Brazing Process And Mechanism Of ZrB₂-SiC Composite Ceramic Using Zr-Ni-Based Filler

ZrB₂-Si C composite ceramic with excellent mechanical properties at high temperatures is widely used in aerospace and other extreme high temperature environments. But its own inherent brittleness greatly limits its application. Therefore, to achieve the robust joining of ZrB₂-Si C composite ceramic has very important significance. In this thesis, high quality joining of ZrB₂-Si C composite ceramic was successfully realized by using Zr-Ni based filler. The effct of brazing parameters and the Mo content of the filler on microstructure and properties of the joints were investigated. The optimal brazing parameters were identified. The interface reaction mechanism was also analyzed.The Zr-Ni filler was introduced into joining the ZrB₂-Si C composite ceramic. The effect of brazing temperature, holding time on the structure, shear strength of the joints were explored. With Zr-Ni active brazes, the typical interface structure was ZrB₂-Si C/Zr₂Ni（matrix） +Zr（s, s） +Zr₂Si+Zr C/ZrB₂-Si C. Zr₂ Si was distributed in the vicinity of the interface and Zr（s, s） uniformly distributed in the matrix Zr₂ Ni. The shear strength of the joint ascenged first and then declined as the brazing temperature increasing and holding time extending. When the brazing temperature was 1190℃ and the holding time was 10 min, the joints achieved the maximum shear strength about 98 Mpa, which is about 61% of the base material shear strength. When the test temperature is 800℃, the shear strength of the joint is 125 MPa, which is above room temperature strength and about 80% of the base material shear strength; when 900℃, the shear strength is 76 MPa. The joining mechanism of ZrB₂-Si C composite ceramic was investigated. With the increase of brazing temperature, Zr-Ni alloy melt and spread on both sides of the matrix. The Zr elements in the liquid brazing alloy react with the Si C in the base material, Zr₂ Si and Zr C are produced. During cooling, Zr₂ Ni and Zr（s, s） were produced by the eutectic reaction of the liquid brazing filler. A stable joint was formed when cooling to the room temperature.The Zr-Ni+Mo filler was introduced into joining the ZrB₂-Si C composite ceramic. The effect of Mo content, brazing temperature, holding time on the structure and shear strength of the joints were explored. The typical interface structure was: ZrB₂-Si C/Zr₂Si+Zr₂Ni+Zr（s, s） +Zr Mo2+Zr C/ZrB₂-Si C, which is similar to the joint with Zr-Ni filler. Adding 5at. %, 10 at. % Mo into the Zr-Ni brazing filler metal to form a composite filler. When the brazing temperature was 1200℃ and the holding time was 10 min, the shear strength of the joints were 115 Mpa and 57 MPa. Mo can reduce the coefficient of thermal expansion of the overall filler alloy, improve its strength. However, Mo also makes the filler poor fluidity, reduces the plasticity and toughness of the filler alloy. Therefore, an appropriate amount of Mo content increases the strength of the joint.Using Zr-Ni+5% Mo composite filler at different brazing temperatures and holding time brazed ZrB₂-Si C composite ceramic, shear strength of the joint first increased and the decreased. When the brazing temperature was 1200℃ and the holding time was 10 min, the joints achieved the maximum shear strength about 115 Mpa, which is about 72% of the base material and increased by 11% compared with the joint using Zr-Ni filler. Best brazing process parameters: brazing temperature 1200℃, heat 10 min, joint strength at room temperature obtained up to 115 MPa. High-temperature shear strength was also achieved compared with the Zr-Ni joint.When the test temperature was 800℃, 900℃, 1000℃, the shear strength of the joint is up to 158 MPa, 133 MPa, 66 MPa, which is about 98%, 83%, 42% of the base material shear strength, repectively.