Study On Ablation Resistance And Thermal Shock Resistance Of ZrB₂ Based Ultra High Temperature Ceramics And Its' Joints

It is of significant importance to develop hypersonic vehicles for space exploration and national security.When the aircraft entries into the atmosphere at a high speed,it will face extreme conditions.The thermal protection structures would suffer from severe tests,such as thermal erosion,oxidation and thermal shock.Ultra-high temperature ceramics?UHTCs?are one of the most promising thermal protection materials for hypersonic vehicles,due to their ability to maintain physical and chemical stability in extreme environments.However,the inherent brittleness and poor thermal shock resistance of UTHCs limit their applications in aerospace engineering.In addition,it is quite necessary to join UHTCs to form complex structures for the thermal protection system of hypersonic vehicles,due to their relative poor manufacture ability.The joined structures must be subjected to complex thermal shock environments in service,and their thermal shock resistance directly determines the service life of hypersonic vehicles.Therefore,how to accurately characterize,evaluate and improve the thermal shock resistance of UHTCs and joined structures has always been a focus and hotspot of UHTCs research.In this paper,ZrB₂-0.5 vol.%SiC and 20 vol.%SiC ceramics were prepared using spark plasma sintering?SPS?method.The mechanical properties were studied by Nanoindentation.In addition,the residual stress the prepared ceramics was measured by Raman spectrum method.The results showed that there existed residual compressive stress in the SiC phase with an average value of 300 MPa.Meanwhile,ZrB₂-0.5 vol.%SiC ceramics were ablated by plasma gun.According to the microstructure morphology and mass changes,it could be concluded that:?1?the damage of the samples under cyclic ablation was more serious than that under single ablation;?2?that micro-cracks formed on the surface of the samples under cyclic ablation would lead to more serious oxidation reaction,since oxygen penetrated into the sample through the micro-cracks.Furthermore,ZrB₂-20 vol.%SiC ceramics were joined with the brazing filler Ni using SPS and hot pressing methods in this paper.The microstructure morphology and elements distribution of ceramic joints were analyzed in detail.The results showed that:?1?the the ceramic joints could be divided into three layers:initial ceramic layer,transition layer and residual Ni layer;?2?the distribution of the elements of the ceramic joints using SPS method was more uniform.And also,the cyclic thermal shock tests of ceramic joints was carried out using the quartz lamp thermal shock test platform.Results showed that:?1?the ceramic joints joined by SPS method had better cyclic thermal shock resistance;?2?the cracks were prone to occur at the interface between the transition layer and the ceramic layer or the residual Ni layer.In summary,the ZrB₂-based ceramics were sintered by SPS method and their mechanical properties and residual stress were tested in this paper.The cyclic ablation tests for the ceramic samples were carried out using a plasma gun.In addition,the cyclic thermal shock resistances of the ZrB₂-based ceramics joints,joined by SPS and hot pressing methods,were tested by the quartz lamp thermal shock test platform,respectively.The research results in this paper would provide a guide for the UHTCs application in the thermal protection systems of hypersonic vehicles.