thermal shock are 427? and 295? of the rod like ZrB₂-SiC composites ZS-2 and ZS-3.ZS-2 showed the better thermal shock resistance.