Deposition and utilization of nano-multilayered brazing filler systems designed for melting point depression

摘要

In this article the preparation and utilization of nano-structured composite films as advanced alternative to eutectic Ag-Cu braze foils are described. The aim is to obtain a system with the same composition but with melting point depression for brazing heat sensitive materials (e.g. ultra-fine grained alloys). Our approach involves the preparation and optimization by sputter deposition of multilayer films with Ag-Cu layers (<12nm) alternated by carbon diffusion barrier layers (<10nm), whose role is to maintain the metallic braze material confined in the nano-size in order to achieve the melting point depression. Films with 20 to 200 repetitions have been produced in both laboratory scale and industrial facilities. It is found that the deposition process and the quality of the films are influenced by substrate temperature, chamber pressure, number of Ag-Cu/C repetitions, and deposition time. With optimized conditions we could obtain films having a melting point depression of 50℃ compared to the bulk eutectic alloy, as confirmed by high temperature XRD measurements. The nano-multilayer structure before and after brazing, thickness and composition have been characterized by TEM, SEM, EDX, X-ray diffraction and ellipsometry. It is found that the layers of eutectic Ag-Cu are nano-crystalline and the carbon barrier layers are amorphous, and both have a controlled thickness and full density throughout the whole film. Brazing tests of steel 1.4301 with 4μm thick nano-multilayer film as braze filler were performed in vacuum. The brazing test demonstrated that the brazing filler extracted from the nano-multilayers wets the steel substrate, however further optimization is in progress in order to achieve joints with quality matching industrial standards.