Enhancement mechanism of ultrasonic vibration on interface bonding strength in laser joining of polymer to ceramic

The current challenge in laser joining polymer and ceramic heterogeneous structures lies in their relatively low mechanical strength, primarily attributed to weak interface bonding. To address this issue, this paper proposes the application of ultrasonic vibration during the laser joining process of PET with both Si3N4 and Ti-coated Si3N4 sheets to enhance the interface bonding strength. Investigating the mechanisms underlying the enhancement of interface bonding strength due to ultrasonic vibration is crucial for improving the overall mechanical strength of the joints. It is found that the physical wetting thermodynamic conditions and chemical bond thermodynamic conditions for laser joining of PET to ceramics were the surface tension of ceramics higher than that of the molten PET and the variation in the Gibbs function less than -24.3 kcal/mol, respectively. Experimental results demonstrated that the bonding interface between Si3N4 sheet and PET exhibits only physical and mechanical bonding, with no evidence of new chemical bonds. However, XPS analysis detected new Ti-C bonds at the bonding interface of Ti-coated Si3N4 and PET. Furthermore, experimental results substantiated that ultrasonic vibration could improve the spreading wetting of PET melt on the Si3N4 surface, resulting in a 17 % increase in the bonded area of the weld seam. Additionally, ultrasonic vibration enhanced the molten PET filling into the blind hole structure on the Si3N4 surface, resulting in an approximately 73 % increase in the filling depth of the blind holes. Lastly, ultrasonic vibration facilitated the chemical reaction between the molten PET and Ti atoms, resulting in an approximately 57 % increase in the thickness of the interface containing Ti-C bonds. It is evident that ultrasonic vibration can bolster the interfacial bonding strength of PET/Si3N4 and PET/Ti-coated Si3N4 in laser joining. Moreover, in comparison to physical bonding, its more pronounced impact on mechanical and chemical bonding is noteworthy.