Effects of electroless nickel plating method for low temperature joining ZnS ceramics

ZnS has been widely used as a material for windows in optical systems. This is because it has a good transmittance in both mid-infrared and far-infrared bands, a stable chemical performance, a moderate linear thermal expansion coefficient and an adequate mechanical strength. ZnS windows need the so-called splice technology implementation to satisfy the integrity of large size applications or special surface structures. Traditional ZnS light windows are mechanically assembled with metal frames. However, the strength of the large-size ZnS formed by mechanical assembly is low. Consequently, an adequate joining technology is needed to overcome the low splice strength and improve the mechanical strength of the assembly. In this paper, we proposed a pre-metallization method by using an electroless plating of Ni on ZnS surface. Subsequently, a low temperature joining by using a Sn63Pb37 solder was completed. By a study of the process and the corresponding mechanism of the electroless nickel plating, a Ni-P layer with a controllable P content and thickness has been formed, and a smooth surface and a good combination with ZnS has been obtained. The microstructure of the Sn63Pb37 and Ni-P/ZnS was analyzed, and the phase composition and element distribution within the joint were determined. The influences of process parameters on the microstructures of the joints have been explored, including the brazing temperature, the time of thermal energy preservation and the thickness of the coating. Finally, the shear strengths of joints under different imposed parameters were optimized leading to the peak soldering temperature of 250 degrees C and the dwell at the peak of 1 min. The fractures were along Ni-P/ZnS ceramic interface, therefore, a successfully low temperature joining process on ZnS ceramics using Ni-P plating and Sn63Pb37 solder had been performed.