Wetting Behaviors Of Sn-(Ag-Cu) Liquids On Metastable Cu, Ni And Ni₇₅Si₁₅B₁₀ Substrates And Their Interfacial Microstructures

The wetting behaviors and interactions at the solid-liquid interface are important physicochemical phenomena during material preparation and processing, which determine the possibility of materials preparation and their service performance to some extent. So far, substrates used for the wetting experiments are generally polycrystalline materials. Few studies have been concerned with the wetting of metastable materials by the molten metals.With the development of science and the progress of material preparation, metallic glass and nano-materials get more and more attentions. As we know, when the grain size of materials drops to nanometer range, the interface/volume ratio, the degree of atomic mismatch and the surface roughness increase. On the other hand, the structural characteristics of metallic glasses are of short-range order, long-range disorder, no grain boundary and lack of atomic periodical arrangement. Amorphous alloys would change into stable states under appropriate circumstances. Structural relaxation would occur at lower temperatures and crystallization at higher temperatures.Because of the structural specificity of nano-materials and metallic glasses, the wetting behavior of molten alloy on these metastable substrates would be different from that on the usual polycrystalline metals. In this study, the main goal is to investigate the wetting behaviors of molten Sn-3.5Ag-0.7Cu on nano Cu and Ni and their interfacial microstructures, and then to compare with those on micron crystal substrates. Besides, the wetting behaviors of molten Sn on Ni-Si-B amorphous substrates and corresponding nano-crystalline substrates obtained by annealing treatment were also investigated. This study is important to reveal the difference in the wetting behaviors between amorphous materials and usual large crystalline materials. It also provides guidance for joining of metallic glass and nano-materials.The main research results are as follows:1. The wettability and the spread ability of molten Sn-3.5Ag-0.7Cu on micron Cu and Ni substrates are better than that on nano-crystalline Cu and Ni substrates. The strong adsorption of the nano-materials surface may account for the larger initial contact angles. The diffusion-limited model fits the experimental data well and the wetting behavior is controlled by the diffusive supply of Sn from the drop bulk to the triple line across the reaction layer.2. As the atomic activity and diffusion rate of the nano-materials are higher than those of the micro-materials, the interface structures of solder/nano Cu and solder/nano Ni show their particularities: (1) Kirkendall cavity can be found at the interface of the Sn-3.5Ag-0.7Cu/nano Cu system while no cavity appears at the interface of the Sn-3.5Ag-0.7Cu/micron Cu system. (2) There is a clear discontinuous phase distribution at the interface of the Sn-3.5Ag-0.7Cu/nano Ni system. The phase preferentially formed at the interface is (Ni_1-xCu_x)₃Sn₄, and then (Cu_1-yNi_y)₆Sn₅ phase grows attaching to the (Ni_1-xCu_x)₃Sn₄ phase. However, it would break away from (Ni_1-xCu_x)₃Sn₄ and enter the molten solder. The phase formed at center of the interface is (Cu_1-yNi_y)₆Sn₅ when the temperature is 521K.3. Annealing treatment of amorphous Ni₇₅Si₁₅B₁₀ at different temperatures for 10 minutes has significant influence on the wetting behavior of Sn/Ni₇₅Si₁₅B₁₀ system. The change of atomic activity and surface structure are key points. The size of Ni-Sn phase and the thickness of reaction layer firstly increase and then decrease with increasing temperature. The increasing diffusivity of the reactant element due to the structural relaxation and crystallization at elevated temperatures is the key point. However, the diffusion ability would decrease with the further growth of the grains.4. When the substrates keep amorphous (T=525-625K), both the initial contact angles and the equilibrium contact angles increase with increasing temperature, the Ni-Sn phases grow larger and the reaction layer turns thicker, consisting with the behavior of the contact angles. However, the faster diffusion of Sn does not contribute to the wettability.Instead, the decrease inγ_SV and the increase inγ_SL may have more significant negative effect on it.