Crystallization Behavior Of CaO-Al₂O₃-SiO₂-TiO₂ Glass-ceramics Filler And Its Application For Joining Alumina Ceramics

Alumina ceramics are widely used in mechanical,electronic,aerospace,petrochemical and other fields because of their excellent mechanical properties,high temperature resistance,corrosion resistance and wear resistance.However,due to the intrinsic brittleness of ceramic materials,it is difficult to process and shape them.The joining technology is an important method to fabricate large size or complex shape ceramic parts.In particular,the joining technology for high temperature applications has become a research hotspot and difficulty in this field.In this paper,a CaO-Al₂O₃-SiO₂-TiO₂?CAST?glass-ceramics filler with high crystallinity was designed and prepared.The crystallization behavior,microstructure evolution mechanism,thermal expansion and softening behavior of the glass-ceramics fillers were studied in detail.The reliable bonding between alumina ceramics was successfully obtained by using the CAST glass-ceramics filler.The influence of joining prameters on the microstructures and mechanical properties of the joints was investigated.The crystallization kinetics study shows that with the increase of TiO₂ content,the activation energy of crystallization of glass-ceramics firstly decreases and then increases while the crystal growth mode changes from two-dimensional crystallization to three-dimensional crystallization.The mimimum vaule of crystallization activation energy?323.59kJ/mol?was obtained when the content of TiO₂ is 15%.Microstructure analysis shows that when the content of TiO₂ is 10%,the crystals precipitated in the glass are CaAl₂Si₂O₈,CaTiSiO₅ and CaSiO₃.However,only CaTiSiO₅or CaAl₂Si₂O₈ can be observed for the glass ceramic with 15 wt%or 20 wt%TiO₂ addition,respectively.Low TiO₂ content results in formation of Ti-contained crystals?CaTiSiO₅?due to phase separation.In contrast,high TiO₂ concentrations lead to precipitation of small titanium oxide crystals that act as nuclei,facilitating the development of CaAl₂Si₂O₈.When the content of TiO₂ is equal to or higher than 15%,the size of precipitated crystals reaches nanometer level.The thermal expansion coefficient of glass-ceramics is importantly related to the type and content of crystalline phase.When the content of TiO₂ is 15%,the thermal expansion coefficients of glass and glass ceramic both are close to that of alumina ceramics.The softening temperature of glass-ceramics severely depends on its crystallization volume fraction.When the TiO₂ content is 15%,the softening temperature increased from 769?for glassy sample to 1027?for crystallized sample due to the highest crystallization volume fraction?69.7%?and the lowest crystallization activation enerty.Therefore,CAST-2 glass is more suitable to join alumina ceramics for high temperature application.During the bonding process,the dissolution of alumina ceramics into filler results in the formation of CaAl₂Si₂O₈ at the interface of joints.With the increase of joining temperature,the content of CaAl₂Si₂O₈ increases gradually.The difference of thermal expansion coefficient between CaAl₂Si₂O₈ and alumina ceramics decreases the bending strength of joints.On the other hand,the low joint temperature?1240??results in the formation of pores in the brazing seam.Therefore,the maximum bending strength of256MPa is obtained when the joining temperature is 1260?.After the crystallization treatment,a large number of CaTiSiO₅ crystals can be precipitated in the joint,which is helpful to improve the working temperature of the joints.The flexural strength of joints at800?can reach 242MPa.In addition,the problem of joining large-size alumina ceramics is further discussed in this paper.The results show that capillary brazing method is helpful to obtain large-size joints without defects.