Technologies And Properties Of ZrO₂ Ceramics Joints Bonded Using Glass Fillers

Zirconia has been widely applied in many fields such as mechanical engineering,electronic technology,aerospace and biomedical applications because of its excellent properties.However,it was difficult to process ZrO₂ceramics into complex shapes due to their high fracture toughness.Joining was an effective approach to produce large or complex shapes ZrO₂ceramics components.Glass brazing was a commonly method used for connecting ceramic materials.The coefficient of thermal expansion（CTE）of common glass filler was lower than that of ZrO₂ceramic.In this paper,on the one hand,Nd₂O₃-Al₂O₃-SiO₂-TiO₂（NAST）glass filler was designed to join ZrO₂ceramics by crystallized itself to improve its coefficient of thermal expansion.On the other hand,alkali metal oxides were used to improve the coefficient of thermal expansion of glass fillers,and Ca O-Al₂O₃-SiO₂-Li₂O（CASL）glass fillers were designed to connect ZrO₂ceramics.The crystallization,thermal expansion and wetting behaviors of two types of glass fillers have been investigated,and the effects of joining process parameters on the microstructure and mechanical properties of the joints were analyzed.The peak crystallization temperature of NAST glass was 1097℃,and the crystallization phase was Nd₂SiO₅.The crystallization mechanism of the glass was surface crystallization.The thermal expansion coefficient of the glass was 5.7×10^-6/℃,and the softening temperature was approximately 900℃.After crystallization treatment,the coefficient of thermal expansion of NAST glass-ceramic was increased to 6.8×10^-6/℃.Meanwhile,the glass filler had good wettability on the ZrO₂ceramic substrates,and the equilibrium wetting angle was about 20°.The experimental results of joining ZrO₂ceramics with NAST glass filler showed that after the joining temperature of the glassy filler rose from 1350℃to 1400℃,the weld seam narrowed and the holes disappeared,but there will be cracks due to the coefficient of thermal expansion mismatch between the glassy filler and ZrO₂ceramics.After the crystallization treatment of the joint,the cracks disappeared,and extended the crystallization insulation time can promote the crystallization of the weld glass,which was conducive to obtain a defect free joint.When the welding temperature was 1400℃,the bending strength of the non-crystallized joint was only 29 MPa.After 60 min of crystallization at 1100℃,the maximum bending strength was 174 MPa of the joint was obtained.The crystal phase of CASL glass with a Li₂O content of 10%was Li Al Si₂O₆.The crystal phases of CASL glass with 12%and 14%Li₂O content were both Li Al Si₂O₆and Ca SiO₃.As the content of Li₂O in glass filler increased from 10%to 14%,the coefficient of thermal expansion of glass filler increased from 8.43×10^-6/℃to 9.13×10^-6/℃,which was closer to the coefficient of thermal expansion of ZrO₂ceramic,but the bending strength of glass decreased from 90.3 MPa to 48.7 MPa.Three types of CASL glass fillers exhibited excellent wetting properties on ZrO₂substrates.The experimental results of used CASL glass fillers to jointed ZrO₂ceramics showed that the joints obtained from the three types of CASL glass fillers were dense and without pores.The CASL glass filler with Li₂O content of 14%had the best match coefficient of thermal expansion,but the strength of the glass was poor,leaded to the reduction of joint strength.When used CASL glass filler with a Li₂O content of 12%and jointed at the optimal wetting temperature,the three-point bending strength of the joint reached 627.8 MPa,which was close to 80%of the strength of the ZrO₂ceramic substrate.